Cardiac electrophysiological remodeling associated with enhanced arrhythmia susceptibility in a canine model of elite exercise

The health benefits of regular physical exercise are well known. Even so, there is increasing evidence that the exercise regimes of elite athletes can evoke cardiac arrhythmias including ventricular fibrillation and even sudden cardiac death (SCD). The mechanism of exercise-induced arrhythmia and SCD is poorly understood. Here, we show that chronic training in a canine model (12 sedentary and 12 trained dogs) that mimics the regime of elite athletes induces electrophysiological remodeling (measured by ECG, patch-clamp, and immunocytochemical techniques) resulting in increases of both the trigger and the substrate for ventricular arrhythmias. Thus, 4 months sustained training lengthened ventricular repolarization (QTc: 237.1±3.4 ms vs. 213.6±2.8 ms, n=12; APD90: 472.8±29.6 ms vs. 370.1±32.7 ms, n=29 vs. 25), decreased transient outward potassium current (6.4±0.5 pA/pF vs. 8.8±0.9 pA/pF at 50 mV, n=54 vs. 42), and increased the short-term variability of repolarization (29.5±3.8 ms vs. 17.5±4.0 ms, n=27 vs. 18). Left ventricular fibrosis and HCN4 protein expression were also enhanced. These changes were associated with enhanced ectopic activity (number of escape beats from 0/hr to 29.7±20.3/hr) in vivo and arrhythmia susceptibility (elicited ventricular fibrillation: 3 of 10 sedentary dogs vs. 6 of 10 trained dogs). Our findings provide in vivo, cellular electrophysiological and molecular biological evidence for the enhanced susceptibility to ventricular arrhythmia in an experimental large animal model of endurance training.

The hidden cardiotoxic effect of rofecoxib on rabbit ventricular preparations

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): National Research, Development and Innovation Office Hungary

Introduction

Unexpected ischaemia-induced cardiac adverse events are major contributors to clinical trial discontinuation and drug attrition, and these effects remain undetected by routine preclinical cardiotoxicity assessment. On the other hand, the proarrhythmic effect of drug candidates is studied in healthy cells, tissues, or exclusively in healthy experimental animals. However, a number of experimental and clinical reports demonstrate that the proarrhythmic effects of most drugs are manifested only in pathological conditions. Thus, the aim is to develop a sensitized animal model that can reliably screen for the arrhythmogenic effects of a compound. Here we show that the selective COX- 2 inhibitor rofecoxib, which has been withdrawn over cardiovascular safety concerns, possesses cardiac electrophysiological adverse effects only revealed during ischaemia/reperfusion, a phenomenon we termed "hidden cardiotoxicity".

Purpose

Our group has previously reported the hidden cardiotoxic effect of rofecoxib on rat ventricular preparations. Given the significant differences in cardiac electrophysiological properties between rats and humans, the human extrapolation of arrhythmological results obtained from rats is limited, so our aim was to investigate the proarrhythmic effect of rofecoxib in a sensitized rabbit model.

Methods

Action potentials were registered from rabbit right ventricular papillary muscles using the conventional microelectrode technique. The following parameters were measured under normoxic and ischaemic conditions in the presence or absence of 10 μM rofecoxib: conduction time, action potential amplitude, action potential duration at 25%, 50%, 75% and 90% repolarization (APD25-90). The effects of rofecoxib upon test ischaemia and reperfusion were investigated.

Results

Rofecoxib (10 μM) did not alter electrophysiological parameters in normoxic conditions. However, following 30 minute ischaemia the APD90 was significantly decreased during reperfusion compared to APD90 in the vehicle-treated group: 161.96 ± 19.55 ms vs. 180.04 ± 23.21 ms (p<0.05, n=5), respectively. Following sI/R, a decrease in Vmax and AMPL parameters indicating impulse conduction velocity was also measured in the 10 μM rofecoxib- treated group, but the differences were not statistically significant.

Conclusions

Under pathological conditions, rofecoxib may increase the incidence of reperfusion arrhythmias due to the mechanism of delayed afterdepolarization (DAD) and reentrant arrhythmias. Consequently, the sensitized rabbit model detailed above may be suitable for investigating the "hidden cardiotoxic" effect of a drug candidate compound. Significant differences were observed in the effect of rofecoxib on repolarization between the rat and rabbit models. However, due to the well- known electrophysiological differences between the two species, the human relevance of the results obtained in rabbits is more reliable.

In vivo and cellular antiarrhythmic and cardiac electrophysiological effects of desethylamiodarone in dog cardiac preparations

BACKGROUND AND PURPOSE

The aim of the present study was to study the antiarrhythmic effects and cellular mechanisms of desethylamiodarone (DEA), the main metabolite of amiodarone (AMIO), following acute and chronic 4-week oral treatments (25-50 mg·kg^-1 ·day^-1 ).

EXPERIMENTAL APPROACH

The antiarrhythmic effects of acute iv. (10 mg·kg^-1 ) and chronic oral (4 weeks, 25 mg·kg^-1 ·day^-1 ) administration of DEA were assessed in carbachol and tachypacing-induced dog atrial fibrillation models. Action potentials were recorded from atrial and right ventricular tissue following acute (10 μM) and chronic (p.o. 4 weeks, 50 mg·kg^-1 ·day^-1 ) DEA application using the conventional microelectrode technique. Ionic currents were measured by the whole cell configuration of the patch clamp technique in isolated left ventricular myocytes. Pharmacokinetic studies were performed following a single intravenous dose (25 mg·kg^-1 ) of AMIO and DEA intravenously and orally. In chronic (91-day) toxicological investigations, DEA and AMIO were administered in the oral dose of 25 mg·kg^-1 ·day^-1 ).

KEY RESULTS

DEA exerted marked antiarrhythmic effects in both canine atrial fibrillation models. Both acute and chronic DEA administration prolonged action potential duration in atrial and ventricular muscle without any changes detected in Purkinje fibres. DEA decreased the amplitude of several outward potassium currents such as I_Kr , I_Ks , I_K1 , I_to , and I_KACh , while the I_CaL and late I_Na inward currents were also significantly depressed. Better drug bioavailability and higher volume of distribution for DEA were observed compared to AMIO. No neutropenia and less severe pulmonary fibrosis was found following DEA compared to that of AMIO administration.

CONCLUSION AND IMPLICATIONS

Chronic DEA treatment in animal experiments has marked antiarrhythmic and electrophysiological effects with better pharmacokinetics and lower toxicity than its parent compound. These results suggest that the active metabolite, DEA, should be considered for clinical trials as a possible new, more favourable option for the treatment of cardiac arrhythmias including atrial fibrillation.

Arrhythmogenic Remodeling in the Failing Heart

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Antiarrhythmic and cardiac electrophysiological effects of SZV-270, a novel compound with combined Class I/B and Class III effects, in rabbits and dogs

Cardiovascular diseases are the leading causes of mortality. Sudden cardiac death is most commonly caused by ventricular fibrillation (VF). Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major cause of stroke and heart failure. Pharmacological management of VF and AF remains suboptimal due to limited efficacy of antiarrhythmic drugs and their ventricular proarrhythmic adverse effects. In this study, the antiarrhythmic and cardiac cellular electrophysiological effects of SZV-270, a novel compound, were investigated in rabbit and canine models. SZV-270 significantly reduced the incidence of VF in rabbits subjected to coronary artery occlusion/reperfusion and reduced the incidence of burst-induced AF in a tachypaced conscious canine model of AF. SZV-270 prolonged the frequency-corrected QT interval, lengthened action potential duration and effective refractory period in ventricular and atrial preparations, blocked I _Kr in isolated cardiomyocytes (Class III effects), and reduced the maximum rate of depolarization (V _max) at cycle lengths smaller than 1000 ms in ventricular preparations (Class I/B effect). Importantly, SZV-270 did not provoke Torsades de Pointes arrhythmia in an anesthetized rabbit proarrhythmia model characterized by impaired repolarization reserve. In conclusion, SZV-270 with its combined Class I/B and III effects can prevent reentry arrhythmias with reduced risk of provoking drug-induced Torsades de Pointes.

Different effects of amiodarone and dofetilide on the dispersion of repolarization between well-coupled ventricular and Purkinje fibers1

Increased transmural dispersion of repolarization is an established contributing factor to ventricular tachyarrhythmias. In this study, we evaluated the effect of chronic amiodarone treatment and acute administration of dofetilide in canine cardiac preparations containing electrotonically coupled Purkinje fibers (PFs) and ventricular muscle (VM) and compared the effects to those in uncoupled PF and VM preparations using the conventional microelectrode technique. Dispersion between PFs and VM was inferred from the difference in the respective action potential durations (APDs). In coupled preparations, amiodarone decreased the difference in APDs between PFs and VM, thus decreasing dispersion. In the same preparations, dofetilide increased the dispersion by causing a more pronounced prolongation in PFs. This prolongation was even more emphasized in uncoupled PF preparations, while the effect in VM was the same. In uncoupled preparations, amiodarone elicited no change on the difference in APDs. In conclusion, amiodarone decreased the dispersion between PFs and VM, while dofetilide increased it. The measured difference in APD between cardiac regions may be the affected by electrotonic coupling; thus, studying PFs and VM separately may lead to an over- or underestimation of dispersion.

Comparison of the effects of IK,ACh, IKr, and INa block in conscious dogs with atrial fibrillation and on action potentials in remodeled atrial trabeculae

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major cause of morbidity and mortality. Traditional antiarrhythmic agents used for restoration of sinus rhythm have limited efficacy in long-term AF and they may possess ventricular proarrhythmic adverse effects, especially in patients with structural heart disease. The acetylcholine receptor-activated potassium channel (I_K,ACh) represents an atrial selective target for future AF management. We investigated the effects of the I_K,ACh blocker tertiapin-Q (TQ), a derivative of the honeybee toxin tertiapin, on chronic atrial tachypacing-induced AF in conscious dogs, without the influence of anesthetics that modulate a number of cardiac ion channels. Action potentials (APs) were recorded from right atrial trabeculae isolated from dogs with AF. TQ significantly and dose-dependently reduced AF incidence and AF episode duration, prolonged atrial effective refractory period, and prolonged AP duration. The reference drugs propafenone and dofetilide, both used in the clinical management of AF, exerted similar effects against AF in vivo. Dofetilide prolonged atrial AP duration, whereas propafenone increased atrial conduction time. TQ and propafenone did not affect the QT interval, whereas dofetilide prolonged the QT interval. Our results show that inhibition of I_K,ACh may represent a novel, atrial-specific target for the management of AF in chronic AF.

Combined inhibition of key potassium currents has different effects on cardiac repolarization reserve and arrhythmia susceptibility in dogs and rabbits

A reliable assessment of the pro-arrhythmic potential for drugs in the development phase remains elusive. Rabbits and dogs are commonly used to create models of pro-arrhythmia, but the differences between them with respect to repolarizing potassium currents are poorly understood. We investigated the incidence of drug-induced torsades de pointes (TdP) and measured conventional ECG parameters and the short-term variability of the QT interval (STVQT) following combined pharmacological inhibition of IK1+IKs and IK1+IKr in conscious dogs and anesthetized rabbits. A high incidence of TdP was observed following the combined inhibition of IK1+IKs in dogs (67% vs. 14% in rabbits). Rabbits exhibited higher TdP incidence after inhibition of IK1+IKr (72% vs. 14% in dogs). Increased TdP incidence was associated with significantly larger STVQT in both models. The relatively different roles of IK1 and IKs in dog and rabbit repolarization reserve should be taken into account when extrapolating the results from animal models of pro-arrhythmia to humans. A stronger repolarization reserve in dogs (likely due to stronger IK1 and IKs), and the more human-like susceptibility to arrhythmia of rabbits argues for the preferred use of rabbits in the evaluation of adverse pro-arrhythmic effects.

Diclofenac prolongs repolarization in ventricular muscle with impaired repolarization reserve

BACKGROUND

The aim of the present work was to characterize the electrophysiological effects of the non-steroidal anti-inflammatory drug diclofenac and to study the possible proarrhythmic potency of the drug in ventricular muscle.

METHODS

Ion currents were recorded using voltage clamp technique in canine single ventricular cells and action potentials were obtained from canine ventricular preparations using microelectrodes. The proarrhythmic potency of the drug was investigated in an anaesthetized rabbit proarrhythmia model.

RESULTS

Action potentials were slightly lengthened in ventricular muscle but were shortened in Purkinje fibers by diclofenac (20 µM). The maximum upstroke velocity was decreased in both preparations. Larger repolarization prolongation was observed when repolarization reserve was impaired by previous BaCl(2) application. Diclofenac (3 mg/kg) did not prolong while dofetilide (25 µg/kg) significantly lengthened the QT(c) interval in anaesthetized rabbits. The addition of diclofenac following reduction of repolarization reserve by dofetilide further prolonged QT(c). Diclofenac alone did not induce Torsades de Pointes ventricular tachycardia (TdP) while TdP incidence following dofetilide was 20%. However, the combination of diclofenac and dofetilide significantly increased TdP incidence (62%). In single ventricular cells diclofenac (30 µM) decreased the amplitude of rapid (I(Kr)) and slow (I(Ks)) delayed rectifier currents thereby attenuating repolarization reserve. L-type calcium current (I(Ca)) was slightly diminished, but the transient outward (I(to)) and inward rectifier (I(K1)) potassium currents were not influenced.

CONCLUSIONS

Diclofenac at therapeutic concentrations and even at high dose does not prolong repolarization markedly and does not increase the risk of arrhythmia in normal heart. However, high dose diclofenac treatment may lengthen repolarization and enhance proarrhythmic risk in hearts with reduced repolarization reserve.

Class I/B antiarrhythmic property of ranolazine, a novel antianginal agent, in dog and human cardiac preparations

The aim of this study was to investigate the cellular electrophysiological effects of ranolazine on action potential characteristics. The experiments were carried out in dog and human cardiac preparations using the conventional microelectrode technique. In dog Purkinje fibres ranolazine produced a concentration- and frequency-dependent depression of the maximum rate of depolarization (V(max)) while action potential duration (APD) was shortened. In dog and human right ventricular papillary muscle ranolazine exerted no significant effect on APD, while it produced, like mexiletine, use-dependent depression of V(max) with relatively fast onset and offset kinetics. In dog midmyocardial preparations the drug did not exert statistically significant effect on repolarization at 10 μM, although a tendency toward prolongation was observed at 20 μM. A moderate lengthening of APD(90) by ranolazine was noticed in canine atrial preparations obtained from dogs in sinus rhythm and in tachypacing induced remodelled preparations. Use-dependent depression of V(max) was more pronounced in atria from dogs in sinus rhythm than those in remodelled atria or in the ventricle. These findings indicate that ranolazine, in addition to its known late sodium current blocking effect, also depresses peak I(Na) with class I/B antiarrhythmic characteristics. Although peak I(Na) inhibition by ranolazine is stronger in the atria, it is also substantial (at fast stimulation frequencies) in ventricular preparations. Ranolazine also decreased the dispersion of ventricular repolarization (the difference in APD(90) values between Purkinje fibres and papillary muscles), which can contribute to the antiarrhythmic property of the drug.

Mechanisms of ventricular rate adaptation as a predictor of arrhythmic risk

Protracted QT interval (QTI) adaptation to abrupt heart rate (HR) changes has been identified as a clinical arrhythmic risk marker. This study investigates the ionic mechanisms of QTI rate adaptation and its relationship to arrhythmic risk. Computer simulations and experimental recordings in human and canine ventricular tissue were used to investigate the ionic basis of QTI and action potential duration (APD) to abrupt changes in HR with a protocol commonly used in clinical studies. The time for 90% QTI adaptation is 3.5 min in simulations, in agreement with experimental and clinical data in humans. APD adaptation follows similar dynamics, being faster in midmyocardial cells (2.5 min) than in endocardial and epicardial cells (3.5 min). Both QTI and APD adapt in two phases following an abrupt HR change: a fast initial phase with time constant < 30 s, mainly related to L-type calcium and slow-delayed rectifier potassium current, and a second slow phase of >2 min driven by intracellular sodium concentration ([Na+]i) dynamics. Alterations in [Na+]i dynamics due to Na+/K+ pump current inhibition result in protracted rate adaptation and are associated with increased proarrhythmic risk, as indicated by action potential triangulation and faster L-type calcium current recovery from inactivation, leading to the formation of early afterdepolarizations. In conclusion, this study suggests that protracted QTI adaptation could be an indicator of altered [Na+]i dynamics following Na+/K+ pump inhibition as it occurs in patients with ischemia or heart failure. An increased risk of cardiac arrhythmias in patients with protracted rate adaptation may be due to an increased risk of early afterdepolarization formation.