Diastolic spontaneous calcium release from the sarcoplasmic reticulum increases beat-to-beat variability of repolarization in canine ventricular myocytes after β-adrenergic stimulation

Rationale: Spontaneous Ca release (SCR) from the sarcoplasmic reticulum can cause delayed afterdepolarizations and triggered activity, contributing to arrhythmogenesis during β-adrenergic stimulation. Excessive beat-to-beat variability of repolarization duration (BVR) is a proarrhythmic marker. Previous research has shown that BVR is increased during intense β-adrenergic stimulation, leading to SCR. Objective: We aimed to determine ionic mechanisms controlling BVR under these conditions. Methods and Results: Membrane potentials and cell shortening or Ca transients were recorded from isolated canine left ventricular myocytes in the presence of isoproterenol. Action-potential (AP) durations after delayed afterdepolarizations were significantly prolonged. Addition of slowly activating delayed rectifier K current (IKs) blockade led to further AP prolongation after SCR, and this strongly correlated with exaggerated BVR. Suppressing SCR via inhibition of ryanodine receptors, Ca2+/calmodulin-dependent protein kinase II inhibition, or by using Mg or flecainide eliminated delayed afterdepolarizations and decreased BVR independent of effects on AP duration. Computational analyses and voltage-clamp experiments measuring L-type Ca current (ICaL) with and without previous SCR indicated that ICaL was increased during Ca-induced Ca release after SCR, and this contributes to AP prolongation. Prolongation of QT, Tpeak-Tend intervals, and left ventricular monophasic AP duration of beats after aftercontractions occurred before torsades de pointes in an in vivo dog model of drug-induced long-QT1 syndrome. Conclusions: SCR contributes to increased BVR by interspersed prolongation of AP duration, which is exacerbated during IKs blockade. Attenuation of Ca-induced Ca release by SCR underlies AP prolongation via increased ICaL. These data provide novel insights into arrhythmogenic mechanisms during β-adrenergic stimulation besides triggered activity and illustrate the importance of IKs function in preventing excessive BVR. © 2012 American Heart Association, Inc.