Scientific Publications

The goal of this work was to investigate the role of t-tubule (TT) remodeling in abnormal Ca cycling in ventricular myocytes of failing dog hearts. Heart failure (HF) was induced using rapid right ventricular pacing. Extensive changes in echocardiographic parameters, including left and right ventricular dilation and systolic dysfunction, diastolic dysfunction, elevated left ventricular filling pressures, and abnormal cardiac mechanics, indicated that severe HF developed. TT loss was extensive when measured as the density of total cell volume, derived from three-dimensional confocal image analysis, and significantly increased the distances in the cell interior to closest cell membrane. Changes in Ca transients indicated increases in heterogeneity of Ca release along the cell length. When critical properties of Ca release variability were plotted as a function of TT organization, there was a complex, nonlinear relationship between impaired calcium release and decreasing TT organization below a certain threshold of TT organization leading to increased sensitivity in Ca release below a TT density threshold of 1.5%. The loss of TTs was also associated with a greater incidence of triggered Ca waves during rapid pacing. Finally, virtually all of these observations were replicated by acute detubulation by formamide treatment, indicating an important role of TT remodeling in impaired Ca cycling. We conclude that TT remodeling itself is a major contributor to abnormal Ca cycling in HF, reducing myocardial performance. The loss of TTs is also responsible for a greater incidence of triggered Ca waves that may play a role in ventricular arrhythmias arising in HF. Three-dimensional analysis of t-tubule density showed t-tubule disruption throughout the whole myocyte in failing dog ventricle. A double-linear relationship between Ca release and t-tubule density displays a steeper slope at t-tubule densities below a threshold value (∼1.5%) above which there is little effect on Ca release (T-tubule reserve). T-tubule loss increases incidence of triggered Ca waves. Chemically induced t-tubule disruption suggests that t-tubule loss alone is a critical component of abnormal Ca cycling in heart failure.

Up to 50% of patients with proximal deep vein thrombosis (DVT) will develop the postthrombotic syndrome characterized by limb swelling and discomfort, hyperpigmentation, skin ulcers, and impaired quality of life. Although catheter-based interventions enabling the restoration of blood flow (RBF) have demonstrated little benefit on postthrombotic syndrome, the impact on the acuity of the thrombus and mechanisms underlying this finding remain obscure. In experimental and clinical studies, we examined whether RBF has a restricted time window for improving DVT resolution.

Angiotensin-converting enzyme 2 (ACE2) and accessory proteases (TMPRSS2 and CTSL) are needed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cellular entry, and their expression may shed light on viral tropism and impact across the body. We assessed the cell-type-specific expression of ACE2, TMPRSS2 and CTSL across 107 single-cell RNA-sequencing studies from different tissues. ACE2, TMPRSS2 and CTSL are coexpressed in specific subsets of respiratory epithelial cells in the nasal passages, airways and alveoli, and in cells from other organs associated with coronavirus disease 2019 (COVID-19) transmission or pathology. We performed a meta-analysis of 31 lung single-cell RNA-sequencing studies with 1,320,896 cells from 377 nasal, airway and lung parenchyma samples from 228 individuals. This revealed cell-type-specific associations of age, sex and smoking with expression levels of ACE2, TMPRSS2 and CTSL. Expression of entry factors increased with age and in males, including in airway secretory cells and alveolar type 2 cells. Expression programs shared by ACE2TMPRSS2 cells in nasal, lung and gut tissues included genes that may mediate viral entry, key immune functions and epithelial-macrophage cross-talk, such as genes involved in the interleukin-6, interleukin-1, tumor necrosis factor and complement pathways. Cell-type-specific expression patterns may contribute to the pathogenesis of COVID-19, and our work highlights putative molecular pathways for therapeutic intervention.

The postthrombotic syndrome is a common, often morbid sequela of venous thrombosis (VT) that arises from thrombus persistence and inflammatory scarring of juxtaposed vein walls and valves. Noninvasive F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) imaging can measure neutrophil inflammation in VT. Here, we hypothesized (1) early fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) VT inflammation can predict subsequent vein wall scarring (VWS) and (2) statin therapy can reduce FDG-PET VT inflammation and subsequent VWS.

Cardiovascular disease (CVD) and its sequelae have long been the leading causes of death and disability in the developed world. Although mortality associated with CVD has been decreasing, due in large part to novel therapeutic options, the rate of decrease has flattened. Thus, there is a great need to investigate alternate therapeutic strategies that can increase efficacy while decreasing adverse effects. Nanomaterials have been widely investigated and have emerged as promising tools for both therapeutic and diagnostic purposes in oncology; however, the potential of nanomaterials has not been extensively explored for cardiovascular medicine. In this review, we focus on recent developments in the field of nanomedicines targeted for CVDs, with a special emphasis on cell membrane-coated nanoparticles (NPs) and their applications.

Gold nanoparticles (AuNPs) have garnered much attention as contrast agents for computerized tomography (CT) because of their facile synthesis and surface functionalization, in addition to their significant X-ray attenuation and minimal cytotoxicity. Cell labeling using AuNPs and tracking of the labeled cells using CT has become a time-efficient and cost-effective method. Actively targeted AuNPs can enhance CT contrast and sensitivity, and further reduce the radiation dosage needed during CT imaging. In this review, we summarize the state-of-the-art use of AuNPs in CT for cell tracking, including the precautionary steps necessary for their use and the difficulty in translating the process into clinical use.

Atrial fibrillation (AF) often arises from structural abnormalities in the left atria (LA). Annotation of the noncoding genome in human LA is limited, as are effects on gene expression and chromatin architecture. Many AF-associated genetic variants reside in noncoding regions; this knowledge gap impairs efforts to understand the molecular mechanisms of AF and cardiac conduction phenotypes.

Neurological disorders including depression, anxiety, post-traumatic stress disorder (PTSD), schizophrenia, autism and epilepsy are associated with an increased incidence of cardiovascular disorders and susceptibility to heart failure. The underlying molecular mechanisms that link neurological disorders and adverse cardiac function are poorly understood. Further, a lack of progress is likely due to a paucity of studies that investigate the relationship between neurological disorders and cardiac electrical activity in health and disease. Therefore, there is an important need to understand the spatiotemporal behavior of neurocardiac mechanisms. This can be advanced through the identification and validation of neurological and cardiac signaling pathways that may be adversely regulated. In this review we highlight how dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, autonomic nervous system (ANS) activity and inflammation, predispose to psychiatric disorders and cardiac dysfunction. Moreover, antipsychotic and antidepressant medications increase the risk for adverse cardiac events, mostly through the block of the human ether-a-go-go-related gene (hERG), which plays a critical role in cardiac repolarization. Therefore, understanding how neurological disorders lead to adverse cardiac ion channel remodeling is likely to have significant implications for the development of effective therapeutic interventions and helps improve the rational development of targeted therapeutics with significant clinical implications.

Smart nanocarriers obtained from bacteria and viruses offer excellent biomimetic properties which has led to significant research into the creation of advanced biomimetic materials. Their versatile biomimicry has application as biosensors, biomedical scaffolds, immobilization, diagnostics, and targeted or personalized treatments. The inherent natural traits of biomimetic and bioinspired bacteria- and virus-derived nanovesicles show potential for their use in clinical vaccines and novel therapeutic drug delivery systems. The past few decades have seen significant progress in the bioengineering of bacteria and viruses to manipulate and enhance their therapeutic benefits. From a pharmaceutical perspective, biomimetics enable the safe integration of naturally occurring bacteria and virus particles to achieve high, stable rates of cellular transfection/infection and prolonged circulation times. In addition, biomimetic technologies can overcome safety concerns associated with live-attenuated and inactivated whole bacteria or viruses. In this review, we provide an update on the utilization of bacterial and viral particles as drug delivery systems, theranostic carriers, and vaccine/immunomodulation modalities.

Brown adipose tissue (BAT) is the primary non-shivering thermogenesis organ in mammals, which plays essential roles in maintaining the body temperature of infants. Although the development of BAT during embryogenesis has been well addressed in rodents, how BAT grows after birth remains unknown. Using mouse interscapular BAT (iBAT) as an example, we studied the cellular and molecular mechanisms that regulate postnatal BAT growth. By analyzing the developmental dynamics of brown adipocytes (BAs), we found that BAs size enlargement partially accounts for iBAT growth. By investigating the BAs cell cycle activities, we confirmed the presence of proliferative BAs in the neonatal mice. Two weeks after birth, most of the BAs exit cell cycle, and the further expansion of the BAT was mainly due to lipogenesis-mediated BAs volume increase. Microscopy and fluorescence-activated cell sorting analyses suggest that most BAs are mononuclear and diploid. Based on the developmental dynamics of brown adipocytes, we propose that the murine iBAT has two different growth phases between birth and weaning: increase of BAs size and number in the first two weeks, and BAs size enlargement thereafter. In summary, our data demonstrate that both lipogenesis and proliferation of BAs contribute to postnatal iBAT growth in mice.

Cardiovascular disease (CVD) is the most common co-morbidity associated with COVID-19 and the fatality rate in COVID-19 patients with CVD is higher compared to other comorbidities, such as hypertension and diabetes. Preliminary data suggest that COVID-19 may also cause or worsen cardiac injury in infected patients through multiple mechanisms such as 'cytokine storm', endotheliosis, thrombosis, lymphocytopenia etc. Autopsies of COVID-19 patients reveal an infiltration of inflammatory mononuclear cells in the myocardium, confirming the role of the immune system in mediating cardiovascular damage in response to COVID-19 infection and also suggesting potential causal mechanisms for the development of new cardiac pathologies and/or exacerbation of underlying CVDs in infected patients. In this review, we discuss the potential underlying molecular mechanisms that drive COVID-19-mediated cardiac damage, as well as the short term and expected long-term cardiovascular ramifications of COVID-19 infection in patients.

It has been reported that Cavin1 deficiency causes lipodystrophy in both humans and mice by affecting lipid metabolism. The ablation of Cavin1 in rodents also causes a significant deviation from Mendelian ratio at weaning in a background-dependent manner, suggesting the presence of undiscovered functions of Cavin1. In the current study, the results show that Cavin1 deficiency causes neonatal death in C57BL/6J mice by dampening the storage and mobilization of glycogen in the liver, which leads to lethal neonatal hypoglycemia. Further investigation by electron microscopy reveals that Cavin1 deficiency impairs the fenestration in liver sinusoidal endothelial cells (LSECs) and impacts the permeability of endothelial barrier in the liver. Mechanistically, Cavin1 deficiency inhibits the RhoA-Rho-associated protein kinase 2-LIM domain kinase-Cofilin signaling pathway and suppresses the dynamics of the cytoskeleton, and eventually causes the reduction of fenestrae in LSECs. In addition, the defect of fenestration in LSECs caused by Cavin1 deficiency can be rescued by treatment with the F-actin depolymerization reagent latrunculin A. In summary, the current study reveals a novel function of Cavin1 on fenestrae formation in LSECs and liver glycogen metabolism, which provide an explanation for the neonatal death of Cavin1 null mice and a potential mechanism for metabolic disorders in patients with Cavin1 mutation.

Intercalated discs (ICD), specific cell-to-cell contacts that connect adjacent cardiomyocytes, ensure mechanical and electrochemical coupling during contraction of the heart. Mutations in genes encoding ICD components are linked to cardiovascular diseases. Here, we show that loss of Xinβ, a newly-identified component of ICDs, results in cardiomyocyte proliferation defects and cardiomyopathy. We uncovered a role for Xinβ in signaling via the Hippo-YAP pathway by recruiting NF2 to the ICD to modulate cardiac function. In Xinβ mutant hearts levels of phosphorylated NF2 are substantially reduced, suggesting an impairment of Hippo-YAP signaling. Cardiac-specific overexpression of YAP rescues cardiac defects in Xinβ knock-out mice-indicating a functional and genetic interaction between Xinβ and YAP. Our study reveals a molecular mechanism by which cardiac-expressed intercalated disc protein Xinβ modulates Hippo-YAP signaling to control heart development and cardiac function in a tissue specific manner. Consequently, this pathway may represent a therapeutic target for the treatment of cardiovascular diseases.

Atrial fibrillation (AF) is the most common heart rhythm disorder in adults and a major cause of stroke. Unfortunately, current treatments of AF are suboptimal because they are not targeted to the molecular mechanisms underlying AF. Using a highly novel gene therapy approach in a canine, rapid atrial pacing model of AF, we demonstrate that NADPH oxidase 2 (NOX2) generated oxidative injury causes upregulation of a constitutively active form of acetylcholine-dependent K current (), called ; this is an important mechanism underlying not only the genesis, but also the perpetuation of electric remodeling in the intact, fibrillating atrium.

The mechanisms underlying atrial-selective prolongation of effective refractory period (ERP) and suppression of atrial fibrillation (AF) by NS8593 and UCL1684, small conductance calcium-activated potassium (SK) channel blockers, are poorly defined. The purpose of the study was to confirm the effectiveness of these agents to suppress AF and to probe the underlying mechanisms. Transmembrane action potentials and pseudoelectrocardiograms were recorded from canine isolated coronary-perfused canine atrial and ventricular wedge preparations. Patch clamp techniques were used to record sodium channel current (INa) in atrial and ventricular myocytes and human embryonic kidney cells. In both atria and ventricles, NS8593 (3-10 µM) and UCL1684 (0.5 µM) did not significantly alter action potential duration, suggesting little to no SK channel inhibition. Both agents caused atrial-selective: (1) prolongation of ERP secondary to development of postrepolarization refractoriness, (2) reduction of Vmax, and (3) increase of diastolic threshold of excitation (all are sodium-mediated parameters). NS8593 and UCL1684 significantly reduced INa density in human embryonic kidney cells as well as in atrial but not in ventricular myocytes at physiologically relevant holding potentials. NS8593 caused a shift of steady-state inactivation to negative potentials in atrial but not ventricular cells. NS8593 and UCL1684 prevented induction of acetylcholine-mediated AF in 6/6 and 8/8 preparations, respectively. This anti-AF effect was associated with strong rate-dependent depression of excitability. The SK channel blockers, NS8593 and UCL1684, are effective in preventing the development of AF due to potent atrial-selective inhibition of INa, causing atrial-selective prolongation of ERP secondary to induction of postrepolarization refractoriness.

The human heart requires a complex ensemble of specialized cell types to perform its essential function. A greater knowledge of the intricate cellular milieu of the heart is critical to increase our understanding of cardiac homeostasis and pathology. As recent advances in low-input RNA sequencing have allowed definitions of cellular transcriptomes at single-cell resolution at scale, we have applied these approaches to assess the cellular and transcriptional diversity of the nonfailing human heart.

Long QT syndrome (LQTS) is a rare genetic disorder and a major preventable cause of sudden cardiac death in the young. A causal rare genetic variant with large effect size is identified in up to 80% of probands (genotype positive) and cascade family screening shows incomplete penetrance of genetic variants. Furthermore, a proportion of cases meeting diagnostic criteria for LQTS remain genetically elusive despite genetic testing of established genes (genotype negative). These observations raise the possibility that common genetic variants with small effect size contribute to the clinical picture of LQTS. This study aimed to characterize and quantify the contribution of common genetic variation to LQTS disease susceptibility.

We have identified a novel form of abnormal Ca wave activity in normal and failing dog atrial myocytes which occurs during the action potential (AP) and is absent during diastole. The goal of this study was to determine if triggered Ca waves affect cellular electrophysiological properties.

Synapse-associated protein 97 (SAP97) is a scaffolding protein crucial for the functional expression of several cardiac ion channels and therefore proper cardiac excitability. Alterations in the functional expression of SAP97 can modify the ionic currents underlying the cardiac action potential and consequently confer susceptibility for arrhythmogenesis. In this study, we generated a murine model for inducible, cardiac-targeted Sap97 ablation to investigate arrhythmia susceptibility and the underlying molecular mechanisms. Furthermore, we sought to identify human SAP97 () variants that were associated with inherited arrhythmogenic disease. The murine model of cardiac-specific Sap97 ablation demonstrated several ECG abnormalities, pronounced action potential prolongation subject to high incidence of arrhythmogenic afterdepolarizations and notable alterations in the activity of the main cardiac ion channels. However, no mutations were found in 40 unrelated cases of genetically elusive long QT syndrome (LQTS). Instead, we provide the first evidence implicating a gain of function in human mutation resulting in an increase in Kv4.3 current () as a novel, potentially pathogenic substrate for Brugada syndrome (BrS). In conclusion, joins a growing list of genes encoding ion channel interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. Dysfunction in these critical components of cardiac excitability can potentially result in fatal cardiac disease. The gene encoding SAP97 () joins a growing list of genes encoding ion channel-interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. In this study we provide the first data supporting encoded SAP97's candidacy as a minor Brugada syndrome susceptibility gene.

Genome-wide association studies have uncovered over a 100 genetic loci associated with atrial fibrillation (AF), the most common arrhythmia. Many of the top AF-associated loci harbor key cardiac transcription factors, including PITX2, TBX5, PRRX1, and ZFHX3. Moreover, the vast majority of the AF-associated variants lie within noncoding regions of the genome where causal variants affect gene expression by altering the activity of transcription factors and the epigenetic state of chromatin. In this review, we discuss a transcriptional regulatory network model for AF defined by effector genes in Genome-wide association studies loci. We describe the current state of the field regarding the identification and function of AF-relevant gene regulatory networks, including variant regulatory elements, dose-sensitive transcription factor functionality, target genes, and epigenetic states. We illustrate how altered transcriptional networks may impact cardiomyocyte function and ionic currents that impact AF risk. Last, we identify the need for improved tools to identify and functionally test transcriptional components to define the links between genetic variation, epigenetic gene regulation, and atrial function.

() is a highly pathogenic facultative anaerobe that in some instances resides as an intracellular bacterium within macrophages and cancer cells. This pathogen can establish secondary infection foci, resulting in recurrent systemic infections that are difficult to treat using systemic antibiotics. Here, we use reconstructed apoptotic bodies (ReApoBds) derived from cancer cells as "nano decoys" to deliver vancomycin intracellularly to kill by targeting inherent "eat me" signaling of ApoBds. We prepared ReApoBds from different cancer cells (SKBR3, MDA-MB-231, HepG2, U87-MG, and LN229) and used them for vancomycin delivery. Physicochemical characterization showed ReApoBds size ranges from 80 to 150 nm and vancomycin encapsulation efficiency of 60 ± 2.56%. We demonstrate that the loaded vancomycin was able to kill intracellular efficiently in an model of infected RAW-264.7 macrophage cells, and U87-MG (p53-wt) and LN229 (p53-mt) cancer cells, compared to free-vancomycin treatment ( < 0.001). The vancomycin loaded ReApoBds treatment in infected macrophages showed a two-log-order higher CFU reduction than the free-vancomycin treatment group. studies revealed that ReApoBds can specifically target macrophages and cancer cells. Vancomycin loaded ReApoBds have the potential to kill intracellular infection in macrophages and cancer cells.

Heart failure remains a major cause of hospitalization and death worldwide. Heart failure can be caused by abnormalities in the epigenome resulting from dysregulation of histone-modifying enzymes. While chromatin enzymes catalyzing lysine acetylation and methylation of histones have been the topic of many investigations, the role of arginine methyltransferases has been overlooked. In an effort to understand regulatory mechanisms implicated in cardiac hypertrophy and heart failure, we assessed the expression of protein arginine methyltransferases (PRMTs) in the left ventricle of failing human hearts and control hearts. Our results show a significant up-regulation of protein arginine methyltransferase 6 (PRMT6) in failing human hearts compared to control hearts, which also occurs in the early phase of cardiac hypertrophy in mouse hearts subjected to pressure overload hypertrophy induced by trans-aortic constriction (TAC), and in neonatal rat ventricular myocytes (NRVM) stimulated with the hypertrophic agonist phenylephrine (PE). These changes are associated with a significant increase in arginine 2 asymmetric methylation of histone H3 (H3R2Me2a) and reduced lysine 4 tri-methylation of H3 (H3K4Me3) observed both in NRVM and . Importantly, forced expression of PRMT6 in NRVM enhances the expression of the hypertrophic marker, atrial natriuretic peptide (ANP). Conversely, specific silencing of PRMT6 reduces ANP protein expression and cell size, indicating that PRMT6 is critical for the PE-mediated hypertrophic response. Silencing of PRMT6 reduces H3R2Me2a, a mark normally associated with transcriptional repression. Furthermore, evaluation of cardiac contractility and global ion channel activity in live NRVM shows a striking reduction of spontaneous beating rates and prolongation of extra-cellular field potentials in cells expressing low-level PRMT6. Altogether, our results indicate that PRMT6 is a critical regulator of cardiac hypertrophy, implicating H3R2Me2a as an important histone modification. This study identifies PRMT6 as a new epigenetic regulator and suggests a new point of control in chromatin to inhibit pathological cardiac remodeling.

Hand hygiene is of utmost importance as it may be contaminated easily from direct contact with airborne microorganism droplets from coughs and sneezes. Particularly in situations like pandemic outbreak, it is crucial to interrupt the transmission chain of the virus by the practice of proper hand sanitization. It can be achieved with contact isolation and strict infection control tool like maintaining good hand hygiene in hospital settings and in public. The success of the hand sanitization solely depends on the use of effective hand disinfecting agents formulated in various types and forms such as antimicrobial soaps, water-based or alcohol-based hand sanitizer, with the latter being widely used in hospital settings. To date, most of the effective hand sanitizer products are alcohol-based formulations containing 62%-95% of alcohol as it can denature the proteins of microbes and the ability to inactivate viruses. This systematic review correlated with the data available in Pubmed, and it will investigate the range of available hand sanitizers and their effectiveness as well as the formulation aspects, adverse effects, and recommendations to enhance the formulation efficiency and safety. Further, this article highlights the efficacy of alcohol-based hand sanitizer against the coronavirus.

The RASopathies are a group of genetic disorders that result from germline pathogenic variants affecting RAS-mitogen activated protein kinase (MAPK) pathway genes. RASopathies share RAS/MAPK pathway dysregulation and share phenotypic manifestations affecting numerous organ systems, causing lifelong and at times life-limiting medical complications. RASopathies may benefit from precision medicine approaches. For this reason, the Sixth International RASopathies Symposium focused on exploring precision medicine. This meeting brought together basic science researchers, clinicians, clinician scientists, patient advocates, and representatives from pharmaceutical companies and the National Institutes of Health. Novel RASopathy genes, variants, and animal models were discussed in the context of medication trials and drug development. Attempts to define and measure meaningful endpoints for treatment trials were discussed, as was drug availability to patients after trial completion.

Developmental and/or epileptic encephalopathies (DEEs) are a group of devastating genetic disorders, resulting in early-onset, therapy-resistant seizures and developmental delay. Here we report on 22 individuals from 15 families presenting with a severe form of intractable epilepsy, severe developmental delay, progressive microcephaly, visual disturbance and similar minor dysmorphisms. Whole exome sequencing identified a recurrent, homozygous variant (chr2:64083454A > G) in the essential UDP-glucose pyrophosphorylase (UGP2) gene in all probands. This rare variant results in a tolerable Met12Val missense change of the longer UGP2 protein isoform but causes a disruption of the start codon of the shorter isoform, which is predominant in brain. We show that the absence of the shorter isoform leads to a reduction of functional UGP2 enzyme in neural stem cells, leading to altered glycogen metabolism, upregulated unfolded protein response and premature neuronal differentiation, as modeled during pluripotent stem cell differentiation in vitro. In contrast, the complete lack of all UGP2 isoforms leads to differentiation defects in multiple lineages in human cells. Reduced expression of Ugp2a/Ugp2b in vivo in zebrafish mimics visual disturbance and mutant animals show a behavioral phenotype. Our study identifies a recurrent start codon mutation in UGP2 as a cause of a novel autosomal recessive DEE syndrome. Importantly, it also shows that isoform-specific start-loss mutations causing expression loss of a tissue-relevant isoform of an essential protein can cause a genetic disease, even when an organism-wide protein absence is incompatible with life. We provide additional examples where a similar disease mechanism applies.

It is well known that heart failure (HF) typically coexists with atrial fibrillation (AF). However, until now, no clear mechanism has been established that relates HF to AF. In this study, we apply a multiscale computational framework to establish a mechanistic link between atrial myocyte structural remodeling in HF and AF. Using a spatially distributed model of calcium (Ca) signaling, we show that disruption of the spatial relationship between L-type Ca channels (LCCs) and ryanodine receptors results in markedly increased Ca content of the sarcoplasmic reticulum (SR). This increase in SR load is due to changes in the balance between Ca entry via LCCs and Ca extrusion due to the sodium-calcium exchanger after an altered spatial relationship between these signaling proteins. Next, we show that the increased SR load in atrial myocytes predisposes these cells to subcellular Ca waves that occur during the action potential (AP) and are triggered by LCC openings. These waves are common in atrial cells because of the absence of a well-developed t-tubule system in most of these cells. This distinct spatial architecture allows for the presence of a large pool of orphaned ryanodine receptors, which can fire and sustain Ca waves during the AP. Finally, we incorporate our atrial cell model in two-dimensional tissue simulations and demonstrate that triggered wave generation in cells leads to electrical waves in tissue that tend to fractionate to form wavelets of excitation. This fractionation is driven by the underlying stochasticity of subcellular Ca waves, which perturbs AP repolarization and consequently induces localized conduction block in tissue. We outline the mechanism for this effect and argue that it may explain the propensity for atrial arrhythmias in HF.

Myocardial recovery from ischemia-reperfusion (IR) is shaped by the interaction of many signaling pathways and tissue repair processes, including the innate immune response. We and others previously showed that sustained expression of the transcriptional co-activator yes-associated protein (YAP) improves survival and myocardial outcome after myocardial infarction. Here, we asked whether transient YAP expression would improve myocardial outcome after IR injury. After IR, we transiently activated YAP in the myocardium with modified mRNA encoding a constitutively active form of YAP (aYAP modRNA). Histological studies 2 d after IR showed that aYAP modRNA reduced cardiomyocyte (CM) necrosis and neutrophil infiltration. 4 wk after IR, aYAP modRNA-treated mice had better heart function as well as reduced scar size and hypertrophic remodeling. In cultured neonatal and adult CMs, YAP attenuated HO- or LPS-induced CM necrosis. TLR signaling pathway components important for innate immune responses were suppressed by YAP/TEAD1. In summary, our findings demonstrate that aYAP modRNA treatment reduces CM necrosis, cardiac inflammation, and hypertrophic remodeling after IR stress.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are used for many applications including safety pharmacology. However, a deficiency or complete absence of several K currents suggests repolarization reserve is low in hiPSC-CMs. We determined whether a dual I and I activator can improve repolarization reserve in hiPSC-CMs resulting in a more electrophysiologically mature phenotype.

Atrial fibrillation (AF) is the most common heart rhythm disorder and a major cause of stroke. Unfortunately, current therapies for AF are suboptimal, largely because the molecular mechanisms underlying AF are poorly understood. Since the autonomic nervous system is thought to increase vulnerability to AF, we used a rapid atrial pacing (RAP) canine model to investigate the anatomic and electrophysiological characteristics of autonomic remodeling in different regions of the left atrium. RAP led to marked hypertrophy of parent nerve bundles in the posterior left atrium (PLA), resulting in a global increase in parasympathetic and sympathetic innervation throughout the left atrium. Parasympathetic fibers were more heterogeneously distributed in the PLA when compared with other left atrial regions; this led to greater fractionation and disorganization of AF electrograms in the PLA. Computational modeling revealed that heterogeneously distributed parasympathetic activity exacerbates sympathetic substrate for wave break and reentry. We further discovered that levels of nerve growth factor (NGF) were greatest in the left atrial appendage (LAA), where AF was most organized. Preferential NGF release by the LAA - likely a direct function of frequency and regularity of atrial stimulation - may have important implications for creation of a vulnerable AF substrate.

More than 90% of individuals with Noonan syndrome (NS) with mutations clustered in the CR2 domain of RAF1 present with severe and often lethal hypertrophic cardiomyopathy (HCM). The signaling pathways by which NS RAF1 mutations promote HCM remain elusive, and so far, there is no known treatment for NS-associated HCM.

Obstructive sleep apnea (OSA) affects an estimated 20% of adults worldwide and has been associated with electrical and structural abnormalities of the atria, although the molecular mechanisms are not well understood. Here, we used two-dimensional polyacrylamide gel electrophoresis (2D PAGE) coupled with nanoliquid chromatography-tandem mass spectrometry (nanoLC-MS/MS) to investigate the proteins that are dysregulated in the atria from severe and moderate apnea when compared to control. We found enzymes involved in the glycolysis, beta-oxidation, electron transport chain and Krebs cycle to be down-regulated. The data suggested that the dysregulated proteins may play a role in atrial pathology developing via chronic obstructive apnea and hypoxia. Our results are consistent with our previous 1D-PAGE and nanoLC-MS/MS study (Channaveerappa et al, J Cell Mol Med. 2017), where we found that some aerobic and anaerobic glycolytic and Krebs cycle enzymes were down-regulated, suggesting that apnea may be a result of paucity of oxygen and production of ATP and reducing equivalents (NADH). The 2D-PAGE study not only complements our current study, but also advances our understanding of the OSA. The complete mass spectrometry data are available via ProteomeXchange with identifier PXD011181.

Heart valve disease is a major clinical problem worldwide. Cardiac valve development and homeostasis need to be precisely controlled. Hippo signaling is essential for organ development and tissue homeostasis, while its role in valve formation and morphology maintenance remains unknown. VGLL4 is a transcription cofactor in vertebrates and we found it was mainly expressed in valve interstitial cells at the post-EMT stage and was maintained till the adult stage. Tissue specific knockout of VGLL4 in different cell lineages revealed that only loss of VGLL4 in endothelial cell lineage led to valve malformation with expanded expression of YAP targets. We further semi-knockout YAP in VGLL4 ablated hearts, and found hyper proliferation of arterial valve interstitial cells was significantly constrained. These findings suggest that VGLL4 is important for valve development and manipulation of Hippo components would be a potential therapy for preventing the progression of congenital valve disease.

Extrasystoles arising from the muscular sleeves associated with the pulmonary veins (PV), superior vena cava (SVC), and coronary sinus (CS) are known to precipitate atrial fibrillation (AF). The late sodium channel current (I ) inhibitor ranolazine has been reported to exert antiarrhythmic effects in canine PV and SVC sleeves by suppressing late phase 3 early and delayed after depolarization (EAD and DAD)-induced triggered activity induced by parasympathetic and/or sympathetic stimulation. The current study was designed to extend our existing knowledge of the electrophysiological and pharmacologic properties of canine CS preparations and assess their response to inhibition of late I following autonomic stimulation.

Noonan syndrome with multiple lentigines (NSML), formerly known as LEOPARD Syndrome, is a rare autosomal dominant disorder. Approximately 90% of NSML cases are caused by missense mutations in the PTPN11 gene which encodes the protein tyrosine phosphatase SHP2. A human induced pluripotent stem cell (iPSC) line was generated using peripheral blood mononuclear cells (PBMCs) from a patient with NSML that carries a gene mutation of p.Q510P on the PTPN11 gene using non-integrating Sendai virus technique. This iPSC line offers a useful resource to study the disease pathophysiology and a cell-based model for drug development to treat NSML.

Early repolarization syndrome (ERS) is an inherited cardiac arrhythmia syndrome associated with sudden cardiac death. Approaches to therapy are currently very limited. This study probes the mechanisms underlying the electrocardiographic and arrhythmic manifestation of experimental models of ERS and of the ameliorative effect of radiofrequency ablation.

When an atrial cell is paced rapidly, calcium (Ca) waves can form on the cell boundary and propagate to the cell interior. These waves are referred to as "triggered waves" because they are initiated by Ca influx from the L-type Ca channel and occur during the action potential. However, the consequences of triggered waves in atrial tissue are not known. Here, we develop a phenomenological model of Ca cycling in atrial myocytes that accounts for the formation of triggered waves. Using this model, we show that a fundamental requirement for triggered waves to induce abnormal electrical activity in tissue is that these waves must be synchronized over large populations of cells. This is partly because triggered waves induce a long action potential duration (APD) followed by a short APD. Thus, if these events are not synchronized between cells, then they will on average cancel and have minimal effects on the APD in tissue. Using our computational model, we identify two distinct mechanisms for triggered wave synchronization. The first relies on cycle length (CL) variability, which can prolong the CL at a given beat. In cardiac tissue, we show that CL prolongation leads to a substantial amplification of APD because of the synchronization of triggered waves. A second synchronization mechanism applies in a parameter regime in which the cell exhibits stochastic alternans in which a triggered wave fires, on average, only every other beat. In this scenario, we identify a slow synchronization mechanism that relies on the bidirectional feedback between the APD in tissue and triggered wave initiation. On large cables, this synchronization mechanism leads to spatially discordant APD alternans with spatial variations on a scale of hundreds of cells. We argue that these spatial patterns can potentially serve as an arrhythmogenic substrate for the initiation of atrial fibrillation.

Brugada syndrome (BrS) is an inherited disease associated with ST elevation in the right precordial leads, polymorphic ventricular tachycardia (PVT), and sudden cardiac death in adults. Mutations in the cardiac sodium channel account for a large fraction of BrS cases. BrS manifests in the right ventricle (RV), which led us to examine the biophysical and molecular properties of sodium channel in myocytes isolated from the left (LV) and right ventricle. Patch clamp was used to record sodium current (I ) in single canine RV and LV epicardial (epi) and endocardial (endo) myocytes. Action potentials were recorded from multicellular preparations and single cells. mRNA and proteins were determined using quantitative RT-PCR and Western blot. Although LV wedge preparations were thicker than RV wedges, transmural ECG recordings showed no difference in the width of the QRS complex or transmural conduction time. Action potential characteristics showed RV epi and endo had a lower V compared with LV epi and endo cells. Peak I density was significantly lower in epi and endo RV cells compared with epi and endo LV cells. Recovery from inactivation of I in RV cells was slightly faster and half maximal steady-state inactivation was more positive. β2 and β4 mRNA was detected at very low levels in both ventricles, which was confirmed at the protein level. Our observations demonstrate that V and Na current are smaller in RV, presumably due to differential Na 1.5/β subunit expression. These results provide a potential mechanism for the right ventricular manifestation of BrS.

Brugada phenocopies (BrPs) are clinical entities that differ in etiology from true congenital Brugada syndrome but have identical electrocardiographic (ECG) patterns. Hyperkalemia is known to be one of the causes of BrP. The aim of this study was to determine the clinical characteristics and evolution of hyperkalemia-induced BrP. Data from 27 cases of hyperkalemia-induced BrP were collected from the International Registry at www.brugadaphenocopy.com. Data were extracted from publications. Of the 27 patients included in the analysis, 18 (67%) were male; mean age was 53 ± 15 years (range 31 to 89). Mean serum potassium concentration was 7.45 ± 0.89 mmol/L. Type-1 Brugada ECG pattern was observed in 21 cases (78%), whereas 6 cases (22%) showed a type-2 Brugada ECG pattern. The Brugada ECG pattern resolved once the hyperkalemia was corrected, with no arrhythmic events. Estimated time to resolution was 7 ± 3 hours. In 4 cases (16%), a concurrent metabolic abnormality was detected: 3 (11%) presented with acidosis, 2 (7%) with hyponatremia, 1 (4%) with hypocalcaemia, 1 (4%) with hyperphosphatemia, and 1 (4%) with hyperglycemia. In 7 cases (26%), provocative testing using sodium channel blockers was performed, and all failed to reproduce a BrS ECG pattern (BrP class A). Additionally, no sudden cardiac death or malignant ventricular arrhythmias were detected. Hyperkalemia was found a common cause of BrP in our International Registry. The Brugada ECG pattern appears to occur at high serum potassium concentrations (>6.5 mmol/L). The ECG normalizes within hours of correcting the electrolyte imbalance. Importantly, hyperkalemia-induced BrP has not been associated with sudden cardiac death or ventricular arrhythmia.

Typical Brugada ECG pattern is the keystone in the diagnosis of Brugada syndrome. However, the exact prevalence remains unclear, especially in Asia. The present study was designed to systematically evaluate the prevalence of spontaneous Brugada ECG pattern recorded at standard leads.

Abnormal intracellular Ca2+ cycling contributes to triggered activity and arrhythmias in the heart. We investigated the properties and underlying mechanisms for systolic triggered Ca2+ waves in left atria from normal and failing dog hearts.

Obstructive sleep apnoea (OSA) affects 9-24% of the adult population. OSA is associated with atrial disease, including atrial enlargement, fibrosis and arrhythmias. Despite the link between OSA and cardiac disease, the molecular changes in the heart which occur with OSA remain elusive. To study OSA-induced cardiac changes, we utilized a recently developed rat model which closely recapitulates the characteristics of OSA. Male Sprague Dawley rats, aged 50-70 days, received surgically implanted tracheal balloons which were inflated to cause transient airway obstructions. Rats were given 60 apnoeas per hour of either 13 sec. (moderate apnoea) or 23 sec. (severe apnoea), 8 hrs per day for 2 weeks. Controls received implants, but no inflations were made. Pulse oximetry measurements were taken at regular intervals, and post-apnoea ECGs were recorded. Rats had longer P wave durations and increased T wave amplitudes following chronic OSA. Proteomic analysis of the atrial tissue homogenates revealed that three of the nine enzymes in glycolysis, and two proteins related to oxidative phosphorylation, were down regulated in the severe apnoea group. Several sarcomeric and pro-hypertrophic proteins were also up regulated with OSA. Chronic OSA causes proteins changes in the atria which suggest impairment of energy metabolism and enhancement of hypertrophy.

The goal of this study was to assess the effects of N-methyl-d-aspartate (NMDA) receptors activation on heart rate variability (HRV) and susceptibility to atrial fibrillation (AF).

This study sought to evaluate the phenotypic and functional expression of an apparent hotspot mutation associated with short QT syndrome (SQTS).

Pharmacological therapy for congestive heart failure (CHF) with ventricular arrhythmia is limited. In the study, our aim was to evaluate the effects of Chinese traditional medicine Shensong Yangxin capsules (SSYX) on heart rhythm and function in CHF patients with frequent ventricular premature complexes (VPCs).

Brugada syndrome (BrS) is an inherited primary arrhythmia disorder leading to sudden cardiac arrest. , encoding the α-subunit of the cardiac sodium channel (Nav1.5), is the most common pathogenic gene of BrS. An implantable cardioverter defibrillator (ICD) is the standard treatment for secondary prevention. This study aimed to evaluate association of the variant with this cardiac conduction disturbance and appropriate ICD shock therapy in Thai symptomatic BrS patients with ICD implants.

The collar of the pulmonary vein (PV) is the focal point for the initiation of atrial arrhythmias, but the mechanisms underlying how PV cells differ from neighboring left atrial tissue are unclear. We examined the biophysical and molecular properties of I in cells isolated from the canine pulmonary sleeve and compared the properties to left atrial tissue. PV and left atrial myocytes were isolated and patch clamp techniques were used to record I. Action potential recordings from either tissue type were made using high-resistance electrodes. mRNA was determined using quantitative RT-PCR and proteins were determined by Western blot. Analysis of the action potential characteristics showed that PV tissue had a lower Vmax compared with left atrial tissue. Fast I showed that current density was slightly lower in PV cells compared with LA cells (-96 ± 18.7 pA/pF vs. -120 ± 6.7 pA/pF, respectively, p < 0.05). The recovery from inactivation of I in PV cells was slightly slower but no marked difference in steady-state inactivation was noted. Analysis of late I during a 225-ms pulse showed that late I was significantly smaller in PV cells compared to LA cells at all measured time points into the pulse. These results suggest PV cells have lower density of both peak and late I. Molecular analysis of Nav1.5 and the four beta subunits showed lower levels of Nav1.5 as well as Navβ1 subunits, confirming the biophysical findings. These data show that a lower density of I may lead to depression of excitability and predispose the PV collar to re-entrant circuits under pathophysiological conditions.

This study sought to test the hypothesis that elimination of sites of abnormal repolarization, via epicardial RFA, suppresses the electrocardiographic and arrhythmic manifestations of BrS.

Hypertrophic cardiomyopathy (HCM) patients with early repolarization (ER) pattern are at higher risk of ventricular arrhythmia, yet the genetic background of this situation has not been well investigated. Here we report novel trigenic mutations detected in a Chinese family of obstructive HCM with ER and short QT syndrome (SQTS).

Left stellate ganglion (LSG) hyperactivity promotes ischemia induced ventricular arrhythmia (VA). Blocking the Nav1.8 channel decreases neuron activity. Therefore, the present study aimed to investigate whether blocking the Nav1.8 channel with its specific blocker A-803467 in the LSG reduces sympathetic activity and exerts anti-arrhythmic effects. Forty canines were divided into dimethylsulfoxide (DMSO) group and 10 mM, 15 mM, and 20 mM A-803467 groups. A volume of 0.1 ml of A-803467 or DMSO was injected into the LSG. The ventricular electrophysiological parameters, LSG function were measured before and 30 min after the injection. VA was assessed for 60 min after ischemia and then LSG tissues were collected for molecular biological experiments. Compared with DMSO, concentration-dependent prolonged action potential duration and effective refractory period, decreased LSG function were identified after A-803467 treatment. Moreover, the severity of ischemia induced VA was decreased in A-803467 groups. Furthermore, decreased nerve growth factor, decreased c-fos and increased sympathetic neuron apoptosis were found in the LSG after A-803467 injection. In conclusion, blocking the Nav1.8 channel could significantly attenuate ischemia-induced VA, primarily by suppressing LSG activity.

The E23K variant of the Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel has been implicated in cardiac remodeling. However, the effects of E23K variant on ventricular electrophysiology and arrhythmogenesis remain unclear.

Perturbations in the CACNA1C-encoded L-type calcium channel α-subunit have been linked recently to heritable arrhythmia syndromes, including Timothy syndrome, Brugada syndrome, early repolarization syndrome, and long QT syndrome. These heritable arrhythmia syndromes may serve as a pathogenic basis for autopsy-negative sudden unexplained death in the young (SUDY). However, the contribution of CACNA1C mutations to SUDY is unknown.

Catestatin (CST), which is emerging as a novel cardiac modulator, can protect the heart against excessive sympathetic drive in hypertensive cardiomyopathy. The aim of this study is to investigate whether exogenous CST decreases excessive cardiac sympathetic drive and improves autonomic function and exerts cardioprotective effects in myocardial infarction (MI) rats. Rats were divided into a sham group, MI group, and MI plus CST (MI + CST) group. Four weeks later, the autonomic function of the animals was assessed by analyzing heart rate variability (HRV) and measuring plasma catecholamine. Cardiac function was evaluated via echocardiography. Electrophysiological characteristics were assessed in Langendorff-perfused hearts. Compared to the MI group, the chronic administration of CST significantly increased the standard deviation of normal-normal intervals (P < .01) and low-frequency (LF) and high-frequency (HF) HRV and decreased the ratio of LF-HF HRV (P < .01 for all). Additionally, the level of plasma catecholamine was reduced in the MI + CST group compared to the MI group (P < .01). Treatment with CST significantly increased ejection fraction (EF) and fraction shorting (FS) and significantly decreased the left ventricular end-systolic diameter and left ventricular end-diastolic diameter at 28 days postmyocardial infraction (P < .05 for all). After MI, the ventricular repolarization duration, such as QTc intervals and action potential duration (APD) at 90% repolarization, was prolonged, and this prolongation could be decreased by CST (P < .05 for all). The CST also increased the threshold of ADP alternans (P < .01). Moreover, ventricular arrhythmias were induced in 83% of the MI group but only 33% of the MI + CST group (P < .05). These results suggested that the chronic administration of CST plays a role in cardioprotection in MI rats, which may function by decreasing the cardiac sympathetic drive and improving autonomic function.

Low-level carotid baroreflex stimulation (LL-CBS) appears to have a potential antiarrhythmogenic effect.

T-box 18 (TBX18) plays a crucial role in the formation and development of the head of the sinoatrial node. The objective of this study was to induce adipose-derived stem cells (ADSCs) to produce pacemaker-like cells by transfection with the TBX18 gene. A recombinant adenovirus vector carrying the human TBX18 gene was constructed to transfect ADSCs. The ADSCs transfected with TBX18 were considered the TBX18-ADSCs. The control group was the GFP-ADSCs. The transfected cells were co-cultured with neonatal rat ventricular cardiomyocytes (NRVMs). The results showed that the mRNA expression of TBX18 in TBX18-ADSCs was significantly higher than in the control group after 48 h and 7 days. After 7 days of co-culturing with NRVMs, there was no significant difference in the expression of the myocardial marker cardiac troponin I (cTnI) between the two groups. RT-qPCR and western blot analysis showed that the expression of HCN4 was higher in the TBX18-ADSCs than in the GFP-ADSCs. The If current was detected using the whole cell patch clamp technique and was blocked by the specific blocker CsCl. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSCMs) showed approximately twice the current density compared with the ADSCs. Our study indicated that the TBX18 gene induces ADSCs to differentiate into pacemaker‑like cells in the cardiac microenvironment. Although further experiments are required in order to assess safety and efficacy prior to implementation in clinical practice, this technique may provide new avenues for the clinical therapy of bradycardia.

Ganglionated plexus have been developed as additional ablation targets to improve the outcome of atrial fibrillation (AF) besides pulmonary vein isolation. Recent studies implicated an intimate relationship between neuronal sodium channel Na1.8 (encoded by SCN10A) and AF. The underlying mechanism between Na1.8 and AF remains unclear. This study aimed to determine the role of Na1.8 in cardiac electrophysiology in an acute AF model and explore possible therapeutic targets.

The basic defect in long-QT syndrome type III (LQT3) is an excessive inflow of sodium current during phase 3 of the action potential caused by mutations in the SCN5A gene. Most sodium channel blockers reduce the early (peak) and late components of the sodium current (I and I), but ranolazine preferentially reduces I. We, therefore, evaluated the effects of ranolazine in LQT3 caused by the D1790G mutation in SCN5A.

CC chemokine receptor 9 (CCR9), which is a unique receptor for CC chemokine ligand (CCL25), is mainly expressed on lymphocytes, dendritic cells (DCs) and monocytes/macrophages. CCR9 mediates the chemotaxis of inflammatory cells and participates in the pathological progression of inflammatory diseases. However, the role of CCR9 in the pathological process of myocardial infarction (MI) remains unexplored; inflammation plays a key role in this process. Here, we used CCR9 knockout mice to determine the functional significance of CCR9 in regulating post-MI cardiac remodeling and its underlying mechanism. MI was induced by surgical ligation of the left anterior descending coronary artery in CCR9 knockout mice and their CCR9+/+ littermates. Our results showed that the CCR9 expression levels were up-regulated in the hearts of the MI mice. Abrogation of CCR9 improved the post-MI survival rate and left ventricular (LV) dysfunction and decreased the infarct size. In addition, the CCR9 knockout mice exhibited attenuated inflammation, apoptosis, structural and electrical remodeling compared with the CCR9+/+ MI mice. Mechanistically, CCR9 mainly regulated the pathological response by interfering with the NF-κB and MAPK signaling pathways. In conclusion, the data reveal that CCR9 serves as a novel modulator of pathological progression following MI through NF-κB and MAPK signaling.

Phenotypic overlap of type 3 long QT syndrome (LQT3), Brugada syndrome (BrS), cardiac conduction disease (CCD), and sinus node dysfunction (SND) is observed with SCN5A mutations. SCN5A-E1784K is the most common mutation associated with BrS and LQTS3. The present study examines the genotype-phenotype relationship in a large family carrying SCN5A-E1784K and SCN5A-H558R polymorphism.

In Brugada syndrome (BrS), spontaneous type 1 electrocardiogram (ECG) is an established risk marker for fatal arrhythmias whereas drug-induced type 1 ECG shows a relatively benign prognosis. No study has analyzed the prognosis of fever-induced type 1 ECG (F-type1) in a large BrS cohort.

Early repolarization syndrome (ERS) is associated with polymorphic ventricular tachycardia (PVT) and ventricular fibrillation, leading to sudden cardiac death.

Brugada syndrome (BrS) is a rare heritable ventricular arrhythmia. Genetic defects in SCN5A, a gene that encodes the α-subunit of the sodium ion channel Nav1.5, are present in 15-30% of BrS cases. SCN5A remains by far, the highest yielding gene for BrS. We studied a young male who presented with syncope at age 11. This proband was screened for possible disease causing SCN5A mutations. The inheritance pattern was also examined amongst his first-degree family members.

A loss of repolarization reserve due to downregulation of K(+) currents has been observed in cultured ventricular myocytes. A similar reduction of K(+) currents is well documented under numerous pathophysiological conditions. We examined the extent of K(+) current downregulation in cultured canine cardiac myocytes and determined whether a dual K(+) current activator can normalize K(+) currents and restore action potential (AP) configuration.

Atrial-selective inhibition of cardiac sodium channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study was designed to examine the basis for the atrial-selective actions of Wenxin Keli.

The fast transient outward potassium current (Ito,f) plays a critical role in the electrical and contractile properties of the myocardium. Ito,f channels are formed by the co-assembly of the pore-forming α-subunits, Kv4.2 and Kv4.3, together with the accessory β-subunit KChIP2. Reductions of Ito,f are common in the diseased heart, which is also associated with enhanced stimulation of β-adrenergic receptors (β-ARs). We used cultured neonatal rat ventricular myocytes to examine how chronic β-AR stimulation decreases Ito,f. To determine which downstream pathways mediate these Ito,f changes, adenoviral infections were used to inhibit CaMKIIδc, CaMKIIδb, calcineurin, or nuclear factor κB (NF-κB). We observed that chronic β-AR stimulation with isoproterenol (ISO) for 48 h reduced Ito,f along with mRNA expression of all three of its subunits (Kv4.2, Kv4.3, and KChIP2). Inhibiting either CaMKIIδc nor CaMKIIδb did not prevent the ISO-mediated Ito,f reductions, even though CaMKIIδc and CaMKIIδb clearly regulated Ito,f and the mRNA expression of its subunits. Likewise, calcineurin inhibition did not prevent the Ito,f reductions induced by β-AR stimulation despite strongly modulating Ito,f and subunit mRNA expression. In contrast, NF-κB inhibition partly rescued the ISO-mediated Ito,f reductions in association with restoration of KChIP2 mRNA expression. Consistent with these observations, KChIP2 promoter activity was reduced by p65 as well as β-AR stimulation. In conclusion, NF-κB, and not CaMKIIδ or calcineurin, partly mediates the Ito,f reductions induced by chronic β-AR stimulation. Both mRNA and KChIP2 promoter data suggest that the ISO-induced Ito,f reductions are, in part, mediated through reduced KChIP2 transcription caused by NF-κB activation.

The Ca(2+)-independent transient outward K(+) current (I(to)) plays a critical role in underlying phase 1 of repolarization of the cardiac action potential and, as a result, is central to modulating excitation-contraction coupling and propensity for arrhythmia. Additionally, I(to) and its molecular constituents are consistently reduced in cardiac hypertrophy and heart failure. In this review, we discuss the physiological role of I(to) as well as the molecular basis of this current in human and canine hearts, in which I(to) has been thoroughly studied. In particular, we discuss the role of Ito; in the action potential and the mechanisms by which I(to) modulates excitation-contraction coupling. We also describe the effects of mutations in the subunits constituting the Ito channel as well as the role of I(to) in the failing myocardium. Finally, we review pharmacological modulation of I(to) and discuss the evidence supporting the hypothesis that restoration of I(to) in the setting of heart failure may be therapeutically beneficial by enhancing excitation-contraction coupling and cardiac function.

The differential response of atrial and ventricular cells to late sodium channel current (late INa) inhibition has not been thoroughly investigated. The aim of the present study was to compare the atrioventricular differences in electrophysiological actions of GS-458967, a potent late INa blocker.

The inward rectifier potassium current, IK1, contributes to the terminal phase of repolarization of the action potential (AP), as well as the value and stability of the resting membrane potential. Regional variation in IK1 has been noted in the canine heart, but the biophysical properties have not been directly compared. We examined the properties and functional contribution of IK1 in isolated myocytes from ventricular, atrial and Purkinje tissue. APs were recorded from canine left ventricular midmyocardium, left atrial and Purkinje tissue. The terminal rate of repolarization of the AP in ventricle, but not in Purkinje, depended on changes in external K(+) ([K(+)]o). Isolated ventricular myocytes had the greatest density of IK1 while atrial myocytes had the lowest. Furthermore, the outward component of IK1 in ventricular cells exhibited a prominent outward component and steep negative slope conductance, which was also enhanced in 10 mM [K(+)]o. In contrast, both Purkinje and atrial cells exhibited little outward IK1, even in the presence of 10 mM [K(+)]o, and both cell types showed more persistent current at positive potentials. Expression of Kir2.1 in the ventricle was 76.9-fold higher than that of atria and 5.8-fold higher than that of Purkinje, whereas the expression of Kir2.2 and Kir2.3 subunits was more evenly distributed in Purkinje and atria. Finally, AP clamp data showed distinct contributions of IK1 for each cell type. IK1 and Kir2 subunit expression varies dramatically in regions of the canine heart and these regional differences in Kir2 expression likely underlie regional distinctions in IK1 characteristics, contributing to variations in repolarization in response to in [K(+)]o changes.

Atrioventricular nodal reentrant tachycardia (AVNRT) may coexist with Brugada syndrome (BrS).

The availability of safe and effective drugs for the management of atrial fibrillation (AF) remains an unmet medical need.

Short QT syndrome (SQTS) is a rare arrhythmogenic inherited heart disease. Diagnosis can be challenging in subjects with slightly shortened QT interval at electrocardiogram. In this study we compared the QT interval behaviour during exercise in a cohort of SQTS patients with a control group, to evaluate the usefulness of exercise test in the diagnosis of SQTS.

Perhaps the most mature area of multi-scale systems biology is the modelling of the heart. Current models are grounded in over fifty years of research in the development of biophysically detailed models of the electrophysiology (EP) of cardiac cells, but one aspect which is inadequately addressed is the incorporation of uncertainty and physiological variability. Uncertainty quantification (UQ) is the identification and characterisation of the uncertainty in model parameters derived from experimental data, and the computation of the resultant uncertainty in model outputs. It is a necessary tool for establishing the credibility of computational models, and will likely be expected of EP models for future safety-critical clinical applications. The focus of this paper is formal UQ of one major sub-component of cardiac EP models, the steady-state inactivation of the fast sodium current, INa. To better capture average behaviour and quantify variability across cells, we have applied for the first time an 'individual-based' statistical methodology to assess voltage clamp data. Advantages of this approach over a more traditional 'population-averaged' approach are highlighted. The method was used to characterise variability amongst cells isolated from canine epi and endocardium, and this variability was then 'propagated forward' through a canine model to determine the resultant uncertainty in model predictions at different scales, such as of upstroke velocity and spiral wave dynamics. Statistically significant differences between epi and endocardial cells (greater half-inactivation and less steep slope of steady state inactivation curve for endo) was observed, and the forward propagation revealed a lack of robustness of the model to underlying variability, but also surprising robustness to variability at the tissue scale. Overall, the methodology can be used to: (i) better analyse voltage clamp data; (ii) characterise underlying population variability; (iii) investigate consequences of variability; and (iv) improve the ability to validate a model. To our knowledge this article is the first to quantify population variability in membrane dynamics in this manner, and the first to perform formal UQ for a component of a cardiac model. The approach is likely to find much wider applicability across systems biology as current application domains reach greater levels of maturity.

Timothy syndrome (TS) is a rare multisystem genetic disorder characterized by a myriad of abnormalities, including QT prolongation, syndactyly, and neurologic symptoms. The predominant genetic causes are recurrent de novo missense mutations in exon 8/8A of the CACNA1C-encoded L-type calcium channel; however, some cases remain genetically elusive.

The study investigates how increased Ito, as mediated by the activator NS5806, affects excitation-contraction coupling in chronic heart failure (HF). We hypothesized that restoring spike-and-dome morphology of the action potential (AP) to a healthy phenotype would be insufficient to restore the intracellular Ca(2) (+) transient (CaT), due to HF-induced remodelling of Ca(2+) handling.

BrS is an inherited sudden cardiac death syndrome. Less than 35% of BrS probands have genetically identified pathogenic variants. Recent evidence has implicated SCN10A, a neuronal sodium channel gene encoding Nav1.8, in the electrical function of the heart.

There is a critical need for safer and more effective pharmacological management of atrial fibrillation (AF) in the setting of heart failure (HF).

Patients with short QT syndrome (SQTS) have an increased risk for atrial tachyarrhythmias, ventricular tachyarrhythmias, and/or sudden cardiac death. PQ segment depression (PQD) is related to atrial fibrillation and carries a poor prognosis in the setting of acute inferior myocardial infarction and is a well-defined electrocardiographic (ECG) marker of acute pericarditis.

The aim of this study was to test the hypothesis that late potentials and fractionated electrogram activity are due to delayed depolarization within the anterior aspects of right ventricular (RV) epicardium in experimental models of Brugada syndrome (BrS).

Heart failure (HF) is associated with development of AF and life-threatening ventricular tachycardia and fibrillation (VT/VF). Vulnerability to development of AF and VT/VF at different stages of HF and the underlying pathophysiological mechanisms are poorly defined. The present study was designed to determine the time-course of development of electrical and structural remodelling of the atria and ventricles, and their contribution to induction of AF and VT/VF in a canine model of HF.

Early repolarization pattern in the ECG has been associated with increased risk for ventricular tachycardia/fibrillation (VT/VF), particularly when manifest in inferior leads. This study examines the mechanisms underlying VT/VF in early repolarization syndrome (ERS). Transmembrane action potentials (APs) were simultaneously recorded from 2 epicardial sites and 1 endocardial site of coronary-perfused canine left-ventricular (LV) wedge preparations, together with a pseudo-ECG. Transient outward current (Ito) was recorded from epicardial myocytes isolated from the inferior and lateral LV of the same heart. J wave area (pseudo-ECG), epicardial AP notch magnitude and index were larger in inferior vs. lateral wall preparations at baseline and after exposure to provocative agents (NS5806+verapamil+acetylcholine (ACh)). Ito density was greater in myocytes from inferior vs. lateral wall (18.4 ± 2.3pA/pF vs. 11.6 ± 2.0pA/pF; p<0.05). A combination of NS5806 (7 μM) and verapamil (3 μM) or pinacidil (4 μM), used to pharmacologically model the genetic defects responsible for ERS, resulted in prominent J-point and ST-segment elevation. ACh (3 μM), simulating increased vagal tone, precipitated phase-2-reentry-induced polymorphic VT/VF. Using identical protocols, inducibility of arrhythmias was 3-fold higher in inferior vs. lateral wedges. Quinidine (10 μM) or isoproterenol (1 μM) restored homogeneity and suppressed VT/VF. Our data support the hypothesis that 1) ERS is caused by a preferential accentuation of the AP notch in the LV epicardium; 2) this repolarization defect is accentuated by elevated vagal tone; 3) higher intrinsic levels of Ito account for the greater sensitivity of the inferior LV wall to development of VT/VF; and 4) quinidine and isoproterenol exert ameliorative effects by reversing the repolarization abnormality.

Genetic defects in KCNJ8, encoding the Kir6.1 subunit of the ATP-sensitive K(+) channel (I(K-ATP)), have previously been associated with early repolarization (ERS) and Brugada (BrS) syndromes. Here we test the hypothesis that genetic variants in ABCC9, encoding the ATP-binding cassette transporter of IK-ATP (SUR2A), are also associated with both BrS and ERS.

Hypothermia has been reported to induce ventricular tachycardia and fibrillation (VT/VF) in patients with early repolarization (ER) pattern. This study examines the cellular mechanisms underlying VT/VF associated with hypothermia in an experimental model of ER syndrome and examines the effectiveness of quinidine, cilostazol, and milrinone to prevent hypothermia-induced arrhythmias.

Hypertrophic cardiomyopathy (HCM) is the most common monogenic cardiac disorder encountered in the clinic. Data relative to the electrophysiologic characteristics and pharmacologic responsiveness of human tissues and cells isolated from patients with HCM are rare. As a consequence, cellular mechanisms underlying arrhythmogenicity are poorly understood.

Developmental changes in the electrical characteristics of the ventricular myocardium are not well defined. This study examines the contribution of inwardly rectifying K(+) current (IK1), transient outward K(+) current (Ito), delayed rectifier K(+) currents (IKr and IKs) and sodium channel current (INa) to repolarization in the canine neonate myocardium.

Brugada syndrome is an inherited disease associated with vulnerability to ventricular tachycardia and sudden cardiac death in young adults. Milrinone and cilostazol, oral phosphodiesterase (PDE) type III inhibitors, have been shown to increase L-type calcium channel current (ICa) and modestly increase heart rate by elevating the level of intracellular cyclic adenosine monophosphate.

The ability to recapitulate mature adult phenotypes is critical to the development of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) as models of disease. The present study examines the characteristics of the transient outward current (Ito) and its contribution to the hiPSC-CM action potential (AP).

Previous studies have shown that late sodium channel current (INa) blockers such as ranolazine can exert antiarrhythmic effects by suppressing early and delayed afterdepolarization (EAD and DAD)-induced triggered activity.

Wenxin Keli (WK), a Chinese herb extract, is reported to be effective in the treatment of atrial and ventricular cardiac arrhythmias. Recent studies suggest that WK inhibits the transient potassium outward current (I(to)).

The anti-arrhythmic efficacy of the late sodium channel current (late I(Na)) inhibition has been convincingly demonstrated in the ventricles, particularly under conditions of prolonged ventricular repolarization. The value of late I(Na) block in the setting of atrial fibrillation (AF) remains poorly investigated. All sodium channel blockers inhibit both peak and late I(Na) and are generally more potent in inhibiting late vs. early I(Na). Selective late I(Na) block does not prolong the effective refractory period (ERP), a feature common to practically all anti-AF agents. Although the late I(Na) blocker ranolazine has been shown to be effective in suppression of AF, it is noteworthy that at concentrations at which it blocks late I(Na) in the ventricles, it also potently blocks peak I(Na) in the atria, thus causing rate-dependent prolongation of ERP due to development of post-repolarization refractoriness. Late I(Na) inhibition in atria is thought to suppress intracellular calcium (Ca(i))-mediated triggered activity, secondary to a reduction in intracellular sodium (Na(i)). However, agents that block late I(Na) (ranolazine, amiodarone, vernakalant, etc) are also potent atrial-selective peak I(Na) blockers, so that the reduction of Na(i) loading in atrial cells by these agents can be in large part due to the block of peak I(Na). The impact of late I(Na) inhibition is reduced by the abbreviation of the action potential that occurs in AF patients secondary to electrical remodeling. It stands to reason that selective late I(Na) block may contribute more to inhibition of Ca(i)-mediated triggered activity responsible for initiation of AF in clinical pathologies associated with a prolonged atrial APD (such as long QT syndrome). Additional studies are clearly needed to test this hypothesis.

We studied principal neurons from canine intracardiac (IC) ganglia to determine whether large-conductance calcium-activated potassium (BK) channels play a role in their excitability. We performed whole cell recordings in voltage- and current-clamp modes to measure ion currents and changes in membrane potential from isolated canine IC neurons. Whole cell currents from these neurons showed fast- and slow-activated outward components. Both current components decreased in the absence of calcium and following 1-2 mM tetraethylammonium (TEA) or paxilline. These results suggest that BK channels underlie these current components. Single-channel analysis showed that BK channels from IC neurons do not inactivate in a time-dependent manner, suggesting that the dynamic of the decay of the fast current component is akin to that of intracellular calcium. Immunohistochemical studies showed that BK channels and type 2 ryanodine receptors are coexpressed in IC principal neurons. We tested whether BK current activation in these neurons occurred via a calcium-induced calcium release mechanism. We found that the outward currents of these neurons were not affected by the calcium depletion of intracellular stores with 10 mM caffeine and 10 μM cyclopiazonic acid. Thus, in canine intracardiac neurons, BK currents are directly activated by calcium influx. Membrane potential changes elicited by long (400 ms) current injections showed a tonic firing response that was decreased by TEA or paxilline. These data strongly suggest that the BK current present in canine intracardiac neurons regulates action potential activity and could increase these neurons excitability.

The electrophysiological heterogeneity that exists across the ventricular wall in the mammalian heart has long been recognized, but remains an area that is incompletely understood. Experimental studies of the mechanisms of arrhythmogenesis in the whole heart often examine the epicardial surface in isolation and thereby disregard transmural electrophysiology. Significant heterogeneity exists in the electrophysiological properties of cardiomyocytes isolated from different layers of the ventricular wall, and given that regional heterogeneities of membrane repolarization properties can influence the electrophysiological substrate for re-entry, the diversity of cell types and characteristics spanning the ventricular wall is important in the study of arrhythmogenesis. For these reasons, coronary-perfused left ventricular wedge preparations have been developed to permit the study of transmural electrophysiology in the intact ventricle. Since the first report by Yan and Antzelevitch in 1996, electrical recordings from the transmural surface of canine wedge preparations have provided a wealth of data regarding the cellular basis for the electrocardiogram, the role of transmural heterogeneity in arrhythmogenesis, and differences in the response of the different ventricular layers to drugs and neurohormones. Use of the wedge preparation has since been expanded to other species and more recently it has also been widely used in optical mapping studies. The isolated perfused wedge preparation has become an important tool in cardiac electrophysiology. In this review, we detail the methodology involved in recording both electrical and optical signals from the coronary-perfused wedge preparation and review the advances in cardiac electrophysiology achieved through study of the wedge.

Remodeling of ion channel expression is well established in heart failure (HF). We determined the extent to which I(to) is reduced in tachypacing-induced HF and assessed the ability of an I(to) activator (NS5806) to recover this current.

The action of AVE0118 to prolong effective refractory period (ERP) in atria but not in ventricles is thought to be due to its inhibition of IKur. However, in nonremodeled atria, AVE0118 prolongs ERP but not action potential duration (APD70-90), which can be explained with the inhibition of sodium but not potassium channel current. ERP, APD, and the maximum rate of increase of the AP upstroke (Vmax) were measured in the canine-isolated coronary-perfused right atrial and in superfused ventricular tissue preparations. Whole-cell patch-clamp techniques were used to measure sodium channel current in HEK293 cells stably expressing SCN5A. AVE0118 (5-10 μM) prolonged ERP (P < 0.001) but not APD70 and decreased Vmax (by 15%, 10 μM, P < 0.05; n = 10 for each). Ventricular ERP, APD90, and Vmax were not changed significantly by 10 μM AVE0118 (all P = ns; n = 7). AVE0118 effectively suppressed acetylcholine-mediated persistent atrial fibrillation. AVE0118 (10 μM) reduced peak current amplitude of SCN5A-WT current by 36.5% ± 6.6% (P < 0.01; n = 7) and shifted half-inactivation voltage (V0.5) of the steady-state inactivation curve from -89.9 ± 0.5 to -96.0 ± 0.9 mV (P < 0.01; n = 7). Our data suggest that AVE0118-induced prolongation of atrial, but not ventricular ERP, is due largely to atrial-selective depression of sodium channel current, which likely contributes to the effectiveness of AVE0118 to suppress atrial fibrillation.

Cardiac sodium channel β-subunit mutations have been associated with several inherited cardiac arrhythmia syndromes.

Tricyclic antidepressants are known to induce cardiac arrhythmias at therapeutic or supratherapeutic doses. The tricyclic antidepressant, amitriptyline, is reported to induce ST segment elevation in the right precordial electrocardiogram (ECG) leads, thus unmasking Brugada syndrome (BrS). The mechanism by which antidepressants induce the BrS phenotype and associated sudden death is not well established.

Several clinical trials have shown that vernakalant is effective in terminating recent onset atrial fibrillation (AF). The electrophysiological actions of vernakalant are not fully understood.

Adenosine triphosphate (ATP)-sensitive potassium cardiac channels consist of inward-rectifying channel subunits Kir6.1 or Kir6.2 (encoded by KCNJ8 or KCNJ11) and the sulfonylurea receptor subunits SUR2A (encoded by ABCC9).

In addition to extrasystoles of pulmonary vein (PV) origin, those arising from the superior vena cava (SVC) can precipitate atrial fibrillation (AF). The present study evaluates the electrophysiological properties of canine SVC sleeve preparations and the effect of ranolazine on late phase 3 early and delayed afterdepolarization (EAD and DAD)-induced triggered activity in SVC sleeves and compares SVC and PV sleeve electrophysiological properties.

Vernakalant (VER) is a relatively atrial-selective antiarrhythmic drug capable of blocking potassium and sodium currents in a frequency- and voltage-dependent manner. Ranolazine (RAN) is a sodium-channel blocker shown to exert antiarrhythmic effects in pulmonary vein (PV) sleeves. dl-Sotalol (SOT) is a β-blocker commonly used in the rhythm-control treatment of atrial fibrillation. This study evaluated the electrophysiological and antiarrhythmic effects of VER, RAN, and SOT in canine PV sleeve preparations in a blinded fashion.

Ranolazine has been shown to produce atrial-selective depression of sodium channel-dependent parameters and suppress atrial fibrillation (AF) in a variety of experimental models. The present study contrasts the effects of ranolazine and those of a clinically used anti-AF class IC agent, propafenone. Electrophysiological and anti-AF effects of propafenone and ranolazine were compared at clinically relevant concentrations (i.e., 0.3-1.5 and 1-10 μM, respectively) in canine isolated coronary-perfused atrial and ventricular preparations. Transmembrane action potential and pseudo-ECG were recorded. Both ranolazine and propafenone produced atrial-selective prolongation of action potential duration. Propafenone depressed sodium channel-mediated parameters [maximum rate of rise of the action potential upstroke (V(max)), conduction time, and diastolic threshold of excitation] and induced postrepolarization refractoriness to a greater degree than ranolazine, and these effects, unlike those induced by ranolazine, were not or only mildly atrial-selective at normal rates (cycle length 500 ms). At fast pacing rates, however, the effects of propafenone on V(max) and conduction time became atrial-selective, because of the elimination of diastolic interval in atria, but not in ventricles. Propafenone (1.5 μM) and ranolazine (10.0 μM) were effective in preventing the initiation of persistent acetylcholine-mediated AF (6/7 and 9/11 atria, respectively), its termination (8/10 and 8/12 atria, respectively), and subsequent reinduction (8/8 and 7/8 atria, respectively). Thus, propafenone and ranolazine both suppress AF, but ranolazine, unlike propafenone, does it with minimal effects on ventricular myocardium, suggesting a reduced potential for promoting ventricular arrhythmias.

Wenxin Keli is a Chinese herb extract reported to be of benefit in the treatment of cardiac arrhythmias, cardiac inflammation, and heart failure.

In the United States, sudden cardiac death accounts for an estimated 300,000 to 350,000 cases each year, with 80,000 presenting as the first manifestation of a preexisting, sometimes unrecognized, coronary artery disease. Acute myocardial infarction (AMI)-induced ventricular fibrillation frequently occurs without warning, often leading to death within minutes in patients who do not receive prompt medical attention. Identification of patients at risk for AMI-induced lethal ventricular arrhythmias remains an unmet medical need. Recent studies suggest that a genetic predisposition may significantly contribute to the vulnerability of the ischemic myocardium to ventricular tachycardia/ventricular fibrillation. Numerous experimental models have been developed for the purpose of advancing our understanding of the mechanisms responsible for the development of cardiac arrhythmias in the setting of ischemia and with the aim of identifying antiarrhythmic therapies that could be of clinical benefit. While our understanding of the mechanisms underlying AMI-induced ventricular arrhythmias is coming into better focus, the risk stratification of patients with AMI remains a major challenge. This review briefly discusses our current state of knowledge regarding the mechanisms of ischemic ventricular arrhythmias and their temporal distribution as revealed by available experimental models, how these correlate with the clinical syndromes, as well as prospective prophylactic therapies for the prevention and treatment of ischemia-induced life-threatening arrhythmias.

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Development of safe and effective pharmacological treatments for AF is one of the greatest unmet medical needs facing our society. In spite of significant progress in non-pharmacological AF treatments (largely due to the use of catheter ablation techniques), anti-arrhythmic agents (AADs) remain first line therapy for rhythm control management of AF for most AF patients. When considering efficacy, safety and tolerability, currently available AADs for rhythm control of AF are less than optimal. Ion channel inhibition remains the principal strategy for termination of AF and prevention of its recurrence. Practical clinical experience indicates that multi-ion channel blockers are generally more optimal for rhythm control of AF compared to ion channel-selective blockers. Recent studies suggest that atrial-selective sodium channel block can lead to safe and effective suppression of AF and that concurrent inhibition of potassium ion channels may potentiate this effect. An important limitation of the ion channel block approach for AF treatment is that non-electrical factors (largely structural remodeling) may importantly determine the generation of AF, so that "upstream therapy", aimed at preventing or reversing structural remodeling, may be required for effective rhythm control management. This review focuses on novel pharmacological targets for the rhythm control management of AF.

Postoperative atrial fibrillation (AF) is a major complication of cardiothoracic surgery, leading to significant consequences, including a higher rate of stroke, longer hospital stays and increased costs. Amiodarone is among the most widely used agents for prevention of postoperative AF. Ranolazine, a US FDA-approved antianginal agent, has been shown to effectively, safely prevent and terminate nonpostoperative AF in both experimental and clinical studies. In a recent publication, Miles and colleagues directly compared the efficacy and safety of amiodarone and ranolazine for prevention of postoperative AF in 393 patients. The patients were pretreated with amiodarone and ranolaizne for >1 week and 1 day, respectively, and the treatment continued for 10-14 days after surgery. Following coronary artery bypass grafting (CABG), AF occurred in 26.5% of patients taking amiodarone and in 17.5% of patients taking ranolazine (34% reduction; p < 0.035). No differences in adverse events between the two groups of patients were recorded. The results of this retrospective nonrandomized single-center study indicate that ranolazine may be used to effectively and safely prevent postoperative AF. These results need to be confirmed in a larger randomized study. If confirmed, ranolazine may be a good choice for preventing AF in patients undergoing CABG.

KCNE1 encodes an auxiliary subunit of cardiac potassium channels. Loss-of-function variations in this gene have been associated with the LQT5 form of the long QT syndrome (LQTS), secondary to reduction of I(Ks) current. We present a case in which a D85N rare polymorphism in KCNE1 is associated with an LQT2 phenotype.

Atrial-selective inhibition of cardiac Na(+) channel current (I(Na)) and I(Na)-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study examined the basis for the atrial-selective actions of ranolazine. Whole cell I(Na) was recorded at 15°C in canine atrial and ventricular myocytes and in human embryonic kidney (HEK)-293 cells expressing SCN5A. Tonic block was negligible at holding potentials from -140 to -100 mV, suggesting minimal drug interactions with the closed state. Trains of 40 pulses were elicited over a range of holding potentials to determine use-dependent block. Guarded receptor formalism was used to analyze the development of block during pulse trains. Use-dependent block by ranolazine increased at more depolarized holding potentials, consistent with an interaction of the drug with either preopen or inactivated states, but was unaffected by longer pulse durations between 5 and 200 ms, suggesting a weak interaction with the inactivated state. Block was significantly increased at shorter diastolic intervals between 20 and 200 ms. Responses in atrial and ventricular myocytes and in HEK-293 cells displayed a similar pattern. Ranolazine is an open state blocker that unbinds from closed Na(+) channels unusually fast but is trapped in the inactivated state. Kinetic rates of ranolazine interactions with different states of atrial and ventricular Na(+) channels were similar. Our data suggest that the atrial selectivity of ranolazine is due to a more negative steady-state inactivation curve, less negative resting membrane potential, and shorter diastolic intervals in atrial cells compared with ventricular cells at rapid rates.

Block of Na(+) channel conductance by ranolazine displays marked atrial selectivity that is an order of magnitude higher that of other class I antiarrhythmic drugs. Here, we present a Markovian model of the Na(+) channel gating, which includes activation-inactivation coupling, aimed at elucidating the mechanisms underlying this potent atrial selectivity of ranolazine. The model incorporates experimentally observed differences between atrial and ventricular Na(+) channel gating, including a more negative position of the steady-state inactivation curve in atrial versus ventricular cells. The model assumes that ranolazine requires a hydrophilic access pathway to the channel binding site, which is modulated by both activation and inactivation gates of the channel. Kinetic rate constants were obtained using guarded receptor analysis of the use-dependent block of the fast Na(+) current (I(Na)). The model successfully reproduces all experimentally observed phenomena, including the shift of channel availability, the sensitivity of block to holding or diastolic potential, and the preferential block of slow versus fast I(Na.) Using atrial and ventricular action potential-shaped voltage pulses, the model confirms significantly greater use-dependent block of peak I(Na) in atrial versus ventricular cells. The model highlights the importance of action potential prolongation and of a steeper voltage dependence of the time constant of unbinding of ranolazine from the atrial Na(+) channel in the development of use-dependent I(Na) block. Our model predictions indicate that differences in channel gating properties as well as action potential morphology between atrial and ventricular cells contribute equally to the atrial selectivity of ranolazine. The model indicates that the steep voltage dependence of ranolazine interaction with the Na(+) channel at negative potentials underlies the mechanism of the predominant block of I(Na) in atrial cells by ranolazine.

Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on peak and late Na(+) current (I(Na)) in canine ventricular cells. Epicardial and endocardial myocytes were isolated, and patch-clamp techniques were used to record I(Na). Action potential recordings from left ventricular wedges exposed to acidic Tyrode solution showed a widening of the QRS complex, indicating slowing of transmural conduction. In myocytes, exposure to acidic conditions resulted in a 17.3 ± 0.9% reduction in upstroke velocity. Analysis of fast I(Na) showed that current density was similar in epicardial and endocardial cells at normal pH (68.1 ± 7.0 vs. 63.2 ± 7.1 pA/pF, respectively). Extracellular acidosis reduced the fast I(Na) magnitude by 22.7% in epicardial cells and 23.1% in endocardial cells. In addition, a significant slowing of the decay (time constant) of fast I(Na) was observed at pH 6.6. Acidosis did not affect steady-state inactivation of I(Na) or recovery from inactivation. Analysis of late I(Na) during a 500-ms pulse showed that the acidosis significantly reduced late I(Na) at 250 and 500 ms into the pulse. Using action potential clamp techniques, application of an epicardial waveform resulted in a larger late I(Na) compared with when an endocardial waveform was applied to the same cell. Acidosis caused a greater decrease in late I(Na) when an epicardial waveform was applied. These results suggest acidosis reduces both peak and late I(Na) in both cell types and contributes to the depression in cardiac excitability observed under ischemic conditions.

Ranolazine is a Food and Drug Administration-approved antianginal agent. Experimental and clinical studies have shown that ranolazine has antiarrhythmic effects in both ventricles and atria. In the ventricles, ranolazine can suppress arrhythmias associated with acute coronary syndrome, long QT syndrome, heart failure, ischemia, and reperfusion. In atria, ranolazine effectively suppresses atrial tachyarrhythmias and atrial fibrillation (AF). Recent studies have shown that the drug may be effective and safe in suppressing AF when used as a pill-in-the pocket approach, even in patients with structurally compromised hearts, warranting further study. The principal mechanism underlying ranolazine's antiarrhythmic actions is thought to be primarily via inhibition of late I(Na) in the ventricles and via use-dependent inhibition of peak I(Na) and I(Kr) in the atria. Short- and long-term safety of ranolazine has been demonstrated in the clinic, even in patients with structural heart disease. This review summarizes the available data regarding the electrophysiologic actions and antiarrhythmic properties of ranolazine in preclinical and clinical studies.

Angiotensin-converting enzyme (ACE) inhibitors and angiotensin II-receptor blockers (ARBs) are prototypes of "upstream" therapy for the management of atrial fibrillation (AF). Ectopic activity arising from the PV sleeves plays a prominent role in the development of AF.

The L-type cardiac calcium channel (LTCC) plays a prominent role in the electric and mechanical function of the heart. Mutations in the LTCC have been associated with a number of inherited cardiac arrhythmia syndromes, including Timothy, Brugada, and early repolarization syndromes. Elucidation of the genetic defects associated with these syndromes has led to a better understanding of molecular and cellular mechanisms and the development of novel therapeutic approaches to dealing with the arrhythmic manifestations. This review provides an overview of the molecular structure and function of the LTCC, the genetic defects in these channels known to contribute to inherited disorders, and the underlying molecular and cellular mechanisms contributing to the development of life-threatening arrhythmias.

The purpose of this study was to determine the electrophysiologic effects of simvastatin in canine pulmonary vein (PV) sleeve preparations.

Mutations in KCNJ2, the gene encoding the human inward rectifier potassium channel Kir2.1 (IK1 or IKir2.1), have been identified in Andersen-Tawil syndrome. Andersen-Tawil syndrome is a multisystem inherited disease exhibiting periodic paralysis, cardiac arrhythmias, and dysmorphic features at times mimicking catecholaminergic polymorphic ventricular tachycardia.

L-type calcium channel (LTCC) mutations have been associated with Brugada syndrome (BrS), short QT (SQT) syndrome, and Timothy syndrome (LQT8). Little is known about the extent to which LTCC mutations contribute to the J-wave syndromes associated with sudden cardiac death.

Long QT syndrome (LQTS) is an inherited disorder characterized by prolonged QT intervals and potentially life-threatening arrhythmias. Mutations in 12 different genes have been associated with LQTS. Here we describe a patient with LQTS who has a mutation in KCNQ1 as well as a polymorphism in KCNH2. The proband (MMRL0362), a 32-year-old female, exhibited multiple ventricular extrasystoles and one syncope. Her ECG (QT interval corrected for heart rate (QTc) = 518ms) showed an LQT2 morphology in leads V4-V6 and LQT1 morphology in leads V1-V2. Genomic DNA was isolated from lymphocytes. All exons and intron borders of 7 LQTS susceptibility genes were amplified and sequenced. Variations were detected predicting a novel missense mutation (V110I) in KCNQ1, as well as a common polymorphism in KCNH2 (K897T). We expressed wild-type (WT) or V110I Kv7.1 channels in CHO-K1 cells cotransfected with KCNE1 and performed patch-clamp analysis. In addition, WT or K897T Kv11.1 were also studied by patch clamp. Current-voltage (I-V) relations for V110I showed a significant reduction in both developing and tail current densities compared with WT at potentials >+20 mV (p < 0.05; n = 8 cells, each group), suggesting a reduction in IKs currents. K897T- Kv11.1 channels displayed a significantly reduced tail current density compared with WT-Kv11.1 at potentials >+10 mV. Interestingly, channel availability assessed using a triple-pulse protocol was slightly greater for K897T compared with WT (V0.5 = -53.1 ± 1.13 mV and -60.7 ± 1.15 mV for K897T and WT, respectively; p < 0.05). Comparison of the fully activated I-V revealed no difference in the rectification properties between WT and K897T channels. We report a patient with a loss-of-function mutation in KCNQ1 and a loss-of-function polymorphism in KCNH2. Our results suggest that a reduction of both IKr and IKs underlies the combined LQT1 and LQT2 phenotype observed in this patient.

The aim of this study was to evaluate the effectiveness of a combination of dronedarone and ranolazine in suppression of atrial fibrillation (AF).

To review the electrophysiologic effects of antiarrhythmic agents in pulmonary veins (PV) sleeve preparations.

Dronedarone is approved by the U.S. Food and Drug Administration for the treatment of patients with atrial fibrillation (AF) as a safe alternative to amiodarone. There are no full-length published reports describing the effectiveness of acute dronedarone use against AF in experimental or clinical studies.

Recent development of drugs for the treatment of atrial fibrillation (AF) has focused on atrial selective agents. We examined the atrioventricular differences in sodium channel block of the antiarrhythmic agent AZD1305 in atria and ventricles of anesthetized dogs in vivo, canine isolated arterially perfused preparations in vitro, and isolated myocytes using whole-cell patch-clamp techniques. AZD1305 did not change heart rate or blood pressure in vivo but prolonged action potential duration and increased effective refractory period, diastolic threshold of excitation, and conduction time preferentially in atria both in vitro and in vivo. AZD1305 reduced the maximum rate of rise of the action potential upstroke (V(max)) predominantly in atria (-51% +/- 10% in atria vs. -31% +/- 23% in ventricles; 3 microM; cycle length = 500 milliseconds). Fast sodium current (I(Na)) was blocked by AZD1305 to a greater degree in atrial versus ventricular myocytes (particularly tonic inhibition). In coronary-perfused right atria, AZD1305 very effectively prevented induction of persistent acetylcholine-mediated AF and, in a different set of atria, terminated persistent AF (in 5 of 5 and 7 of 8 atria, respectively). In conclusion, AZD1305 exerts atrial predominant sodium channel-blocking effects in vitro and in vivo and effectively suppresses AF.

The objective of this study was to examine the electrophysiologic and antiarrhythmic effects of the new antiarrhythmic agent tert-butyl (2-[7-[2-(4-cyano-2-fluorophenoxy)ethyl]-9-oxa-3,7-diazabicyclo[3.3.1]non3-yl]ethyl)carbamate (AZD1305) in canine pulmonary vein (PV) sleeve preparations isolated from untreated and long-term amiodarone-treated animals. Ectopic activity arising from PV sleeves plays a prominent role in the development of atrial fibrillation (AF). Delayed afterdepolarizations (DADs) and late phase 3 early afterdepolarizations (EADs), originating from the PV have been proposed as potential triggers in initiation of AF. Action potentials were recorded from canine superfused PV sleeves using standard microelectrode techniques. Acetylcholine (1 microM), isoproterenol (1 microM), or their combination was used to induce EADs, DADs, and triggered activity (TA). The effects of AZD1305 (0.1-10 microM) were evaluated in PV sleeve preparations isolated from untreated and amiodarone-treated (40 mg/kg daily for 6 weeks) dogs. AZD1305 (0.1-10 microM, 30 min) significantly prolonged action potential duration and reduced excitability. Abbreviating basic cycle length from 1000 to 300 ms resulted in a decrease of V(max) from 314 +/- 79 to 251 +/- 55 V/s (Delta = -20%) in control and from 177 +/- 53 to 76.5 +/- 33 V/s (Delta = -57%) after AZD1305 (n = 6, p < 0.05). AZD1305 markedly attenuated or suppressed DADs and DAD-induced TA, but not late phase 3 EADs. AZD1305-induced attenuation of excitability, leading to activation failure at much longer cycle lengths, was much more pronounced in PV from amiodarone-treated dogs. Potent effects of AZD1305 to depress excitability, prolong action potential duration, and suppress DAD-induced triggered activity in canine PV sleeve preparations may be effective in suppressing triggers responsible for the genesis of AF and other atrial arrhythmias.

Identification of infants at risk for sudden arrhythmic death remains one of the leading challenges of modern medicine. We present a family in which a common polymorphism (single nucleotide polymorphism) inherited from the father, combined with a stop codon mutation inherited from the mother (both asymptomatic), led to 2 cases of sudden infant death.

The J wave, also referred to as an Osborn wave, is a deflection immediately following the QRS complex of the surface ECG. When partially buried in the R wave, the J wave appears as J-point elevation or ST-segment elevation. Several lines of evidence have suggested that arrhythmias associated with an early repolarization pattern in the inferior or mid to lateral precordial leads, Brugada syndrome, or arrhythmias associated with hypothermia and the acute phase of ST-segment elevation myocardial infarction are mechanistically linked to abnormalities in the manifestation of the transient outward current (I(to))-mediated J wave. Although Brugada syndrome and early repolarization syndrome differ with respect to the magnitude and lead location of abnormal J-wave manifestation, they can be considered to represent a continuous spectrum of phenotypic expression that we propose be termed J-wave syndromes. This review summarizes our current state of knowledge concerning J-wave syndromes, bridging basic and clinical aspects. We propose to divide early repolarization syndrome into three subtypes: type 1, which displays an early repolarization pattern predominantly in the lateral precordial leads, is prevalent among healthy male athletes and is rarely seen in ventricular fibrillation survivors; type 2, which displays an early repolarization pattern predominantly in the inferior or inferolateral leads, is associated with a higher level of risk; and type 3, which displays an early repolarization pattern globally in the inferior, lateral, and right precordial leads, is associated with the highest level of risk for development of malignant arrhythmias and is often associated with ventricular fibrillation storms.

This study was designed to quantitate transseptal dispersion of repolarization (DR) and delineate its role in arrhythmogenesis using the calcium agonist BayK 8644 to mimic the gain of function of calcium channel current responsible for Timothy syndrome.

Inherited loss of function mutations in SCN5A have been linked to overlapping syndromes including cardiac conduction disease and Brugada syndrome (BrS). The mechanisms responsible for the development of one without the other are poorly understood.

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Currently available antiarrhythmic drugs (AADs), although highly effective in acute cardioversion of paroxysmal AF, are generally only moderately successful in long-term maintenance of sinus rhythm. The use of AADs is often associated with an increased risk of ventricular proarrhythmia, extracardiac toxicity, and exacerbation of concomitant diseases such as heart failure. AF is commonly associated with intracardiac and extracardiac disease, which can modulate the efficacy and safety of AAD therapy. In light of the multifactorial intracardiac and extracardiac causes of AF generation, current development of anti-AF agents is focused on modulation of ion channel activity as well as on upstream therapies that reduce structural substrates. The available data indicate that multiple ion channel blockers exhibiting potent inhibition of peak I(Na) with relatively rapid unbinding kinetics, as well as inhibition of late I(Na) and I(Kr), may be preferable for the management of AF when considering both safety and efficacy.

Amiodarone and ranolazine have been characterized as inactivated- and activated-state blockers of cardiac sodium channel current (I(Na)), respectively, and shown to cause atrial-selective depression of I(Na)-related parameters. This study tests the hypothesis that their combined actions synergistically depress I(Na)-dependent parameters in atria but not ventricles.

Safe and effective pharmacologic management of atrial fibrillation (AF) is one of the greatest challenges facing an aging society. Currently available pharmacologic strategies for rhythm control of AF are associated with ventricular arrhythmias and in some cases multi-organ toxicity. Consequently, drug development has focused on atrial-selective agents such as IKur blockers. Recent studies suggest that IKur block alone may be ineffective for suppression of AF and may promote AF in healthy hearts. Recent experimental studies have demonstrated other important electrophysiologic differences between atrial and ventricular cells, particularly with respect to sodium channel function, and have identified sodium channel blockers that exploit these electrophysiologic distinctions. Atrial-selective sodium channel blockers, such as ranolazine and amiodarone, effectively suppress and/or prevent the induction of AF in experimental models, while producing little to no effect on ventricular myocardium. These findings suggest that atrial-selective sodium channel block may be a fruitful new strategy for the management of AF.

Although sinus node bradycardia is a very common clinical condition, the cellular mechanisms contributing to abnormal sinus node function are not clearly delineated. In recent publications, mutations in the hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channels have been associated with sinus bradycardia. These channels are thought to be crucial in generating the spontaneous sinus node action potential, in accelerating the heart rate during sympathetic drive, and decelerating heart rate during vagal stimulation. Humans carrying HCN4 mutations indeed display significant bradycardia. Recent studies generating HCN4 knock out mice suggested that although HCN4 is crucial in early development, other mechanisms may also play a role in the accelerated heat rate achieved during sympathetic drive. In this review, we focus on genotype-phenotype correlation of these mutations and discuss the relative contribution of various ion channels to sinus node function. We also discuss the importance of HCN in treating clinical conditions such as brady- and tachycardia.

Atrial fibrillation (AF) is a growing clinical problem, increasing in prevalence as the population of the United States and countries around the world ages. Intensive research aimed at improving prevention, diagnosis, and treatment of AF is ongoing. Although the use and efficacy of catheter ablation-based approaches in AF treatment have increased significantly in the last decade, pharmacological agents remain the first-line therapy for rhythm management of AF. Currently available anti-AF agents are generally only moderately effective and associated with extracardiac toxicity and/or a risk for development of life-threatening ventricular arrhythmias. Included among current investigational strategies for improving the effectiveness and safety of anti-AF drugs is the development of (1) Agents that produce atrial-specific or predominant inhibition of I(Kur), I(K-ACh), or I(Na); (2) "Upstream therapies" that effect nonion channel targets that reduce atrial structural remodeling, hypertrophy, dilatation, inflammation, oxidative injury, etc; (3) Derivatives of "old" anti-AF drugs with an improved safety pharmacological profile; and (4) Gap junction therapy aimed at improving conduction without affecting sodium channels. This review focuses on new pharmacological approaches under investigation for the treatment of AF.

To examine the effects of chronic amiodarone on the electrophysiology of canine pulmonary vein (PV) sleeve preparations and left ventricular wedge preparation.

Brugada syndrome, characterized by ST-segment elevation in the right precordial ECG leads and the development of life-threatening ventricular arrhythmias, has been associated with mutations in 6 different genes. We identify and characterize a mutation in a new gene.

The pharmacological approach to therapy of atrial fibrillation (AF) is often associated with adverse effects resulting in the development of ventricular arrhythmias. As a consequence, much of the focus in recent years has been on development of atrial-selective agents. Atrial-selective sodium channel blockers have recently been shown to exist and be useful in the management of AF. This review summarizes the available data relative to current therapies, focusing on our understanding of the actions of atrial selective sodium channel blockers in suppressing and preventing the induction of AF and electrophysiological properties that confer atrial-selectivity to these antifibrillatory drugs.

Recent studies have demonstrated an association between mutations in CACNA1c or CACNB2b and Brugada syndrome (BrS). Previously described mutations all caused a loss of function secondary to a reduction of peak calcium current (I(Ca)). We describe a novel CACNB2b mutation associated with BrS in which loss of function is caused by accelerated inactivation of I(Ca). The proband, a 32 year old male, displayed a Type I ST segment elevation in two right precordial ECG leads following a procainamide challenge. EP study was positive with induction of polymorphic VT/VF. Interrogation of implanted ICD revealed brief episodes of very rapid ventricular tachycardia. He was also diagnosed with vasovagal syncope. Genomic DNA was isolated from lymphocytes. All exons and intron borders of 15 ion channel genes were amplified and sequenced. The only mutation uncovered was a missense mutation (T11I) in CACNB2b. We expressed WT or T11I CACNB2b in TSA201 cells co-transfected with WT CACNA1c and CACNA2d. Patch clamp analysis showed no significant difference between WT and T11I in peak I(Ca) density, steady-state inactivation or recovery from inactivation. However, both fast and slow decays of I(Ca) were significantly faster in mutant channels between 0 and + 20 mV. Action potential voltage clamp experiments showed that total charge was reduced by almost half compared to WT. We report the first BrS mutation in CaCNB2b resulting in accelerated inactivation of L-type calcium channel current. Our results suggest that the faster current decay results in a loss-of-function responsible for the Brugada phenotype

Although amiodarone is one of the most effective pharmacologic agents used in clinical management of atrial fibrillation (AF), little is known about its differential effects in atrial and ventricular myocardium.

Block of ultrarapid delayed rectified potassium current (I(Kur)), present in atria but not in ventricles, is thought to be a promising approach for atrial-specific therapy of atrial fibrillation (AF). However, it has been shown that I(Kur) block may abbreviate atrial repolarization and that loss-of-function mutations in KCNA5, which encodes K(v) 1.5 channels responsible for I(Kur), is associated with familial AF.

The Brugada syndrome, first described as a new clinical entity in 1992, is widely recognized today as a form of inherited sudden cardiac arrest. The past 16 years witnessed a progressive increase in the number of reported cases and a dramatic proliferation of articles serving to define the clinical, genetic, cellular, ionic, and molecular aspects of the disease. This article provides a brief overview of recent advances in our understanding of the clinical presentation and molecular and cellular mechanisms and an update of existing controversies.

Brugada syndrome has been linked to mutations in SCN5A. Agents that dissociate slowly from the sodium channel such as flecainide and ajmaline unmask the Brugada syndrome electrocardiogram and precipitate ventricular tachycardia/fibrillation. Lidocaine, an agent with rapid dissociation kinetics, has previously been shown to exert no effect in patients with Brugada syndrome. We characterized a novel double mutation of SCN5A (V232I in DI-S4+L1308F in DIII-S4) identified in a rare case of lidocaine (1 mg/kg)-induced Brugada syndrome. We studied lidocaine blockade of I(Na) generated by wild-type and V232I+L1308F mutant cardiac sodium channels expressed in mammalian TSA201 cells using patch clamp techniques. Despite no significant difference in steady-state gating parameters between V232I+L1308F and wild-type sodium currents at baseline, use-dependent inhibition of I(Na) by lidocaine was more pronounced in V232I+L1308F versus wild-type (73.0+/-0.1% versus 18.23+/-0.04% at 10 micromol/L measured at 10 Hz, respectively). A dose of 10 micromol/L lidocaine also caused a more negative shift of steady-state inactivation in V232I+L1308F versus wild-type (-14.1+/-0.3 mV and -4.8+/-0.3 mV, respectively). The individual mutations produced a much less accentuated effect. We report the first case of lidocaine-induced Brugada electrocardiogram phenotype. The double mutation in SCN5A, V232I, and L1308F alters the affinity of the cardiac sodium channel for lidocaine such that the drug assumes Class IC characteristics with potent use-dependent block of the sodium channel. Our results demonstrate an additive effect of the 2 missense mutations to sensitize the sodium channel to lidocaine. These findings suggest caution when treating patients carrying such genetic variations with Class I antiarrhythmic drugs.

The arrhythmogenic effects of hyperthermia have been highlighted in the Brugada syndrome but remain largely unexplored in other arrhythmic syndromes. The present study examines the effect of hyperthermia on transmural dispersion of action potential duration (TD-APD), early afterdepolarization (EAD) activity, and torsade de pointes (TdP) under long-QT conditions.

The risk of developing severe ventricular arrhythmias and/or organ toxicity by currently available drugs used to treat atrial fibrillation (AF) has prompted the development of atrial-selective antiarrhythmic agents. Until recently the principal focus has been on development of agents that selectively inhibit the ultra-rapid delayed rectifier outward potassium channels (I Kur), taking advantage of the presence of these channels in atria but not ventricles. Recent experimental studies have demonstrated important atrioventricular differences in biophysical properties of the sodium channel and have identified sodium channel blockers such as ranolazine and chronic amiodarone that appear to take advantage of these electrophysiologic distinctions and act to specifically or predominantly depress sodium channel-mediated parameters in "healthy" canine atria versus ventricles. Atrial-selective/predominant sodium channel blockers such as ranolazine effectively suppress AF in experimental models of AF involving canine isolated right atrial preparations at concentrations that produce little to no effect on ventricular electrophysiologic parameters. These findings point to atrial-selective sodium channel block as a new strategy for the management of AF. The present review examines our current understanding of atrioventricular distinctions between atrial and ventricular sodium channels and our understanding of the basis for atrial selectively of the sodium channel blockers. A major focus will be on the ability of the atrial-selective sodium channel blocking properties of these agents, possibly in conjunction with I Kur and/or I Kr blocking properties, to suppress and prevent the reinduction of AF.

Ectopic activity arising from the pulmonary veins (PV) plays a prominent role in the development of atrial fibrillation (AF).

A greater depression of the action potential (AP) of the ventricular epicardium (Epi) versus endocardium (Endo) is readily observed in experimental models of acute ischemia and Brugada syndrome. Endo and Epi differences in transient outward K(+) current and/or ATP-sensitive K(+) channel current are believed to contribute to the differential response. The present study tested the hypothesis that the greater sensitivity of Epi is due in part to its functionally distinct early fast Na(+) current (I(Na)). APs were recorded from isolated Epi and Endo tissue slices and coronary-perfused wedge preparations before and after exposures to elevated extracellular K(+) concentration ([K(+)](o); 6-12 mM). I(Na) was recorded from Epi and Endo myocytes using whole cell patch-clamp techniques. In tissue slices, increasing [K(+)](o) to 12 mM reduced V(max) to 51.1 +/- 5.3% and 26.8 +/- 9.6% of control in Endo (n = 9) and Epi (n = 14), respectively (P < 0.05). In wedge preparations (n = 12), the increase in [K(+)](o) caused selective depression of Epi APs and transmural conduction slowing and block. I(Na) density was not significantly different between Epi (n = 14) and Endo (n = 15) cells, but Epi cells displayed a more negative half-inactivation voltage [-83.6 +/- 0.1 and -75.5 +/- 0.3 mV for Epi (n = 16) and Endo (n = 16), respectively, P < 0.05]. Our data suggest that reduced I(Na) availability in ventricular Epi may contribute to its greater sensitivity to electrical depression and thus may contribute to the R-ST segment changes observed under a variety of clinical conditions including acute myocardial ischemia, severe hyperkalemia, and Brugada syndrome.

Short QT syndrome (SQTS) is a primary electrical disease of the heart associated with a high risk of sudden cardiac death. A gain-of-function in I(Kr), due to a mutation in KCNH2, underlies SQT1.

Antiarrhythmic drug therapy remains the principal approach for suppression of atrial fibrillation (AF) and flutter (AFl) and prevention of their recurrence. Among the current strategies for suppression of AF/AFl is the development of antiarrhythmic agents that preferentially affect atrial, rather than ventricular electrical parameters. Inhibition of the ultrarapid delayed rectifier potassium current (IKur), present in the atria, but not in the ventricles, is an example of an atrial-selective approach. Our recent study examined the hypothesis that sodium channel characteristics differ between atrial and ventricular cells and that atrial-selective sodium channel block is another effective strategy for the management of AF. We have demonstrated very significant differences in the inactivation characteristics of atrial versus ventricular sodium channels and a striking atrial selectivity for the action of ranolazine, an inactivated-state sodium channel blocker, to produce use-dependent block of the sodium channels, leading to depression of excitability, development of post-repolarization refractoriness (PRR), and suppression of AF. Lidocaine and chronic amiodarone, both predominantly inactivated-state sodium channel blockers, also produced a preferential depression of sodium channel-dependent parameters (VMax conduction velocity, diastolic threshold of excitation, and PRR) in the atria. Propafenone, a predominantly open-state sodium channel blocker, produced similar changes of electrophysiological parameters, which were was not atrial-selective. The ability of ranolazine, chronic amiodarone, and propafenone to prolong the atrial action potential potentiated their ability to suppress AF in coronary-perfused canine atrial preparations.

T-wave alternans (TWA) is characterized by beat to beat alteration in the amplitude, polarity and/or morphology of the electrocardiographic T wave. TWA has been reported in patients with the Brugada syndrome (BS) and is thought to be associated with an increased risk for development of VT/VF. The cellular mechanisms involved are not well-defined and are the subject of this investigation.

A number of antipsychotic and antidepressant drugs are known to increase the risk of ventricular arrhythmias and sudden cardiac death. Based largely on a concern over QT prolongation and the development of life-threatening arrhythmias, a number of antipsychotic drugs have been temporarily or permanently withdrawn from the market or their use restricted. Some antidepressants and antipsychotics have been linked to QT prolongation and the development of Torsade de pointes arrhythmias, whereas others have been associated with a Brugada syndrome phenotype and the development of polymorphic ventricular arrhythmias. This review examines the mechanisms and predisposing factors underlying the development of cardiac arrhythmias, and sudden cardiac death, associated with antidepressant and antipsychotic drugs in clinical use.

Previous studies indicate that action potential duration (APD) alternans is initiated in the endocardial (END) and midmyocardial (MID) regions rather than the epicardium (EPI) in the canine left ventricle (LV). This study examines regional differences in the rate dependence of Ca(2+) transient characteristics under conditions that give rise to APD and associated T wave alternans. The role of the sarcoplasmic reticulum (SR) was further evaluated by studying Ca(2+) transient characteristics in myocytes isolated from neonates, where an organized SR is poorly developed. All studies were performed in cells and tissues isolated from the canine LV. Isolated canine ENDO, MID, and EPI LV myocytes were either field stimulated or voltage clamped, and Ca(2+) transients were measured by confocal microscopy. In LV wedge preparations, increasing the basic cycle length (BCL) from 800 to 250 ms caused alternans to appear mainly in the ENDO and MID region; alternans were not observed in EPI under these conditions. Ca(2+) transient alternans developed in response to rapid pacing, appearing in EPI cells at shorter BCL compared with MID and ENDO cells (BCL=428 +/- 17 vs. 517 +/- 29 and 514 +/- 21, respectively, P < 0.05). Further increases in pacing rate resulted in the appearance of subcellular alternans of Ca(2+) transient amplitude, which also appeared in EPI at shorter BCL than in ENDO and MID cells. Ca(2+) transient alternans was not observed in neonate myocytes. We conclude that 1) there are distinct regional differences in the vulnerability to rate-dependent Ca(2+) alternans in dog LV that may be related to regional differences in SR function and Ca(2+) cycling; 2) the development of subcellular Ca(2+) alternans suggests the presence of intracellular heterogeneities in Ca(2+) cycling; and 3) the failure of neonatal cells to develop Ca(2+) alternans provides further support that SR Ca(2+) cycling is a major component in the development of these phenomena.

This review examines the role of spatial electrical heterogeneity within the ventricular myocardium on the function of the heart in health and disease. The cellular basis for transmural dispersion of repolarization (TDR) is reviewed, and the hypothesis that amplification of spatial dispersion of repolarization underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies is evaluated. The role of TDR in long QT, short QT, and Brugada syndromes, as well as catecholaminergic polymorphic ventricular tachycardia (VT), is critically examined. In long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells; in Brugada syndrome, however, it is thought to be due to selective abbreviation of the APD of the right ventricular epicardium. Preferential abbreviation of APD of the endocardium or epicardium appears to be responsible for the amplification of TDR in short QT syndrome. In catecholaminergic polymorphic VT, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, long QT, short QT, Brugada, and catecholaminergic polymorphic VT syndromes are pathologies with very different phenotypes and etiologies, but they share a common final pathway in causing sudden cardiac death.

Long QT Syndrome (LQTS) is an inherited disorder characterized by prolonged QT intervals and life-threatening polymorphic ventricular tachyarrhythmias. LQT1 caused by KCNQ1 mutations is the most common form of LQTS.

The development of selective atrial antiarrhythmic agents is a current strategy for suppression of atrial fibrillation (AF).

Torsade de pointes (TdP) is a life-threatening arrhythmia that develops as a consequence of a reduction in the repolarization reserve of cardiac cells leading to amplification of electrical heterogeneities in the ventricular myocardium as well as to the development of early after depolarization-induced triggered activity. Electrical heterogeneities within the ventricles are due to differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization that contributes to the inscription of the electrocardiographic T wave. A number of non-antiarrhythmic drugs and antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify spatial dispersion of repolarization, thus creating the substrate for re-entry. This results in a prolongation of the QT interval, abnormal T waves, and development of TdP. Agents that prolong the QT interval but do not cause an increase in transmural dispersion of repolarization (TDR) do not induce TdP, suggesting that QT prolongation is not the sole or optimal determinant for arrhythmogenesis. This article reviews recent advances in our understanding of these mechanisms, particularly the role of TDR in the genesis of drug-induced TdP, and examines how these may guide us towards development of safer drugs.

Ventricular tachycardia (VT) and ventricular fibrillation (VF) complicating Brugada syndrome, a genetic disorder linked to SCN5A mutations, and VF complicating acute myocardial infarction (AMI) both have been linked to phase 2 reentry.

This lecture examines the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization (TDR) underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies, including the long QT, short QT, and Brugada syndromes as well as catecholaminergic polymorphic ventricular tachycardia. In the long QT syndrome, amplification of TDR often is secondary to preferential prolongation of the action potential duration of M cells, whereas in Brugada syndrome, it is thought to be due to selective abbreviation of the action potential duration of right ventricular epicardium. In the short QT syndrome, preferential abbreviation of action potential duration of either endocardium or epicardium appears to be responsible for amplification of TDR. In catecholaminergic polymorphic ventricular tachycardia, reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. Thus, the long QT, short QT, Brugada, and catecholaminergic ventricular tachycardia syndromes are pathologies with very different phenotypes and etiologies. However, these syndromes share a common final pathway in their predisposition to sudden cardiac death.

Timothy syndrome is a multisystem disorder associated with QT interval prolongation and ventricular cardiac arrhythmias. The syndrome has been linked to mutations in Ca(V)1.2 resulting in gain of function of the L-type calcium current (I(Ca,L)). Ranolazine is an antianginal agent shown to exert an antiarrhythmic effect in experimental models of long QT syndrome.

Doctor Wilde, presenting on behalf of himself and Dr Eckardt, discussed the role of invasive and noninvasive tests for risk stratification of Brugada syndrome. Doctor Hiraoka, presenting on behalf of Y. Yokoyama, M. Takagi, N. Aihara, K. Aonuma, and the Japan Idiopathic Ventricular Fibrillation Study Investigators, further discussed the diagnostic criteria for the Brugada syndrome. Doctor Antzelevitch examined the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies including the long QT, short QT, and Brugada syndromes. Doctor Corrado discussed the relationship between channelopathies and heart muscle diseases.

Cardiac ion channelopathies are responsible for an ever-increasing number and diversity of familial cardiac arrhythmia syndromes. We describe a new clinical entity that consists of an ST-segment elevation in the right precordial ECG leads, a shorter-than-normal QT interval, and a history of sudden cardiac death.

Loss-of-function mutations in SCN5A have been associated with the Brugada syndrome. We report the first Brugada syndrome family with compound heterozygous mutations in SCN5A. The proband inherited 1 mutation from each parent and transmitted 1 to each daughter.

First introduced as a new clinical entity in 1992, the Brugada syndrome is associated with a relatively high risk of sudden death in young adults, and occasionally in children and infants. Recent years have witnessed a striking proliferation of papers dealing with the clinical and basic aspects of the disease. Characterized by a coved-type ST-segment elevation in the right precordial leads of the electrocardiogram (ECG), the Brugada syndrome has a genetic basis that thus far has been linked only to mutations in SCN5A, the gene that encodes the alpha-subunit of the sodium channel. The Brugada ECG is often concealed, but can be unmasked or modulated by a number of drugs and pathophysiological states including sodium channel blockers, a febrile state, vagotonic agents, tricyclic antidepressants, as well as cocaine and propranolol intoxication. Average age at the time of initial diagnosis or sudden death is 40 +/- 22, with the youngest patient diagnosed at 2 days of age and the oldest at 84 years. This review provides an overview of the clinical, genetic, molecular, and cellular aspects of the Brugada syndrome, incorporating the results of two recent consensus conferences. Controversies with regard to risk stratification and newly proposed pharmacologic strategies are discussed.

One hundred years after Willem Einthoven first recorded the electrocardiogram (ECG), physicians and scientists are still debating the cellular basis for the various waves of the ECG. In this review, our focus is on the cellular basis for the J, T, and U waves of the ECG. The J wave and T wave are thought to arise as a consequence of voltage gradients that develop as a result of the electrical heterogeneities that exist within the ventricular myocardium. The presence of a prominent action potential notch in epicardium but not endocardium gives rise to a voltage gradient during ventricular activation that inscribes the J wave. Transmural and apico-basal voltage gradients developing as a result of difference in the time course of repolarization of the epicardial, M, and endocardial cell action potentials, and the more positive plateau potential of the M cell contribute to inscription of the T wave. Amplification of these heterogeneities results in abnormalities of the J wave and T wave, leading to the development of the Brugada, long QT, and short QT syndromes. The basis for the U wave has long been a matter of debate. One theory attributes the U wave to mechanoelectrical feedback. A second theory ascribes it to voltage gradients within ventricular myocardium and a third to voltage gradients between the ventricular myocardium and the His-Purkinje system. Although direct evidence in support of any of these three hypotheses is lacking, recent studies involving the short QT syndrome have generated renewed interest in the mechanoelectrical hypothesis.

The autonomic nervous system has been implicated in several arrhythmogenic diseases, including long QT syndrome type 3 (LQT3) and Brugada syndrome. Scarce information on the cellular components of the intrinsic cardiac ganglia from higher mammals has limited our understanding of the role of the autonomic nervous system in such diseases.

Ventricular myocardium in larger mammals is composed of three distinct cell types: epicardial, M, and endocardial cells. Epicardial and M cell, but not endocardial cell, action potentials have a prominent I(to)-mediated notch. M cells are distinguished from the other cell types in that they display a smaller I(Ks), but a larger late I(Na) and I(Na-Ca). These ionic differences may account for the longer action potential duration (APD) and steeper APD-rate relationship of the M cell. The difference in the time course of repolarization of phase 1 and phase 3 contributes to the inscription of the electrocardiographic J wave and T wave, respectively. These repolarization gradients are modulated by electrotonic interactions, [K(+)](o), and agents or mutations that alter net repolarizing current. An increase in late I(Na), as occurring under a variety of pathophysiological states or in response to certain toxins, leads to a preferential prolongation of the M cell action potential, thus prolonging the QT interval and increasing transmural dispersion of repolarization (TDR), which underlies the development of torsade de pointes (TdP) arrhythmias. Agents that reduce late I(Na) are effective in reducing TDR and suppressing TdP. A reduction in peak I(Na) or an increase in net repolarizing current in the early phases of the action potential can lead to a preferential abbreviation of the action potential of epicardium in the right ventricle, and thus the development of a large TDR, phase 2 reentry, and polymorphic ventricular tachycardia associated with the Brugada syndrome.

Dimethyl lithospermate B (dmLSB) is an extract of Danshen, a traditional Chinese herbal remedy, which slows inactivation of INa, leading to increased inward current during the early phases of the action potential (AP). We hypothesized that this action would be antiarrhythmic in the setting of Brugada syndrome.

Early (EAD) and delayed (DAD) afterdepolarizations-induced triggered activity is capable of initiating and maintaining cardiac arrhythmias. EAD-induced triggered responses are traditionally thought to be involved in the generation of ventricular arrhythmias under long QT conditions and are precipitated by bradycardia or long pauses. In contrast, DAD-induced triggered activity commonly underlies arrhythmias precipitated by tachycardia. Spontaneous release of calcium from the sarcoplasmic reticulum (SR) secondary to cellular calcium overload induces DADs and some forms of EADs. Recent studies from our laboratory have uncovered a novel mechanism giving rise to triggered activity, termed "late-phase 3 EAD," which combines properties of both EAD and DAD, but has its own unique character. Late-phase 3 EAD-induced triggered extrasystoles represent a new concept of arrhythmogenesis in which abbreviated repolarization permits "normal SR calcium release" to induce an EAD-mediated closely coupled triggered response, particularly under conditions permitting intracellular calcium loading. This review briefly describes the mechanisms and properties of late-phase 3 EADs, how they differ from conventional EADs and DADs, as well as their role in the initiation of cardiac arrhythmias, such as atrial fibrillation.

Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).)

This review examines the hypothesis that amplification of spatial dispersion of repolarization in the form of transmural dispersion of repolarization (TDR) underlies the development of life-threatening ventricular arrhythmias associated with inherited ion channelopathies including the long QT, short QT and Brugada syndromes as well as catecholaminergic polymorphic ventricular tachycardia. In the long QT syndrome, amplification of TDR is often secondary to preferential prolongation of the action potential duration (APD) of M cells, whereas in the Brugada syndrome, it is thought to be because of selective abbreviation of the APD of right ventricular epicardium. Preferential abbreviation of APD of either endocardium or epicardium appears to be responsible for amplification of TDR in the short QT syndrome. In catecholaminergic polymorphic VT, the reversal of the direction of activation of the ventricular wall is responsible for the increase in TDR. In conclusion, the long QT, short QT, Brugada and catecholaminergic VT syndromes are pathologies with very different phenotypes and aetiologies, but which share a common final pathway in causing sudden death.

Resynchronization therapy involving right ventricular endocardial and left ventricular epicardial pacing improves cardiac output, quality of life, and New York Heart Association functional class in patients with congestive heart failure. Although a great deal of attention has been directed at showing the mechanical benefits and in fine-tuning the biventricular pacing configuration and protocol, little attention has been focused on the consequences of reversing the direction of activation of the left ventricular wall. Recent basic science and clinical studies have shown a proarrhythmic effect of reversing the direction of activation of the left ventricular wall. Reversal of the normal activation sequence prolongs the QT interval and increases the existing transmural dispersion of repolarization, creating the substrate and trigger for re-entrant arrhythmias under long QT conditions. A number of case reports of R-on-T extrasystoles and ventricular tachyarrhythmia induction as a result of biventricular pacing support this observation, and raise concern that biventricular pacing may be proarrhythmic in select cases, particularly when associated with a prolonged QT interval. Our focus in this review is on current understanding of transmural heterogeneity of repolarization that exists across the left ventricular wall, how this dispersion of repolarization is amplified as a consequence of reversal of the normal activation sequence, and how these basic experimental findings may apply to patients receiving cardiac resynchronization therapy.

We describe a genetic basis for atrial fibrillation and short QT syndrome in utero. Heterologous expression of the mutant channel was used to define the physiological consequences of the mutation.

Torsades de pointes (TdP) is a potentially lethal arrhythmia that develops as a consequence of amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization responsible for inscription of the ECG T wave. Antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify the intrinsic spatial dispersion of repolarization, thus creating the substrate for the development of reentry. The result is prolongation of the QT interval, abnormal T waves, and development of polymorphic reentrant ventricular tachycardia displaying characteristics of TdP. Prolongation of the QT interval apparently is not the sole determinant of a drug's potential to cause TdP. Agents that do not increase transmural dispersion of repolarization have little or no potential to induce TdP despite any ability to prolong the QT interval. In addition, drugs such as amiodarone and sodium pentobarbital can cause large QT prolongations but, by reducing transmural dispersion of repolarization, may reduce the likelihood of TdP. Arterially perfused wedge preparations of canine left ventricle can be used to explore the role of transmural dispersion of repolarization in the genesis of TdP. The purpose of this article is to review recent advances that have improved our understanding of these mechanisms, particularly the role of transmural dispersion of repolarization, in the genesis of drug-induced TdP and to examine how these advances can guide us toward the development of safer and more effective drugs.

ST-Segment elevation is a common electrocardiogram (ECG) manifestation of acute transmural myocardial ischemia in leads facing the injury. Acute myocardial ischemia involving the right-ventricular (RV) outflow tract is known to induce a Brugada-like ECG. In this paper, we examined the electrophysiological bases for the similarities between the ECG characteristics of the Brugada syndrome model induced by terfenadine (5 mumol/L) and the ECG manifestations of the acute transmural no-flow ischemia model.

Heterogeneity of transmural ventricular repolarization in the heart has been linked to a variety of arrhythmic manifestations. Electrical heterogeneity in ventricular myocardium is due to ionic distinctions among the three principal cell types: Endocardial, M and Epicardial cells. A reduction in net repolarizing current generally leads to a preferential prolongation of the M cell action potential. An increase in net repolarizing current can lead to a preferential abbreviation of the action potential of right ventricular epicardium or left ventricular endocardium. These changes can result in amplification of transmural heterogeneities of repolarization and thus predispose to the development of potentially lethal reentrant arrhythmias. The long QT, short QT, Brugada and catecholaminergic VT syndromes are all examples of pathologies that have very different phenotypes and aetiologies, but share a common final pathway in causing sudden death via amplification transmural or other spatial dispersion of repolarization within the ventricular myocardium. These same mechanisms are likely to be responsible for life-threatening arrhythmias in a variety of other cardiomyopathies ranging from heart failure and hypertrophy, which may involve mechanisms very similar to those operative in long QT syndrome, to ischaemia and infarction, which may involve mechanisms more closely resembling those responsible for the Brugada syndrome.

Short QT syndrome (SQTS) is characterized by ventricular arrhythmias and sudden death. One form of SQTS is caused by mutation N588K in human ether-a-go-go-related gene (HERG). In this study we sought to determine the potential role of N588K in arrhythmias.

Tamoxifen has been reported to directly activate large conductance calcium-activated potassium (KCa) channels through the KCa beta1 subunit, suggesting a cardio-protective role of this compound. The present study using knock-out (KO) mice for the KCa channel beta1 subunit was aimed at understanding the molecular mechanisms of the effects of tamoxifen on arterial smooth muscle KCa channels. Single channel studies were conducted in excised patches from cerebral artery myocytes from both wild-type and KO animals. The present data demonstrated that tamoxifen can inhibit arterial KCa channels due to a major decrease in channel open probability (P(o)), a mechanism different from the reduction in single channel amplitude reported previously and also observed in the present work. A tamoxifen-induced decrease in P(o) was present in arterial KCa channels from both wild-type and beta1 KO animals. This inhibition was concentration-dependent and partially reversible with a half-maximal concentration constant IC(50) of 2.6 microm. The effect of tamoxifen was actually dual Single channel kinetic analysis showed that tamoxifen shortens both mean closed time and mean open time; the latter is probably due to an intermediate duration voltage-independent blocking mechanism. Thus, tamoxifen block would predominate when KCa channel P(o) is >0.1-0.2, limiting the maximum P(o), whereas a leftward shift in voltage or Ca(2+) activation curves can be observed for P(o) values lower than those values. This dual effect of tamoxifen appears to be independent of the beta1 subunit. The molecular specificity of tamoxifen, or eventually other xenoestrogen derivatives, for the KCa channel beta1 subunit is uncertain.

Ventricular myocardium in larger mammals has been shown to be comprised of three distinct cell types: epicardial, M, and endocardial. Epicardial and M cell action potentials differ from endocardial cells with respect to the morphology of phase 1. These cells possess a prominent I(to)-mediated notch responsible for the "spike and dome" morphology of the epicardial and M cell response. M cells are distinguished from the other cell types in that they display a smaller I(Ks), but a larger late I(Na) and I(Na-Ca). These ionic distinctions underlie the longer action potential duration (APD) and steeper APD-rate relationship of the M cell. Difference in the time course of repolarization of phase 1 and phase 3 are responsible for the inscription of the electrocardiographic J wave and T wave, respectively. These repolarization gradients are sensitively modulated by electrotonic communication among the three cells types, [K(1)](o), and the presence of drugs that either reduce or augment net repolarizing current. A reduction in net repolarizing current generally leads to a preferential prolongation of the M cell action potential, responsible for a prolongation of the QT interval and an increase in transmural dispersion of repolarization (TDR), which underlies the development of torsade de pointes arrhythmias. An increase in net repolarizing current can lead to a preferential abbreviation of the action potential of epicardium in the right ventricle (RV), and endocardium in the left ventricle (LV). These actions also lead to a TDR that manifests as the Brugada syndrome in RV and the short QT syndrome in LV.

Although the role of action potential duration restitution (APD-R) in the initiation and maintenance of ventricular fibrillation (VF) has been the subject of numerous investigations, its role in the generation of atrial fibrillation (AF) is less well studied. The cellular and ionic basis for coarse versus fine AF is not well delineated.

Mutations in the ryanodine 2 receptor (RyR2) gene have been identified in patients with catecholaminergic polymorphic ventricular tachycardia. We examined the cellular basis for the ECG features and arrhythmia mechanisms using low-dose caffeine to mimic the defective calcium homeostasis encountered under these conditions.

Atrial fibrillation remains one of the most challenging arrhythmias for the clinician and basic researcher. Different approaches have been undertaken to improve its understanding; from the development of animal models to the analysis of genetic backgrounds in individuals with familial and acquired forms of the disease. In the last few years, a large body of evidence has shown that alterations in ionic currents are involved in the disease. However, it has not been until recently, with the genetic link between mutations in proteins responsible for these ionic currents and the familial disease, that we have been given the final evidence that atrial fibrillation can also be primarily an ion channelopathy. Despite the limited prevalence of the inherited diseases, it has been shown before that the knowledge gained in their study will be helpful in dealing with the most common acquired forms of the disease. Therefore, as data keep unraveling, clinicians can expect that soon better therapeutic and preventive options for atrial fibrillation will emerge from basic science.

Since its introduction as a clinical entity in 1992, the Brugada syndrome has progressed from being a rare disease to one that is second only to automobile accidents as a cause of death among young adults in some countries. Electrocardiographically characterized by a distinct ST-segment elevation in the right precordial leads, the syndrome is associated with a high risk for sudden cardiac death in young and otherwise healthy adults, and less frequently in infants and children. Patients with a spontaneously appearing Brugada ECG have a high risk for sudden arrhythmic death secondary to ventricular tachycardia/fibrillation. The ECG manifestations of Brugada syndrome are often dynamic or concealed and may be unmasked or modulated by sodium channel blockers, a febrile state, vagotonic agents, alpha-adrenergic agonists, beta-adrenergic blockers, tricyclic or tetracyclic antidepressants, a combination of glucose and insulin, hypo- and hyperkalemia, hypercalcemia, and alcohol and cocaine toxicity. In recent years, an exponential rise in the number of reported cases and a striking proliferation of articles defining the clinical, genetic, cellular, ionic, and molecular aspects of the disease have occurred. The report of the first consensus conference, published in 2002, focused on diagnostic criteria. The present report, which emanated from the second consensus conference held in September 2003, elaborates further on the diagnostic criteria and examines risk stratification schemes and device and pharmacological approaches to therapy on the basis of the available clinical and basic science data.

The Brugada syndrome (BS) is characterized by ST segment elevation in the right precordial leads and sudden cardiac death. The disease is linked to mutations in SCN5A in approximately 20% of cases. We collected a large family with BS and have identified a novel intronic mutation. We performed the clinical, genetic, molecular and biophysical characterization of this disease-causing mutation. With direct sequencing we identified an intronic insertion of TGGG 5 bp from the end of the Exon 27 of SCN5A. For transcript analysis, we investigated Epstein-Barr-transformed lymphoblastoid cell lines from patients and controls. Total RNA was extracted and RT-PCR experiments were performed to analyze the splicing patterns in exon 27 and 28. We identified two bands, one of the expected size and the other which showed a 96 bp deletion in exon 27, leading to a 32 amino acid in-frame deletion involving segments 2 and 3 of Domain IV of the SCN5A protein. This finding indicates that the intronic mutation creates a cryptic splice site inside Exon 27. Biophysical analysis using whole-cell patch-clamp techniques showed a complete loss of function of the mutated channels when heterologously expressed. In summary, this is the first report of a dysfunctional sodium channel created by an intronic mutation giving rise to cryptic splice site activation in SCN5A in a family with the BS. The deletion of fragments of segments 2 and 3 of Domain IV leads to complete loss of function, consistent with the biophysical data found in several mutations causing BS.

The short QT syndrome is a newly described clinical entity characterized by the presence of a short QT interval associated with cardiac tachyarrhythmias including sudden cardiac death at a young age in otherwise healthy individuals. A genetic basis has been identified linking the disease to mutations in KCNH2 in the familial forms and a mutation in KCNQ1 in a sporadic form of the disease.

Since its introduction as a clinical entity in 1992, the Brugada syndrome has progressed from being a rare disease to one that is second only to automobile accidents as a cause of death among young adults in some countries. Electrocardiographically characterized by a distinct ST-segment elevation in the right precordial leads, the syndrome is associated with a high risk for sudden cardiac death in young and otherwise healthy adults, and less frequently in infants and children. Patients with a spontaneously appearing Brugada ECG have a high risk for sudden arrhythmic death secondary to ventricular tachycardia/fibrillation. The ECG manifestations of Brugada syndrome are often dynamic or concealed and may be unmasked or modulated by sodium channel blockers, a febrile state, vagotonic agents, alpha-adrenergic agonists, beta-adrenergic blockers, tricyclic or tetracyclic antidepressants, a combination of glucose and insulin, hypo- and hyperkalemia, hypercalcemia, and alcohol and cocaine toxicity. In recent years, an exponential rise in the number of reported cases and a striking proliferation of articles defining the clinical, genetic, cellular, ionic, and molecular aspects of the disease have occurred. The report of the first consensus conference, published in 2002, focused on diagnostic criteria. The present report, which emanated from the second consensus conference held in September 2003, elaborates further on the diagnostic criteria and examines risk stratification schemes and device and pharmacological approaches to therapy on the basis of the available clinical and basic science data.

Since its introduction as a new clinical entity in 1992, the Brugada syndrome has attracted great interest because of its high incidence in many parts of the world and its association with high risk for sudden death in infants, children, and young adults. Recent years have witnessed an exponential rise in the number of reported cases and a striking proliferation of articles serving to define the clinical, genetic, cellular, ionic, and molecular aspects of the disease. A consensus report published in 2002 delineated diagnostic criteria for the syndrome. A second consensus conference was held in September 2003. This review provides an in-depth overview of the clinical, genetic, molecular, and cellular aspects of the Brugada syndrome, incorporating the results of the two consensus conferences, and the numerous clinical and basic publications on the subject. The proposed terminology, diagnostic criteria, risk stratification schemes, and device and pharmacologic approach to therapy discussed are based on available clinical and basic studies and should be considered a work-in-progress that will without doubt require fine-tuning as confirmatory data from molecular studies and prospective trials become available.

The short-QT syndrome is a new clinical entity characterized by corrected QT intervals <300 ms and a high incidence of ventricular tachycardia (VT) and fibrillation (VF). Gain-of-function mutations in the gene for outward potassium currents have been shown to underlie the congenital syndrome. The present study examined the cellular basis of VT/VF in an experimental model associated with short QT intervals created with a potassium channel activator.

Brugada syndrome is increasingly being recognized in clinical medicine. What started as an electrocardiographic curiosity has become an important focus of attention for individuals working in the different disciplines related to sudden cardiac death, from basic scientists to clinical cardiac electrophysiologists. In just 12 years, since the description of the disease, clinically relevant information is continuously being provided to physicians to help protect the individuals with Brugada syndrome to the best of our ability. And this information has been gathered thanks to the effort of hundreds of basic scientists, physicians and patients who continue to give their time, effort and data to help understand how the electrocardiographic pattern may cause sudden cardiac death. There are still many unanswered questions, both at the clinical and basic field. However, with the further collection of data, the longer follow-up and the continued interest from the basic science world we will have the necessary tools to the successful unraveling of the disease.

The Brugada syndrome is an arrhythmogenic disease caused in part by mutations in the cardiac sodium channel gene, SCN5A. The electrocardiographic pattern characteristic of the syndrome is dynamic and is often absent in affected individuals. Sodium channel blockers are effective in unmasking carriers of the disease. However, the value of the test remains controversial.

The cellular basis for the monophasic action potential (MAP) has long been a matter of debate. At the center of the controversy is the issue as to which of the two electrodes is the recording electrode and which is the indifferent electrode. The present study is designed to address this issue.

Ranolazine is a novel antianginal agent capable of producing anti-ischemic effects at plasma concentrations of 2 to 6 microM without a significant reduction of heart rate or blood pressure. This review summarizes the electrophysiologic properties of ranolazine. Ranolazine significantly blocks I(Kr) (IC(50) = 12 microM), late I(Na), late I(Ca), peak I(Ca), I(Na-Ca) (IC(50) = 5.9, 50, 296, and 91 microM, respectively) and I(Ks) (17% at 30 microM), but causes little or no inhibition of I(to) or I(K1). In left ventricular tissue and wedge preparations, ranolazine produces a concentration-dependent prolongation of action potential duration (APD) in epicardium, but abbreviation of APD of M cells, leading to either no change or a reduction in transmural dispersion of repolarization (TDR). The result is a modest prolongation of the QT interval. Prolongation of APD and QT by ranolazine is fundamentally different from that of other drugs that block I(Kr) and induce torsade de pointes in that APD prolongation is rate-independent (ie, does not display reverse rate-dependent prolongation of APD) and is not associated with early after depolarizations, triggered activity, increased spatial dispersion of repolarization, or polymorphic ventricular tachycardia. Torsade de pointes arrhythmias were not observed spontaneously nor could they be induced with programmed electrical stimulation in the presence of ranolazine at concentrations as high as 100 microM. Indeed, ranolazine was found to possess significant antiarrhythmic activity, acting to suppress the arrhythmogenic effects of other QT-prolonging drugs. Ranolazine produces ion channel effects similar to those observed after chronic exposure to amiodarone (reduced late I(Na), I(Kr), I(Ks), and I(Ca)). Ranolazine's actions to reduce TDR and suppress early after depolarization suggest that in addition to its anti-anginal actions, the drug possesses antiarrhythmic activity.

Ranolazine is a novel antianginal agent capable of producing antiischemic effects at plasma concentrations of 2 to 6 micromol/L without reducing heart rate or blood pressure. The present study examines its electrophysiological effects in isolated canine ventricular myocytes, tissues, and arterially perfused left ventricular wedge preparations.

We hypothesized that a combination of I(Na) and I(Ca) blockade may be more effective in causing loss of the epicardial action potential (AP) dome and precipitating the Brugada syndrome (BS). The present study was designed to test this hypothesis in an in vitro model of BS.

The role of electrical heterogeneity in development of cardiac arrhythmias is well recognized. The extent to which transmembrane action potential (TAP) heterogeneity contributes to the normal electrophysiology of well-oxygenated atria is not well defined. The principal objective of the present study was to define regional and transmural differences in characteristics of the TAP in isolated superfused and arterially perfused canine right atrial (RA) preparations under baseline, rapidly activating delayed rectifier K(+) current (I(Kr)) block, and combined block of ultrarapid delayed rectifier and transient outward K(+) current (I(Kur)/I(to) block). Superfused preparations that survived generally displayed a triangle-shaped TAP. Exceptions included cells from the crista terminalis, where TAPs with a normal plateau could be recorded. In contrast, most TAPs recorded from throughout the perfused RA displayed a spike-and-dome and/or plateau morphology. The perfused RA displayed a heterogeneous distribution of repolarization, V(max), and spike-and-dome morphology along the epicardial and endocardial surfaces as well as transmurally, in the region of the upper crista terminalis. I(Kr) block with E-4031 prolonged repolarization homogeneously in the perfused RA, whereas I(Kur)/I(to) block using low concentrations of 4-aminopyridine abbreviated action potential duration at 90% repolarization heterogeneously, leading to a reduction in dispersion of repolarization. Our data indicate that the electrical heterogeneities, previously described for the canine ventricle, also exist within the atria and that I(Kr) block does not accentuate and I(Kur)/I(to) block reduces RA dispersion of repolarization. Our study also points to major differences in the transmembrane activity recorded using superfused vs. arterially perfused atrial preparations.

Epicardial pacing of the left ventricle (LV) has been shown to prolong the QT interval and predispose to the development of torsade de pointes arrhythmias. The present study examines the cellular basis for QT prolongation and arrhythmogenesis after reversal of the direction of activation of the LV wall.

Although electrical heterogeneity within the ventricular myocardium has been the focus of numerous studies, little attention has been directed to the mechanical correlates. This study examines unloaded cell shortening, Ca(2+) transients, and inward L-type Ca(2+) current (I(Ca,L)) characteristics of epicardial, endocardial, and midmyocardial cells isolated from the canine left ventricle. Unloaded cell shortening was recorded using a video edge detector, Ca(2+) transients were measured in cells loaded with 15 microM fluo-3 AM and voltage and current-clamp recordings were obtained using patch-clamp techniques. Time to peak and latency to onset of contraction were shortest in epicardial and longest in endocardial cells; midmyocardial cells displayed an intermediate time to peak. When contraction was elicited using uniform voltage-clamp square waves, epicardial versus endocardial distinctions persisted and midmyocardial cells displayed a time to peak comparable to that of epicardium. The current-voltage relationship for I(Ca,L) and fluorescence-voltage relationship were similar in the three cell types when quantitated using square pulses. However, peak I(Ca,L) and total charge were significantly larger when an epicardial versus endocardial action potential waveform was used to elicit the current under voltage-clamp conditions. Sarcoplasmic reticulum Ca(2+) content, assessed by rapid application of caffeine, was largest in epicardial cells and contributed to a faster time to peak. Our data point to important differences in calcium homeostasis and mechanical function among the three ventricular cell types. These differences serve to synchronize contraction across the ventricular wall. Although these distinctions are conferred in part by differences in electrical characteristics of the three cell types, intrinsic differences in excitation-contraction coupling are evident.

Atrial fibrillation (AF) is the most common arrhythmia seen in medical practice. Despite the overall advance in the treatment of the cardiac dysrhythmias with the introduction of radiofrequency ablation, therapeutic options in AF have remained largely unchanged and aimed at controlling the heart rate and anticoagulation. New surgical and ablation techniques are being developed. While promising, they are still extremely laborious and available only to a handful of patients. The limited success in the therapy of AF is in part due to our poor understanding of its molecular pathophysiology. Molecular research of AF has focused on 2 main fields, identification of the genes that play a role in the initiation of the disease and altered gene expression during the disease state. These studies are aimed at identifying not only the triggering factors in the acute form but also those that perpetuate the arrhythmia and convert it into a chronic form. Advances in genetics and molecular biology will likely give new insights into the development of the disease and improve our understanding and therapeutic options.

Sudden cardiac death takes the lives of more than 300 000 Americans annually. Malignant ventricular arrhythmias occurring in individuals with structurally normal hearts account for a subgroup of these sudden deaths. The present study describes the genetic basis for a new clinical entity characterized by sudden death and short-QT intervals in the ECG.

Brugada syndrome is characterized by sudden death secondary to malignant arrhythmias and the presence of ST segment elevation in leads V(1) to V(3) of patients with structurally normal hearts. This ECG pattern often is concealed but can be unmasked using potent sodium channel blockers. Like congenital long QT syndrome type 3 (LQT3) and sudden unexpected death syndrome, Brugada syndrome has been linked to mutations in SCN5A.

Cisapride, a gastrointestinal prokinetic agent, was recently withdrawn from the market because of its propensity to induce torsade de pointes (TdP) arrhythmias. The present study examines the electrophysiological actions of cisapride in the isolated arterially perfused canine left ventricular wedge preparation.

Brugada syndrome, first described as a new clinical entity by Pedro and Josep Brugada in 1992, has attracted great interest because of its high prevalence in many regions of the world and its association with high risk for sudden death. The syndrome has captured the attention of the cardiac electrophysiology community because it serves as a paradigm for our understanding of the role of spatial dispersion of repolarization in the development of cardiac arrhythmias. The past decade has witnessed an exponential rise in the number of reported cases and a striking proliferation of papers serving to define the clinical, genetic, cellular, ionic and molecular aspects of this disease. This brief review summarizes the key clinical and experimental milestones that have brought us to our current understanding and approach to therapy of the Brugada syndrome.

Atrial fibrillation (AF) at times recurs immediately after termination of the arrhythmia. The mechanism(s) responsible for the extrasystole that reinduces AF is largely unknown. We hypothesized that abbreviation of action potential duration (APD) would permit very rapid rates of excitation, known to induce intracellular calcium loading, which in turn could promote delayed and/or early afterdepolarizations (EADs).

An intriguing new clinical entity characterized by ST-segment elevation in the right precordial electrocardiographic leads and a high incidence of sudden death in individuals with structurally normal hearts was described by Pedro and Josep Brugada in 1992. The past decade has witnessed an exponential rise in the number of reported cases and a dramatic proliferation of papers serving to define the clinical, genetic, cellular, ionic, and molecular aspects of this disease. The purpose of this brief review is to chronicle the historical highlights that have brought us to our present understanding of Brugada syndrome.

The slowly activating delayed rectifier potassium current (IKs) contributes prominently to ventricular repolarization of the cardiac action potential. Development of a selective IKs blocker is important for the elucidation of the physiologic and pathophysiologic relevance of IKs and the development of antiarrhythmic strategies. HMR 1556 [(3R,4S)-(+)-N-[3-hydroxy-2,2-dimethyl-6-(4,4,4-trifluorobutoxy) chroman-4-yl]-N-methylmethanesulfonamide] is a new chromanol derivative developed as a selective IKs blocker. Chromanol 293B, the most specific IKs blocker currently available, also inhibits the transient outward current (Ito). HMR 1556 was examined for its effects on IKs compared with rapidly activating delayed rectifier (IKr), inward rectifier (IK1), Ito, and L-type calcium (ICa.L) currents in canine left ventricular myocytes. HMR 1556 (0.5-500 nM ) inhibited IKs in a concentration-dependent manner (IC50 of 10.5 nM, compared with chromanol 293B's IC50 of 1.8microM). Inhibition of Ito was observed only at relatively high concentrations (IC50 of 33.9 microM comparable to chromanol 293B's IC of 38 microM). High concentrations of HMR 1556 also inhibited ICa.L (IC of 27.5 microM) and IKr (IC50 of 12.6 microM) while IK1 was unaffected. Our results indicate that HMR 1556 is superior to chromanol 293B in its potency and specificity for inhibition of IKs, making it a valuable experimental tool and a potential therapeutic agent.

The Brugada syndrome has gained wide recognition throughout the world and today is believed to be responsible for 4% to 12% of all sudden deaths and approximately 20% of deaths in patients with structurally normal hearts. The incidence of the disease is on the order of 5 per 10 000 inhabitants and, apart from accidents, is the leading cause of death of men under the age of 50 in regions of the world where the inherited syndrome is endemic. This minireview briefly summarizes the progress made over the past decade in our understanding of the clinical, genetic, cellular, ionic, and molecular aspects of this disease.

The Brugada syndrome displays an autosomal dominant mode of transmission with low penetrance. Despite equal genetic transmission of the disease, the clinical phenotype is 8 to 10 times more prevalent in males than in females. The basis for this intriguing sex-related distinction is unknown. The present study tests the hypothesis that the disparity in expression of the Brugada phenotype is a result of a more prominent I(to)-mediated action potential notch in the right ventricular (RV) epicardium of males versus females.

Previous studies from our laboratory demonstrated (1) a much larger I(Ks) and (2) inability to induce early afterdepolarization (EAD) activity in epicardial and endocardial cells versus M cells. This study tests the hypothesis that these two characteristics are interrelated.

QT prolongation is commonly associated with life-threatening torsade de pointes arrhythmias that develop as a consequence of the amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization responsible for the inscription of the electrocardiographic T wave. Agents and conditions that reduce net repolarizing current amplify the intrinsic spatial dispersion of repolarization, thus creating the substrate for the development of re-entry. The result is a prolongation of the QT interval, abnormal T waves, and development of polymorphic re-entrant ventricular tachycardia displaying characteristics of torsades de pointes. These conditions also predispose M cells and Purkinje fibers to develop early afterdepolarization-induced extrasystoles, which are thought to trigger episodes of torsades de pointes. Agents that prolong the QT interval but do not increase transmural dispersion of repolarization are not capable of inducing torsades de pointes. The available data suggest that that the principal problem with the long QT syndrome is not long QT intervals but rather the dispersion of repolarization that often accompanies prolongation of the QT interval.

Recent studies of the molecular basis of the long QT syndrome (LQTS) have advanced our understanding of the mechanisms responsible for the abnormal prolongation of ventricular repolarization and revealed associations between LQTS and other primary electrical diseases of the heart such as Brugada syndrome. The role of DNA single nucleotide polymorphisms in acquired LQTS and differences between the Romano-Ward and Jervell-Lange-Nielsen forms of congenital LQTS are gradually coming into focus. In this brief review, our goal is to summarize the molecular mechanisms proposed to underlie the susceptibility to arrhythmias in LQTS and discuss the direction of current and future research.

A growing number of cardiomyopathies have been shown to result in a reduction in both I(Kr) and I(Ks) yet little is known about the electrophysiologic and ECG characteristics of combined I(Kr) and I(Ks) block.

Recent studies have established the presence of three distinct cell types in the ventricular myocardium: epicardial, M and endocardial cells. Epicardial and M cell action potentials differ from endocardial cells with respect to the morphology of phase 1. These cells possess a prominent transient outward current (I(to))-mediated notch responsible for the 'spike and dome' morphology of the epicardial and M cell response. M cells are distinguished from the other cell types in that they display a smaller slowly activating delayed rectifier current (I(Ks)), but a larger late sodium current (late I(Na)) and sodium-calcium exchange current (I(Na-Ca)). These ionic distinctions underlie the longer action potential duration (APD) and steeper APD-rate relationship of the M cell, which is more pronounced in the presence of antiarrhythmic agents with class III actions. The preferential prolongation of the M cell action potential results in the development of a transmural dispersion of repolarization (TDR), which can be estimated from the electrocardiogram (ECG) as the interval between the peak and the end of the T wave (QTpeak-QTend interval). Using the canine arterially perfused ventricular wedge model, transmembrane action potentials of the various cardiac cell types can be correlated to the waveforms of the ECG, providing insight into the cellular etiology of ECG abnormalities. Two congenital syndromes of sudden cardiac death that have been modeled using this technique are the long QT and Brugada syndromes. The long QT syndrome has been linked to 5 gene mutations on chromosomes 3, 7, 11, and 21. Mutations in the cardiac sodium channel SCN5A have been linked to families with a history of the Brugada syndrome. Although the etiologies of these two syndromes are different, lethal arrhythmias in both are thought to arise due to amplification of intrinsic electrical heterogeneities. Similar mechanisms are likely responsible for life-threatening arrhythmias in a variety of other cardiomyopathies ranging from heart failure and hypertrophy, which involve mechanisms similar to those operative in LQTS, to ischemia and infarction, which may involve mechanisms more closely resembling those responsible for the Brugada syndrome.

Action potentials and whole cell sodium current were recorded in canine epicardial, midmyocardial, and endocardial myocytes in normal sodium at 37 degrees C. Tetrodotoxin (TTX) reduced the action potential duration of midmyocardial cells to a greater degree than either epicardial or endocardial cells. Whole cell recordings in potassium-free and very-low-chloride solutions revealed a slowly decaying current that was completely inhibited by 5 microM TTX or replacement of external and internal sodium with the impermeant cation N-methyl-D-glucamine. Late sodium current density at 0 mV was 47% greater in midmyocardial cells and averaged -0.532 +/- 0.058 pA/pF in endocardial, -0.463 +/- 0.068 pA/pF in epicardial, and -0.785 +/- 0.070 pA/pF in midmyocardial cells. Neither the frequency dependence of late sodium current nor its recovery from inactivation exhibited transmural differences. After a 4.5-s pulse to -30 mV, late sodium current recovered with a single time constant of 140 ms. We conclude that a larger late sodium conductance in midmyocardial cells will favor longer action potentials in these cells. More importantly, drugs that slow inactivation of sodium channels will produce a nonuniform response across the ventricular wall that is proarrhythmic.

Regional differences in electrical properties of cardiac cells contribute to the normal function of the heart as well as to the inscription of the J wave and T wave of the ECG. Amplification of these electrical heterogeneities can lead to the development of life-threatening cardiac arrhythmias and sudden death. A number of ionic distinctions have been shown to contribute to the different action potential morphologies of epicardial, M and endocardial ventricular cells as well as to the distinctive responses of these three cell types to pharmacological agents and pathophysiological states (for reviews see Antzelevitch et al. 1999; Antzelevitch & Dumaine, 2000).

Drugs that selectively inhibit the slowly activating component of the delayed rectifier potassium current (I(Ks)) are being considered as possible antiarrhythmic agents, because they produce more prolongation of action potential duration at fast rates with less transmural dispersion of repolarization compared with blockers of the rapidly activating component (I(Kr)). Although the chromanol derivative chromanol 293B has been shown to be relatively selective in blocking I(Ks) in some species, its selectivity is far from established.

The mechanisms responsible for active cardiac arrhythmias are generally divided into two major categories: (1) enhanced or abnormal impulse formation and (2) reentry. Reentry can be subdivided into three subcategories: (1) circus movement, (2) reflection, and (3) Phase 2 reentry. Reentry occurs when a propagating impulse fails to die out after normal activation of the heart and persists to re-excite the heart after expiration of the refractory period. Evidence implicating reentry as a mechanism of cardiac arrhythmias stems back to the turn of the century. Amplification of intrinsic electrical heterogeneities provides the substrate responsible for developing Phase 2 and circus movement reentry, which underlie ventricular tachycardia in the long QT and Brugada syndromes.

This study examines the effects of nicorandil, a K(+) channel opener, on transmural dispersion of repolarization (TDR) and induction of torsade de pointes (TdP) under conditions mimicking the LQT1, LQT2, and LQT3 forms of the congenital long-QT syndrome (LQTS).

This study examines the amplitude of sodium-calcium exchange current (I(NaCa)) in epicardial, midmyocardial, and endocardial canine ventricular myocytes. Whole cell currents were recorded at 37( degrees )C using standard or perforated-patch voltage-clamp techniques in the absence of potassium, calcium-activated chloride, and sodium-pump currents. I(NaCa) was triggered by release of calcium from the sarcoplasmic reticulum or by rapid removal of external sodium. I(NaCa) was large in midmyocardial myocytes and significantly smaller in endocardial myocytes, regardless of the method used to activate I(NaCa). I(NaCa) at -80 mV was -0.316 +/- 0. 013, -0.293 +/- 0.016, and -0.210 +/- 0.007 pC/pF, respectively, in midmyocardial, epicardial, and endocardial myocytes when activated by the calcium transient. When triggered by sodium removal, peak I(NaCa) was 0.74 +/- 0.04, 0.57 +/- 0.04, and 0.50 +/- 0.03 pA/pF, respectively, in midmyocardial, epicardial, and endocardial myocytes. Epicardial I(NaCa) was smaller than midmyocardial I(NaCa) when activated by removal of external sodium but was comparable to epicardial and midmyocardial I(NaCa) when activated by the normal calcium transient, implying possible transmural differences in excitation-contraction coupling. Our results suggest that I(NaCa) differences contribute to transmural electrical heterogeneity under normal and pathological states. A large midmyocardial I(NaCa) may contribute to the prolonged action potential of these cells as well as to the development of triggered activity under calcium-loading conditions.

An early afterdepolarization (EAD)-induced triggered beat is thought to precipitate torsade de pointes (TdP) in the long QT syndrome (LQTS). Previous studies demonstrated the development of EAD activity and dispersion of repolarization under LQT2 (reduced I(Kr)) and LQT3 (augmented late I(Na)), but not LQT1 (reduced I(Ks)), conditions. The present study examines these electrophysiologic characteristics during I(Ks) block.

To define the cellular mechanisms responsible for the development of life-threatening arrhythmias in response to sympathetic activity in the congenital and acquired long QT syndromes (LCQTS).

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the increased production of antibodies reactive with a variety of self and non-self antigens. A number of immunomodulatory therapies have been investigated for the disease process. Intragastric administration of low dose kidney extract (KE) three times weekly for 5 weeks and then weekly until 6 months of age in SLE mice, showed decreased anti-dsDNA antibody levels, less kidney damage and significantly prolonged survival compared with control phosphate buffered saline (PBS)-fed mice. The KE-fed mice also exhibited reduced T cell proliferative response to KE in comparison with PBS-fed controls. Serum isotype distribution of the anti-dsDNA antibodies revealed a marked reduction of IgG1 and IgG3 responses in the KE-fed mice. While the renal inflammatory cell infiltration and expression of interleukin-4 (IL-4) and IL-10 were markedly suppressed, no local enhancement of transforming growth factor-beta (TGF-beta) was detected. Oral administration of low dose KE, however, upregulated expression of IL-2, IFN-gamma and TNF-alpha in the kidneys and suppressed glomerulonephritis. These findings suggest that oral KE affects the disease process in SLE and raise the possibility that oral administration of KE or other potential autoantigens may provide a new approach for the treatment of SLE.

The Brugada syndrome is a major cause of sudden death, particularly among young men of Southeast Asian and Japanese origin. The syndrome is characterized electrocardiographically by an ST-segment elevation in V1 through V3 and a rapid polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. Our group recently linked the disease to mutations in SCN5A, the gene encoding for the alpha subunit of the cardiac sodium channel. When heterologously expressed in frog oocytes, electrophysiological data recorded from the Thr1620Met missense mutant failed to adequately explain the electrocardiographic phenotype. Therefore, we sought to further characterize the electrophysiology of this mutant. We hypothesized that at more physiological temperatures, the missense mutation may change the gating of the sodium channel such that the net outward current is dramatically augmented during the early phases of the right ventricular action potential. In the present study, we test this hypothesis by expressing Thr1620Met in a mammalian cell line, using the patch-clamp technique to study the currents at 32 degrees C. Our results indicate that Thr1620Met current decay kinetics are faster when compared with the wild type at 32 degrees C. Recovery from inactivation was slower for Thr1620Met at 32 degrees C, and steady-state activation was significantly shifted. Our findings explain the features of the ECG of Brugada patients, illustrate for the first time a cardiac sodium channel mutation of which the arrhythmogenicity is revealed only at temperatures approaching the physiological range, and suggest that some patients may be more at risk during febrile states.

The Brugada syndrome is characterized by marked ST-segment elevation in the right precordial ECG leads and is associated with a high incidence of sudden and unexpected arrhythmic death. Our study examines the cellular basis for this syndrome.

The present study was designed to examine regional differences in the response of alpha 1 adrenoceptor stimulation in the canine ventricle.

The discovery and characterization of the M cell, a unique cell type residing in the deep layers of the ventricular myocardium, has opened a new door in our understanding of the electrophysiology and pharmacology of the heart in both health and disease. The hallmark of the M cell is the ability of its action potential to prolong much more than that of other ventricular myocardial cells in response to a slowing of rate and/or in response to agents that act to prolong action potential duration. Our goal in this review is to provide a comprehensive characterization of the M cell, its contribution to transmural heterogeneity, and its role in the normal electrical function of the heart, in the inscription of the ECG (particularly the T wave), and in the development of QT dispersion, T wave alternans, long QT intervals, and cardiac arrhythmias, such as torsades de pointes. Our secondary goal is to address the controversy that has arisen relative to the functional importance of the M cell in the normal heart. The controversy derives largely from the failure of some investigators to demonstrate transmural heterogeneity of repolarization in the dog in vivo under control conditions and after administration of quinidine. The inability to demonstrate transmural heterogeneity under these conditions may be due to the use of bipolar recording techniques that, in our experience, seriously underestimate transmural dispersion of repolarization (TDR). The use of sodium pentobarbital and alpha-chloralose as anesthesia also is problematic, because these agents reduce or eliminate TDR by affecting a variety of ion channel currents. Finally, attempts to amplify transmural dispersion of repolarization with an agent such as quinidine must take into account that relatively high concentrations can result in effects opposite to those desired due to drug inhibition of multiple ion channels. These observations may explain the inability of earlier studies to detect the M cell.

Brugada syndrome is characterized by ST segment elevation in the right precordial leads, V1-V3 (unrelated to ischemia or structural disease), normal QT intervals, apparent right bundle branch block, and sudden cardiac death, particularly in men of Asian origin. An autosomal dominant mode of inheritance with variable expression has been described. The only gene thus far linked to the Brugada syndrome is the cardiac sodium channel gene, SCN5A. The possible cellular and ionic basis for these features of the Brugada syndrome are discussed. Strong sodium channel block, among other modalities, has been shown to be capable of inducing epicardial and transmural dispersion of repolarization, thus providing the substrate for the development of phase 2 and circus movement reentry, which underlies ventricular tachycardia/ventricular fibrillation.

T-wave alternans (TWA), an ECG phenomenon characterized by beat-to-beat alternation of the morphology, amplitude, and/or polarity of the T wave, is commonly observed in the acquired and congenital long-QT syndromes (LQTS). This study examines the cellular and ionic basis for TWA induced by rapid pacing under conditions mimicking the LQT3 form of the congenital LQTS in an arterially perfused canine left ventricular wedge preparation.

Sodium pentobarbital is widely used for anesthesia in experimental studies as well as in clinics, and it is known to prevent the development of torsades de pointes (TdP) in in vivo models of the long QT syndrome (LQTS).

The contributions of electrogenic sodium/calcium exchange current (INaCa), calcium-activated chloride conductance [ICl(Ca)], and calcium-activated nonselective cation conductance to delayed afterdepolarizations (DAD) were examined. Nonselective cation channels were absent in canine M cells, since inhibition of INaCa and ICl(Ca) eliminated all calcium-activated currents without abolishing cell shortening. After the cells were treated with isoproterenol and ouabain to increase calcium loading, INaCa was 168 +/- 30 x 10(-3) pC/pF and ICl(Ca) was 114 +/- 24 x 10(-3) pC/pF. Transient overlapping inward and outward currents were evoked positive to the chloride reversal potential (ECl). Outward current was chloride sensitive, and inward current was blocked by replacement of external sodium with lithium. When ECl was -50 mV, triggered activity occurred in normal external sodium and persisted after inhibition of INaCa. Steps to -80 mV revealed oscillating inward currents in normal sodium and chloride, which persisted after inhibition of INaCa. When ECl was equal to -113 mV, ICl(Ca) opposed INaCa at the resting potential. DAD occurred in normal sodium, and inhibition of outward ICl(Ca) provoked triggered activity. We conclude that INaCa represents approximately 60% of the total calcium-activated current at resting potentials but that both INaCa and ICl(Ca) work in concert to cause DAD in calcium-overloaded cells.

Much of the characterization of the M cell to date has been accomplished using isolated tissues and cells. This study uses an arterially perfused wedge preparation to examine the characteristics and distribution of M cells within the anterior wall of the canine left ventricle under more physiological conditions.

This study probes the cellular basis for the T wave under baseline and long-QT (LQT) conditions using an arterially perfused canine left ventricular (LV) wedge preparation, which permits direct temporal correlation of cellular transmembrane and ECG events.

This study examines the cellular basis for the phenotypic appearance of broad-based T waves, increased transmural dispersion of repolarization (TDR), and torsade de pointes (TdP) induced by beta-adrenergic agonists under conditions mimicking the LQT1 form of the congenital long-QT syndrome.

Precipitation of torsades de pointes (TdP) has been shown to be associated with acceleration of heart rate in both experimental and clinical studies. To gain insight into the cellular mechanism(s) responsible for the initiation of acceleration-induced TdP, we studied the effect of acceleration of pacing rate in canine left ventricular epicardial, M region, endocardial, and Purkinje fiber preparations pretreated with E-4031, an IKr blocker known to induce the long QT syndrome and TdP.

Experimental systemic lupus erythematosus (SLE)-like disease was induced in BALB/c mice by immunization with heparan sulfate, the major glycosaminoglycan of glomerular basement membrane. Following booster injections with heparan sulfate (HS), high levels of anti-HS, anti-dsDNA, and anti-cardiolipin antibodies were detected in the sera of the immunized mice. An enzyme-linked immunospot (ELISPOT) assay indicted that IgG anti-HS and anti-dsDNA antibody-secreting cells were present in the kidneys and most likely contributed to antibody localization. Antibodies eluted from the kidneys of immunized mice were found to react strongly with HS and dsDNA when tested in vitro. The HS-immunized mice developed moderate to severe levels of proteinuria. Histologic examination of kidneys from HS-immunized mice revealed deposition of immunoglobulin in the kidneys. Our results describe the induction of SLE-like disease in normal mice following immunization with HS. This experimental model may be useful for understanding the immunologic basis for autoimmunity to HS.

Recent clinical studies have reported a greater effectiveness of sodium channel block with mexiletine to abbreviate the QT interval in patients with the chromosome 3 variant (SCN5A, LQT3) of the long QT syndrome (LQTS) than those with the chromosome 7 form of the disease (HERG, LQT2), suggesting the possibility of gene-specific therapy for the two distinct forms of the congenital LQTS. Experimental studies using the arterially perfused left ventricular wedge preparation have confirmed these clinical observations on the QT interval but have gone on to further demonstrate a potent effect of mexiletine to reduce dispersion of repolarization and prevent torsades de pointes (TdP) in both LQT2 and LQT3 models. A differential action of sodium channel block on the three ventricular cell types is thought to mediate these actions of mexiletine. This study provides a test of this hypothesis by examining the effects of mexiletine in isolated canine ventricular epicardial, endocardial, and M region tissues under conditions that mimic the SCN5A and HERG gene defects.

The contributions of the 4-aminopyridine (4-AP)-sensitive transient outward potassium conductance (Ito1) and the calcium-activated chloride conductance (ICl(Ca)] to cardiac action potentials were investigated in canine ventricular myocytes. Action potentials or currents were recorded at 37 degrees C using standard whole cell or amphotericin B perforated-patch-clamp techniques. Inhibition of Ito1 by 1 mM 4-AP prolonged phase 1 repolarization, elevated the action potential notch, and depressed the plateau. Action potential voltage clamp revealed that 4-AP blocked a rapidly decaying outward current during phase 1 without affecting plateau or diastolic currents. These results suggested that depression of the plateau was not a direct result of Ito1 inhibition but followed from delayed phase 1 repolarization. Calcium current (ICa) at the peak of the action potential dome was reduced 60 +/- 4% when the rate of phase 1 repolarization was reduced. ICl(Ca) measured by action potential clamp reversed over the course of the action potential. Chloride fluxes associated with outward and inward components of the 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid-sensitive current were +130 +/- 17 and -184 +/- 20 (pA.ms)/pF, respectively. The effects of selective inhibition of ICl(Ca) on the action potential were dependent on the rate of early repolarization and the prominence of the notch. Inhibition of ICl(Ca) elevated the plateau and slightly abbreviated action potential duration when the notch was prominent. When repolarization was prolonged and the notch was shallow, inhibition of ICl(Ca) elevated the notch and the plateau and abbreviated duration. We have shown that Ito1 and ICl(Ca) contribute to canine ventricular action potentials. The extent of overlap between Ito1 and ICl(Ca) during the action potential is largely determined by the amplitude of Ito1 and the depth of the notch. Regional differences in the density of Ito1, or interventions that moderate phase 1 repolarization by reducing this current, will have considerable effect on the time course of ICa and calcium-dependent conductances.

This study examines the contribution of transmural heterogeneity of transmembrane activity to phenotypic T-wave patterns and the effects of pacing and of sodium channel block under conditions mimicking HERG and SCN5A defects linked to the congenital long-QT syndrome (LQTS).

Lysis of aortic endothelial cells (EC) by neutrophils from spontaneously hypertensive rats (SHR) was investigated using a nonradioactive cytotoxicity assay. Interleukin-1-activated EC, but not unstimulated EC, were effective target cells for lysis by SHR neutrophils. Supernatants from activated neutrophil did not exert a cytotoxic effect on EC. Inhibitors of reactive oxygen species did not affect the cytotoxicity of neutrophils on EC. In contrast, inhibitors of serine protease and elastase markedly inhibited the cytotoxicity of neutrophils on EC. Antibodies against the endothelial cell surface ligands ICAM-1 (CD54) and E-selectin (CD62E) inhibited the adhesion and cytotoxicity of activated neutrophils on EC. The cytotoxicity of neutrophils required direct cell-to-cell contact because separating them with a microporous membrane abrogated the neutrophil-mediated cytotoxic activity. These results demonstrate that SHR neutrophils possess potent cytotoxicity against cytokine-activated EC. Neutrophil-mediated damage of EC could contribute to organ damage in hypertension under conditions of local or systemic activation of neutrophils.

We have previously demonstrated that arterial antigens derived from the aorta of spontaneously hypertensive rats (SHRs) stimulate arterial antigen-reactive T cell clones established from the spleens of SHR to proliferate and release cytokines. To identify immunogenic protein components associated with the arterial wall, arterial antigen-reactive T cell clones were tested against arterial antigens separated by SDS-PAGE and transferred to nitrocellulose. The greatest T cell reactivity was obtained with protein bands of molecular weight 66 kDa, 50 kDa, and 45 kDa. T cell clones reactive against the 50 and 45 kDa antigens from gels failed to respond to proteins of other molecular weight (M(r)) separated under reducing or nonreducing conditions, suggesting that these molecules are not subunits of larger proteins and may represent monomeric antigens polymerized into the arterial wall. These data suggest that certain epitopes of arterial wall antigens are immunogenic. T cells activated with these immunogenic epitopes could initiate or perpetuate vasculitis in the arteries of hypertensive rats.

Tetrodotoxin (TTX) or substitution of external Na+ reduces the 4-aminopyridine-sensitive transient outward current (Ito1) in rat ventricular myocytes. We investigated the outcome of reducing external sodium on the kinetics, gating, and selectivity of Ito1 with a dual-patch electrode technique to record whole cell currents and transmembrane potentials independently of the voltage clamp in canine midmyocardial cells. Steps from -80 to 0 mV produced overlapping inward sodium and outward potassium currents, accompanied by a loss of voltage control associated with activation of INa. Substitution of external Na+ or application of TTX abolished INa, restored voltage control, and reduced Ito1. Inactivation of INa with a 10-ms prestep to -45 mV decreased Ito1 to the same extent as external Na+ substitution. The kinetics, gating, and selectivity of Ito1 recorded after inactivation of INa were unaffected by drastic reductions in external Na+. Our findings suggest that a larger Ito1 in the presence of normal external Na+ is due to 1) transient loss of voltage control and concomitant changes in activation of Ito1 and/or 2) facilitation of an outward current by intracellular Na+. We conclude that reduction of external sodium has no direct effect on the kinetics or gating of Ito1, non does Na+ contribute to current flow through Ito1 channels in canine midmyocardial cells.

This study sought to elucidate the cellular and ionic basts for erythromycin-induced long QT syndrome.

Autoantigen-reactive T cells might play an important role in the pathogenesis of systemic lupus erythematosus (SLE). Autoantigen-reactive T cell clones were generated from spleens of NZB x NZW F1 (BWF1) and normal control BALB/c mice with interleukin-2 (IL-2), a procedure that selects for in vivo activated antigen-reactive T cells. The antigen-specificity of the T cell clones was tested by using a panel of candidate autoantigens. The T cell clones from BWF1 mice but not those from BALB/c mice proliferated against heparan sulfate, the major glycosaminoglycan of glomerular basement membrane. None of the clones proliferated against dsDNA or cardiolipin. All the heparan sulfate-reactive T cell clones had the ability to selectively augment the production of IgG anti-dsDNA autoantibodies. When cultured with either heparan sulfate or Concanavalin A, the T cell clones produced high levels of IL-4 and IL-5 with no detectable IL-2 or IFN-gamma. In contrast, T cell clones derived from BALB/c mice augmented the production of total polyclonal IgG but not the production of anti-dsDNA antibodies. These studies indicate the existence of heparan sulfate-reactive T cells in BWF1 mice. Characterization of heparan sulfate-reactive T cells that could selectively augment anti-dsDNA production will permit the design of targeted and antigen-specific therapy.

Myocardial ischemia results in an early phase of arrhythmias that primarily involves reentrant mechanisms. However, the trigger that initiates reentry remains unknown. We studied the initiation of reentry attending the development of electrical heterogeneity during simulated ischemia in isolated canine right ventricular subepicardium.

Transmural heterogeneities of repolarizing currents underlie prominent differences in the electrophysiology and pharmacology of ventricular epicardial, endocardial, and M cells in a number of species. The degree to which heterogeneities exist between the right and left ventricles is not well appreciated. The present study uses standard microelectrode and whole cell patch-clamp techniques to contrast the electrophysiological characteristics and pharmacological responsiveness of tissues and myocytes isolated from right (RVE) and left canine ventricular epicardium (LVE). RVE and LVE studied under nearly identical conditions displayed major differences in the early repolarizing phases of the action potential. The magnitude of phase 1 in RVE was nearly threefold that in LVE: 28.7 +/- 6.2 vs. 10.6 +/- 4.1 mV (basic cycle length = 2,000 ms). Phase 1 in RVE was also more sensitive to alterations of the stimulation rate and to 4-aminopyridine (4-AP), suggesting a much greater contribution of the transient outward current (I(to) 1) in RVE than in LVE. The combination of 4-AP plus ryanodine, low chloride, or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (chloride channel blocker) completely eliminated the notch and all rate dependence of the early phases of the action potential, making RVE and LVE indistinguishable. At +70 mV, RVE myocytes displayed peak I(to) 1 densities between 28 and 37 pA/pF. LVE myocytes included cells with similar I(to) 1 densities (thought to represent subsurface cells) but also cells with much smaller current levels (thought to represent surface cells). Average peak I(to) 1 density was significantly smaller in LVE than in RVE at voltages more than or equal to +10 mV. Our data point to prominent differences in the magnitude of the I(to) 1-mediated action potential notch in cells at the surface of RVE compared with the LVE and suggest that important distinctions may exist in the response of these two tissues to pharmacological agents and pathophysiological states, as previously demonstrated for epicardium and endocardium. Our findings also suggest that a calcium-activated outward current contributes to the early repolarization phase in RVE and LVE and that the influence of this current, although small, is more important in the left ventricle.

Recent studies have described the presence of M cells in the deep layers of the canine and human ventricle displaying electrophysiologic and pharmacologic features different from those of epicardial (EPI) and endocardial (ENDO) cells. The M cell is distinguished electrophysiologically by the ability of its action potential to prolong disproportionately to that of other myocardial cells with slowing of the stimulation rate and pharmacologically by its unique sensitivity to Class III antiarrhythmic agents. The present study was designed to test the hypothesis that similar cells are present in the guinea pig ventricle.

Because the number and characteristics of delayed rectifier K+ current (IK) components vary between species, the role of each component in the action potential and modulation by class III agents is uncertain. To address these issues, IK was assessed in adult isolated canine ventricular and and atrial myocytes by using whole-cell and perforated-patch techniques. IK components were characterized by using two complementary approaches: a kinetic approach (based on biexponential fits to deactivating tail currents) and a pharmacological approach approach (using the methanesulfonanilide compound E-4031). In ventricular myocytes, two exponential tail current components were distinguished; these components differed in the voltage and time dependence of activation and the effect of lower (K+). Both kinetic components contributed equally to peak tail current amplitude (measured at -35 mV) after a single 300-ms pulse to 5 mV, simulating an action potential. By use of E-4031, rapidly and slowly activating components described kinetically were identified. The activation kinetics and rectification properties of canine IKr and IKs are qualitatively similar to those described previously for guinea pigs. In contrast, canine IKr and IKs deactivation kinetics differed markedly from those found in guinea pigs, with canine IKr deactivating slowly (time constant tau, 2 to 3 s near -35 mV) and IKs deactivating rapidly (tau, 150 ms near -35 mV and decreasing to 30 ms near -85 mV). E-4031 elicited reverse rate-dependent effects (greater drug-induced prolongation of the action potential at slower stimulation rates); this effect is inconsistent with the hypothesis attributing reverse rate dependence to incomplete IKs deactivation during rapid stimulation (due to rapid deactivation of canine IKs). Two IK components with characteristics comparable to those found in ventricular myocytes were also observed in atrial myocytes. In conclusion, (1) IKr- and IKs-like components of IK are present in canine atrial and ventricular myocytes, with deactivation kinetics strikingly different from those found in guinea pigs, and (2) the rapid deactivation kinetics of canine IKs do not support its role in reverse rate dependence with class III agents in this species.

The presence of autoantibodies directed against arterial antigens in serum samples from spontaneously hypertensive rats and related controls that included Wistar-Kyoto and Sprague-dawley rats were assayed by enzyme-linked immunosorbent assay and immunoblotting technique. Circulating immunoglobulin G antibodies reactive against arterial antigen, as measured by enzyme-linked immunosorbent assay, could be detected in serum samples of 26 of 30 spontaneously hypertensive rats (87%) and 8 of 30 (27%) Wistar kyoto rats. These antibodies (Abs) were not detectable either by enzyme-linked immunosorbent assay or immunoblotting in sera from Sprague-dawley rats. The arterial antigen-reactive antibody was antigen specific, because the binding reactivity was absorbed by arterial antigen but not by fibroblasts or peripheral blood mononuclear cells. Immunoglobulin G arterial antigen-reactive antibody was significantly higher in adult spontaneously hypertensive rats with established hypertension, compared with young prehypertensive rats or normotensive wistar kyoto rats. Immunoblotting of spontaneously hypertensive rats sera revealed reactivity of arterial antigen-reactive antibody against arterial antigen ranging in size from 20 to 97 kDa. Sera from Wistar kyoto rats recognized arterial antigen ranging in size from 40 to 90 kDa. A significant correlation (p < 0.004) was found between adult spontaneously hypertensive rats with established hypertension and the presence of arterial antigen-reactive antibody reactivity against arterial antigen of 20, 69 and 97 kDa. Antibody directed against a 20 kDa arterial antigen was detected in both young prehypertensive rats and adult rats with established hypertension but not in Wistar kyoto or Sprague-dawley rats. Antibodies directed against both 69 and 97 kDa arterial antigens were detected only in spontaneously hypertensive rats sera. These data show that the pattern of arterial antigen-reactive antibody reactivity in sera of hypertensive rats in heterogeneous, and suggest that arterial antigen-reactive antibody directed against few arterial antigens may be involved in the pathogenesis of hypertension in the spontaneously hypertensive rat.

The J wave is a deflection that appears in the ECG as a late delta wave following the QRS or as a small secondary R wave (R'). Also referred to as an Osborn wave, the J wave has been observed in the ECG of animals and humans for more than four decades, yet the mechanism underlying its manifestation is poorly understood. The present study investigates the cellular basis for the J wave using an isolated arterially perfused preparation consisting of a wedge of canine right or left ventricle.

Recent studies have described the existence of M cells in the deep structures of the canine and human ventricle. The present study was designed to further characterize the M cell with respect to its distribution across the canine left ventricular free wall and the dependence of its action potential on [K+]o.

Recent studies have described regional differences in the electrophysiology and pharmacology of ventricular myocardium in canine, feline, rat, guinea pig, and human hearts. In this study, we use standard microelectrode and whole-cell patch-clamp techniques to examine the characteristics of the action potential and the delayed rectifier K+ current (IK) in epicardial, M region (deep subepicardial to midmyocardial), and endocardial cells isolated from the canine left ventricle. Cells from the M region displayed much longer action potential durations (APDs) at slow rates. At a basic cycle length of 4 s, APD measured at 90% repolarization was 358 +/- 16 (mean +/- SEM), 262 +/- 12, and 287 +/- 11 ms in cells from the M region, epicardium, and endocardium, respectively. Steady state APD-rate relations were steeper in cells from the M region. In complete Tyrode's solution, IK was smaller in myocytes from the M region when compared with those isolated from the epicardium or endocardium. Further characterization of IK was conducted in a Na(+)-, K(+)-, and Ca(2+)-free bath solution to isolate the slowly activating component of the delayed rectifier (IKs) from the rapidly activating component (IKr). IKs was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to -20 mV, IKs tail current density was 1.99 +/- 0.30 pA/pF (mean +/- SEM) in epicardial cells, 1.83 +/- 0.18 pA/pF in endocardial cells, and 0.92 +/- 0.14 pA/pF in M cells. Voltage dependence and time course of activation and deactivation of IKs were similar in the three cell types. The relative contribution of IKr and IKs among the three cell types was examined by using 6 mmol/L [K+]o Tyrode's solution with and without E-4031, a highly selective blocker of IKr. An E-4031-sensitive current was observed in the presence but not in the absence of extracellular K+. This rapidly activating component showed characteristics similar to those of IKr as described in rabbit and cat ventricular cells. Deactivation of IKr was significantly slower than that of IKs. IKr (E-4031-sensitive component) tail current density was similar in the three cell types, whereas IKs (E-4031-insensitive component) tail current density was significantly smaller in the M cells. Our results suggest that the distinctive phase-3 repolarization features of M cells are due in part to a lesser contribution of IKs and that this distinction may also explain why M cells are the main targets for agents that prolong APD in ventricular myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)

Delayed rectifier K+ current (IK) was studied in isolated myocytes from canine left ventricular epicardium and midmyocardium using whole cell patch-clamp techniques. IK density during activation was greater in epicardial vs. midmyocardial myocytes [1.06 +/- 0.09 vs. 0.66 +/- 0.09 pA/pF (SE); P < 0.01] measured during 3-s depolarizing pulses to +25 mV. IK density was greater in epicardial myocytes at all times examined (range 150 ms to 3 s, +25 mV) and on termination of 3-s test pulses (potentials +5 to +65 mV). Greater IK density could not be explained by differences in activation kinetics or voltage dependence of activation. Two components of IK (IK,r and IK,s) have been described in guinea pig myocytes (Sanguinetti, M. C., and N. Jurkiewicz, J. Gen. Physiol. 96: 192-214, 1990). To assess whether differences in IK density could be ascribed to IK,r or IK,s, tail currents were fit to the sum of two decaying exponentials, with each component analogous to IK,s and IK,r based on sensitivity to E-4031 and rectification properties. Greater tail current density in epicardial myocytes was due to greater IK,s with no discernible difference in IK,r. These results suggest that regional differences in IK density in left ventricular epicardium are due to a larger IK,s component, which contributes to ventricular electrical heterogeneity and may reflect the differential expression of the IsK channel.

The contribution of chloride and potassium to the 4-aminopyridine (4-AP)-resistant transient outward current was investigated in dog cardiac myocytes. Whole cell currents were recorded at 37 degrees C in single cells dissociated from epicardial and midmyocardial regions of the canine ventricle. Sodium-calcium exchange current and voltage-dependent transient outward potassium current (IA) were blocked in sodium-free solutions containing 2 mM 4-AP; sodium channels were inactivated by the -50-mV holding potential. When patch pipettes contained 0.4-0.8 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, voltage-clamp steps over the range -20 to +50 mV activated an inward calcium current (ICa) and a Ca(2+)-activated chloride current [ICl(Ca)]. ICl(Ca) was blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), or reduction of external chloride. Independent of the presence of potassium, the reversal potential of the SITS-sensitive current varied with extracellular chloride, as predicted for a chloride-selective conductance. The bell-shaped current-voltage relation of ICl(Ca) has a threshold of -20 mV and a peak at +40 mV. No evidence could be found for a Ca(2+)-activated potassium current or a Ca(2+)-activated nonspecific cation current under these conditions. ICl(Ca) contributed to oscillatory inward currents at diastolic potentials in cells superfused by isoproterenol and high Ca2+, suggesting a role for this current in triggered arrhythmias associated with delayed afterdepolarizations. In the normal heart, ICl(Ca) is likely to contribute to rate- and rhythm-dependent repolarization of the cardiac action potential.

Total boron concentrations in Drosophila changed during development and aging. The highest concentration of boron was found during the egg stage, followed by a decline during the larval stages. Newly emerged flies contained 35.5 ppm boron. During the adult stage the boron concentration increased by 52% by 9 weeks of age. Adding excess dietary boron during the adult stage decreased the median life span by 69% at 0.01 M sodium borate and by 21% at 0.001 M sodium borate. Lower concentrations gave small but significant increases in life span. Supplementing a very low boron diet with 0.00025 M sodium borate improved life span by 9.5%. The boron contents of young and old mouse tissues were similar to those of Drosophila and human samples. Boron supplements of 4.3 and 21.6 ppm in the drinking water, however, did not significantly change the life span of old mice fed a diet containing 31.1 ppm boron.

M cells and transitional cells residing in the deep structures of the ventricular free walls are distinguished by the ability of their action potentials to prolong disproportionately to those of other ventricular cells at relatively slow rates. This feature of the M cell due, at least in part, to a smaller contribution of the slowly activating component of the delayed rectifier current (IKs) is thought to contribute to the unique pharmacologic responsiveness of M cells, making them the primary targets in ventricular myocardium for agents that cause action potential prolongation and induce early and delayed afterdepolarizations and triggered activity. Previous studies dealt exclusively with the characteristics and distribution of M cells in the canine right and left ventricular free wall near the base of the ventricles. The present study uses standard microelectrode techniques to define their behavior and distribution in the apical region of the ventricular wall as well as in the endocardial structures of the ventricle, including the interventricular septum, papillary muscles, and trabeculae.

Elevated intracellular calcium activity is thought to play an important role in arrhythmia induction, particularly during ischemia and reperfusion. Delayed after-depolarization-induced triggered activity and intracellular communication problems are thought to be responsible.

Recent findings point to an important heterogeneity in the electrical behavior of cells spanning the ventricular wall as well as important differences in the response of the various cell types to cardioactive drugs and pathophysiologic states. These observations have permitted a fine tuning and, in some cases, a reevaluation of basic concepts of arrhythmia mechanisms. This brief review examines the implications of some of these new findings within the scope of what is already known about early and delayed afterdepolarizations and triggered activity and discusses the possible relevance of these mechanisms to clinical arrhythmias.

The effect of protein tyrosine kinase (PTK) inhibition on spontaneously hypertensive rat vascular smooth muscle contraction was examined in this study. By using isolated aortic strips, it was found that platelet-derived growth factor (PDGF) (0.1-1 nM), an activator of PTKs, elicited contraction with an EC50 of 0.25 +/- 0.08 nM. Treatment with tyrphostin (0.02-200 microM), a selective inhibitor of PTKs, caused a significant rightward shift of the concentration-response curves (P < .05). The IC50 for tyrphostin in the spontaneously hypertensive rat was calculated to be 9.5 +/- 4.2 microM. Tyrphostin also inhibited contractile activity in normotensive control Wistar-Kyoto rat aorta with an IC50 of 0.24 +/- 0.09 microM. Tyrphostin inhibited PDGF-induced contraction over a range of calcium concentrations, suggesting that it may oppose contraction through inhibition of calcium influx via PDGF-induced receptor-operated channel. However, KCl-mediated voltage-operated calcium channels were largely unaffected by tyrphostin, because it was unable to relax aortae which had been partially depolarized. Tyrphostin also had no significant antagonistic effect on contraction induced by phenylephrine or phorbol-12,13-dibutyrate. For both of these agents, contraction is mediated through activation of protein kinase C, which further alludes to the specificity of tyrphostin for PTKs. Treatment with 1 nM PDGF caused a significant stimulation of particulate/membrane PTK activity in the aorta. Tyrphostin attenuated PDGF-induced PTK activity in a concentration-dependent manner. The data suggest that PTKs may play a role in vascular smooth muscle contraction and that specific inhibition of PTK activity results in vasorelaxation. Furthermore, the results suggest that vasoconstriction and vascular smooth muscle cell proliferation may share common biochemical signalling pathways.

Acute ischemia is known to produce more severe electrophysiological disturbances in canine ventricular epicardium than endocardium, although the mechanism for the differential sensitivity is still unresolved. Recent studies have demonstrated the presence of a prominent transient outward current (Ito) in ventricular epicardium but not endocardium. The present study was designed to test the hypothesis that the differential sensitivity of these two tissues to ischemia results, at least in part, from a more prominent Ito in epicardium than in endocardium.

Benzo(a)pyrene (BaP), a major environmental pollutant and component of cigarette smoke, is both carcinogenic and atherogenic in experimental models. We investigated the effect of long-term administration of BaP on atherogenesis in both atherosclerosis-susceptible White Carneau (WC) and atherosclerosis-resistant Show Racer (SR) pigeons. The number and size of arterial lesions in the brachiocephalic arteries in WC and SR females but not males were significantly enhanced after long-term dosing with BaP. Metabolic activation appears to be required for BaP atherogenicity, since benzo(e)pyrene (BeP), a noncarcinogenic analogue of BaP, did not enhance lesion development. Studies with 3H-BaP revealed no significant differences between male and female or between WC and SR pigeons in the arterial distribution of BaP and/or its metabolites. There were no consistent differences in blood pressure or plasma cholesterol levels between breeds or sexes. However, chronic administration of BaP did result in complete infertility in female birds, concomitant with grossly visible changes in ovarian appearance. These results clearly show that long-term dosing with BaP alters ovarian structure and function in treated birds, at the same time aggravating the development of arterial lesions. Thus, BaP-induced atherogenicity in female pigeons may be a consequence of an alteration in estrogen production or of antiestrogenic properties of BaP at the level of the arterial wall and may serve as a highly useful animal model to examine the well-known rapid development of atherosclerosis in postmenopausal women.

It has been postulated that hypertension in the spontaneously hypertensive rat (SHR) results from autoimmune damage to the SHR vasculature. The objective of this study was to isolate autoreactive T-cells specific for arterial antigens, and to characterize these cells.

Pinacidil is known to augment a time-independent outward current in cardiac tissues by activating the ATP-regulated potassium channels. Activation of this current, IK-ATP, is thought to be responsible for increased potassium permeability in ischemia. The contribution of IK-ATP activation to arrhythmogenesis and the role of activation of this current in suppression of arrhythmias are areas of great interest and debate. Because electrical depression attending myocardial ischemia is more accentuated in ventricular epicardium than in endocardium, we endeavored to contrast the effects of pinacidil-induced IK-ATP activation on the electrophysiology of canine ventricular epicardium and endocardium.

CDF1 and C57BL/6J male mice were acutely dosed with Adriamycin (ADR), and total cardiac malondialdehyde (MDA) quantitated following isolation by modification of previously developed procedures. Cardiac MDA content in CDF1 mice increased significantly 5 days following ADR dosing as reported by others, but was unchanged in C57BL/6J mice. ADR-induced mortality and a significant loss in cardiac weight 2-3 days after treatment was similar in both strains. Cardiac lipid hydroperoxide (LH) content was also unchanged in C57BL/6J mice dosed acutely with ADR. However, hepatic LH content increased rapidly following treatment with ADR, reaching maximal level 1 day following treatment before returning to below untreated levels 24 hours later. Studies with genetically acatalasemic C57BL/6J mice showed that neither cardiac nor hepatic lipid hydroperoxide content in ADR-dosed animals is affected by tissue catalase levels. These results demonstrate that C57BL/6J mouse heart is refractory to ADR-induced lipid peroxidation (LP) although overall mortality from the drug is unaffected, and do not support the hypothesis that ADR-induced mortality in mice is a consequence of cardiac LP.

The life span of adult Drosophila melanogaster fruit flies changed when they were fed two different photosensitizers. Methylene blue decreased the median life span by 49% when present in the food at a concentration of 0.001 M. Another photosensitizer, riboflavin, produced no changes in life span under the same conditions of a 12:12 h light/dark cycle at a daytime light intensity of 1000 lux. Flies exposed to constant darkness lived 43.2% longer than those exposed to constant light at a light intensity of 2000 lux. Under these conditions, riboflavin increased the life span of the flies exposed to constant light by as much as 25%. We conclude that riboflavin confers some degree of protection against the effects of constant light exposure. The completely different results obtained with riboflavin and methylene blue suggest a possible mechanism for photoageing involving photodynamic action mediated through the production of singlet oxygen.

The life span of Drosophila melanogaster (Oregon R) males was found to be proportional to the logarithm of the iron content of the diet. Life span was also shown to be proportional to the rate of iron accumulation for Drosophila, mice and man. The total body iron content was found to correlate with the total calcium content of adult Drosophila. Iron content during the developmental stages, however, remained relatively constant and did not change with changes in the calcium concentrations. Dietary tea (Camellia sinensis) extracts were found to inhibit the ageing-related accumulation of iron and to prolong the life span of Drosophila by as much as 21.4%. It is concluded that iron accumulation is a significant factor contributing to senescence.

Recent studies from our laboratory involving syncytial preparations have delineated electrophysiological distinctions between epicardium, endocardium, and a unique population of cells in the deep subepicardial to midmyocardial layers (M region) of the canine ventricle. In the present study, we used standard microelectrode, single microelectrode switch voltage-clamp, and whole-cell patch-clamp techniques to examine transmembrane action potentials, steady-state current-voltage relations, and the 4-aminopyridine-sensitive transient outward current (Ito1) in myocytes enzymatically dissociated from discrete layers of the free wall of the canine left ventricle. Action potential characteristics of myocytes isolated from the epicardium, M region, and endocardium were very similar to those previously observed in syncytial preparations isolated from the respective regions of the ventricular wall. A prominent spike and dome was apparent in myocytes from epicardium and the M region but not in myocytes from endocardium. Action potential duration-rate relations were considerably more pronounced in cells isolated from the M region. Current-voltage relations recorded from cells of epicardial, M region, and endocardial origin all displayed an N-shaped configuration with a prominent negative slope-conductance region. The magnitude of the inward rectifier K+ current (IK1) was 392 +/- 86, 289 +/- 65, and 348 +/- 115 pA in epicardial, M region, and endocardial myocytes, respectively, when defined as steady-state current blocked by 10 mM Cs+. Similar levels were obtained when IK1 was defined as the steady-state difference current measured in the presence (6 mM) and absence of extracellular K+. Ito1 was significantly greater in epicardial and M region myocytes than in endocardial myocytes. At a test potential of +70 mV (holding potential, -80 mV), Ito1 amplitude was 4,203 +/- 2,370, 3,638 +/- 1,135, and 714 +/- 286 pA in epicardial, M region, and endocardial cells, respectively. No significant differences were observed in the voltage dependence of inactivation of Ito1 in the three cell types. The time course of reactivation of Ito1 was slower in cells from the M region compared with either epicardial or endocardial cells. Our data suggest that prominent heterogeneity exists in the electrophysiology of cells spanning the canine ventricular wall and that differences in the intensity of the transient outward current contribute importantly, but not exclusively, to this heterogeneity. These findings should advance our understanding of basic heart function and the ionic bases for the electrocardiographic J wave, T wave, U wave, and long QT intervals as well as improve our understanding of some of the complex factors contributing to the development of cardiac arrhythmias.

The effects of recombinant human interleukin-2 covalently linked to polyethylene glycol (PEG-IL-2) or interleukin-2 (IL-2) on hypertension and in vitro suppressor T cell function in the spontaneously hypertensive rats (SHR) were investigated. Male young prehypertensive (4 weeks old) SHRs and adult (10 weeks old) SHRs with established hypertension were injected with low (5,000 units (u)/kg) or high (50,000-100,000 u/kg) dose of PEG-IL-2 or IL-2 as a single bolus or repeated injections. Systolic blood pressure was measured twice weekly using the tail-cuff technique. Systolic blood pressure in the PEG-IL-2 or IL-2 treated animals, irrespective of age, dose, or route of injection, did not differ significantly from that measured in vehicle-treated controls over a 10 week period. Mean arterial pressure measured by intra-arterial catheter was 159 +/- 7 mm Hg 10 weeks after treatment with repeated injections of 5,000 u/kg of PEG-IL-2 and 158 +/- 9 mm Hg in vehicle-treated controls. All rats injected with IL-2 had IL-2-specific IgG antibody in their sera. None of the PEG-IL-2 treated rats had any detectable anti-IL-2 antibodies in their sera. Thus, PEG-IL-2 showed far less immunogenicity than IL-2. Suppressor T (Ts) cells generated from adult SHR spleen cells failed to suppress pokeweed mitogen (PWM)-driven immunoglobulin G (IgG) synthesis. PEG-IL-2 or IL-2 supplementation both in vitro and in vivo restored the ability of adult SHR to generate Ts cells able to inhibit IgG synthesis. Our data suggest that PEG-IL-2 or IL-2 administration does correct a prominent defective Ts cell activity found in adult SHR, but that correction of this immune abnormality is not attended by an attenuation of hypertension.

Oscillations of membrane potential that attend or follow the cardiac action potential and depend on preceding transmembrane activity for their manifestation are known as afterdepolarizations. Early afterdepolarizations (EADs) interrupt or retard repolarization of the cardiac action potential, whereas delayed afterdepolarizations (DADs) arise after full repolarization. EADs and DADs can give rise to spontaneous action potentials or triggered activity believed to be responsible for a variety of cardiac arrhythmias. Recent studies from our laboratory have highlighted differences in the electrophysiology and pharmacology of three functionally distinct myocardial cell types found in the canine ventricle. Epicardial, M region, and endocardial tissues and cells show distinct, sometimes opposite, responses to a variety of drugs, including those capable of inducing EADs and DADs.

We recently reported that sodium channel block can produce opposite effects on action potential duration (APD) and refractoriness in epicardial versus endocardial tissues of the canine ventricle. In addition, strong sodium channel current inhibition was found to cause loss of the action potential dome in epicardium but not endocardium, thus inducing a marked dispersion of repolarization and refractoriness between epicardium and endocardium as well as among neighboring epicardial sites. The marked heterogeneity that evolves under these conditions provides a substrate for the development of arrhythmias. Flecainide was found to induce extrasystolic activity more readily than other sodium blockers. The present study contrasts the electrophysiological actions of flecainide in canine ventricular epicardium and endocardium and examines the characteristics of flecainide-induced arrhythmias in epicardial sheets of canine ventricle.

Since it is unclear how the concentration of dolichol fractions change with ageing in mammals, we have examined the changes in another organism, Drosophila. Dolichol extracted from Drosophila melanogaster was found to consist of three fractions composed of 15, 16 and 17 isoprene units. The total dolichol content of female flies maintained at 25 degrees C increased with ageing between 0 and 64 days of adult age but the change was not significant. The total dolichol content of male flies decreased with ageing but the decrease was not significant. The relative amounts of the three different dolichol fractions in both male and female flies also failed to show any significant ageing-related change. The greatest amount of dolichol was found in the 16 isoprene unit fraction representing 67.2% of the total dolichol in male flies and 65.4% in female flies. Increased dietary dolichol had little or no influence on the life span of Drosophila when given either during the developmental or adult stages.

Suppressor T cell function in the spontaneously hypertensive rat (SHR) and normotensive Wistar Kyoto (WKY) rats was analyzed using syngeneic mixed lymphocyte reaction (SMLR) and concanavalin A (Con A) activation. A depressed SMLR was found in adult SHR but not in adult WKY. IL-2 synthesized by SHR was 40-fold lower than that of WKY, and the suppressor T cells generated in the SMLR were incapable of suppressing IgG synthesis. Precursors of cells that can be activated by Con A to become functional suppressor cells are reduced in adult SHR. Supernatant fluids derived from Con A-activated spleen cells from adult SHR failed to significantly inhibit IgG synthesis by cultures of syngeneic spleen cells compared to supernatant fluids from young SHR or WKY Con A-activated spleen cells. However, spleen cells from both adult SHR and WKY proliferated strongly and released equivalent amounts of IL-2 in response to Con A. Addition of exogenous IL-2 to the SMLR cultures in vitro restored the ability of SHR T cells to respond in the SMLR, with generation of cells capable of suppressing IgG synthesis. Administration of SHR with IL-2 in vivo also restored the suppressor T cell function in the SMLR. These results suggest a defective suppressor T cell activation and loss of suppressor T cell activity as the SHR age.

The vulnerable period (VP) is an interval of time during the cardiac cycle within which premature stimulation may lead to trains of responses (one: many stimulus-response coupling). Although the VP parallels the recovery of sodium channel availability, modulators of its boundaries remain unclear. Numerical studies of a uniform cable demonstrated that reduction in sodium channel availability increased the range of premature stimuli, resulting in unidirectional block, a precursor of reentrant activation. Consequently, we hypothesized that the kinetics of use-dependent sodium channel blockade could reflect one dimension of a drug's proarrhythmic potential. In strips from guinea pig right ventricle, we probed the boundaries of the VP in the presence of use-dependent sodium channel antagonists utilizing a train of stimuli followed by a premature stimulus. Under drug-free conditions when the sites of drive and premature stimulation were the same, the VP was less than 4 ms in duration. When the drive and premature sites were different, the drug-free VP was greater than 5 ms in 22 of 24 preparations and 0 in the other two, with an average VP duration of 16 +/- 10 ms (mean +/- SD). In the presence of 1 microM moricizine, VP = 17 +/- 4 ms; 12 microM moricizine, VP = 35 +/- 4 ms; 3 microM flecainide, VP = 50 +/- 17 ms; and 4 microM quinidine, VP = 2 +/- 1 ms. These results suggest that residual unsuppressed premature ventricular contractions (PVCs) in the presence of some class 1 drugs have a greater potential for initiating a proarrhythmic response than PVCs in the absence of a class 1 drug.

We examined the effects of in vivo administration of prolactin on growth-related gene expression in aorta. Optimal mRNA expression for both the proto-oncogene, c-myc and ornithine decarboxylase occurred at 22 mg/kg prolactin. For c-myc, this was seen as early as 15 min. Prolactin-induced ornithine decarboxylase mRNA expression began at 1 h. The results confirm work showing induction of these genes by prolactin in tissues where it is mitogenic and supports a role for prolactin in the trophic response of vascular smooth muscle.

Reports have suggested that the fast inward sodium current (INa) in cardiac tissues may be modulated by beta-adrenergic stimulation and that such modulation may affect conduction in the setting of myocardial ischemia and infarction. However, many of these studies have used dissociated myocytes or broken cell preparations, whose responses need not necessarily reflect those of syncytial preparations. To investigate further the possibility that beta-adrenergic stimulation of INa may differ in various preparations, we compared the effects of the beta-agonist isoproterenol (ISO) on syncytial canine Purkinje fibers and ventricular muscle preparations, as well as isolated ventricular myocytes. Alterations of the maximum rate of rise of the action potential upstroke (Vmax) were used as an index of changes of INa. ISO (1 microM) had no effect on Vmax of upstrokes of normally polarized (fast responses) or partially depolarized (elevated [K+]o, depressed fast responses) syncytial ventricular muscle preparations or Purkinje fibers. In contrast, lower concentrations of ISO (0.5-1.0 microM) modulated Vmax of isolated ventricular myocytes, depending on the technique used to monitor transmembrane potential. When 2.7 M KCl-filled microelectrodes were used, ISO reduced Vmax of partially depolarized myocytes without affecting Vmax of normally polarized myocytes. However, when myocytes were dialyzed using patch pipettes, ISO reduced Vmax of partially depolarized myocytes and increased Vmax of normally polarized myocytes, effecting a hyperpolarized shift of the normalized inactivation curve relating Vmax to resting membrane potential. The different beta-adrenergic responses of syncytial preparations and nondialyzed and dialyzed myocytes suggest that differences in the ionic or metabolic condition of the preparations likely alter cAMP-dependent responses and channel phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Early afterdepolarizations (EADs) are membrane oscillations that interrupt or retard the repolarization phase of the cardiac action potential, whereas delayed afterdepolarizations (DADs) are oscillations that arise after full repolarization. When EADs and DADs are sufficiently large to depolarize the cell membrane to its voltage threshold, they give rise to triggered action potentials, which are believed to underlie some forms of extrasystolic activity and tachyarrhythmias. EAD- and DAD-induced triggered activity have been described and well characterized in isolated Purkinje fibers exposed to a wide variety of drugs, but are rarely seen in syncytial preparations of ventricular myocardium. These results are inconsistent with those of in vivo studies or experiments involving enzymatically dissociated myocytes. In the present study, we used the cardiotonic agent acetylstrophanthidin (AcS) and the calcium channel agonist Bay K 8644 to provide evidence in support of the hypothesis that induction of prominent EADs, DADs, and triggered activity occurs in a select population of cells in ventricular myocardium. The data indicate that EADs, DADs, and triggered activity produced by digitalis and Bay K 8644 are limited to or more readily induced in the deep subepicardial cell layers of the canine ventricle (M cells). Afterdepolarization-induced triggered activity was never observed in the epicardial or endocardial layers.

Using microelectrode techniques we compared the effects of tetrodotoxin (TTX, 2-3 microM), DL-propranolol (1-3 micrograms/ml), and flecainide acetate (10-15 microM) on isolated canine ventricular epicardial (epicardium) and endocardial (endocardium) tissues. Propranolol, TTX, and flecainide decreased Vmax and phase 0 amplitude in a use-dependent manner in both tissues. The effects of propranolol were slow to develop and wash out. TTX and propranolol always abbreviated action potential duration in endocardium. Action potential duration was abbreviated by 23.8 +/- 5.6 msec after propranolol (1 microgram/ml, basic cycle length [BCL] = 1,000 msec) and 10.8 +/- 12.9 msec after TTX (2 microM, BCL = 1,000 msec). In epicardium, the reduction of phase 0 and 1 amplitudes led to a slowing of the second action potential upstroke and an increase in the amplitude of phase 2. This accentuation of the notch resulted in a paradoxical prolongation of the epicardial action potential. Action potential duration was prolonged 34.4 +/- 11.3 msec after 4 hours of exposure to propranolol (1 microgram/ml, BCL = 1,000 msec), 11.1 +/- 6.3 msec after 15 minutes of exposure to TTX (2 microM, BCL = 1,000 msec), and 19.9 +/- 8.2 msec after 25-45 minutes of exposure to flecainide (15 microM, BCL = 500 msec). With stronger sodium block, phase 1 terminated at more negative potentials, the second upstroke often failed to appear, and an all-or-none repolarization ensued causing a marked abbreviation of the epicardial action potential. In some epicardial preparations, we observed marked abbreviation at some sites but prolongation at other sites after sodium blockade with flecainide. The dispersion of repolarization was often attended by reentrant activity. The differential response of epicardium and endocardium to sodium blockade was not observed when the preparations were pretreated with 4-aminopyridine or ryanodine, agents known to diminish the transient outward current and epicardial notch. Acceleration-induced prolongation of refractoriness was observed after sodium blockade in epicardium but not in endocardium. Postrepolarization refractoriness also occurred in epicardium but not in endocardium after TTX, propranolol, or flecainide exposure. The data indicate that propranolol, TTX, and flecainide, via their action to block sodium current, may exert opposite effects on action potential duration and refractoriness in cells spanning the ventricular wall. The presence of the transient outward current in epicardium but not in endocardium appears to contribute importantly to these differences.(ABSTRACT TRUNCATED AT 400 WORDS)

Recent studies have shown that canine ventricular epicardium and endocardium differ with respect to electrophysiological characteristics and pharmacological responsiveness and that these differences are in large part due to the presence of a prominent transient outward current Ito and a spike-and-dome morphology of the action potential in epicardium but not endocardium. In attempting to quantitate these differences and assess their gradation across the ventricular wall, we encountered a subpopulation of cells in the deep subepicardial layers with electrophysiological characteristics different from those of either epicardium or endocardium. These cells, which we have termed M cells, display a spike-and-dome morphology typical of epicardium but a maximal rate of rise of the action potential upstroke that is considerably greater than that of either epicardium or endocardium. Using the restitution of the amplitude of phase 1 of the action potential as a marker for the reactivation of Ito, we showed M cells to possess a prominent 4-aminopyridine-sensitive Ito with a reactivation time course characterized by two components with fast and slow time constants. The rate dependence of action potential duration of M cells was considerably more accentuated than that of epicardium or endocardium and more akin to that of Purkinje fibers (not observed histologically in this region). Phase 4 depolarization was never observed in M cells, not even after exposure to catecholamines and/or low [K+]o. In summary, our study presents evidence for the existence of a unique subpopulation of cells in the deep subepicardium of the canine left and right ventricles with electrophysiological features intermediate between those of conducting and myocardial cells. Although their function is unknown, M cells may facilitate conduction in epicardium and are likely to influence or mediate the manifestation of electrocardiographic J waves, T waves, U waves, and long QT intervals and contribute importantly to arrhythmogenesis.

Drosophila melanogaster (Oregon R) males were exposed to visible light intensities varying from 0.3 to 7300 lux at environmental temperatures of 30, 35 and 37 degrees C, on a 12-h light/dark diurnal rhythm. At 30 degrees C reducing the light exposure from 7300 to 4 lux increased the median life span by 141%. At 35 degrees C reducing the light exposure from 4650 to 0.3 lux increased the life span by 389%. At 37 degrees C a reduction from 6580 to 0.3 lux increased life span by 453%. Even dim light (65 lux) affected life span in a negative manner. Two phases of response to light were identified, with a slow change in life span occurring below 400 lux and a more rapid rate of change above 400 lux. We conclude that visible light may be a major factor in the ageing process for Drosophila and that photochemical effects may contribute to senescence in other organisms. Possible alternative reasons for the effect of light on the life span of Drosophila include changes in body temperature, physical activity and oxygen consumption.

A prolongation of the ventricular effective refractory period in response to cholinergic agonists or vagal stimulation has been demonstrated in a number of in vivo animal models. However, exposure of isolated myocardial tissues obtained from these hearts to as much as 10(-4) M acetylcholine has been shown to produce essentially no change in action potential duration or effective refractory period. The discrepancy between the in vivo and in vitro findings generally has been explained on the basis of accentuated antagonism, whereby parasympathetic agonists exert their influence through antagonism of the effects of beta-adrenergic tone in vivo. The fact that acetylcholine exerts little if any direct effect on the electrical activity of ventricular myocardium, although well accepted, is based exclusively on studies performed using endocardial preparations. Our recent demonstration of major electrophysiological differences between canine ventricular endocardium and epicardium prompted us to examine the effects of acetylcholine and the role of accentuated antagonism in these two tissue types. Using standard microelectrode techniques, we show that acetylcholine (10(-7)-10(-5) M) has little if any effect in canine ventricular endocardium but a pronounced effect to either prolong or markedly abbreviate action potential duration and effective refractory period in epicardium. These effects of acetylcholine on epicardium are attended by an accentuation of the spike and dome morphology of the action potential, are readily reversed with atropine, fail to appear when epicardium is pretreated with the transient outward current blocker 4-aminopyridine, are accentuated in the presence of isoproterenol (10(-7) to 5 x 10(-6) M), and persist in the presence of propranolol. Isoproterenol-induced abbreviation of action potential duration and effective refractory period is also shown to be more pronounced in epicardium than in endocardium; equimolar concentrations of acetylcholine completely antagonize the effects of isoproterenol in endocardium and epicardium. We conclude that acetylcholine exerts important direct effects on the electrical response of canine ventricular myocardium, which are accentuated in the presence of beta-adrenergic agonists. Our findings suggest the differential response of epicardium and endocardium to acetylcholine is due to the presence of a transient outward current-mediated spike and dome morphology in the epicardial action potential. Finally, the data suggest that acetylcholine may exert antiarrhythmic as well as arrhythmogenic effects through its actions to alter conduction and refractoriness.

Total boron concentrations in Drosophila changed during development and ageing. The highest concentration of boron was found during the egg stage followed by a decline during the larval stages. Newly emerged flies contained 35.5 ppm boron. During the adult stage the boron concentration increased by 52% by 9 weeks of age. Adding excess dietary boron during the adult stage decreased the median life span by 69% at 0.01 M sodium borate and by 21% at 0.001 M sodium borate. Lower concentrations gave small but significant increases in life span. Supplementing a very low boron diet with 0.00025 M sodium borate improved life span by 9.5%. The boron contents of young and old mouse tissues were similar to those of Drosophila and human samples. We conclude that moderate levels of dietary boron may have a general protective effect in biological systems. The mechanism of this effect at present remains unknown.

Recent reports indicate that cyclooxygenase inhibitors such as aspirin may facilitate the release of interleukin-2 from thymic T cells. We have previously reported that aspirin has antihypertensive effects in the standard animal model of essential hypertension, the spontaneously hypertensive rat (SHR). Because the SHR has been reported to be deficient in T cells, it was of interest to determine whether the course of hypertension in this model could be altered by interleukin-2, the T cell growth factor. A single bolus of interleukin-2 (5,000 units/kg s.c.) prevented the increase of blood pressure in the young SHR and lowered pressure to normotensive levels in the well-established hypertensive adult SHR. In the latter, the effects of a single dose have been found to persist for at least 6 months with no toxic or untoward effects apparent. Blood pressure in Goldblatt, single-kidney wistar-kyoto rats, a model of renal hypertension, was not affected by interleukin-2.

Tetramethylpyrazine is extracted from Rhizoma ligustici wallichii, an herb used in the Chinese medicine Chung Chong. Both herb and extract have been used in the treatment of anginal pain and stroke. Animal studies in the West have shown that tetramethylpyrazine improves coronary blood flow, is short acting, and has a low toxicity. There are no clinical or animal studies on the uterine effects of tetramethylpyrazine. We present results of a preliminary study with isolated uterine strips from rats. We found that tetramethylpyrazine, in a dose-dependent manner (0.6 to 20 micrograms/ml), reduced uterine diastolic tone and inhibited the response to oxytocin (0.02 to 0.32 micrograms/ml). Higher concentrations of tetramethylpyrazine were needed to block the uterine responses to prostaglandin E2 (0.01 to 0.1 microgram/ml). On the basis of clinical and folk experience in the Far East, tetramethylpyrazine appears to have fewer systemic effects in human beings than have the beta-adrenergic agonists or calcium channel blockers. We suggest that studies in the whole animal and in the clinic might provide reasons to use tetramethylpyrazine to reduce uterine contractions and tone in pregnant women at term.

Previous studies have provided evidence for an important contribution of the transient outward current to the electrical activity of canine ventricular epicardium, but not endocardium. The present study examines the characteristics of action potential duration and refractoriness in these two tissue types. The time and rate dependence of changes in action potential duration and refractoriness observed in epicardium were significantly more accentuated than in endocardium. The restitution of action potential duration in epicardium paralleled the restitution of phase 1 amplitude of the action potential in this tissue. The correlation between phase 1 amplitude and action potential duration recorded from a large number of epicardial and endocardial preparations was significant under both steady state and restitution conditions. 4-Aminopyridine, a transient outward current blocker, decreased the time dependence of phase 1 amplitude and concomitantly decreased the time dependence of action potential duration in epicardium. 4-Aminopyridine abbreviated the action potential duration of epicardium at slow stimulation rates but had little effect or prolonged it at fast rates or after premature stimulation. (The availability of a transient outward current is relatively small after premature stimulation.) The data support the hypothesis that the prominent presence of a transient outward current in epicardium, but not endocardium, contributes to the differences in the time and rate dependence of action potential duration and refractoriness in the two tissue types. The results also demonstrate the effect of an outward current to prolong the action potential and the effect of an outward current blocker to abbreviate the action potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Reflection is a subclass of reentrant cardiac arrhythmias in which reexcitation of the heart occurs as a result of to and fro electrotonically mediated transmission of impulses across a narrow zone of impaired conductivity. Although relatively well characterized in ventricular tissues, the reflection mechanism has not been studied in atrial tissues. In this study we examine the possibility of reflected reentry in segmentally depressed atrial tissues and evaluate conduction characteristics in these preparations. Narrow strips of atrial pectinate muscle or crista terminalis (canine and calf) were placed in a three-chambered bath and the central segment was superfused with an isotonic sucrose solution or an "ischemic" Tyrode's solution. Proximal to distal conduction across the 1.0- to 1.2-mm wide ischemic gap showed step delays as long as 210 ms. Reflected reentry was readily demonstrable when prominent step delays occurred during anterograde conduction of the impulse across the gap. Progressive acceleration of the stimulation rate resulted in progressively greater impairment of anterograde conduction until complete block occurred. The incidence and patterns of reflected reentry were therefore a sensitive function of the stimulation rate. Other features exhibited by these preparations include a slow recovery of excitability following the action potential, postrepolarization refractoriness, and electrotonic inhibition and summation. Our data suggest that the characteristics of conduction and reflection in segmentally depressed atrial tissues are qualitatively similar to those in ventricular tissues. The presence of electrotonic inhibition in atrial may also help to explain the functionally inexcitable zone seen in the vortex of the leading circle model of atrial flutter.

Early afterdepolarization (EAD)-induced triggered activity is thought to contribute to the cardiac arrhythmogenic effects of several class I antiarrhythmic agents. The combination of quinidine therapy, bradycardia, and hypokalemia is known to predispose to torsade de pointes, which is a form of atypical polymorphous ventricular tachycardia commonly associated with long QT intervals. Recent clinical reports have shown suppression of quinidine-induced torsade de pointes with intravenous administration of magnesium sulfate. To provide further understanding of these relations, we used standard microelectrode techniques to examine the time course of quinidine-induced action potential prolongation, EAD, and triggered activity development and the dependence of these changes on [K+]0, [Mg2+]0, and stimulation frequency in isolated Purkinje fiber preparations exposed to low concentrations of the drug. At slow stimulation rates, the quinidine-induced increase of action potential duration was slow to develop and failed to reach a steady state after 3 hours of exposure to the drug. EAD and EAD-induced triggered activity generally became apparent 70-90 minutes after adding the drug. Quinidine produced triggered activity in 10 of 22 preparations superfused with Tyrode's solution containing normal [K+]0 (3.5-4.0 mM) and in six other preparations when [K+]0 was reduced. In the presence of normal [K+]0, two types of EAD and triggered activity were distinguished. In four of 10 preparations, this activity arose from phase 2 of the action potential; in eight of 10, it was associated with phase 3; and in two experiments, both types were present in the same preparation. The incidence of both forms of triggered responses depended greatly on the rate of stimulation. Triggered activity arising from phase 3 was always manifest at rates considerably slower than those giving rise to phase 2 activity. Both forms of triggered activity were sensitive to changes in the extracellular concentration of potassium and magnesium. Lower-than-normal levels of these electrolytes facilitated the manifestation of triggered activity, whereas elevated levels suppressed or caused a shift in the frequency-dependence of the activity. Phase 2, but not phase 3, EADs were abolished in response to increased [Mg2+]0. The data show a clear congruity between the conditions that predispose to torsade de pointes in the clinic and the conditions under which quinidine may induce triggered activity and marked action potential prolongation in isolated Purkinje fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Total aluminum concentrations increased with ageing in the liver and kidney of male C57BL/6J mice, remained unchanged in brain and heart, and decreased with ageing in femur and lung for mice ranging in age from 56 to 1186 days. Ligating one kidney did not significantly increase aluminum concentrations in the various organs. Feeding 1 X 10(-2) M aluminum chloride (270 ppm Al) in the drinking water beginning at 604 days of age decreased the average life span by 6.7%. We conclude that very little aluminum accumulation occurs with ageing in the organs tested in this study, in spite of a high dietary intake. Other organs might show a change. Only one aluminum concentration was used in this study which accelerated the rate of ageing as indicated by a change in the survival curve. The effect of higher or lower aluminum concentrations remains to be seen.

1. We previously showed that chronic exposure to aspirin (100 mg kg-1 daily, by mouth) is effective in preventing the onset of hypertension in young (28-84 day old) spontaneously hypertensive rats (SHRs). This is contrary to what others have reported using older SHRs. 2. Renal prostaglandin F2 alpha was also reduced in young SHRs and Wistar-Okamoto strain rats (WKYs) exposed to aspirin. 3. In the present study we extended the period of aspirin treatment in young rats to beyond 84 days of age. We found that aspirin lost its antihypertensive effect in SHR and WKY rats at 110 +/- 7 days of age regardless of whether the exposure to aspirin had begun at age 28, 49 or 87 days. 4. We conclude that the loss of antihypertensive effect of aspirin in the SHR and in older WKY rats, is determined by some factor(s) probably not related to prostaglandin F2 alpha, which reaches full expression in the 110 +/- 7 day old rat, or is fully dissipated at this age. 5. The anti-PGF2 alpha activity of aspirin in the SHR and WKY rat was short-lived and apparently unrelated in time to the antihypertensive effect of aspirin.

Previous studies have denied the presence of a transient outward current (Ito) in ventricular myocardium of dog, sheep, and calf. Using conventional microelectrode techniques, we provide evidence for a significant contribution of Ito to epicardial, but not endocardial, activity of canine ventricular myocardium. The epicardial action potential when compared with that of endocardium shows a smaller phase 0 amplitude, a much more prominent phase 1, and a phase 2 amplitude that is greater than that of phase 0. Epicardial action potentials, unlike those of endocardium, display a "spike and dome" morphology that becomes progressively more accentuated at slower stimulation rates. Using the restitution of phase 1 amplitude as a marker for the process responsible for the spike and dome phenomenon, we were able to delineate two exponential components: 1) a slow component that recovers with a time constant of 350-570 msec and 2) a fast component with a time constant of 41-85 msec. The slow component was largely abolished by 1-5 mM 4-aminopyridine, an Ito blocker. The fast component was diminished by 4-aminopyridine, but it was also inhibited by ryanodine and by Sr2+ replacement of Ca2+, which are interventions known to inhibit the Ca2+-activated component of Ito. Following 4-aminopyridine and Sr2+ or ryanodine treatment, the epicardial responses more closely resembled those of endocardium. In summary, the data demonstrate a marked heterogeneity of active membrane properties in canine ventricular muscle. These observations may aid in understanding the basis for rate-dependent changes in the T wave of the ECG, supernormal conduction in ventricular muscle, the greater sensitivity of epicardium to ischemia, and the rate dependence of some cardiac arrhythmias.

Subsidiary atrial automaticity generated by cardiac fibers of the rabbit tricuspid valve was studied in vitro with the use of standard microelectrode techniques. The time course of diastolic depolarization of leading pacemaker fibers was characteristically biphasic over a potential range of -80 to -55 mV, with a brief primary phase of relatively steep slope and a longer secondary phase of lesser slope. The regenerative upstroke of pacemaker potentials was suppressed by cobalt ions and varied with changes in extracellular Ca2+ concentration in a manner suggesting a slow channel process. The time course of each diastolic phase, estimated by measuring slope, varied in a reproducible manner with changes in membrane potential elicited with long duration current pulses. Cesium ions reduced the slope of both diastolic phases at all potentials with greater effects on the primary diastolic phase, but spontaneous activity in the absence of current pulses was not significantly altered. Furthermore, pacemaker activity varied with changes in extracellular K+ concentration and in response to barium ions in manners suggesting the presence of a decaying K+ conductance. These experiments demonstrate the utility of valve tissue as a model of subsidiary atrial pacemaker activity and suggest the presence of current systems qualitatively similar to those observed in the sinoatrial node.

Certain temporal patterns of A-V and V-A transmission in experimental preparations resemble phenomena attributed to "supernormal" conduction in the clinic. Detailed study of the properties of the A-V transmission system in such experiments reveals alternative explanations in which supernormality is clearly eliminated. By application of similar principles, supernormality can be eliminated as a factor in most if not all of the published examples. Three major categories can be discerned: (1) occult 2:1 A-V block, in which an idioventricular beat "retracts" an otherwise refractory barrier within the A-V node; (2) alternation between dissociated intranodal transmission pathways; and (3) "ventriculophasic" (vagal) depression of nodal conductivity.

Bone fractures create five problems that must be resolved: bleeding, risk of infection, hypoxia, disproportionate strain, and inability to bear weight. There have been enormous advancements in our understanding of the molecular mechanisms that resolve these problems after fractures, and in best clinical practices of repairing fractures. We put forth a modern, comprehensive model of fracture repair that synthesizes the literature on the biology and biomechanics of fracture repair to address the primary problems of fractures. This updated model is a framework for both fracture management and future studies aimed at understanding and treating this complex process. This model is based upon the fracture acute phase response (APR), which encompasses the molecular mechanisms that respond to injury. The APR is divided into sequential stages of "survival" and "repair." Early in convalescence, during "survival," bleeding and infection are resolved by collaborative efforts of the hemostatic and inflammatory pathways. Later, in "repair," avascular and biomechanically insufficient bone is replaced by a variable combination of intramembranous and endochondral ossification. Progression to repair cannot occur until survival has been ensured. A disproportionate APR-either insufficient or exuberant-leads to complications of survival (hemorrhage, thrombosis, systemic inflammatory response syndrome, infection, death) and/or repair (delayed- or non-union). The type of ossification utilized for fracture repair is dependent on the relative amounts of strain and vascularity in the fracture microenvironment, but any failure along this process can disrupt or delay fracture healing and result in a similar non-union. Therefore, incomplete understanding of the principles herein can result in mismanagement of fracture care or application of hardware that interferes with fracture repair. This unifying model of fracture repair not only informs clinicians how their interventions fit within the framework of normal biological healing but also instructs investigators about the critical variables and outputs to assess during a study of fracture repair.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) hold promise for therapeutic applications. To serve these functions, the hiPSC-CM must recapitulate the electrophysiologic properties of native adult cardiomyocytes. This study examines the electrophysiologic characteristics of hiPSC-CM between 11 and 121 days of maturity. Embryoid bodies (EBs) were generated from hiPS cell line reprogrammed with Oct4, Nanog, Lin28 and Sox2. Sharp microelectrodes were used to record action potentials (AP) from spontaneously beating clusters (BC) micro-dissected from the EBs (n = 103; 37°C) and to examine the response to 5 µM E-4031 (n = 21) or BaCl(2) (n = 22). Patch-clamp techniques were used to record I(Kr) and I(K1) from cells enzymatically dissociated from BC (n = 49; 36°C). Spontaneous cycle length (CL) and AP characteristics varied widely among the 103 preparations. E-4031 (5 µM; n = 21) increased Bazett-corrected AP duration from 291.8±81.2 to 426.4±120.2 msec (p<0.001) and generated early afterdepolarizations in 8/21 preparations. In 13/21 BC, E-4031 rapidly depolarized the clusters leading to inexcitability. BaCl(2), at concentrations that selectively block I(K1) (50-100 µM), failed to depolarize the majority of clusters (13/22). Patch-clamp experiments revealed very low or negligible I(K1) in 53% (20/38) of the cells studied, but presence of I(Kr) in all (11/11). Consistent with the electrophysiological data, RT-PCR and immunohistochemistry studies showed relatively poor mRNA and protein expression of I(K1) in the majority of cells, but robust expression of I(Kr.) In contrast to recently reported studies, our data point to major deficiencies of hiPSC-CM, with remarkable diversity of electrophysiologic phenotypes as well as pharmacologic responsiveness among beating clusters and cells up to 121 days post-differentiation (dpd). The vast majority have a maximum diastolic potential that depends critically on I(Kr) due to the absence of I(K1). Thus, efforts should be directed at producing more specialized and mature hiPSC-CM for future therapeutic applications.

The lead content of mouse femurs increased by 83% between 76 and 958 days of age with values ranging from 0.192 to 1.78 ng Pb/mg dry weight. These values are remarkably lower than in previous reports for the lead content of bone. The lead content of mouse liver showed no aging-related trend with values ranging from 0.00823 to 0.0149 ng/mg dry weight. Bone density, calcium and collagen content were not related to the lead content. We conclude that while bone lead content is very low in mice, it increases with aging but does not appear to be related to the osteopenia which develops in the C57BL/6J male mouse.

Male C57BL/6J mice ranging in age from 50 to 1186 days were used to measure total calcium and calmodulin concentrations. The increase in calcium between 0 and 1,000 days of age was 260% for kidney, followed by brain (189%), heart (173.5%), lung (106.5%) and liver (78.5%). Calcium in femur declined by 28.2%. The calmodulin content of liver increased with ageing. Both liver and kidney calmodulin concentrations declined early in life followed by ageing-related increases. Brain, lung and heart calmodulin concentrations did not change significantly with ageing. We conclude that changes in calcium homeostasis are not reflected in calmodulin changes. The loss of calcium in bone is consistent with the occurrence of osteoporosis in ageing C57 mice.

Arrhythmia is a disease that is caused by abnormal electrical activity in the heart rate or rhythm. It is the major cause of cardiovascular morbidity and mortality. Although several antiarrhythmic drugs have been used in clinic for decades, their application is often limited by their adverse effects. As a result, natural drugs, which have fewer side effects, are now being used to treat arrhythmias. We searched for all articles on the role of biologically active ingredients from natural drug treatments for arrhythmias in different mechanisms in PubMed. This study reviews 19 natural drug therapies, with 18 active ingredient therapies, such as alkaloids, flavonoids, saponins, quinones, and terpenes, and two kinds of traditional Chinese medicine compound (Wenxin-Keli and Shensongyangxin), all of which have been studied and reported as having antiarrhythmic effects. The primary focus is the proposed antiarrhythmic mechanism of each natural drug agent. . We stress persistent vigilance on the part of the provider in discussing the use of natural drug agents to provide a solid theoretical foundation for further research on antiarrhythmia drugs.

The Brugada syndrome (BS) has been linked to mutations in SCN5A. Despite equal hereditary transmission of the mutation between the sexes, the syndrome is 8 to 10 times more likely to occur in males. As recently reported, SCN5A mutations such as G1406R lead to development of BS phenotype principally in males and conduction disease phenotype in females. We hypothesized that these differences may be related to a larger transient outward current (Ito)-mediated right ventricular (RV) epicardial (Epi) action potential (AP) notch in males versus females, resulting in a higher incidence of all-or-none repolarization at the end of phase 1 and phase 2 re-entry (P2R) when challenged with sodium and calcium channel block. Using canine RV wedge preparations, we developed an experimental model of the BS using terfenadine to depress the AP dome in RV Epi.

Regional differences in repolarization characteristics of distinct cell types are responsible for the inscription of the J wave and T wave of the electrocardiogram (ECG). Amplification of these electrical heterogeneities contributes to the development of a variety of cardiac arrhythmias. This brief review examines the ionic and cellular basis for these heterogeneities and their role in the Brugada and long-QT syndromes. Both cases involve an accentuation of transmural dispersion of repolarization (TDR). In the case of the Brugada syndrome. TDR is accentuated as a result of a preferential abbreviation of the right ventricular epicardial action potential, whereas in the long-QT syndrome, accentuation of TDR is secondary to a preferential prolongation of the action potential of the M cell.

An early repolarization (ER) pattern in the ECG, distinguished by J-point elevation, slurring of the terminal part of the QRS and ST-segment elevation has long been recognized and considered to be a benign electrocardiographic manifestation. Experimental studies conducted over a decade ago suggested that some cases of ER may be associated with malignant arrhythmias. Validation of this hypothesis was provided by recent studies demonstrating that an ER pattern in the inferior or inferolateral leads is associated with increased risk for life-threatening arrhythmias, termed ER syndrome (ERS). Because accentuated J waves characterize both Brugada syndrome (BS) and ERS, these syndromes have been grouped under the term "J wave syndromes". ERS and BS share similar ECG characteristics, clinical outcomes and risk factors, as well as a common arrhythmic platform related to amplification of I(to)-mediated J waves. Although BS and ERS differ with respect to the magnitude and lead location of abnormal J wave manifestation, they can be considered to represent a continuous spectrum of phenotypic expression. Although most subjects exhibiting an ER pattern are at minimal to no risk, mounting evidence suggests that careful attention should be paid to subjects with "high risk" ER. The challenge ahead is to be able to identify those at risk for sudden cardiac death. Here I review the clinical and genetic aspects as well as the cellular and molecular mechanisms underlying the J wave syndromes.

This study probes the cellular basis for ischemia-induced ST-segment elevation with the isolated arterially perfused canine ventricular wedge preparation. Transmembrane action potentials (AP) from epicardial (Epi) and endocardial (Endo) regions, a pseudo-electrocardiogram (ECG), and 5 intramural unipolar electrograms were simultaneously recorded at a basic cycle length of 800 or 2,000 ms. Global ischemia was induced by an abrupt interruption of coronary flow for 30 minutes. Under control conditions, the ST segment was isoelectric because of the absence of voltage gradients at the level of AP plateau among the cells spanning the ventricular wall. Global ischemia could cause an all-or-none repolarization at the end of phase 1 of the AP in Epi but not Endo leading to ST-segment elevation and extrasystolic activity secondary to phase 2 re-entry. In the majority of preparations, global ischemia resulted in a progressive increase in transmural conduction time after 25 to 30 minutes of interruption of flow caused by a step delay of impulse transmission in the midmyocardium. The ECG assumed a "tombstone" configuration. Correlation of the APs and ECG activity revealed that the apparent severe ST-segment elevation encountered under these conditions is actually a markedly prolonged R wave. In control, Endo repolarized after Epi yielding upright T waves in the ECG. After 30 minutes of ischemia Epi repolarized after Endo causing reversal of repolarization gradients and T-wave inversion. The ischemia-induced electrophysiologic changes returned to nearly control values within 5 minutes of reperfusion. Our results indicate that 2 distinctly different mechanisms involving 1) loss of the epicardial action potential dome and 2) markedly delayed transmural conduction underlie the apparent ST-segment elevation encountered during acute ischemia.

Sudden cardiac death accounts for 19% of sudden deaths in children between 1 and 13 years of age and 30% of sudden deaths that occur between 14 and 21 years of age. The incidence of sudden cardiac death displays 2 peaks: one between 45 and 75 years of age, as a result of coronary artery disease, and the other between birth and 6 months of age, caused by sudden infant death syndrome. The role of cardiac arrhythmias in sudden infant death syndrome has long been a matter of debate and the role of cardiac arrhythmias in children in general is not well defined. Recent findings point to a contribution of primary electrical diseases of the heart including the Brugada and long QT syndromes to sudden death in infants and children. Mutations in SCN5A and HERG and KvLQT1 have been shown to be associated with life-threatening arrhythmias and long QT intervals in young infants. These mutations cause changes in sodium and potassium currents that amplify intrinsic electrical heterogeneities within the heart, thus providing a substrate as well as a trigger for the development of reentrant arrhythmias, including Torsade de Pointes (TdP), commonly associated with the long QT syndrome (LQTS). Mutations in SCN5A have also been shown to cause the sodium channel to turn off prematurely and thus to set the stage for the development of a rapid polymorphic ventricular tachycardia/ventricular fibrillation in patients with the Brugada Syndrome. In LQTS, ion channel mutations cause a preferential prolongation of the M cell action potential that contributes to the development of long QT intervals, wide-based or notched T waves, and a large transmural dispersion of repolarization, which provides the substrate for the development of TdP. An early afterdepolarization-induced triggered beat is thought to provide the extrasystole that precipitates TdP. In the Brugada syndrome, mutations in SCN5A reduce sodium current density, causing premature repolarization of the epicardial action potential due to an all or none repolarization at the end of phase 1. The loss of the action potential dome in epicardium, but not endocardium, creates a dispersion of repolarization across the ventricular wall, resulting in a transmural voltage gradient that manifests in the electrocardiogram (ECG) as an ST-segment elevation and in the development of a vulnerable window during which reentry can be induced. Under these conditions, loss of the action potential dome at some epicardial sites but not others gives rise to phase 2 reentry, which provides an extrasystole capable of precipitating ventricular tachycardia/ventricular fibrillation (or rapid TdP). The practical importance of identifying infants and children with Brugada and LQTS syndromes lies in the fact that most deaths due to these congenital defects can be prevented. A simple ECG is often sufficient to permit diagnosis and thus to prevent the development of life-threatening arrhythmic events. Mass ECG screening of neonates and children however has been the subject of debate focused on issues ranging from the emotional impact of dealing with false positives to those concerning socio-economic and medico-legal factors. These issues are discussed in other articles. These concerns notwithstanding, it is important that we continue to question whether the economic inefficiencies of current screening methodologies supersede the value of a young life.

The Brugada syndrome is characterized by ST-segment elevation in the right precordial leads, V1-V3 (unrelated to ischemia or structural disease), normal QT intervals, RBBB pattern, and sudden cardiac death, particularly in men of Asian origin. An autosomal dominant mode of inheritance with variable penetrance is generally observed. The only gene mutations thus far linked to the Brugada Syndrome appear in the alpha subunit of the gene that encodes for the cardiac sodium channel, SCN5A. An outward shift in the balance of currents contributing to phase 1 of the right ventricular action potential is thought to underline to electrocardiographic manifestation of the syndrome. Strong sodium channel block, among other modalities, can accentuate the action potential notch in right ventricular epicardial cells, eventually leading to loss of the action potential dome. This results in the development of a large dispersion of repolarization within epicardium as well as between epicardium and endocardium, providing the substrate for the development of phase 2 and cirus movement reentry, which underline VT/VF. Therapy is directed at restoring the balance of current via inhibition of the transient outward current, Ito, and/or stimulation of inward calcium using beta adrenergic agonists, among several strategies.

Recent studies have shown that ventricular myocardium is composed of at least 3 electrophysiologically distinct cell types: epicardial, endocardial, and M cells. Action potentials recorded from epicardial and M cells, unlike those recorded from endocardium, display a spike-and-dome morphology, the result of a prominent transient outward current-mediated phase 1. M cells are distinguished from endocardial and epicardial cells by the ability of their action potential to prolong disproportionately in response to a slowing of rate and/or to agents with class III actions. This intrinsic electrical heterogeneity contributes to the inscription of the electrocardiogram as well as to the development of a variety of cardiac arrhythmias. The transmural dispersion of repolarization is in large part responsible for the inscription of the J wave and T wave of the electrocardiogram. Because full repolarization of epicardium defines the peak of the T wave and that of the M cells, the end of the T wave, the interval between the peak and the end of the T wave provides a valuable index of transmural dispersion of repolarization. Differences in the response of the 3 cell types to pharmacologic agents and/or pathophysiological states often results in amplification of intrinsic electrical heterogeneities, thus providing a substrate as well as a trigger for the development of reentrant arrhythmias, including torsade de pointes (TdP) commonly associated with the long QT syndrome (LQTS) and the polymorphic ventricular tachycardia/fibrillation encountered in patients with the Brugada syndrome. Early repolarization of the epicardial action potential results in abnormal abbreviation of action potential duration due to an all-or-none repolarization at the end of phase 1 of the epicardial action potential. The loss of the action potential dome in epicardium but not endocardium gives rise to a large dispersion of repolarization across the ventricular wall, resulting in a transmural voltage gradient that manifests in the electrocardiogram as an ST segment elevation (or idiopathic J wave). Under these conditions, heterogeneous repolarization of the epicardial action potential gives rise to phase 2 reentry, which provides an extrasystole capable of precipitating ventricular tachycardia/fibrillation (or rapid TdP). Experimental models displaying these phenomena show electrocardiographic characteristics similar to those of the Brugada syndrome as well as those encountered during acute ischemia. Transmural dispersion of repolarization is also greatly amplified in LQTS. Disproportionate prolongation of the M-cell action potential contributes to the development of long QT intervals, wide-based or notched T waves, and a large transmural dispersion of repolarization, which provides the substrate for the development of a polymorphic ventricular tachycardia closely resembling torsade de pointes. An early afterdepolarization-induced triggered beat is thought to provide the extrasystole that precipitates TdP. Pharmacologic models of the LQT1, LQT2 and LQT3 forms of LQTS mimic the distinctive electrocardiographic, electrophysiologic, and pharmacologic responses observed in patients with these 3 different genetic syndromes. In LQTS, as in the Brugada syndrome, a mutation in an ion channel gene (in some cases the same gene--SCN5A) is responsible for the development of a large transmural dispersion of repolarization, which serves to provide the arrhythmogenic substrate tha can lead to sudden death.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are used for safety pharmacology and to investigate genetic diseases affecting cardiac ion channels. It is unclear whether adult myocytes or hiPSC-CMs are the better platform for cardiac safety pharmacology. We examined the biophysical and molecular properties of I in adult myocytes and hiPSC-CMs.

Torsade de Pointes (TdP) is an atypical ventricular arrhythmias associated with the acquired and congenital forms of the long QT syndrome. The substrate for the arrhythmia develops as a consequence of the amplification of electrical heterogeneities intrinsic to the ventricular myocardium. These heterogeneities exist because of differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization responsible for the inscription of the electrocardiographic T wave. A wide variety of drugs are capable of reducing net repolarizing current and thus amplifying the intrinsic spatial dispersion of repolarization, so as to create the substrate for the development of re-entry. The result is a prolongation of the QT interval, abnormal T waves, and development of polymorphic re-entrant ventricular tachycardia displaying characteristics of TdP. Recent studies demonstrate that prolongation of the QT interval is not the sole determinant of the potential of a drug to cause TdP; agents that do not increase transmural dispersion of repolarization have little or no potential to induce the arrhythmia despite their ability to prolong the QT interval. Moreover, drugs such as sodium pentobarbital, which reduce transmural dispersion of repolarization, can diminish the likelihood of TdP, despite their ability to prolong the QT interval.

Free uric acid concentrations declined with aging in male Oregon R Drosophila melanogaster by 59% or more between 0 and 50 days of adult age. Free xanthine concentrations increased between 0 and 5 days of age and declined by 75% between 5 and 50 days of age. Xanthine oxidase activity was maximal for newly emerged flies and then declined rapidly reaching a minimum at 9 days of age. After 9 days of age xanthine concentrations may be the limiting factor for the production of uric acid by xanthine oxidase in aging fruit flies. Declining uric acid concentrations may represent a loss of antioxidant potential in aging Drosophila.

The Brugada syndrome is a congenital syndrome of sudden cardiac death first described as a new clinical entity in 1992. Electrocardiographically characterized by a distinct coved-type ST segment elevation in the right precordial leads, the syndrome is associated with a high risk for sudden cardiac death in young and otherwise healthy adults, and less frequently in infants and children. The ECG manifestations of the Brugada syndrome are often dynamic or concealed and may be revealed or modulated by sodium channel blockers. The syndrome may also be unmasked or precipitated by a febrile state, vagotonic agents, alpha-adrenergic agonists, beta-adrenergic blockers, tricyclic or tetracyclic antidepressants, a combination of glucose and insulin, and hypokalemia, as well as by alcohol and cocaine toxicity. An implantable cardioverter-defibrillator (ICD) is the most widely accepted approach to therapy. Pharmacological therapy aimed at rebalancing the currents active during phase 1 of the right ventricular action potential is used to abort electrical storms, as an adjunct to device therapy, and as an alternative to device therapy when use of an ICD is not possible. Isoproterenol and cilostazol boost calcium channel current, and drugs like quinidine inhibit the transient outward current, acting to diminish the action potential notch and thus suppress the substrate and trigger for ventricular tachycardia/fibrillation (VT/VF).