Abstract
Introduction: Loss-of function mutations in HERG potassium channels in Long QT2 syndrome are associated with sudden cardiac death triggered by sympathetic surge. In various models of cardiac disease changes in calcium (Ca) homeostasis, associated with upregulated plasmalemmal Na/Ca exchanger (NCX) and/or hyperactive ryanodine receptors (RyRs), have been linked to increased risk of arrhythmia. Methods and Results: The aims of this study were to investigate possible changes in Ca homeostasis in cardiomyocytes derived from from the hearts of LQT2 rabbits and to determine whether similar changes contribute to the triggering of early afterdepolarizations (EADs) characteristic of LQT2 myocytes under β-adrenergic stimulation. Parameters of Ca handling were measured in ventricular myocytes enzymatically isolated from LQT2 and littermate control hearts using confocal Ca imaging and whole cell patch clamp. Ca imaging revealed no changes in amplitude of Ca transients in intact myocytes and sarcoplasmic reticulum (SR) Ca content assessed by measuring amplitude of transients evoked by rapid application of caffeine. A two-fold increase in rate of decay of caffeine-induced Ca transients in LQT2 myocytes vs. controls indicated increased NCX activity. Experiments in saponin-permeabilized cells using a SR-entrapped low-affinity Ca indicator revealed ~2-fold increase in RyR-mediated SR Ca leak in LQT2 myocytes in comparison with controls. In contrast to controls, current-clamped LQT2 myocytes exposed to β-adrenergic agonist isoproterenol exhibited prolonged plateau of action potentials (AP) at -10 - 0 mV accompanied by aberrant Ca releases and phase 3 EADs. Importantly, complete exhaustion of SR [Ca] by incubation of LQT2 cells with SERCA inhibitor thapsigargin or 10 mM caffeine, shortened APD and completely abolished EADs. Conclusion: Functional changes in RyR and NCX underlie preserved systolic Ca transients in LQT2 myocytes under basal conditions and may contribute to stress-induced arrhythmia by maintaining the plateau of AP at voltages ideal for reactivation of plasmalemmal Ca channels and generation of EADs.
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