Inhibition of the Na+/Ca2+ exchanger suppresses torsades de pointes in an intact heart model of long QT syndrome-2 and long QT syndrome-3

Mechanisms underlying the spontaneous termination of torsades de pointes in an experimental model of long QT syndrome

BACKGROUND

Torsades de pointes (TdP) represent a complex polymorphic ventricular tachycardia. While the triggering mechanisms of early afterdepolarization and increased dispersion of repolarization are well investigated, the sudden self-limiting termination remains poorly understood.

OBJECTIVE

The purpose of this study was to perform analysis of TdP to investigate factors causing spontaneous termination.

METHODS

We used a large data set from Langendorff experiments in isolated rabbit hearts in which drug-induced QT prolongation, bradycardia, and hypokalemia provoke TdP. We included 427 episodes with typical TdP characteristics of polymorphic self-terminating beats and twisting QRS complexes occurring in the presence of abnormal QT prolongation due to various different QT-prolonging drugs. The use of 8 monophasic action potential catheters allowed the characterization of action potential duration, configuration, and dispersion of repolarization beyond the capabilities of the surface electrocardiogram. To identify possible mechanisms of arrhythmia termination, the initial, midpoint, and terminal 3 ventricular complexes were analyzed for each episode.

RESULTS

An abrupt decrease in spatial dispersion over the course of a TdP episode was identified as a precursor for termination of TdP. Within the last 3 beats, a sudden significant decrease in the dispersion of repolarization was observed as a predictor of termination. In parallel, there was a decrease in action potential duration (action potential duration at 90% repolarization) before termination. Also, a change in action potential configuration (action potential duration at 90% repolarization/action potential duration at 50% repolarization ratio) in terms of the loss of action potential dome with a restitution of action potential triangulation was observed.

CONCLUSION

In >400 TdP episodes, homogenization of myocardial repolarization with the recovery of an action potential configuration determines the termination of TdP episodes.

Relevance of mexiletine in the era of evolving antiarrhythmic therapy of ventricular arrhythmias

Despite impressive developments in the field of ventricular arrhythmias, there is still a relevant number of patients with ventricular arrhythmias who require antiarrhythmic drug therapy and may, e.g., in otherwise drug and/or ablation refractory situations, benefit from agents known for decades, such as mexiletine. Through its capability of blocking fast sodium channels in cardiomyocytes, it has played a minor to moderate antiarrhythmic role throughout the recent decades. Nevertheless, certain patients with structural heart disease suffering from drug-refractory, i.e., mainly amiodarone refractory ventricular arrhythmias, as well as those with selected forms of congenital long QT syndrome (LQTS) may nowadays still benefit from mexiletine. Here, we outline mexiletine's cellular and clinical electrophysiological properties. In addition, the application of mexiletine may be accompanied by various potential side effects, e.g., nausea and tremor, and is limited by several drug-drug interactions. Thus, we shed light on the current therapeutic role of mexiletine for therapy of ventricular arrhythmias and discuss clinically relevant aspects of its indications based on current evidence.

α7 nicotinic receptors play a role in regulation of cardiac hemodynamics

The α7 nicotinic receptors (NR) have been confirmed in the heart but their role in cardiac functions has been contradictory. To address these contradictory findings, we analyzed cardiac functions in α7 NR knockout mice (α7-/-) in vivo and ex vivo in isolated hearts. A standard limb leads electrocardiogram was used, and the pressure curves were recorded in vivo, in Arteria carotis and in the left ventricle, or ex vivo, in the left ventricle of the spontaneously beating isolated hearts perfused following Langedorff's method. Experiments were performed under basic conditions, hypercholinergic conditions, and adrenergic stress. The relative expression levels of α and β NR subunits, muscarinic receptors, β1 adrenergic receptors, and acetylcholine life cycle markers were determined using RT-qPCR. Our results revealed a prolonged QT interval in α7-/- mice. All in vivo hemodynamic parameters were preserved under all studied conditions. The only difference in ex vivo heart rate between genotypes was the loss of bradycardia in prolonged incubation of isoproterenol-pretreated hearts with high doses of acetylcholine. In contrast, left ventricular systolic pressure was lower under basal conditions and showed a significantly higher increase during adrenergic stimulation. No changes in mRNA expression were observed. In conclusion, α7 NR has no major effect on heart rate, except when stressed hearts are exposed to a prolonged hypercholinergic state, suggesting a role in acetylcholine spillover control. In the absence of extracardiac regulatory mechanisms, left ventricular systolic impairment is revealed.

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Ca2+ mishandling in heart failure: Potential targets

Ca²⁺ mishandling is a common feature in several cardiovascular diseases such as heart failure (HF). In many cases, impairment of key players in intracellular Ca²⁺ homeostasis has been identified as the underlying mechanism of cardiac dysfunction and cardiac arrhythmias associated with HF. In this review, we summarize primary novel findings related to Ca²⁺ mishandling in HF progression. HF research has increasingly focused on the identification of new targets and the contribution of their role in Ca²⁺ handling to the progression of the disease. Recent research studies have identified potential targets in three major emerging areas implicated in regulation of Ca²⁺ handling: the innate immune system, bone metabolism factors and post-translational modification of key proteins involved in regulation of Ca²⁺ handling. Here, we describe their possible contributions to the progression of HF.

Daphnetin Preconditioning Decreases Cardiac Injury and Susceptibility to Ventricular Arrhythmia following Ischaemia-Reperfusion through the TLR4/MyD88/NF-Κb Signalling Pathway

BACKGROUND/AIMS

Daphnetin (7,8-dihydroxycoumarin, DAP) exhibits various bioactivities, such as anti-inflammatory and antioxidant activities. However, the role of DAP in myocardial ischaemia/reperfusion (I/R) injury and I/R-related arrhythmia is still uncertain. This study aimed to investigate the mechanisms underlying the effects of DAP on myocardial I/R injury and electrophysiological properties in vivo and in vitro.

METHODS

Myocardial infarct size was measured by triphenyltetrazolium chloride staining. Cardiac function was assessed by echocardiographic and haemodynamic analyses. The levels of creatine kinase-MB, lactate dehydrogenase, malondialdehyde, superoxide dismutase, interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNF-α) were detected using commercial kits. Apoptosis was measured by terminal deoxynucleotidyl-transferase-mediated dUTP nick-end labelling staining and flow cytometry. The viability of H9c2 cells was determined by the Cell Counting Kit-8 assay. In vitro, the levels of IL-6 and TNF-α were measured by quantitative PCR. The expression levels of proteins associated with apoptosis, inflammation, and the Toll-like receptor 4/myeloid differentiation factor 88/nuclear factor kappa B (TLR4/MyD88/NF-κB) signalling pathway were detected by Western blot analysis. The RR, PR, QRS, and QTc intervals were assessed by surface ECG. The 90% action potential duration (APD90), threshold of APD alternans, and ventricular tachycardia inducibility were measured by the Langendorff perfusion technique.

RESULTS

DAP preconditioning decreased myocardial I/R injury and hypoxia/reoxygenation (H/R) injury in cells. DAP preconditioning improved cardiac function after myocardial I/R injury. DAP preconditioning also suppressed apoptosis, attenuated oxidative stress, and inhibited inflammatory responses in vivo and in vitro. Furthermore, DAP preconditioning decreased the susceptibility to ventricular arrhythmia after myocardial I/R. Finally, DAP preconditioning inhibited the expression of TLR4, MyD88, and phosphorylated NF-κB (p-NF-κB)/P65 in mice subjected to I/R and cells subjected to H/R.

CONCLUSIONS

DAP preconditioning protected against myocardial I/R injury and decreased susceptibility to ventricular arrhythmia by inhibiting the TLR4/MyD88/NF-κB signalling pathway.

SAR340835, a Novel Selective Na+/Ca2+ Exchanger Inhibitor, Improves Cardiac Function and Restores Sympathovagal Balance in Heart Failure

In failing hearts, Na⁺/Ca²⁺ exchanger (NCX) overactivity contributes to Ca²⁺ depletion, leading to contractile dysfunction. Inhibition of NCX is expected to normalize Ca²⁺ mishandling, to limit afterdepolarization-related arrhythmias, and to improve cardiac function in heart failure (HF). SAR340835/SAR296968 is a selective NCX inhibitor for all NCX isoforms across species, including human, with no effect on the native voltage-dependent calcium and sodium currents in vitro. Additionally, it showed in vitro and in vivo antiarrhythmic properties in several models of early and delayed afterdepolarization-related arrhythmias. Its effect on cardiac function was studied under intravenous infusion at 250,750 or 1500 µg/kg per hour in dogs, which were either normal or submitted to chronic ventricular pacing at 240 bpm (HF dogs). HF dogs were infused with the reference inotrope dobutamine (10 µg/kg per minute, i.v.). In normal dogs, NCX inhibitor increased cardiac contractility (dP/dt_max) and stroke volume (SV) and tended to reduce heart rate (HR). In HF dogs, NCX inhibitor significantly and dose-dependently increased SV from the first dose (+28.5%, +48.8%, and +62% at 250, 750, and 1500 µg/kg per hour, respectively) while significantly increasing dP/dt_max only at 1500 (+33%). Furthermore, NCX inhibitor significantly restored sympathovagal balance and spontaneous baroreflex sensitivity (BRS) from the first dose and reduced HR at the highest dose. In HF dogs, dobutamine significantly increased dP/dt_max and SV (+68.8%) but did not change HR, sympathovagal balance, or BRS. Overall, SAR340835, a selective potent NCX inhibitor, displayed a unique therapeutic profile, combining antiarrhythmic properties, capacity to restore systolic function, sympathovagal balance, and BRS in HF dogs. NCX inhibitors may offer new therapeutic options for acute HF treatment. SIGNIFICANCE STATEMENT: HF is facing growing health and economic burden. Moreover, patients hospitalized for acute heart failure are at high risk of decompensation recurrence, and no current acute decompensated HF therapy definitively improved outcomes. A new potent, Na⁺/Ca²⁺ exchanger inhibitor SAR340835 with antiarrhythmic properties improved systolic function of failing hearts without creating hypotension, while reducing heart rate and restoring sympathovagal balance. SAR340835 may offer a unique and attractive pharmacological profile for patients with acute heart failure as compared with current inotrope, such as dobutamine.

Oxysterols Modulate the Acute Effects of Ethanol on Hippocampal N-Methyl-d-Aspartate Receptors, Long-Term Potentiation, and Learning

Ethanol is a noncompetitive inhibitor of N-methyl-d-aspartate receptors (NMDARs) and acutely disrupts hippocampal synaptic plasticity and learning. In the present study, we examined the effects of oxysterol positive allosteric modulators (PAMs) of NMDARs on ethanol-mediated inhibition of NMDARs, block of long-term potentiation (LTP) and long-term depression (LTD) in rat hippocampal slices, and defects in one-trial learning in vivo. We found that 24S-hydroxycholesterol and a synthetic oxysterol analog, SGE-301, overcame effects of ethanol on NMDAR-mediated synaptic responses in the CA1 region but did not alter acute effects of ethanol on LTD; the synthetic oxysterol, however, overcame acute inhibition of LTP. In addition, both oxysterols overcame persistent effects of ethanol on LTP in vitro, and the synthetic analog reversed defects in one-trial inhibitory avoidance learning in vivo. These results indicate that effects of ethanol on both LTP and LTD arise by complex mechanisms beyond NMDAR antagonism and that oxysterol NMDAR PAMS may represent a novel approach for preventing and reversing acute ethanol-mediated changes in cognition.

The role of calcium homeostasis remodeling in inherited cardiac arrhythmia syndromes

Sudden cardiac death due to malignant ventricular arrhythmias remains the major cause of mortality in the postindustrial world. Defective intracellular Ca²⁺ homeostasis has been well established as a key contributing factor to the enhanced propensity for arrhythmia in acquired cardiac disease, such as heart failure or diabetic cardiomyopathy. More recent advances provide a strong basis to the emerging view that hereditary cardiac arrhythmia syndromes are accompanied by maladaptive remodeling of Ca²⁺ homeostasis which substantially increases arrhythmic risk. This brief review will focus on functional changes in elements of Ca²⁺ handling machinery in cardiomyocytes that occur secondary to genetic mutations associated with catecholaminergic polymorphic ventricular tachycardia, and long QT syndrome.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Innate Immune Receptors, Key Actors in Cardiovascular Diseases

Cardiovascular diseases (CVDs) are the leading cause of death in the industrialized world. Most CVDs are associated with increased inflammation that arises mainly from innate immune system activation related to cardiac damage. Sustained activation of the innate immune system frequently results in maladaptive inflammatory responses that promote cardiovascular dysfunction and remodeling. Much research has focused on determining whether some mediators of the innate immune system are potential targets for CVD therapy. The innate immune system has specific receptors-termed pattern recognition receptors (PRRs)-that not only recognize pathogen-associated molecular patterns, but also sense danger-associated molecular signals. Activation of PRRs triggers the inflammatory response in different physiological systems, including the cardiovascular system. The classic PRRs, toll-like receptors (TLRs), and the more recently discovered nucleotide-binding oligomerization domain-like receptors (NLRs), have been recently proposed as key partners in the progression of several CVDs (e.g., atherosclerosis and heart failure). The present review discusses the key findings related to the involvement of TLRs and NLRs in the progression of several vascular and cardiac diseases, with a focus on whether some NLR subtypes (nucleotide-binding oligomerization domain, leucine rich repeat and pyrin domain-containing receptor 3 and nucleotide-binding oligomerization domain-containing protein 1) can be candidates for the development of new therapeutic strategies for several CVDs.

Chronic intermittent tachypacing by an optogenetic approach induces arrhythmia vulnerability in human engineered heart tissue

AIMS

Chronic tachypacing is commonly used in animals to induce cardiac dysfunction and to study mechanisms of heart failure and arrhythmogenesis. Human induced pluripotent stem cells (hiPSC) may replace animal models to overcome species differences and ethical problems. Here, 3D engineered heart tissue (EHT) was used to investigate the effect of chronic tachypacing on hiPSC-cardiomyocytes (hiPSC-CMs).

METHODS AND RESULTS

To avoid cell toxicity by electrical pacing, we developed an optogenetic approach. EHTs were transduced with lentivirus expressing channelrhodopsin-2 (H134R) and stimulated by 15 s bursts of blue light pulses (0.3 mW/mm2, 30 ms, 3 Hz) separated by 15 s without pacing for 3 weeks. Chronic optical tachypacing did not affect contractile peak force, but induced faster contraction kinetics, shorter action potentials, and shorter effective refractory periods. This electrical remodelling increased vulnerability to tachycardia episodes upon electrical burst pacing. Lower calsequestrin 2 protein levels, faster diastolic depolarization (DD) and efficacy of JTV-519 (46% at 1 µmol/L) to terminate tachycardia indicate alterations of Ca2+ handling being part of the underlying mechanism. However, other antiarrhythmic compounds like flecainide (69% at 1 µmol/L) and E-4031 (100% at 1 µmol/L) were also effective, but not ivabradine (1 µmol/L) or SEA0400 (10 µmol/L).

CONCLUSION

We demonstrated a high vulnerability to tachycardia of optically tachypaced hiPSC-CMs in EHT and the effective termination by ryanodine receptor stabilization, sodium or hERG potassium channel inhibition. This new model might serve as a preclinical tool to test antiarrhythmic drugs increasing the insight in treating ventricular tachycardia.

Multiple Dynamical Mechanisms of Phase-2 Early Afterdepolarizations in a Human Ventricular Myocyte Model: Involvement of Spontaneous SR Ca 2+ Release

Early afterdepolarization (EAD) is known to cause lethal ventricular arrhythmias in long QT syndrome (LQTS). In this study, dynamical mechanisms of EAD formation in human ventricular myocytes (HVMs) were investigated using the mathematical model developed by ten Tusscher and Panfilov (Am J Physiol Heart Circ Physiol 291, 2006). We explored how the rapid (IKr) and slow (IKs) components of delayed-rectifier K+ channel currents, L-type Ca2+ channel current (ICa L), Na+/Ca2+ exchanger current (INCX), and intracellular Ca2+ handling via the sarcoplasmic reticulum (SR) contribute to initiation, termination and modulation of phase-2 EADs during pacing in relation to bifurcation phenomena in non-paced model cells. Parameter-dependent dynamical behaviors of the non-paced model cell were determined by calculating stabilities of equilibrium points (EPs) and limit cycles, and bifurcation points to construct bifurcation diagrams. Action potentials (APs) and EADs during pacing were reproduced by numerical simulations for constructing phase diagrams of the paced model cell dynamics. Results are summarized as follows: (1) A modified version of the ten Tusscher-Panfilov model with accelerated ICaL inactivation could reproduce bradycardia-related EADs in LQTS type 2 and β-adrenergic stimulation-induced EADs in LQTS type 1. (2) Two types of EADs with different initiation mechanisms, ICaL reactivation-dependent and spontaneous SR Ca2+ release-mediated EADs, were detected. (3) Termination of EADs (AP repolarization) during pacing depended on the slow activation of IKs. (4) Spontaneous SR Ca2+ releases occurred at higher Ca2+ uptake rates, attributable to the instability of steady-state intracellular Ca2+ concentrations. Dynamical mechanisms of EAD formation and termination in the paced model cell are closely related to stability changes (bifurcations) in dynamical behaviors of the non-paced model cell, but they are model-dependent. Nevertheless, the modified ten Tusscher-Panfilov model would be useful for systematically investigating possible dynamical mechanisms of EAD-related arrhythmias in LQTS.

Acquired Long QT Syndrome and Electrophysiology of Torsade de Pointes

Congenital long QT syndrome (LQTS) has been the most investigated cardiac ion channelopathy. Although congenital LQTS remains the domain of cardiologists, cardiac electrophysiologists and specialised centres, the much more frequently acquired LQTS is the domain of physicians and other members of healthcare teams required to make therapeutic decisions. This paper reviews the electrophysiological mechanisms of acquired LQTS, its ECG characteristics, clinical presentation, and management. The paper concludes with a comprehensive review of the electrophysiological mechanisms of torsade de pointes.

Mutation of the Na+/K+-ATPase Atp1a1a.1 causes QT interval prolongation and bradycardia in zebrafish

The genetic underpinnings that orchestrate the vertebrate heart rate are not fully understood yet, but of high clinical importance, since diseases of cardiac impulse formation and propagation are common and severe human arrhythmias. To identify novel regulators of the vertebrate heart rate, we deciphered the pathogenesis of the bradycardia in the homozygous zebrafish mutant hiphop (hip) and identified a missense-mutation (N851K) in Na⁺/K⁺-ATPase α1-subunit (atp1a1a.1). N851K affects zebrafish Na⁺/K⁺-ATPase ion transport capacity, as revealed by in vitro pump current measurements. Inhibition of the Na⁺/K⁺-ATPase in vivo indicates that hip rather acts as a hypomorph than being a null allele. Consequently, reduced Na⁺/K⁺-ATPase function leads to prolonged QT interval and refractoriness in the hip mutant heart, as shown by electrocardiogram and in vivo electrical stimulation experiments. We here demonstrate for the first time that Na⁺/K⁺-ATPase plays an essential role in heart rate regulation by prolonging myocardial repolarization.

Selective late sodium current inhibitor GS-458967 suppresses Torsades de Pointes by mostly affecting perpetuation but not initiation of the arrhythmia

Background and Purpose

Enhanced late sodium current (late I_Na) in heart failure and long QT syndrome type 3 is proarrhythmic. This study investigated the antiarrhythmic effect and mode of action of the selective and potent late I_Na inhibitor GS‐458967 (GS967) against Torsades de Pointes arrhythmias (TdP) in the chronic atrioventricular block (CAVB) dog.

Experimental Approach

Electrophysiological and antiarrhythmic effects of GS967 were evaluated in isolated canine ventricular cardiomyocytes and CAVB dogs with dofetilide‐induced early afterdepolarizations (EADs) and TdP, respectively. Mapping of intramural cardiac electrical activity in vivo was conducted to study effects of GS967 on spatial dispersion of repolarization.

Key Results

GS967 (IC₅₀~200nM) significantly shortened repolarization in canine ventricular cardiomyocytes and sinus rhythm (SR) dogs, in a concentration and dose‐dependent manner. In vitro, despite addition of 1μM GS967, dofetilide‐induced EADs remained present in 42% and 35% of cardiomyocytes from SR and CAVB dogs, respectively. Nonetheless, GS967 (787±265nM) completely abolished dofetilide‐induced TdP in CAVB dogs (10/14 after dofetilide to 0/14 dogs after GS967), while single ectopic beats (sEB) persisted in 9 animals. In vivo mapping experiments showed that GS967 significantly reduced spatial dispersion of repolarization: cubic dispersion was significantly decreased from 237±54ms after dofetilide to 123±34ms after GS967.

Conclusion and Implications

GS967 terminated all dofetilide‐induced TdP without completely suppressing EADs and sEB in vitro and in vivo, respectively. The antiarrhythmic mode of action of GS967, through the reduction of spatial dispersion of repolarization, seems to predominantly impede the perpetuation of arrhythmic events into TdP rather than their initiating trigger.

Acquired long QT syndrome and torsade de pointes

Since its initial description by Jervell and Lange-Nielsen in 1957, the congenital long QT syndrome (LQTS) has been the most investigated cardiac ion channelopathy. Although congenital LQTS continues to remain the domain of cardiologists, cardiac electrophysiologists, and specialized centers, the by far more frequent acquired drug-induced LQTS is the domain of all physicians and other members of the health care team who are required to make therapeutic decisions. This report will review the electrophysiological mechanisms of LQTS and torsade de pointes, electrocardiographic characteristics of acquired LQTS, its clinical presentation, management, and future directions in the field.

Antiarrhythmic effect of vernakalant in an experimental model of Long-QT-syndrome

Aims

The antiarrhythmic drug vernakalant exerts antiarrhythmic effects in atrial fibrillation. Recent experimental data suggest interactions with the late sodium current and antiarrhythmic effects in ventricular arrhythmias. We aimed at investigating whether treatment with vernakalant reduces polymorphic ventricular tachycardia (VT) in an experimental model of Long-QT-syndrome (LQTS).

Methods and results

Twenty-nine isolated rabbit hearts were assigned to two groups and treated with erythromycin (300 µM, n = 15) or veratridine (0.5 µM, n = 14) after obtaining baseline data. Thereafter, vernakalant (10 µM) was additionally infused. Infusion of erythromycin or veratridine significantly increased action potential duration (APD90) and QT interval. Erythromycin and veratridine also significantly augmented spatial dispersion of repolarization (erythromycin: +43 ms; veratridine: +55 ms, P < 0.01, respectively) and temporal dispersion of repolarization. After lowering extracellular [K+] in bradycardic hearts, 11 of 15 erythromycin-treated hearts and 4 of 14 veratridine-treated hearts showed early afterdepolarizations and subsequent polymorphic VT. Additional treatment with vernakalant resulted in a significant reduction of spatial dispersion of spatial dispersion in both groups (erythromycin: -32 ms; veratridine: -35 ms, P < 0.05 each) and a stabilization of temporal dispersion. After additional treatment with vernakalant, only 5 of 15 erythromycin-treated hearts (P = 0.07) and 1 of 14 veratridine-treated hearts (P = 0.32) presented polymorphic VT.

Conclusion

Vernakalant has antiarrhythmic effects in this experimental model of acquired LQTS. A reduction of spatial dispersion of repolarization and a stabilization of temporal dispersion in hearts showing polymorphic VT represent the major underlying electrophysiological mechanisms.

Physiological genomics identifies genetic modifiers of long QT syndrome type 2 severity

Congenital long QT syndrome (LQTS) is an inherited channelopathy associated with life-threatening arrhythmias. LQTS type 2 (LQT2) is caused by mutations in KCNH2, which encodes the potassium channel hERG. We hypothesized that modifier genes are partly responsible for the variable phenotype severity observed in some LQT2 families. Here, we identified contributors to variable expressivity in an LQT2 family by using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and whole exome sequencing in a synergistic manner. We found that iPSC-CMs recapitulated the clinical genotype-phenotype discordance in vitro. Importantly, iPSC-CMs derived from the severely affected LQT2 patients displayed prolonged action potentials compared with cells from mildly affected first-degree relatives. The iPSC-CMs derived from all patients with hERG R752W mutation displayed lower IKr amplitude. Interestingly, iPSC-CMs from severely affected mutation-positive individuals exhibited greater L-type Ca2+ current. Whole exome sequencing identified variants of KCNK17 and the GTP-binding protein REM2, providing biologically plausible explanations for this variable expressivity. Genome editing to correct a REM2 variant reversed the enhanced L-type Ca2+ current and prolonged action potential observed in iPSC-CMs from severely affected individuals. Thus, our findings showcase the power of combining complementary physiological and genomic analyses to identify genetic modifiers and potential therapeutic targets of a monogenic disorder. Furthermore, we propose that this strategy can be deployed to unravel myriad confounding pathologies displaying variable expressivity.

Ryanodine-receptor inhibition by dantrolene effectively suppresses ventricular arrhythmias in an ex vivo model of long-QT syndrome

AIMS

A significant antiarrhythmic potential of ryanodine receptor inhibition was reported in experimental studies. The aim of the present study was to assess potential antiarrhythmic effects of dantrolene in an experimental whole-heart model of drug-induced long-QT syndrome (LQTS).

METHODS

In 12 isolated rabbit hearts, long-QT-2-syndrome was simulated by infusion of erythromycin (300 μM). Twelve rabbit hearts were treated with veratridine (0.5 μM) to mimic long-QT-3-syndrome.

RESULTS

Monophasic action potentials and ECG showed a significant prolongation of QT-interval (+71 ms, P < 0.01) and action potential duration (APD, +43 ms, P < 0.01) after infusion of erythromycin as compared with baseline. Similar results were obtained in veratridine-treated hearts (QT-interval: +43 ms, P < 0.01; APD: +36 ms, P < 0.01). Both erythromycin (+36 ms, P < 0.05) and veratridine (+38 ms) significantly increased dispersion of repolarization. Additional infusion of dantrolene (20 μM) did not significantly alter QT-interval and APD but resulted in a significant reduction of dispersion of repolarization (erythromycin group: -33 ms, P < 0.05; veratridine group: -29 ms, P < 0.05). Lowering of potassium concentration resulted in the occurrence of early afterdepolarizations (EAD) and polymorphic ventricular tachycardia (VT) in 9 of 12 erythromycin-treated hearts (175 episodes) and 8 of 12 veratridine-treated hearts (66 episodes). Additional infusion of dantrolene significantly reduced occurrence of polymorphic VT and resulted in occurrence of EAD and polymorphic VT in 1 of 12 erythromycin-treated hearts (18 episodes) and 1 of 12 veratridine-treated hearts (3 episodes).

CONCLUSION

Inhibition of the ryanodine receptor by dantrolene significantly reduced occurrence of polymorphic VT in drug-induced LQTS. A significant reduction of spatial dispersion of repolarization represents a major antiarrhythmic mechanism. These results imply that dantrolene may represent a promising antiarrhythmic option in drug-induced LQTS.

The Effects of SEA0400 on Ca2+ Transient Amplitude and Proarrhythmia Depend on the Na+/Ca2+ Exchanger Expression Level in Murine Models

Background/Objective: The cardiac Na⁺/Ca²⁺ exchanger (NCX) has been identified as a promising target to counter arrhythmia in previous studies investigating the benefit of NCX inhibition. However, the consequences of NCX inhibition have not been investigated in the setting of altered NCX expression and function, which is essential, since major cardiac diseases (heart failure/atrial fibrillation) exhibit NCX upregulation. Thus, we here investigated the effects of the NCX inhibitor SEA0400 on the Ca²⁺ transient amplitude and on proarrhythmia in homozygous NCX overexpressor (OE) and heterozygous NCX knockout (hetKO) mice compared to corresponding wild-types (WT_OE/WT_hetKO). Methods/Results: Ca²⁺ transients of field-stimulated isolated ventricular cardiomyocytes were recorded with fluo-4-AM or indo-1-AM. SEA0400 (1 μM) significantly reduced NCX forward mode function in all mouse lines. SEA0400 (1 μM) significantly increased the amplitude of field-stimulated Ca²⁺ transients in WT_OE, WT_hetKO, and hetKO, but not in OE (% of basal; OE = 98.7 ± 5.0; WT_OE = 137.8 ± 5.2^*; WT_hetKO = 126.3 ± 6.0^*; hetKO = 140.6 ± 12.8^*; ^*p < 0.05 vs. basal). SEA0400 (1 μM) significantly reduced the number of proarrhythmic spontaneous Ca²⁺ transients (sCR) in OE, but increased it in WT_OE, WT_hetKO and hetKO (sCR per cell; basal/+SEA0400; OE = 12.5/3.7; WT_OE = 0.2/2.4; WT_hetKO = 1.3/8.8; hetKO = 0.2/5.5) and induced Ca²⁺ overload with subsequent cell death in hetKO. Conclusion: The effects of SEA0400 on Ca²⁺ transient amplitude and the occurrence of spontaneous Ca²⁺ transients as a proxy measure for inotropy and cellular proarrhythmia depend on the NCX expression level. The antiarrhythmic effect of SEA0400 in conditions of increased NCX expression promotes the therapeutic concept of NCX inhibition in heart failure/atrial fibrillation. Conversely, in conditions of reduced NCX expression, SEA0400 suppressed the NCX function below a critical level leading to adverse Ca²⁺ accumulation as reflected by an increase in Ca²⁺ transient amplitude, proarrhythmia and cell death. Thus, the remaining NCX function under inhibition may be a critical factor determining the inotropic and antiarrhythmic efficacy of SEA0400.

A model of cardiac ryanodine receptor gating predicts experimental Ca2+-dynamics and Ca2+-triggered arrhythmia in the long QT syndrome

Abnormal Ca²⁺ handling is well-established as the trigger of cardiac arrhythmia in catecholaminergic polymorphic ventricular tachycardia and digoxin toxicity, but its role remains controversial in Torsade de Pointes (TdP), the arrhythmia associated with the long QT syndrome (LQTS). Recent experimental results show that early afterdepolarizations (EADs) that initiate TdP are caused by spontaneous (non-voltage-triggered) Ca²⁺ release from Ca²⁺-overloaded sarcoplasmic reticulum (SR) rather than the activation of the L-type Ca²⁺-channel window current. In bradycardia and long QT type 2 (LQT2), a second, non-voltage triggered cytosolic Ca²⁺ elevation increases gradually in amplitude, occurs before overt voltage instability, and then precedes the rise of EADs. Here, we used a modified Shannon-Puglisi-Bers model of rabbit ventricular myocytes to reproduce experimental Ca²⁺ dynamics in bradycardia and LQT2. Abnormal systolic Ca²⁺-oscillations and EADs caused by SR Ca²⁺-release are reproduced in a modified 0-dimensional model, where 3 gates in series control the ryanodine receptor (RyR2) conductance. Two gates control RyR2 activation and inactivation and sense cytosolic Ca²⁺ while a third gate senses luminal junctional SR Ca²⁺. The model predicts EADs in bradycardia and low extracellular [K⁺] and cessation of SR Ca²⁺-release terminate salvos of EADs. Ca²⁺-waves, systolic cell-synchronous Ca²⁺-release, and multifocal diastolic Ca²⁺ release seen in subcellular Ca²⁺-mapping experiments are observed in the 2-dimensional version of the model. These results support the role of SR Ca²⁺-overload, abnormal SR Ca²⁺-release, and the subsequent activation of the electrogenic Na⁺/Ca²⁺-exchanger as the mechanism of TdP. The model offers new insights into the genesis of cardiac arrhythmia and new therapeutic strategies.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

[Ventricular tachyarrhythmia as a side effect of pharmacotherapy]

Ventricular tachyarrhythmia is a severe and life-threatening potential side effect of pharmacotherapy. Substances with proarrhythmic potential belong to various groups of medication. Apart from antiarrhythmic agents, especially antibiotics and psychiatric drugs are worth mentioning owing to their broad application. Interaction with cardiac potassium channels is the most important reason for drug-induced ventricular tachyarrhythmia. Over 20 years of research in animal models and clinical studies have uncovered the underlying mechanisms. Findings in this field of research have also made a contribution to the understanding of genetic long QT syndromes. Clinical concerns that take drug interactions into account have been neglected due to the mechanistic research approach. For daily clinical practice, combination therapy of several potentially arrhythmogenic drugs is of predominant concern especially in situations when the therapeutic regime is changing such as admission to the hospital, admission to an intensive care unit or consultation of a new specialist. Especially in these situations, considerations about the arrhythmogenic potential of additionally administered drugs should be paid explicit attention. Additional concern should be paid to the fact that several proarrhythmogenic agents are metabolized over single pathways and are therefore prone to drug interactions that can severely raise the drug concentration and as a result arrhythmogenic potential.

Low proarrhythmic potential of citalopram and escitalopram in contrast to haloperidol in an experimental whole-heart model

In several case reports proarrhythmic effects of citalopram and escitalopram have been reported. Systematic analyses on prorarrhythmic effects of these drugs are not yet available. The aim of the present study was to investigate if application of citalopram, escitalopram or haloperidol provokes polymorphic ventricular tachycardia in a sensitive model of proarrhythmia. In isolated rabbit hearts monophasic action potentials and ECG showed a significant QT-prolongation after application of citalopram (2µM: +47ms, 4µM: +56ms, P<0.05) accompanied by an increase of action potential duration (APD) but not dispersion of repolarization. Reduced potassium concentration in bradycardic AV-blocked hearts provoked early afterdepolarizations (EAD) in 2 of 12 hearts but no polymorphic ventricular tachycardia (pVT). Application of escitalopram also increased QT-interval (2µM: +3ms, 4µM: +30ms, P<0.05) and APD without effects on dispersion. 3 of 10 hearts showed EAD and pVT in 2 of 10 hearts (32 episodes). The results were compared to 12 rabbits treated with haloperidol which led to an increase in QT-interval (1µM:+62ms; 2µM:+96ms; P<0.01), APD and dispersion (1µM:+15ms, 2µM:+40ms; P<0.01) and induced EAD in all 12 and pVT in 10 of 12 hearts (152 episodes). Citalopram and escitalopram demonstrated a rather safe electrophysiologic profile despite significant QT prolongation. In contrast, haloperidol led to significant increase of dispersion of repolarization while this parameter remained stable under the influence of citalopram or escitalopram. These results imply that application of citalopram or escitalopram is not as proarrhythmic as some case reports might suggest while haloperidol is torsadogenic.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

The link between abnormal calcium handling and electrical instability in acquired long QT syndrome--Does calcium precipitate arrhythmic storms?

Release of Ca(2+) ions from sarcoplasmic reticulum (SR) into myocyte cytoplasm and their binding to troponin C is the final signal form myocardial contraction. Synchronous contraction of ventricular myocytes is necessary for efficient cardiac pumping function. This requires both shuttling of Ca(2+) between SR and cytoplasm in individual myocytes, and organ-level synchronization of this process by means of electrical coupling among ventricular myocytes. Abnormal Ca(2+) release from SR causes arrhythmias in the setting of CPVT (catecholaminergic polymorphic ventricular tachycardia) and digoxin toxicity. Recent optical mapping data indicate that abnormal Ca(2+) handling causes arrhythmias in models of both repolarization impairment and profound bradycardia. The mechanisms involve dynamic spatial heterogeneity of myocardial Ca(2+) handling preceding arrhythmia onset, cell-synchronous systolic secondary Ca(2+) elevation (SSCE), as well as more complex abnormalities of intracellular Ca(2+) handling detected by subcellular optical mapping in Langendorff-perfused hearts. The regional heterogeneities in Ca(2+) handling cause action potential (AP) heterogeneities through sodium-calcium exchange (NCX) activation and eventually overwhelm electrical coupling of the tissue. Divergent Ca(2+) dynamics among different myocardial regions leads to temporal instability of AP duration and - on the patient level - in T wave lability. Although T-wave alternans has been linked to cardiac arrhythmias, non-alternans lability is observed in pre-clinical models of the long QT syndrome (LQTS) and CPVT, and in LQTS patients. Analysis of T wave lability may provide a real-time window on the abnormal Ca(2+) dynamics causing specific arrhythmias such as Torsade de Pointes (TdP).

Lessons learned from multi-scale modeling of the failing heart

Heart failure constitutes a major public health problem worldwide. Affected patients experience a number of changes in the electrical function of the heart that predispose to potentially lethal cardiac arrhythmias. Due to the multitude of electrophysiological changes that may occur during heart failure, the scientific literature is complex and sometimes ambiguous, perhaps because these findings are highly dependent on the etiology, the stage of heart failure, and the experimental model used to study these changes. Nevertheless, a number of common features of failing hearts have been documented. Prolongation of the action potential (AP) involving ion channel remodeling and alterations in calcium handling have been established as the hallmark characteristics of myocytes isolated from failing hearts. Intercellular uncoupling and fibrosis are identified as major arrhythmogenic factors. Multi-scale computational simulations are a powerful tool that complements experimental and clinical research. The development of biophysically detailed computer models of single myocytes and cardiac tissues has contributed greatly to our understanding of processes underlying excitation and repolarization in the heart. The electrical, structural, and metabolic remodeling that arises in cardiac tissues during heart failure has been addressed from different computational perspectives to further understand the arrhythmogenic substrate. This review summarizes the contributions from computational modeling and simulation to predict the underlying mechanisms of heart failure phenotypes and their implications for arrhythmogenesis, ranging from the cellular level to whole-heart simulations. The main aspects of heart failure are presented in several related sections. An overview of the main electrophysiological and structural changes that have been observed experimentally in failing hearts is followed by the description and discussion of the simulation work in this field at the cellular level, and then in 2D and 3D cardiac structures. The implications for arrhythmogenesis in heart failure are also discussed including therapeutic measures, such as drug effects and cardiac resynchronization therapy. Finally, the future challenges in heart failure modeling and simulation will be discussed.

Calcium release near L-type calcium channels promotes beat-to-beat variability in ventricular myocytes from the chronic AV block dog

Beat-to-beat variability of ventricular repolarization (BVR) has been proposed as a strong predictor of Torsades de Pointes (TdP). BVR is also observed at the myocyte level, and a number of studies have shown the importance of calcium handling in influencing this parameter. The chronic AV block (CAVB) dog is a model of TdP arrhythmia in cardiac hypertrophy, and myocytes from these animals show extensive remodeling, including of Ca(2+) handling. This remodeling process also leads to increased BVR. We aimed to determine the role that (local) Ca(2+) handling plays in BVR. In isolated LV myocytes an exponential relationship was observed between BVR magnitude and action potential duration (APD) at baseline. Inhibition of Ca(2+) release from sarcoplasmic reticulum (SR) with thapsigargin resulted in a reduction of [Ca(2+)]i, and of both BVR and APD. Increasing ICaL in the presence of thapsigargin restored APD but BVR remained low. In contrast, increasing ICaL with preserved Ca(2+) release increased both APD and BVR. Inhibition of Ca(2+) release with caffeine, as with thapsigargin, reduced BVR despite maintained APD. Simultaneous inhibition of Na(+)/Ca(2+) exchange and ICaL decreased APD and BVR to similar degrees, whilst increasing diastolic Ca(2+). Buffering of Ca(2+) transients with BAPTA reduced BVR for a given APD to a greater extent than buffering with EGTA, suggesting subsarcolemmal Ca(2+) transients modulated BVR to a larger extent than the cytosolic Ca(2+) transient. In conclusion, BVR in hypertrophied dog myocytes, at any APD, is strongly dependent on SR Ca(2+) release, which may act through modulation of the l-type Ca(2+) current in a subsarcolemmal microdomain.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.

EXPERIMENTAL APPROACH

Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.

KEY RESULTS

Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.

CONCLUSIONS AND IMPLICATIONS

Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Drug-induced proarrhythmia: risk factors and electrophysiological mechanisms

Drug-induced ventricular tachyarrhythmias can be caused by cardiovascular drugs, noncardiovascular drugs, and even nonprescription agents. They can result in arrhythmic emergencies and sudden cardiac death. If a new arrhythmia or aggravation of an existing arrhythmia develops during therapy with a drug at a concentration usually considered not to be toxic, the situation can be defined as proarrhythmia. Various cardiovascular and noncardiovascular drugs can increase the occurrence of polymorphic ventricular tachycardia of the 'torsade de pointes' type. Antiarrhythmic drugs, antimicrobial agents, and antipsychotic and antidepressant drugs are the most important groups. Age, female sex, and structural heart disease are important risk factors for the occurrence of torsade de pointes. Genetic predisposition and individual pharmacodynamic and pharmacokinetic sensitivity also have important roles in the generation of arrhythmias. An increase in spatial or temporal dispersion of repolarization and a triangular action-potential configuration have been identified as crucial predictors of proarrhythmia in experimental models. These studies emphasized that sole consideration of the QT interval is not sufficient to assess the proarrhythmic risk. In this Review, we focus on important triggers of proarrhythmia and the underlying electrophysiological mechanisms that can enhance or prevent the development of torsade de pointes.

Synchronous systolic subcellular Ca2+-elevations underlie ventricular arrhythmia in drug-induced long QT type 2

BACKGROUND

Repolarization delay is a common clinical problem, which can promote ventricular arrhythmias. In myocytes, abnormal sarcoplasmic reticulum Ca(2+)-release is proposed as the mechanism that causes early afterdepolarizations, the cellular equivalent of ectopic-activity in drug-induced long-QT syndrome. A crucial missing link is how such a stochastic process can overcome the source-sink mismatch to depolarize sufficient ventricular tissue to initiate arrhythmias.

METHODS AND RESULTS

Optical maps of action potentials and Ca(2+)-transients from Langendorff rabbit hearts were measured at low (150×150 μm(2)/pixel) and high (1.5×1.5 μm(2)/pixel) resolution before and during arrhythmias. Drug-induced long QT type 2, elicited with dofetilide inhibition of IKr (the rapid component of rectifying K+ current), produced spontaneous Ca(2+)-elevations during diastole and systole, before the onset of arrhythmias. Diastolic Ca(2+-)waves appeared randomly, propagated within individual myocytes, were out-of-phase with adjacent myocytes, and often died-out. Systolic secondary Ca(2+-)elevations were synchronous within individual myocytes, appeared 188±30 ms after the action potential-upstroke, occurred during high cytosolic Ca(2+) (40%-60% of peak-Ca(2+)-transients), appeared first in small islands (0.5×0.5 mm(2)) that enlarged and spread throughout the epicardium. Synchronous systolic Ca(2+-)elevations preceded voltage-depolarizations (9.2±5 ms; n=5) and produced pronounced Spatial Heterogeneities of Ca(2+)-transient-durations and action potential-durations. Early afterdepolarizations originating from sites with the steepest gradients of membrane-potential propagated and initiated arrhythmias. Interestingly, more complex subcellular Ca(2+)-dynamics (multiple chaotic Ca(2+)-waves) occurred during arrhythmias. K201, a ryanodine receptor stabilizer, eliminated Ca(2+)-elevations and arrhythmias.

CONCLUSIONS

The results indicate that systolic and diastolic Ca(2+)-elevations emanate from sarcoplasmic reticulum Ca(2+)-release and systolic Ca(2+)-elevations are synchronous because of high cytosolic and luminal-sarcoplasmic reticulum Ca(2+), which overcomes source-sink mismatch to trigger arrhythmias in intact hearts.

In silico assessment of drug safety in human heart applied to late sodium current blockers

Drug-induced action potential (AP) prolongation leading to Torsade de Pointes is a major concern for the development of anti-arrhythmic drugs. Nevertheless the development of improved anti-arrhythmic agents, some of which may block different channels, remains an important opportunity. Partial block of the late sodium current (I(NaL)) has emerged as a novel anti-arrhythmic mechanism. It can be effective in the settings of free radical challenge or hypoxia. In addition, this approach can attenuate pro-arrhythmic effects of blocking the rapid delayed rectifying K(+) current (I(Kr)). The main goal of our computational work was to develop an in-silico tool for preclinical anti-arrhythmic drug safety assessment, by illustrating the impact of I(Kr)/I(NaL) ratio of steady-state block of drug candidates on "torsadogenic" biomarkers. The O'Hara et al. AP model for human ventricular myocytes was used. Biomarkers for arrhythmic risk, i.e., AP duration, triangulation, reverse rate-dependence, transmural dispersion of repolarization and electrocardiogram QT intervals, were calculated using single myocyte and one-dimensional strand simulations. Predetermined amounts of block of I(NaL) and I(Kr) were evaluated. "Safety plots" were developed to illustrate the value of the specific biomarker for selected combinations of IC(50)s for I(Kr) and I(NaL) of potential drugs. The reference biomarkers at baseline changed depending on the "drug" specificity for these two ion channel targets. Ranolazine and GS967 (a novel potent inhibitor of I(NaL)) yielded a biomarker data set that is considered safe by standard regulatory criteria. This novel in-silico approach is useful for evaluating pro-arrhythmic potential of drugs and drug candidates in the human ventricle.

Effects of ranolazine on torsades de pointes tachycardias in a healthy isolated rabbit heart model

PURPOSE

Torsades de pointes (TdP) tachycardias are triggered, polymorphic ventricular arrhythmias arising from early afterdepolarizations (EADs) and increased dispersion of repolarization. Ranolazine is a new agent which reduces pathologically elevated late INa but also IKr . Aim of this study was to evaluate the effects of ranolazine in a validated isolated Langendorff-perfused rabbit heart model.

METHODS

TdP was reproducibly induced with d-sotalol (10(-4)  mol/L) and low potassium (K) (1.0 mmol/L for 5 min, pacing at CL 1000 ms). In 10 hearts, ECG and 8 epi- and endocardial monophasic action potentials were recorded. Action potential duration (APD) was measured at 90% repolarization and dispersion defined as APD max-min.

RESULTS

D-sotalol prolonged APD90 and increased dispersion of APD90 , simultaneously causing EADs and induction of TdP. The combination of d-sotalol and two concentrations of ranolazine did not increase dispersion of ventricular APD90 as compared to vehicle. Ranolazine at 5 μmol/L did not cause additional induction of EADs and/or TdP but also did not significantly suppress arrhythmogenic triggers. The higher concentration of ranolazine (10 μmol/L) in combination with d-sotalol caused further prolongation of APD90 , at the same time reduction in APD90 dispersion. In parallel, the incidence of EADs was reduced and an antitorsadogenic effect was seen.

CONCLUSIONS

In the healthy isolated rabbit heart (where late INa is not elevated), ranolazine does not cause proarrhythmia but exerts antiarrhythmic effects in a dose-dependent manner against d-sotalol/low K-induced TdP. This finding-despite additional APD prolongation-supports the safety of a combined use of both drugs and merits clinical investigation.

Cardiac fibrosis as a determinant of ventricular tachyarrhythmias

Animal and emerging clinical studies have demonstrated that increased ventricular fibrosis in a setting of reduced repolarization reserve promotes early afterdepolarizations (EADs) and triggered activity that can initiate ventricular tachycardia and ventricular fibrillation (VT/VF). Increased ventricular fibrosis plays a key facilitatory role in allowing oxidative and metabolic stress-induced EADs to manifest as triggered activity causing VT/VF. The lack of such an arrhythmogenic effect by the same stressors in normal non-fibrotic hearts highlights the importance of fibrosis in the initiation of VT/VF. These findings suggest that antifibrotic therapy combined with therapy designed to increase ventricular repolarization reserve may act synergistically to reduce the risk of sudden cardiac death.

Toll-like receptor 4 activation promotes cardiac arrhythmias by decreasing the transient outward potassium current (Ito) through an IRF3-dependent and MyD88-independent pathway

Cardiac arrhythmias are one of the main causes of death worldwide. Several studies have shown that inflammation plays a key role in different cardiac diseases and Toll-like receptors (TLRs) seem to be involved in cardiac complications. In the present study, we investigated whether the activation of TLR4 induces cardiac electrical remodeling and arrhythmias, and the signaling pathway involved in these effects. Membrane potential was recorded in Wistar rat ventricle. Ca(2+) transients, as well as the L-type Ca(2+) current (ICaL) and the transient outward K(+) current (Ito), were recorded in isolated myocytes after 24 h exposure to the TLR4 agonist, lipopolysaccharide (LPS, 1 μg/ml). TLR4 stimulation in vitro promoted a cardiac electrical remodeling that leads to action potential prolongation associated with arrhythmic events, such as delayed afterdepolarization and triggered activity. After 24 h LPS incubation, Ito amplitude, as well as Kv4.3 and KChIP2 mRNA levels were reduced. The Ito decrease by LPS was prevented by inhibition of interferon regulatory factor 3 (IRF3), but not by inhibition of interleukin-1 receptor-associated kinase 4 (IRAK4) or nuclear factor kappa B (NF-κB). Extrasystolic activity was present in 25% of the cells, but apart from that, Ca(2+) transients and ICaL were not affected by LPS; however, Na(+)/Ca(2+) exchanger (NCX) activity was apparently increased. We conclude that TLR4 activation decreased Ito, which increased AP duration via a MyD88-independent, IRF3-dependent pathway. The longer action potential, associated with enhanced Ca(2+) efflux via NCX, could explain the presence of arrhythmias in the LPS group.

Genetic association study of QT interval highlights role for calcium signaling pathways in myocardial repolarization

The QT interval, an electrocardiographic measure reflecting myocardial repolarization, is a heritable trait. QT prolongation is a risk factor for ventricular arrhythmias and sudden cardiac death (SCD) and could indicate the presence of the potentially lethal Mendelian Long QT Syndrome (LQTS). Using a genome-wide association and replication study in up to 100,000 individuals we identified 35 common variant QT interval loci, that collectively explain ∼8-10% of QT variation and highlight the importance of calcium regulation in myocardial repolarization. Rare variant analysis of 6 novel QT loci in 298 unrelated LQTS probands identified coding variants not found in controls but of uncertain causality and therefore requiring validation. Several newly identified loci encode for proteins that physically interact with other recognized repolarization proteins. Our integration of common variant association, expression and orthogonal protein-protein interaction screens provides new insights into cardiac electrophysiology and identifies novel candidate genes for ventricular arrhythmias, LQTS,and SCD.

New antiarrhythmic targets to control intracellular calcium handling

Sudden cardiac death due to ventricular arrhythmias is a major problem. Drug therapies to prevent SCD do not provide satisfying results, leading to the demand for new antiarrhythmic strategies. New targets include Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII), the Na/Ca exchanger (NCX), the Ryanodine receptor (RyR, and its associated protein FKBP12.6 (Calstabin)) and the late component of the sodium current (I_Na-Late), all related to intracellular calcium (Ca²⁺) handling. In this review, drugs interfering with these targets (SEA-0400, K201, KN-93, W7, ranolazine, sophocarpine, and GS-967) are evaluated and their future as clinical compounds is considered. These new targets prove to be interesting; however more insight into long-term drug effects is necessary before clinical applicability becomes reality.

Electrophysiological profile of vernakalant in an experimental whole-heart model: the absence of proarrhythmia despite significant effect on myocardial repolarization

AIM

The most recent European Society of Cardiology (ESC) update on atrial fibrillation has introduced vernakalant (VER) for pharmacological cardioversion of atrial fibrillation. The aim of the present study was to investigate the safety profile of VER in a sensitive model of proarrhythmia.

METHODS AND RESULTS

In 36 Langendorff-perfused rabbit hearts, VER (10, 30 µM, n = 12); ranolazine (RAN, 10, 30 µM, n = 12), or sotalol (SOT, 50; 100 µM, n = 12) were infused after obtaining baseline data. Monophasic action potentials and a 12-lead electrocardiogram showed a significant QT prolongation after application of VER as compared with baseline (10 µM: +25 ms, 30 µM: +50 ms, P < 0.05) accompanied by an increase of action potential duration (APD). The increase in APD90 was accompanied by a more marked increase in effective refractory period (ERP) leading to a significant increase in post-repolarization refractoriness (PRR, 10 µM: +30 ms, 30 µM: +36 ms, P < 0.05). Vernakalant did not affect the dispersion of repolarization. Lowered potassium concentration in bradycardic hearts did not provoke early afterdepolarizations (EADs) or polymorphic ventricular tachycardia (pVT). Comparable results were obtained with RAN. Hundred micromolars of SOT led to an increase in QT interval (+49 ms) and APD90 combined with an increased ERP and PRR (+23 ms). In contrast to VER, 100 µM SOT led to a significant increase in dispersion of repolarization and to the occurrence of EAD in 10 of 12 and pVT in 8 of 12 hearts.

CONCLUSION

In the present study, application of VER and SOT led to a comparable prolongation of myocardial repolarization. Both drugs increased the PRR. However, VER neither affect the dispersion of repolarization nor induce EAD and therefore did not cause proarrhythmia.

The arrhythmogenic consequences of increasing late INa in the cardiomyocyte

This review presents the roles of cardiac sodium channel NaV1.5 late current (late INa) in generation of arrhythmic activity. The assumption of the authors is that proper Na(+) channel function is necessary to the maintenance of the transmembrane electrochemical gradient of Na(+) and regulation of cardiac electrical activity. Myocyte Na(+) channels' openings during the brief action potential upstroke contribute to peak INa and initiate excitation-contraction coupling. Openings of Na(+) channels outside the upstroke contribute to late INa, a depolarizing current that persists throughout the action potential plateau. The small, physiological late INa does not appear to be critical for normal electrical or contractile function in the heart. Late INa does, however, reduce the net repolarizing current, prolongs action potential duration, and increases cellular Na(+) loading. An increase of late INa, due to acquired conditions (e.g. heart failure) or inherited Na(+) channelopathies, facilitates the formation of early and delayed afterpolarizations and triggered arrhythmias, spontaneous diastolic depolarization, and cellular Ca(2+) loading. These in turn increase the spatial and temporal dispersion of repolarization time and may lead to reentrant arrhythmias.

Combined Na(+)/Ca(2+) exchanger and L-type calcium channel block as a potential strategy to suppress arrhythmias and maintain ventricular function

BACKGROUND

L-type calcium channel (LTCC) and Na(+)/Ca(2+) exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics.

METHODS AND RESULTS

Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 μmol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic.

CONCLUSIONS

In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.

Computational approaches to understand cardiac electrophysiology and arrhythmias

Cardiac rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. These impulses spread throughout the cardiac muscle to manifest as electrical waves in the whole heart. Regularity of electrical waves is critically important since they signal the heart muscle to contract, driving the primary function of the heart to act as a pump and deliver blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. For more than 50 years, mathematically based models of cardiac electrical activity have been used to improve understanding of basic mechanisms of normal and abnormal cardiac electrical function. Computer-based modeling approaches to understand cardiac activity are uniquely helpful because they allow for distillation of complex emergent behaviors into the key contributing components underlying them. Here we review the latest advances and novel concepts in the field as they relate to understanding the complex interplay between electrical, mechanical, structural, and genetic mechanisms during arrhythmia development at the level of ion channels, cells, and tissues. We also discuss the latest computational approaches to guiding arrhythmia therapy.

Calcium handling and ventricular tachyarrhythmias

Pharmacologic modification of cellular calcium handling recently moved into focus as an alternative for prevention and treatment of ventricular tachyarrhythmias. Calcium overload and spontaneous calcium release from the sarcoplasmatic reticulum are regarded as possible initiations of early and delayed afterdepolarization thereby triggering ventricular arrhythmias. In chronic heart failure, calcium overload is more likely to occur compared with healthy hearts, which is one explantation for the increased vulnerability in this condition. L-type calcium channel, sodium-calcium-exchanger (NCX), and ryanodine receptor are crucial for calcium homeostasis and therefore represent potential targets for antiarrhythmic drug therapy. Experimental studies have proven beneficial effects for all these three mechanisms in prevention and suppression of tachyarrhythmias. However, clinical data is mainly available for the L-type calcium channel inhibitor verapamil. Therefore, it is still a long way to clinical employment of drugs modifying cellular calcium handling for antiarrhythmic therapy.