Arrhythmogenic mechanisms in the isolated perfused hypokalaemic murine heart

Loss of protein phosphatase 2A regulatory subunit PPP2R5A is associated with increased incidence of stress-induced proarrhythmia

Background

Protein phosphatase 2A (PP2A) is a serine/threonine-selective holoenzyme that controls Ca2+ homeostasis and contractility of the heart via dephosphorylation of regulatory proteins. In some genetically modified mouse models with increased arrhythmogenicity, a reduced expression of the regulatory subunit B56α of PP2A was found as a concomitant effect. Whether there is a general correlation between the abundance of B56α and the promotion of cardiac arrhythmogenesis remains unclear.

Methods

The aim of this study was therefore to investigate the role of PP2A-B56α in the propensity for arrhythmic activity in the heart. The experimental analysis of this question has been addressed by using a mouse model with deletion of the PP2A-B56α gene, PPP2R5A (KO), in comparison to wild-type animals (WT). Evidence for arrhythmogenicity was investigated in whole animal, isolated heart and cardiomyocytes by ECG, recording of monophasic action potential (MAP) induced by programmed electrical stimulation (PES), measurement of Ca2+ transients under increased pacing frequencies and determination of total K+ channel currents (I K).

Results

ECG measurements showed a prolongation of QT time in KO vs. WT. KO mice exhibited a higher rate of premature ventricular contractions in the ECG. MAP measurements in Langendorff-perfused KO hearts showed increased episodes of ventricular tachyarrhythmia induced by PES. However, the KO hearts showed values for MAP duration that were similar to those in WT hearts. In contrast, KO showed more myocardial cells with spontaneous arrhythmogenic Ca2+ transient events compared to WT. The whole-cell patch-clamp technique applied to ventricular cardiomyocytes revealed comparable peak potassium channel current densities between KO and WT.

Conclusion

These findings support the assumption that a decrease or even the loss of PP2A-B56α leads to an increased propensity of triggered arrhythmias. This could be based on the increased spontaneous Ca2+ tansients observed.

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

Pinocembrin ameliorates atrial fibrillation susceptibility in rats with anxiety disorder induced by empty bottle stimulation

Background: Anxiety disorder (AD) is the most common mental disorder, which is closely related to atrial fibrillation (AF) and is considered to be a trigger of AF. Pinocembrin has been demonstrated to perform a variety of neurological and cardiac protective effects through its anti-inflammatory and antioxidant activities. The current research aims to explore the antiarrhythmic effect of pinocembrin in anxiety disorder rats and its underlying mechanisms.

Methods: 60 male Sprague-Dawley rats were distributed into four groups: CTL group: control rats + saline; CTP group: control rats + pinocembrin; Anxiety disorder group: anxiety disorder rats + saline; ADP group: anxiety disorder rats + pinocembrin. Empty bottle stimulation was conducted to induce anxiety disorder in rats for 3 weeks, and pinocembrin was injected through the tail vein for the last 2 weeks. Behavioral measurements, in vitro electrophysiological studies, biochemical assays, ELISA, Western blot and histological studies were performed to assess the efficacy of pinocembrin. In addition, HL-1 atrial cells were cultured in vitro to further verify the potential mechanism of pinocembrin.

Results: After 3 weeks of empty bottle stimulation, pinocembrin significantly improved the exploration behaviors in anxiety disorder rats. Pinocembrin alleviated electrophysiological remodeling in anxiety disorder rats, including shortening the action potential duration (APD), prolonging the effective refractory period (ERP), increasing the expression of Kv1.5, Kv4.2 and Kv4.3, decreasing the expression of Cav1.2, and ultimately reducing the AF susceptibility. These effects may be attributed to the amelioration of autonomic remodeling and structural remodeling by pinocembrin, as well as the inhibition of oxidative stress with upregulation of the nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) pathway.

Conclusion: Pinocembrin can reduce AF susceptibility in anxiety disorder rats induced by empty bottle stimulation, with the inhibition of autonomic remodeling, structural remodeling, and oxidative stress. Therefore, pinocembrin is a promising treatment for AF in patients with anxiety disorder.

A Carvedilol Analogue, VK-II-86, Prevents Hypokalaemia-induced Ventricular Arrhythmia through Novel multi-Channel Effects

BACKGROUND AND PURPOSE

QT prolongation and intracellular Ca²⁺ loading with diastolic Ca²⁺ release via ryanodine receptors (RyR2) are the predominant mechanisms underlying hypokalaemia-induced ventricular arrhythmia. We investigated the antiarrhythmic actions of two RyR2 inhibitors: dantrolene and VK-II-86, a carvedilol analogue with no β-blocking activity, in hypokalaemia.

EXPERIMENTAL APPROACH

Surface ECG and ventricular action potentials (APs) were recorded from whole-heart murine Langendorff preparations. Ventricular arrhythmia incidence was compared in hearts perfused with low [K⁺ ], and those pre-treated with dantrolene or VK-II-86. Whole-cell patch clamping was used in murine and canine ventricular cardiomyocytes to study the effects of dantrolene and VK-II-86 on AP parameters in low [K⁺ ] and the effects of VK-II-86 on the inward rectifier current (I_K1 ), late sodium current (I_{Na_L} ) and the L-type Ca²⁺ current (I_Ca ). Effects of VK-II-86 on I_Kr were investigated in transfected HEK-293 cells. A fluorogenic probe quantified the effects of VK-II-86 on oxidative stress in hypokalaemia.

KEY RESULTS

Dantrolene reduced the incidence of ventricular arrhythmias induced by low [K⁺ ] in explanted murine hearts by 94%, whereas VK-II-86 prevented all arrhythmias. VK-II-86 prevented hypokalaemia-induced AP prolongation and depolarization, but did not alter AP parameters in normokalaemia. Hypokalaemia was associated with a significant reduction of I_K1 and I_Kr , and increase in I_Na-L , and I_Ca . VK-II-86 prevented all hypokalaemia-induced changes in ion channel activity and oxidative stress.

CONCLUSIONS AND IMPLICATIONS

VK-II-86 prevents hypokalaemia-induced arrhythmogenesis by normalising calcium homeostasis and repolarization reserve. VK-II-86 may provide an exciting treatment in hypokalaemia and other arrhythmias caused by delayed repolarization or Ca²⁺ overload.

Model Systems for Addressing Mechanism of Arrhythmogenesis in Cardiac Repair

PURPOSE OF REVIEW

Cardiac cell-based therapy represents a promising approach for cardiac repair. However, one of the main challenges is cardiac arrhythmias associated with stem cell transplantation. The current review summarizes the recent progress in model systems for addressing mechanisms of arrhythmogenesis in cardiac repair.

RECENT FINDINGS

Animal models have been extensively developed for mechanistic studies of cardiac arrhythmogenesis. Advances in human induced pluripotent stem cells (hiPSCs), patient-specific disease models, tissue engineering, and gene editing have greatly enhanced our ability to probe the mechanistic bases of cardiac arrhythmias. Additionally, recent development in multiscale computational studies and machine learning provides yet another powerful tool to quantitatively decipher the mechanisms of cardiac arrhythmias. Advancing efforts towards the integrations of experimental and computational studies are critical to gain insights into novel mitigation strategies for cardiac arrhythmias in cell-based therapy.

Immediate and Delayed Response of Simulated Human Atrial Myocytes to Clinically-Relevant Hypokalemia

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K⁺, Na⁺, Ca²⁺, and/or Mg²⁺ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (V_r) and the AP following decreases in plasma K⁺, [K⁺]_o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K⁺]_o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames: (i) Immediately after [K⁺]_o is reduced, the K⁺-sensing mechanism of the background inward rectifier current (I_K1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, V_r and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness. (ii) A second much slower electrophysiological response (developing 5-10 minutes after [K⁺]_o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na⁺]_i causes a change in the outward electrogenic current generated by the Na⁺/K⁺ pump, thereby modifying V_r and AP repolarization and changing the human atrial electrophysiological substrate. In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of I_K1 rectification when analyzing both the mechanisms by which [K⁺]_o regulates V_r and how the AP waveform may contribute to "trigger" mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K⁺]_o can produce effects that are known to promote atrial arrhythmias in human hearts.

Integrated transcriptomics and epigenomics reveal chamber-specific and species-specific characteristics of human and mouse hearts

DNA methylation, chromatin accessibility, and gene expression represent different levels information in biological process, but a comprehensive multiomics analysis of the mammalian heart is lacking. Here, we applied nucleosome occupancy and methylome sequencing, which detected DNA methylation and chromatin accessibility simultaneously, as well as RNA-seq, for multiomics analysis of the 4 chambers of adult and fetal human hearts, and adult mouse hearts. Our results showed conserved region-specific patterns in the mammalian heart at transcriptome and DNA methylation level. Adult and fetal human hearts showed distinct features in DNA methylome, chromatin accessibility, and transcriptome. Novel long noncoding RNAs were identified in the human heart, and the gene expression profiles of major cardiovascular diseases associated genes were displayed. Furthermore, cross-species comparisons revealed human-specific and mouse-specific differentially expressed genes between the atria and ventricles. We also reported the relationship among multiomics and found there was a bell-shaped relationship between gene-body methylation and expression in the human heart. In general, our study provided comprehensive spatiotemporal and evolutionary insights into the regulation of gene expression in the heart.

Multi-omic analyses of the four chambers of the human and mouse heart, including transcriptome, DNA methylation and chromatin accessibility, reveals characteristic patterns of gene regulation at the level of heart regions.

Arrhythmogenic Mechanisms in Hypokalaemia: Insights From Pre-clinical Models

Potassium is the predominant intracellular cation, with its extracellular concentrations maintained between 3. 5 and 5 mM. Among the different potassium disorders, hypokalaemia is a common clinical condition that increases the risk of life-threatening ventricular arrhythmias. This review aims to consolidate pre-clinical findings on the electrophysiological mechanisms underlying hypokalaemia-induced arrhythmogenicity. Both triggers and substrates are required for the induction and maintenance of ventricular arrhythmias. Triggered activity can arise from either early afterdepolarizations (EADs) or delayed afterdepolarizations (DADs). Action potential duration (APD) prolongation can predispose to EADs, whereas intracellular Ca²⁺ overload can cause both EADs and DADs. Substrates on the other hand can either be static or dynamic. Static substrates include action potential triangulation, non-uniform APD prolongation, abnormal transmural repolarization gradients, reduced conduction velocity (CV), shortened effective refractory period (ERP), reduced excitation wavelength (CV × ERP) and increased critical intervals for re-excitation (APD-ERP). In contrast, dynamic substrates comprise increased amplitude of APD alternans, steeper APD restitution gradients, transient reversal of transmural repolarization gradients and impaired depolarization-repolarization coupling. The following review article will summarize the molecular mechanisms that generate these electrophysiological abnormalities and subsequent arrhythmogenesis.

Electrocardiographic manifestations in severe hypokalemia

Hypokalemia is one of the most common electrolyte disturbances in the clinic and it can increase the risk of life-threatening arrhythmias. Electrocardiographic characteristics associated with hypokalemia include dynamic changes in T-wave morphology, ST-segment depression, and U waves, which are often best seen in the mid-precordial leads (V2–V4). The PR interval can also be prolonged along with an increase in the amplitude of the P wave. We report a case of a patient with hypokalemia (1.31 mmol/L) who showed typical electrocardiographic characteristics of hypokalemia.

Energy-Reduced Arrhythmia Termination Using Global Photostimulation in Optogenetic Murine Hearts

Complex spatiotemporal non-linearity as observed during cardiac arrhythmia strongly correlates with vortex-like excitation wavelengths and tissue characteristics. Therefore, the control of arrhythmic patterns requires fundamental understanding of dependencies between onset and perpetuation of arrhythmia and substrate instabilities. Available treatments, such as drug application or high-energy electrical shocks, are discussed for potential side effects resulting in prognosis worsening due to the lack of specificity and spatiotemporal precision. In contrast, cardiac optogenetics relies on light sensitive ion channels stimulated to trigger excitation of cardiomyocytes solely making use of the inner cell mechanisms. This enables low-energy, non-damaging optical control of cardiac excitation with high resolution. Recently, the capability of optogenetic cardioversion was shown in Channelrhodopsin-2 (ChR2) transgenic mice. But these studies used mainly structured and local illumination for cardiac stimulation. In addition, since optogenetic and electrical stimulus work on different principles to control the electrical activity of cardiac tissue, a better understanding of the phenomena behind optogenetic cardioversion is still needed. The present study aims to investigate global illumination with regard to parameter characterization and its potential for cardioversion. Our results show that by tuning the light intensity without exceeding 1.10 mW mm-2, a single pulse in the range of 10–1,000 ms is sufficient to reliably reset the heart into sinus rhythm. The combination of our panoramic low-intensity photostimulation with optical mapping techniques visualized wave collision resulting in annihilation as well as propagation perturbations as mechanisms leading to optogenetic cardioversion, which seem to base on other processes than electrical defibrillation. This study contributes to the understanding of the roles played by epicardial illumination, pulse duration and light intensity in optogenetic cardioversion, which are the main variables influencing cardiac optogenetic control, highlighting the advantages and insights of global stimulation. Therefore, the presented results can be modules in the design of novel illumination technologies with specific energy requirements on the way toward tissue-protective defibrillation techniques.

Long QT Syndrome: A Comprehensive Review of the Literature and Current Evidence

Long QT syndrome (LQT) represents a heterogeneous family of cardiac electrophysiologic disorders characterized by QT prolongation and T-wave abnormalities on the electrocardiogram. It is commonly associated with syncope, however, sudden cardiac death can occur due to torsades de pointes. LQT is a clinical diagnosis and should be suspected in individuals on the basis of clinical presentation, family history and ECG characteristics. Management is focused on the prevention of syncope and ultimately sudden death. Complete cessation of symptoms is the goal. Life-style modification, beta blockers and ICD implantation are the most important therapeutic modalities in proper management of patients with LQT. Awareness should be raised regarding possible circumstances that could increase the risk of QT prolongation. Advanced age, hypokalemia, a history of heart failure, and structural heart disease are often mentioned in this context. Prudent consideration is needed before making a decision to recommend an ICD implantation in a young, active patient. Medical and/or device therapy still represent important therapeutic modalities in the management of patients with LQT with careful clinical judgement for the substrate of patients who will benefit. Insights from benchside to bedside have facilitated progress toward better therapeutic strategies, there also remains a need for tailoring management toward individuals in a mechanism-specific manner to optimize care. In addition, continued progress toward fundamental understanding of mechanisms of ion channel function and drug-channel interaction will guide the development of more effective, mechanism-based molecular agents in the treatment of LQT.

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).

Electrophysiological mechanisms of long and short QT syndromes

The QT interval on the human electrocardiogram is normally in the order of 450 ms, and reflects the summated durations of action potential (AP) depolarization and repolarization of ventricular myocytes. Both prolongation and shortening in the QT interval have been associated with ventricular tachy-arrhythmias, which predispose affected individuals to sudden cardiac death. In this article, the molecular determinants of the AP duration and the causes of long and short QT syndromes (LQTS and SQTS) are explored. This is followed by a review of the recent advances on their arrhythmogenic mechanisms involving reentry and/or triggered activity based on experiments conducted in mouse models. Established and novel clinical risk markers based on the QT interval for the prediction of arrhythmic risk and cardiovascular mortality are presented here. It is concluded by a discussion on strategies for the future rational design of anti-arrhythmic agents.

Mild Hypokalemia and Supraventricular Ectopy Increases the Risk of Stroke in Community-Dwelling Subjects

BACKGROUND AND PURPOSE

Stroke is independently associated with the common conditions of hypokalemia and supraventricular ectopy, and we hypothesize that the combination of excessive supraventricular ectopic activity and hypokalemia has a synergistic impact on the prognosis in terms of stroke in the general population.

METHODS

Subjects (55-75 years old) from the Copenhagen Holter Study cohort (N=671) with no history of atrial fibrillation or stroke were studied-including baseline values of potassium and ambulatory 48-hour Holter monitoring. Excessive supraventricular ectopic activity is defined as ≥30 premature atrial complexes per hour or any episodes of runs of ≥20. Hypokalemia was defined as plasma-potassium ≤3.6 mmol/L. The primary end point was ischemic stroke. Cox models were used.

RESULTS

Hypokalemia was mild (mean, 3.4 mmol/L; range, 2.7-3.6). Hypokalemic subjects were older (67.0±6.94 versus 64.0±6.66 years; P<0.0001) and more hypertensive (165.1±26.1 versus 154.6±23.5 mm Hg; P<0.0001). Median follow-up time was 14.4 years (Q1-Q3, 9.4-14.7 years). The incidence of stroke was significantly higher in the hypokalemic group (hazard ratio, 1.84; 95% confidence interval, 1.04-3.28) after covariate adjustments, as well as in a competing risk analysis with death (hazard ratio, 1.51; 95% confidence interval, 1.12-2.04). Excessive supraventricular ectopic activity was also associated with stroke (hazard ratio, 2.23; 95% confidence interval, 1.33-3.76). The combination of hypokalemia and excessive supraventricular ectopic activity increased the risk of events synergistically. Stroke rate was 93 per 1000 patient-year (P<0.0001) in this group (n=17) compared with 6.9 (n=480); 11 (n=81), and 13 (n=93) per 1000 patient-year in the groups without the combination.

CONCLUSIONS

The combination of hypokalemia and excessive supraventricular ectopy carries a poor prognosis in terms of stroke.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Cardiac disease and arrhythmogenesis: Mechanistic insights from mouse models

The mouse is the second mammalian species, after the human, in which substantial amount of the genomic information has been analyzed. With advances in transgenic technology, mutagenesis is now much easier to carry out in mice. Consequently, an increasing number of transgenic mouse systems have been generated for the study of cardiac arrhythmias in ion channelopathies and cardiomyopathies. Mouse hearts are also amenable to physical manipulation such as coronary artery ligation and transverse aortic constriction to induce heart failure, radiofrequency ablation of the AV node to model complete AV block and even implantation of a miniature pacemaker to induce cardiac dyssynchrony. Last but not least, pharmacological models, despite being simplistic, have enabled us to understand the physiological mechanisms of arrhythmias and evaluate the anti-arrhythmic properties of experimental agents, such as gap junction modulators, that may be exert therapeutic effects in other cardiac diseases. In this article, we examine these in turn, demonstrating that primary inherited arrhythmic syndromes are now recognized to be more complex than abnormality in a particular ion channel, involving alterations in gene expression and structural remodelling. Conversely, in cardiomyopathies and heart failure, mutations in ion channels and proteins have been identified as underlying causes, and electrophysiological remodelling are recognized pathological features. Transgenic techniques causing mutagenesis in mice are extremely powerful in dissecting the relative contributions of different genes play in producing disease phenotypes. Mouse models can serve as useful systems in which to explore how protein defects contribute to arrhythmias and direct future therapy.

Anti-arrhythmic effects of hypercalcemia in hyperkalemic, Langendorff-perfused mouse hearts

The present study examined the ventricular arrhythmic and electrophysiological properties during hyperkalemia (6.3 mM [K⁺] vs. 4 mM in normokalemia) and anti-arrhythmic effects of hypercalcemia (2.2 mM [Ca²⁺]) in Langendorff-perfused mouse hearts. Monophasic action potential recordings were obtained from the left ventricle during right ventricular pacing. Hyperkalemia increased the proportion of hearts showing provoked ventricular tachycardia (VT) from 0 to 6 of 7 hearts during programmed electrical stimulation (Fisher's exact test, P<0.05). It shortened the epicardial action potential durations (APD_x) at 90, 70, 50 and 30% repolarization and ventricular effective refractory periods (VERPs) (analysis of variance, P<0.05) without altering activation latencies. Endocardial APD_x and VERPs were unaltered. Consequently, ∆APD_x (endocardial APD_x-epicardial APD_x) was increased, VERP/latency ratio was decreased and critical intervals for reexcitation (APD₉₀-VERP) were unchanged. Hypercalcemia treatment exerted anti-arrhythmic effects during hyperkalemia, reducing the proportion of hearts showing VT to 1 of 7 hearts. It increased epicardial VERPs without further altering the remaining parameters, returning VERP/latency ratio to normokalemic values and also decreased the critical intervals. In conclusion, hyperkalemia exerted pro-arrhythmic effects by shortening APDs and VERPs. Hypercalcemia exerted anti-arrhythmic effects by reversing VERP changes, which scaled the VERP/latency ratio and critical intervals.

Restitution analysis of alternans using dynamic pacing and its comparison with S1S2 restitution in heptanol-treated, hypokalaemic Langendorff-perfused mouse hearts

Action potential duration (APD) and conduction velocity restitution explain the dependence of these parameters on the previous diastolic interval (DI). It is considered to be an adaptive mechanism for preserving diastole at fast heart rates. Hypokalaemia is known to induce ventricular arrhythmias that could be prevented by heptanol, the gap junction uncoupler, mediated through increases in ventricular refractory period (VERP) without alterations in APDs. The present study investigated alternans and restitution properties during normokalaemia, hypokalaemia alone or hypokalaemia with heptanol (0.1 mM) in Langendorff-perfused mouse hearts using a dynamic pacing protocol. APD90 alternans were elicited in the epicardium and endocardium during normokalaemia. Hypokalaemia increased the amplitudes of epicardial APD90 alternans when basic cycle lengths (BCLs) were ≤65 msec, which was associated with increases in maximum APD90 restitution gradients, critical DIs and APD90 heterogeneity. Heptanol (0.1 mM) did not exacerbate or reduce the APD90 alternans or alter these restitution parameters further. By contrast, endocardial APD90 alternans did not show increases in amplitudes during hypokalaemia at short BCLs studied, and restitution parameters were also unchanged. This was true whether in the presence or absence of 0.1 mM heptanol. The study demonstrates that anti-arrhythmic effects of heptanol exerted during hypokalaemia occurred despite exacerbation of APD90 alternans. This would suggest that even in the presence of arrhythmogenic APD90 alternans, arrhythmias could still be prevented by influencing VERP alone. Restitution data obtained here by dynamic pacing were compared to previous data from S1S2 pacing.

Proarrhythmic and Torsadogenic Effects of Potassium Channel Blockers in Patients

The most common arrhythmia requiring drug treatment is atrial fibrillation (AF), which affects 2 to 5 million Americans and continues to be a major cause of morbidity and increased mortality. Despite recent advances in catheter-based and surgical therapies, antiarrhythmic drugs continue to be the mainstay of therapy for most patients with symptomatic AF. However, many antiarrhythmics block the rapid component of the cardiac delayed rectifier potassium current (IKr) as a major mechanism of action, and marked QT prolongation and pause-dependent polymorphic ventricular tachycardia (torsades de pointes) are major class toxicities.

Insight into specific pro-arrhythmic triggers in Brugada and early repolarization syndromes: results of long-term follow-up

The pro-arrhythmic triggers in Brugada and early repolarization syndromes (BrS, ERS) have not been analyzed systematically except for case reports. We clinically investigated the circumstances which precede/predispose to arrhythmic events in these syndromes during long-term follow-up. A detailed history from the patients/witnesses was taken to investigate the antecedent events in the last few hours that preceded syncope/ventricular fibrillation (VF); medical records, ECG and blood test from the emergency room (ER) were reviewed. 19 patients that fulfilled the investigation criteria were followed up for 71 ± 49 months (34-190 months). Prior to the event (syncope/VF), the patients were partaking different activities in the following decreasing order; drinking alcoholic beverage, having meal, and getting up from sleep, exercise. 3 patients reported mental/physical stress prior to the event and 2 patients developed VF several days after starting oral steroid for treatment of bronchial asthma. In the ER, elevated J-wave amplitude (0.27 ± 0.15 mV) was found with 58 % of the patients having hypokalemia. After electrolyte correction and cessation of steroids, the following day plasma K⁺ (4.2 ± 0.3 mEq/L, P < 0.001) was significantly increased and J-wave amplitude (0.13 ± 0.1 mV, P < 0.001) was remarkably reduced. Three patients were kept on oral spironolactone/potassium supplements. During follow-up for 71 ± 49 (34-190) months, among 4 patients with VF recurrence, one patient developed VF after taking oral steroid. In ERS and BrS, hypokalemia and corticosteroid therapy add substantial pro-arrhythmic effects, but potentially treatable. Stopping steroid therapy and avoiding hypokalemia had excellent long-term outcome.

Ventricular anti-arrhythmic effects of heptanol in hypokalaemic, Langendorff-perfused mouse hearts

Ventricular arrhythmic and electrophysiological properties were examined during normokalaemia (5.2 mM [K⁺]), hypokalaemia (3 mM [K⁺]) or hypokalaemia in the presence of 0.1 or 2 mM heptanol in Langendorff-perfused mouse hearts. Left ventricular epicardial or endocardial monophasic action potential recordings were obtained during right ventricular pacing. Hypokalaemia induced ventricular premature beats (VPBs) in 5 of 7 and ventricular tachycardia (VT) in 6 of 7 hearts (P<0.01), prolonged action potential durations (APD₉₀) from 36.2±1.7 to 55.7±2.0 msec (P<0.01) and shortened ventricular effective refractory periods (VERPs) from 44.5±4.0 to 28.9±3.8 msec (P<0.01) without altering conduction velocities (CVs) (0.17±0.01 m/sec, P>0.05), reducing excitation wavelengths (λ, CV × VERP) from 7.9±1.1 to 5.1±0.3 mm (P<0.05) while increasing critical intervals (CI, APD₉₀-VERP) from −8.3±4.3 to 26.9±2.0 msec (P>0.001). Heptanol (0.1 mM) prevented VT, restored effective refractory period (ERP) to 45.2±2.9 msec without altering CV or APD, returning λ to control values (P>0.05) and CI to 8.4±3.8 msec (P<0.05). Heptanol (2 mM) prevented VPBs and VT, increased ERP to 67.7±7.6 msec (P<0.05), and reduced CV to 0.11±0.1 m/sec (P<0.001) without altering APD (P>0.05), returning λ and CI to control values (P>0.05). Anti-arrhythmic effects of heptanol during hypokalaemia were explicable by ERP changes, scaling λ and CI.

Ambulatory cardiac arrhythmias in relation to mild hypokalaemia and prognosis in community dwelling middle-aged and elderly subjects

AIMS

Severe hypokalaemia can aggravate arrhythmia tendency and prognosis, but less is known about risk of mild hypokalaemia, which is a frequent finding. We examined the associations between mild hypokalaemia and ambulatory cardiac arrhythmias and their prognosis.

METHODS AND RESULTS

Subjects from the cohort of the 'Copenhagen Holter Study' (n = 671), with no history of manifest cardiovascular (CV) disease or stroke, were studied. All had laboratory tests and 48-h ambulatory electrocardiogram (ECG) recording. The median follow-up was 6.3 years. p-Potassium was inversely associated with frequency of premature ventricular complexes (PVCs) especially in combination with diuretic treatment (r = -0.22, P = 0.015). Hypokalaemia was not associated with supraventricular arrhythmias. Subjects at lowest quintile of p-potassium (mean 3.42, range 2.7-3.6 mmol/L) were defined as hypokalaemic. Cardiovascular mortality was higher in the hypokalaemic group (hazard ratio and 95% confidence intervals: 2.62 (1.11-6.18) after relevant adjustments). Hypokalaemia in combination with excessive PVC worsened the prognosis synergistically; event rates: 83 per 1000 patient-year in subjects with both abnormalities, 10 and 15 per 1000 patient-year in those with one abnormality, and 3 per 1000 patient-year in subjects with no abnormality. One variable combining hypokalaemia with excessive supraventricular arrhythmias gave similar results in univariate analysis, but not after multivariate adjustments.

CONCLUSION

In middle-aged and elderly subjects with no manifest heart disease, mild hypokalaemia is associated with increased rate of ventricular but not supraventricular arrhythmias. Hypokalaemia interacts synergistically with increased ventricular ectopy to increase the risk of adverse events.

Impact of hypokalemia on electromechanical window, excitation wavelength and repolarization gradients in guinea-pig and rabbit hearts

Normal hearts exhibit a positive time difference between the end of ventricular contraction and the end of QT interval, which is referred to as the electromechanical (EM) window. Drug-induced prolongation of repolarization may lead to the negative EM window, which was proposed to be a novel proarrhythmic marker. This study examined whether abnormal changes in the EM window may account for arrhythmogenic effects produced by hypokalemia. Left ventricular pressure, electrocardiogram, and epicardial monophasic action potentials were recorded in perfused hearts from guinea-pig and rabbit. Hypokalemia (2.5 mM K(+)) was found to prolong repolarization, reduce the EM window, and promote tachyarrhythmia. Nevertheless, during both regular pacing and extrasystolic excitation, the increased QT interval invariably remained shorter than the duration of mechanical systole, thus yielding positive EM window values. Hypokalemia-induced arrhythmogenicity was associated with slowed ventricular conduction, and shortened effective refractory periods, which translated to a reduced excitation wavelength index. Hypokalemia also evoked non-uniform prolongation of action potential duration in distinct epicardial regions, which resulted in increased spatial variability in the repolarization time. These findings suggest that arrhythmogenic effects of hypokalemia are not accounted for by the negative EM window, and are rather attributed to abnormal changes in ventricular conduction times, refractoriness, excitation wavelength, and spatial repolarization gradients.

Impaired epicardial activation-repolarization coupling contributes to the proarrhythmic effects of hypokalaemia and dofetilide in guinea pig ventricles

AIM

Activation-repolarization coupling refers to the inverse relationship between action potential duration and activation time in myocardial regions along the path of ventricular excitation. This study examined whether the activation-repolarization coupling plays a role in coordinating repolarization times between the right ventricular (RV) and left ventricular (LV) chambers, and if impaired coordination contributes to electrical instability produced by hypokalaemia or dofetilide, a blocker of the delayed rectifier K(+) current.

METHODS

In Langendorff-perfused, isolated guinea pig hearts, six monophasic action potential recording electrodes were attached to RV and LV epicardium. Local activation time and action potential duration (APD90 ) were determined during spontaneous beating, regular pacing and extrasystolic excitation.

RESULTS

In regularly beating hearts, the RV epicardial sites had longer APD90 , but exhibited earlier activation times, as compared to LV sites, which minimized the interventricular difference in repolarization time. Upon extrasystolic stimulation, the APD90 was reduced to a greater extent in RV compared with LV, which translated to a reversed slope of APD90 -to-activation time relationship, and increased spatial repolarization gradients. Hypokalaemia and dofetilide prolonged APD90 , with the effect being greater in LV compared with RV. In hypokalaemic hearts, LV activation was delayed. These changes contributed to increased asynchrony in repolarization times in the LV and RV in both regular and extrasystolic beats, and enhanced susceptibility to tachyarrhythmia.

CONCLUSION

Impaired RV-to-LV activation-repolarization coupling is an important determinant of electrical instability in the setting of non-uniformly prolonged epicardial APD90 or slowed interventricular conduction.

Measurement and interpretation of electrocardiographic QT intervals in murine hearts

Alterations in ECG QT intervals correlate with the risk of potentially fatal arrhythmias, for which transgenic murine hearts are becoming increasingly useful experimental models. However, QT intervals are poorly defined in murine ECGs. As a consequence, several different techniques have been used to measure murine QT intervals. The present work develops a consistent measure of the murine QT interval that correlates with changes in the duration of ventricular myocyte action potentials (APs). Volume-conducted ECGs were compared with simultaneously recorded APs, obtained using floating intracellular microelectrodes in Langendorff-perfused mouse hearts. QT intervals were measured from the onset of the QRS complex. The interval, Q-APR₉₀, measured to the time at 90% AP recovery, was compared with two measures of the QT interval. QT1 was measured to the recovery of the ECG trace to the isoelectric baseline for entirely positive T-waves or to the trough of any negative T-wave undershoot. QT2—used extensively in previous studies—was measured to the return of any ECG trough to the isoelectric baseline. QT1, but not QT2, closely correlated with changes in Q-APR₉₀. These findings were confirmed over a range of pacing rates, in low K⁺ concentration solutions, and in Scn5a+/ΔKPQ hearts used to model human long QT syndrome. Application of this method in whole anesthetized mice similarly demonstrated a prolonged corrected QT (QTc) in Scn5a+/ΔKPQ hearts. We therefore describe a robust method for the determination of QT and QTc intervals that correlate with the duration of ventricular myocyte APs in murine hearts.

Temporal alterations and cellular mechanisms of transmural repolarization during progression of mouse cardiac hypertrophy and failure

AIM

The remodelling of transmural dispersion of repolarization (TDR) in human heart failure (HF) and in different animal models of cardiac hypertrophy or HF remains a controversial topic. We hypothesize that TDR may exhibit temporal alterations, depending on the stage of the disease.

METHODS

We systematically investigated the temporal alterations of TDR during the development of cardiac hypertrophy and HF in the mouse pressure-overload model using electrophysiological and molecular biology techniques.

RESULTS

A progressive prolongation of QT interval and changes in the amplitude of the J wave at 2, 5, 9 and 13 weeks were found in anesthetized aorta-banded mice. Action potential duration (APD) at 90% repolarization (APD90) in subendocardial myocytes of the left ventricular free wall remained unchanged at the hypertrophic stage (2 and 5 weeks), but was significantly prolonged in HF mice at 9 and 13 weeks. However, APD90 in subepicardial myocytes exhibited a significant prolongation at 2 weeks and did not progressively extend from 2 weeks to 13 weeks in banded mice. Thus, non-parallel prolongation of APD in subendocardial and subepicardial myocytes led to a reduction in TDR at hypertrophic stage and an amplification of TDR at HF stage. Further experiments revealed that asynchronous down-regulation of voltage-dependent potassium currents (I(to,f), I(K,slow) and I(ss)) and L-type calcium currents (I(Ca-L)) in subendocardial and subepicardial myocytes may contribute to the dynamic remodelling of transmural APD.

CONCLUSION

The two distinct TDR modes were revealed during the progression of mouse cardiac hypertrophy and failure, indicating that the remodelling of TDR depends on the stage of the disease.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Recurrent ventricular fibrillation related to hypokalemia in early repolarization syndrome

We describe a case of early repolarization syndrome in which augmented J waves were documented during an electrical storm associated with hypokalemia. The patient was referred to our hospital for therapy to treat recurrent ventricular fibrillation (VF). The 12-lead electrocardiogram showed giant J waves associated with hypokalemia during multiple episodes of VF. Although antiarrhythmic agents or deep sedation were not effective for the VF, an intravenous supplementation of potassium completely suppressed the VF with a reduction in the J-wave amplitude. Our report discusses the possible relationship between hypokalemia and VF in early repolarization syndrome.

Ventricular arrhythmogenesis following slowed conduction in heptanol-treated, Langendorff-perfused mouse hearts

Arrhythmogenic effects of slowed action potential conduction produced by the gap junction and sodium-channel inhibitor heptanol (0.1-2 mM) were explored in Langendorff-perfused mouse hearts. Monophasic action potential recordings showed that 2 mM heptanol induced ventricular tachycardia in the absence of triggered activity arising from early or after-depolarizations during regular 8 Hz pacing and programmed electrical stimulation (PES). It also increased activation latencies and ventricular effective refractory periods (VERPs), but did not alter action potential duration (APD), thereby reducing local critical intervals for re-excitation given by APD(90) - VERP. Bipolar electrogram recordings showed that 2 mM heptanol increased electrogram duration (EGD) and ratios of EGDs obtained at the longest to those obtained at the shortest S1S2 intervals studied during PES, suggesting increased dispersion of conduction velocities. These findings show, for the first time in the mouse heart, that slowed conduction induces reversible arrhythmogenic effects despite repolarization abnormalities expected to reduce arrhythmogenicity.

Ca²⁺-induced delayed afterdepolarizations are triggered by dyadic subspace Ca2²⁺ affirming that increasing SERCA reduces aftercontractions

Ca(2+)-induced delayed afterdepolarizations (DADs) are depolarizations that occur after full repolarization. They have been observed across multiple species and cell types. Experimental results have indicated that the main cause of DADs is Ca(2+) overload. The main hypothesis as to their initiation has been Ca(2+) overflow from the overloaded sarcoplasmic reticulum (SR). Our results using 37 previously published mathematical models provide evidence that Ca(2+)-induced DADs are initiated by the same mechanism as Ca(2+)-induced Ca(2+) release, i.e., the modulation of the opening of ryanodine receptors (RyR) by Ca(2+) in the dyadic subspace; an SR overflow mechanism was not necessary for the induction of DADs in any of the models. The SR Ca(2+) level is better viewed as a modulator of the appearance of DADs and the magnitude of Ca(2+) release. The threshold for the total Ca(2+) level within the cell (not only the SR) at which Ca(2+) oscillations arise in the models is close to their baseline level (∼1- to 3-fold). It is most sensitive to changes in the maximum sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump rate (directly proportional), the opening probability of RyRs, and the Ca(2+) diffusion rate from the dyadic subspace into the cytosol (both indirectly proportional), indicating that the appearance of DADs is multifactorial. This shift in emphasis away from SR overload as the trigger for DADs toward a multifactorial analysis could explain why SERCA overexpression has been shown to suppress DADs (while increasing contractility) and why DADs appear during heart failure (at low SR Ca(2+) levels).

Cardiac arrhythmia: a simple conceptual framework

This review presents a simple trigger-substrate model of arrhythmogenesis and its application to the generation of reentrant ventricular arrhythmias. We demonstrate its broad applicability to the understanding of arrhythmic phenomena in a wide range of both hereditary and acquired arrhythmic disorders.

Mechanisms of hypokalemia-induced ventricular arrhythmogenicity

Hypokalemia is a common biochemical finding in cardiac patients and may represent a side effect of diuretic therapy or result from endogenous activation of renin-angiotensin system and high adrenergic tone. Hypokalemia is independent risk factor contributing to reduced survival of cardiac patients and increased incidence of arrhythmic death. Animal studies demonstrate that hypokalemia-induced arrhythmogenicity is attributed to prolonged ventricular repolarization, slowed conduction, and abnormal pacemaker activity. The prolongation of ventricular repolarization in hypokalemic setting is caused by inhibition of outward potassium currents and often associated with increased propensity for early afterdepolarizations. Slowed conduction is attributed to membrane hyperpolarization and increased excitation threshold. Abnormal pacemaker activity is attributed to increased slope of diastolic depolarization in Purkinje fibers, as well as delayed afterdepolarizations caused by Ca2+ overload secondary to inhibition of Na+--K+ pump and stimulation of the reverse mode of the Na+--Ca2+ exchange. Hypokalemia effect on repolarization is not uniform at distinct ventricular sites thereby contributing to amplified spatial repolarization gradients which promote unidirectional conduction block. In hypokalemic heart preparations, the prolongation of action potential may be associated with shortening of effective refractory period, thus increasing the propensity for ventricular re-excitation over late phase of repolarization. Shortened refractoriness and slowed conduction contribute to reduced excitation wavelength thereby facilitating re-entry. The interplay of triggering factors (early and delayed afterdepolarizations, oscillatory prepotentials in Purkinje fibers) and a favorable electrophysiological substrate (unidirectional conduction block, reduced excitation wavelength, increased critical interval for ventricular re-excitation) may account for the mechanism of life-threatening tachyarrhythmias in hypokalemic patients.

Predictive value of electrical restitution in hypokalemia-induced ventricular arrhythmogenicity

The ventricular action potential (AP) shortens exponentially upon a progressive reduction of the preceding diastolic interval. Steep electrical restitution slopes have been shown to promote wavebreaks, thus contributing to electrical instability. The present study was designed to assess the predictive value of electrical restitution in hypokalemia-induced arrhythmogenicity. We recorded monophasic APs and measured effective refractory periods (ERP) at distinct ventricular epicardial and endocardial sites and monitored volume-conducted ECG at baseline and after hypokalemic perfusion (2.5 mM K+ for 30 min) in isolated guinea pig heart preparations. The restitution of AP duration measured at 90% repolarization (APD90) was assessed after premature extrastimulus application at variable coupling stimulation intervals, and ERP restitution was assessed by measuring refractoriness over a wide range of pacing rates. Hypokalemia increased the amplitude of stimulation-evoked repolarization alternans and the inducibility of tachyarrhythmias and reduced ventricular fibrillation threshold. Nevertheless, these changes were associated with flattened rather than steepened APD90 restitution slopes and slowed restitution kinetics. In contrast, ERP restitution slopes were significantly increased in hypokalemic hearts. Although epicardial APD90 measured during steady-state pacing (S1-S1 = 250 ms) was prolonged in hypokalemic hearts, the left ventricular ERP was shortened. Consistently, the epicardial ERP measured at the shortest diastolic interval achieved upon a progressive increase in pacing rate was reduced in the hypokalemic left ventricle. In conclusion, this study highlights the superiority of ERP restitution at predicting increased arrhythmogenicity in the hypokalemic myocardium. The lack of predictive value of APD90 restitution is presumably related to different mode of changes in ventricular repolarization and refractoriness in a hypokalemic setting, whereby APD90 prolongation may be associated with shortened ERP.

Electrophysiological determinants of hypokalaemia-induced arrhythmogenicity in the guinea-pig heart

AIM

Hypokalaemia is an independent risk factor contributing to arrhythmic death in cardiac patients. In the present study, we explored the mechanisms of hypokalaemia-induced tachyarrhythmias by measuring ventricular refractoriness, spatial repolarization gradients, and ventricular conduction time in isolated, perfused guinea-pig heart preparations.

METHODS

Epicardial and endocardial monophasic action potentials from distinct left ventricular (LV) and right ventricular (RV) recording sites were monitored simultaneously with volume-conducted electrocardiogram (ECG) during steady-state pacing and following a premature extrastimulus application at progressively reducing coupling stimulation intervals in normokalaemic and hypokalaemic conditions.

RESULTS

Hypokalaemic perfusion (2.5 mm K(+) for 30 min) markedly increased the inducibility of tachyarrhythmias by programmed ventricular stimulation and rapid pacing, prolonged ventricular repolarization and shortened LV epicardial and endocardial effective refractory periods, thereby increasing the critical interval for LV re-excitation. Hypokalaemia increased the RV-to-LV transepicardial repolarization gradients but had no effect on transmural dispersion of APD(90) and refractoriness across the LV wall. As determined by local activation time recordings, the LV-to-RV transepicardial conduction and the LV transmural (epicardial-to-endocardial) conduction were slowed in hypokalaemic heart preparations. This change was attributed to depressed diastolic excitability as evidenced by increased ventricular pacing thresholds.

CONCLUSION

These findings suggest that hypokalaemia-induced arrhythmogenicity is attributed to shortened LV refractoriness, increased critical intervals for LV re-excitation, amplified RV-to-LV transepicardial repolarization gradients and slowed ventricular conduction in the guinea-pig heart.

Cardiac Na+ current regulation by pyridine nucleotides

RATIONALE

Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na+ current (I(Na)) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism.

OBJECTIVE

Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na(v)1.5).

METHODS AND RESULTS

HEK293 cells stably expressing Na(v)1.5 and rat neonatal cardiomyocytes were used. The influence of NADH/NAD+ on arrhythmic risk was evaluated in wild-type or SCN5A(+/-) mouse heart. A280V GPD1-L caused a 2.48+/-0.17-fold increase in intracellular NADH level (P<0.001). NADH application or cotransfection with A280V GPD1-L resulted in decreased I(Na) (0.48+/-0.09 or 0.19+/-0.04 of control group, respectively; P<0.01), which was reversed by NAD+, chelerythrine, or superoxide dismutase. NAD+ antagonism of the Na+ channel downregulation by A280V GPD1-L or NADH was prevented by a protein kinase (PK)A inhibitor, PKAI(6-22). The effects of NADH and NAD+ were mimicked by a phorbol ester and forskolin, respectively. Increasing intracellular NADH was associated with an increased risk of ventricular tachycardia in wild-type mouse hearts. Extracellular application of NAD+ to SCN5A(+/-) mouse hearts ameliorated the risk of ventricular tachycardia.

CONCLUSIONS

Our results show that Na(v)1.5 is regulated by pyridine nucleotides, suggesting a link between metabolism and I(Na). This effect required protein kinase C activation and was mediated by oxidative stress. NAD+ could prevent this effect by activating PKA. Mutations of GPD1-L may downregulate Na(v)1.5 by altering the oxidized to reduced NAD(H) balance.

Calmodulin kinase II initiates arrhythmogenicity during metabolic acidification in murine hearts

Aim:

The multifunctional signal molecule calmodulin kinase II (CaMKII) has been associated with cardiac arrhythmogenesis under conditions where its activity is chronically elevated. Recent studies report that its activity is also acutely elevated during acidosis. We test a hypothesis implicating CaMKII in the arrhythmogenesis accompanying metabolic acidification.

Methods:

We obtained monophasic action potential recordings from Langendorff-perfused whole heart preparations and single cell action potentials (AP) using whole-cell patch-clamped ventricular myocytes. Spontaneous sarcoplasmic reticular (SR) Ca²⁺release events during metabolic acidification were investigated using confocal microscope imaging of Fluo-4-loaded ventricular myocytes.

Results:

In Langendorff-perfused murine hearts, introduction of lactic acid into the Krebs-Henseleit perfusate resulted in abnormal electrical activity and ventricular tachycardia. The CaMKII inhibitor, KN-93 (2 μm), reversibly suppressed this spontaneous arrhythmogenesis during intrinsic rhythm and regular 8 Hz pacing. However, it failed to suppress arrhythmia evoked by programmed electrical stimulation. These findings paralleled a CaMKII-independent reduction in the transmural repolarization gradients during acidosis, which previously has been associated with the re-entrant substrate under other conditions. Similar acidification produced spontaneous AP firing and membrane potential oscillations in patch-clamped isolated ventricular myocytes when pipette solutions permitted cytosolic Ca²⁺ to increase following acidification. However, these were abolished by both KN-93 and use of pipette solutions that held cytosolic Ca²⁺ constant during acidosis. Acidosis also induced spontaneous Ca²⁺ waves in isolated intact Fluo-4-loaded myocytes studied using confocal microscopy that were abolished by KN-93.

Conclusion:

These findings together implicate CaMKII-dependent SR Ca²⁺ waves in spontaneous arrhythmic events during metabolic acidification.

Models of torsades de pointes: effects of FPL64176, DPI201106, dofetilide, and chromanol 293B in isolated rabbit and guinea pig hearts

INTRODUCTION

For studying the torsades de pointes (TdP) liability of a compound, most high and medium throughput methods use surrogate markers such as HERG inhibition and QT prolongation. In this study, we have tested whether isolated hearts may be modified to allow TdP to be the direct readout.

METHOD

Isolated spontaneously beating rabbit and guinea pig hearts were perfused according to the Langendorff method in hypokalemic (2.1 mM) solution. The in vitro lead II ECG equivalent and the incidence of TdP were monitored for 1 h. In addition, heart rate, QTc, Tp-Te, short-term variability (STV), time to arrhythmia, and time to TdP were also analyzed.

RESULTS

FPL64176, a calcium channel activator; and DPI201106, a sodium channel inactivation inhibitor, produced TdP in isolated rabbit and guinea pig hearts in a concentration dependent manner; guinea pig hearts were 3- to 5-fold more sensitive than rabbit hearts. Both compounds also increased QTc and STV. In contrast, dofetilide, an IKr inhibitor, produced no (or a low incidence of) TdP in both species, in spite of prolongation of QTc intervals. Chromanol 293B, an IKs inhibitor, did not produce TdP in rabbit hearts but elicited TdP concentration dependently in guinea pig hearts even though the compound had no effect on QTc intervals.

CONCLUSION

IKs inhibition appears to be more likely to produce TdP in isolated guinea pig hearts than IKr inhibition. Chromanol 293B did not produce TdP in rabbit hearts presumably due to a low level of IKs channels in the heart. TdP produced in this study was consistent with the notion that its production was a consequence of reduced repolarization reserve, thereby causing rhythmic abnormalities. This isolated, perfused, and spontaneously beating rabbit and guinea pig heart preparation in hypokalemic medium may be useful as a preclinical test model for studying proarrhythmic liability of compounds in new drug development.

Empirical correlation of triggered activity and spatial and temporal re-entrant substrates with arrhythmogenicity in a murine model for Jervell and Lange-Nielsen syndrome

KCNE1 encodes the β-subunit of the slow component of the delayed rectifier K⁺ current. The Jervell and Lange-Nielsen syndrome is characterized by sensorineural deafness, prolonged QT intervals, and ventricular arrhythmogenicity. Loss-of-function mutations in KCNE1 are implicated in the JLN2 subtype. We recorded left ventricular epicardial and endocardial monophasic action potentials (MAPs) in intact, Langendorff-perfused mouse hearts. KCNE1^−/− but not wild-type (WT) hearts showed not only triggered activity and spontaneous ventricular tachycardia (VT), but also VT provoked by programmed electrical stimulation. The presence or absence of VT was related to the following set of criteria for re-entrant excitation for the first time in KCNE1^−/− hearts: Quantification of APD₉₀, the MAP duration at 90% repolarization, demonstrated alterations in (1) the difference, ∆APD₉₀, between endocardial and epicardial APD₉₀ and (2) critical intervals for local re-excitation, given by differences between APD₉₀ and ventricular effective refractory period, reflecting spatial re-entrant substrate. Temporal re-entrant substrate was reflected in (3) increased APD₉₀ alternans, through a range of pacing rates, and (4) steeper epicardial and endocardial APD₉₀ restitution curves determined with a dynamic pacing protocol. (5) Nicorandil (20 µM) rescued spontaneous and provoked arrhythmogenic phenomena in KCNE1^−/− hearts. WTs remained nonarrhythmogenic. Nicorandil correspondingly restored parameters representing re-entrant criteria in KCNE1^−/− hearts toward values found in untreated WTs. It shifted such values in WT hearts in similar directions. Together, these findings directly implicate triggered electrical activity and spatial and temporal re-entrant mechanisms in the arrhythmogenesis observed in KCNE1^−/− hearts.

Drug-induced arrhythmias and sudden cardiac death: implications for the pharmaceutical industry

Following a series of high profile withdrawals from the market, the ability of medications to induce potentially fatal arrhythmias is a significant problem facing the pharmaceutical industry. Current preclinical cardiac safety assays are based on the assumption that blockade of a single repolarizing K(+) channel alone precipitates drug-induced arrhythmias, however, current findings point to a range of more complex arrhythmogenic mechanisms. This review begins by exploring clinical findings and potential mechanisms underlying drug-induced sudden cardiac death and then goes on to assess current and explore future strategies to detect cardiotoxicity at the preclinical stage.

Pharmacological changes in cellular Ca2+ homeostasis parallel initiation of atrial arrhythmogenesis in murine Langendorff-perfused hearts

1. Intracellular Ca(2+) overload has been associated with established atrial arrhythmogenesis. The present experiments went on to correlate acute initiation of atrial arrhythmogenesis in Langendorff-perfused mouse hearts with changes in Ca(2+) homeostasis in isolated atrial myocytes following pharmacological procedures that modified the storage or release of sarcoplasmic reticular (SR) Ca(2+) or inhibited entry of extracellular Ca(2+). 2. Caffeine (1 mmol/L) elicited diastolic Ca(2+) waves in regularly stimulated atrial myocytes immediately following addition. This was followed by a decline in the amplitude of the evoked transients and the disappearance of such diastolic events, suggesting partial SR Ca(2+) depletion. 3. Cyclopiazonic acid (CPA; 0.15 micromol/L) produced more gradual reductions in evoked Ca(2+) transients and abolished diastolic Ca(2+) events produced by the further addition of caffeine. 4. Nifedipine (0.5 micromol/L) produced immediate reductions in evoked Ca(2+) transients. Further addition of caffeine produced an immediate increase followed by a decline in the amplitude of the evoked Ca(2+) transients, without eliciting diastolic Ca(2+) events. 5. These findings correlated with changes in spontaneous and provoked atrial arrhythmogenecity in mouse isolated Langendorf-perfused hearts. Thus, caffeine was pro-arrhythmogenic immediately following but not > 5 min after application and both CPA and nifedipine pretreatment inhibited such arrhythmogenesis. 6. Together, these findings relate acute atrial arrhythmogenesis in intact hearts to diastolic Ca(2+) events in atrial myocytes that, in turn, depend upon a finite SR Ca(2+) store and diastolic Ca(2+) release following Ca(2+)-induced Ca(2+) release initiated by the entry of extracellular Ca(2+).

Scn3b knockout mice exhibit abnormal ventricular electrophysiological properties

We report for the first time abnormalities in cardiac ventricular electrophysiology in a genetically modified murine model lacking the Scn3b gene (Scn3b^−/−). Scn3b^−/− mice were created by homologous recombination in embryonic stem (ES) cells. RT-PCR analysis confirmed that Scn3b mRNA was expressed in the ventricles of wild-type (WT) hearts but was absent in the Scn3b^−/− hearts. These hearts also showed increased expression levels of Scn1b mRNA in both ventricles and Scn5a mRNA in the right ventricles compared to findings in WT hearts. Scn1b and Scn5a mRNA was expressed at higher levels in the left than in the right ventricles of both Scn3b^−/− and WT hearts. Bipolar electrogram and monophasic action potential recordings from the ventricles of Langendorff-perfused Scn3b^−/− hearts demonstrated significantly shorter ventricular effective refractory periods (VERPs), larger ratios of electrogram duration obtained at the shortest and longest S₁–S₂ intervals, and ventricular tachycardias (VTs) induced by programmed electrical stimulation. Such arrhythmogenesis took the form of either monomorphic or polymorphic VT. Despite shorter action potential durations (APDs) in both the endocardium and epicardium, Scn3b^−/− hearts showed ΔAPD₉₀ values that remained similar to those shown in WT hearts. The whole-cell patch-clamp technique applied to ventricular myocytes isolated from Scn3b^−/− hearts demonstrated reduced peak Na⁺ current densities and inactivation curves that were shifted in the negative direction, relative to those shown in WT myocytes. Together, these findings associate the lack of the Scn3b gene with arrhythmic tendencies in intact perfused hearts and electrophysiological features similar to those in Scn5a^+/− hearts.

Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart

The recently described exchange protein directly activated by cAMP (Epac) has been implicated in distinct protein kinase A-independent cellular signalling pathways. We investigated the role of Epac activation in adrenergically mediated ventricular arrhythmogenesis. In contrast to observations in control conditions (n = 20), monophasic action potentials recorded in 2 of 10 intrinsically beating and 5 of 20 extrinsically paced Langendorff-perfused wild-type murine hearts perfused with the Epac activator 8-pCPT-2'-O-Me-cAMP (8-CPT, 1 microM) showed spontaneous triggered activity. Three of 20 such extrinsically paced hearts showed spontaneous ventricular tachycardia (VT). Programmed electrical stimulation provoked VT in 10 of 20 similarly treated hearts (P < 0.001; n = 20). However, there were no statistically significant accompanying changes (P > 0.05) in left ventricular epicardial (40.7 +/- 1.2 versus 44.0 +/- 1.7 ms; n = 10) or endocardial action potential durations (APD(90); 51.8 +/- 2.3 versus 51.9 +/- 2.2 ms; n = 10), transmural (DeltaAPD(90)) (11.1 +/- 2.6 versus 7.9 +/- 2.8 ms; n = 10) or apico-basal repolarisation gradients, ventricular effective refractory periods (29.1 +/- 1.7 versus 31.2 +/- 2.4 ms in control and 8-CPT-treated hearts, respectively; n = 10) and APD(90) restitution characteristics. Nevertheless, fluorescence imaging of cytosolic Ca(2+) levels demonstrated abnormal Ca(2+) homeostasis in paced and resting isolated ventricular myocytes. Epac activation using isoproterenol in the presence of H-89 was also arrhythmogenic and similarly altered cellular Ca(2+) homeostasis. Epac-dependent effects were reduced by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibition with 1 microM KN-93. These findings associate VT in an intact cardiac preparation with altered cellular Ca(2+) homeostasis and Epac activation for the first time, in the absence of altered repolarisation gradients previously implicated in reentrant arrhythmias through a mechanism dependent on CaMKII activity.

Effects of potassium channel openers in the isolated perfused hypokalaemic murine heart

Aim

We explored the anti-arrhythmic efficacy of K⁺ channel activation in the hypokalaemic murine heart using NS1643 and nicorandil, compounds which augment I_Kr and I_KATP respectively.

Methods

Left ventricular epicardial and endocardial monophasic action potentials were compared in normokalaemic and hypokalaemic preparations in the absence and presence of NS1643 (30 μm) and nicorandil (20 μm).

Results

Spontaneously beating hypokalaemic hearts (3 mm K⁺) all elicited early afterdepolarizations (EADs) and episodes of ventricular tachycardia (VT). Perfusion with NS1643 and nicorandil suppressed EADs and VT in 7 of 13 and five of six hypokalaemic hearts. Provoked arrhythmia studies using programmed electrical stimulation induced VT in all hypokalaemic hearts, but failed to do so in 7 of 13 and five of six hearts perfused with NS1643 and nicorandil respectively. These anti-arrhythmic effects were accompanied by reductions in action potential duration at 90% repolarization (APD₉₀) and changes in the transmural gradient of repolarization, reflected in ΔAPD₉₀. NS1643 and nicorandil reduced epicardial APD₉₀ from 68.3 ± 1.1 to 56.5 ± 4.1 and 51.5 ± 1.5 ms, respectively, but preserved endocardial APD₉₀ in hypokalaemic hearts. NS1643 and nicorandil thus restored ΔAPD₉₀ from −9.6 ± 4.3 ms under baseline hypokalaemic conditions to 3.9 ± 4.1 and 9.9 ± 2.1 ms, respectively, close to normokalaemic values.

Conclusion

These findings demonstrate, for the first time, the anti-arrhythmic efficacy of K⁺ channel activation in the setting of hypokalaemia. NS1643 and nicorandil are anti-arrhythmic through the suppression of EADs, reductions in APD₉₀ and restorations of ΔAPD₉₀.

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim

To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3.

Methods

Left ventricular endocardial and epicardial monophasic action potentials and arrhythmogenic tendency were compared in isolated wild type (WT) and Scn5a+/Δ hearts perfused with 0.1 and 1 μm propranolol and paced from the right ventricular epicardium.

Results

All spontaneously beating bradycardic Scn5a+/Δ hearts displayed EADs, triggered beats and ventricular tachycardia (VT; n = 7), events never seen in WT hearts (n = 5). Perfusion with 0.1 and 1 μm propranolol suppressed all EADs, triggered beats and episodes of VT. In contrast, triggering of VT persisted following programmed electrical stimulation in 6 of 12 (50%), one of eight (12.5%), but six of eight (75%) Scn5a+/Δ hearts perfused with 0, 0.1 and 1 μm propranolol respectively in parallel with corresponding alterations in repolarization gradients, reflected in action potential duration (ΔAPD₉₀) values. Thus 0.1 μm propranolol reduced epicardial but not endocardial APD₉₀ from 54.7 ± 1.6 to 44.0 ± 2.0 ms, restoring ΔAPD₉₀ from −3.8 ± 1.6 to 3.5 ± 2.5 ms (all n = 5), close to WT values. However, 1 μm propranolol increased epicardial APD₉₀ to 72.5 ± 1.2 ms and decreased endocardial APD₉₀ from 50.9 ± 1.0 to 24.5 ± 0.3 ms, increasing ΔAPD₉₀ to −48.0 ± 1.2 ms.

Conclusion

These findings empirically implicate EADs in potentially initiating spontaneous arrhythmogenic phenomena and transmural repolarization gradients in the re-entrant substrate that would sustain such activity when provoked by extrasystolic activity in murine hearts modelling human LQT3 syndrome.

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.

Criteria for arrhythmogenicity in genetically-modified Langendorff-perfused murine hearts modelling the congenital long QT syndrome type 3 and the Brugada syndrome

The experiments investigated the applicability of two established criteria for arrhythmogenicity in Scn5a+/Delta and Scn5a+/- murine hearts modelling the congenital long QT syndrome type 3 (LQT3) and the Brugada syndrome (BrS). Monophasic action potentials (APs) recorded during extrasystolic stimulation procedures from Langendorff-perfused control hearts and hearts treated with flecainide (1 microM) or quinidine (1 or 10 microM) demonstrated that both agents were pro-arrhythmic in wild-type (WT) hearts, quinidine was pro-arrhythmic in Scn5a+/Delta hearts, and that flecainide was pro-arrhythmic whereas quinidine was anti-arrhythmic in Scn5a+/- hearts, confirming clinical findings. Statistical analysis confirmed a quadratic relationship between epicardial and endocardial AP durations (APDs) in WT control hearts. However, comparisons between plots of epicardial against endocardial APDs and this reference curve failed to correlate with arrhythmogenicity. Restitution curves, relating APD to diastolic interval (DI), were then constructed for the first time in a murine system and mono-exponential growth functions fitted to these curves. Significant (P<0.05) alterations in the DI at which slopes equalled unity, an established indicator of arrhythmogenicity, now successfully predicted the presence or absence of arrhythmogenicity in all cases. We thus associate changes in the slopes of restitution curves with arrhythmogenicity in models of LQT3 and BrS.