Sex differences in ventricular repolarization: from cardiac electrophysiology to Torsades de Pointes

Gender Disparities in Cardiac Cellular Electrophysiology and Arrhythmia Susceptibility in Human Failing Ventricular Myocytes

Gender disparities in ECG variables and susceptibility to arrhythmia exist. The basis of these sex-related distinctions in cardiac electrophysiology has been extensively studied in various species, but is virtually unexplored in humans. The aim of this study was to clarify the cellular basis of electrophysiological gender disparities in human cardiac myocytes. Human midmyocardial left ventricular myocytes were isolated from explanted hearts of male and female patients in end-stage heart failure at the time of cardiac transplantation. The action potentials, sarcolemmal ion currents, and susceptibility to the generation of early afterdepolarizations were studied using whole-cell patch-clamp methodology. The functional effects of gender disparities in sarcolemmal ion currents were assessed by computer simulations using the Priebe-Beuckelmann or the ten Tusscher-Noble-Noble-Panfilov human ventricular cell models. Female myocytes had significantly longer action potentials and greater susceptibility to early afterdepolarizations than male myocytes. All other action potential parameters (resting membrane potential, amplitude, plateau level, upstroke velocity, maximal velocity of phase-1 and phase-3 repolarization) had similar values for both genders. In female myocytes, the transient outward potassium current (I(to1)) tended to be smaller, while the L-type calcium current (I(Ca,L)) and quasi-steady state current (I(QSS)) tended to be larger. Computer simulations showed that these subtle differences in sarcolemmal ion currents may conspire to cause the observed gender disparities in action potential properties. Female failing myocytes have longer action potentials and a greater susceptibility to early afterdepolarizations than male failing myocytes. These gender disparities may be due to slightly larger depolarizing I(Ca,L) in conjunction with slightly smaller repolarizing I(QSS) and I(to1) in female myocytes.

Review of the predictive value of the Langendorff heart model (Screenit system) in assessing the proarrhythmic potential of drugs

Prolongation of the QTc interval of the electrocardiogram (ECG) is used as a surrogate marker for a rare, but life threatening, ventricular arrhythmia known as torsades de pointes (TdP). However, the clear link between QTc prolongation and the arrhythmogenic risk has not been demonstrated unequivocally. In the present review article, we examine (a) the current understanding of electrophysiological and pharmacological mechanisms linking changes in action potential (AP) properties with proarrhythmia and (b) the value of the isolated, paced Langendorff-perfused female rabbit heart model (Screenit system) in predicting the torsadogenic potential of drugs in man. The Screenit system records monophasic action potentials (MAPs) from which the following parameters are evaluated: action potential duration (APD), conduction, instability (indicative of beat to beat APD variability), triangulation (indicative of changes of Phase 3 repolarization), and reverse-use dependency (indicating that the APD is more prolonged at slow heart rates). So far, over 16,000 experiments have been conducted, including approximately 300 dedicated tests to evaluate, in a blinded manner, approximately 70 clinically used drugs. The drugs tested covered a wide range of compounds from various pharmacological and chemical classes with clinical torsadogenic propensity, as well as drugs without the latter effect in clinical settings. Overall, the Screenit system and its associated analysis classified the drugs based on their effects on AP morphology and conduction and additionally identified, in a qualitative manner, drugs clinically associated with TdP. Such an identification is based on the triangulation, reverse-use dependency, and instability of the AP, as well as on the direct indexes of proarrhythmia such as early afterdepolarization (EADs), ventricular tachycardia (VT), and ventricular fibrillation (VF). Overall, drugs that readily induce arrhythmia and/or EADs and/or causes triangulation, reverse-use dependency, and/or instability and/or a chaotic Poincaré plot in a range of concentrations likely to be achieved in man is likely to cause TdP in man, eventually. Only if none of these elements is present, at concentrations well exceeding the free therapeutic plasma concentration, can one expect that the drug will probably be devoid of torsadonenicity. Therefore, this in vitro model provides detailed information on the overall profile of drug-induced electrophysiological effects. In combination with other in vitro and in vivo repolarization assays and with pharmacokinetic data in man, it is a valuable tool to establish an integrated cardiovascular risk assessment of pharmaceutical compounds.