Sarcoplasmic reticulum Ca²⁺ release is both necessary and sufficient for SK channel activation in ventricular myocytes

Abstract

SK channels are upregulated in human patients and animal models of heart failure (HF). However, their activation mechanism and function in ventricular myocytes remain poorly understood. We aim to test the hypotheses that activation of SK channels in ventricular myocytes requires Ca(2+) release from sarcoplasmic reticulum (SR) and that SK currents contribute to reducing triggered activity. SK2 channels were overexpressed in adult rat ventricular myocytes using adenovirus gene transfer. Simultaneous patch clamp and confocal Ca(2+) imaging experiments in SK2-overexpressing cells demonstrated that depolarizations resulted in Ca(2+)-dependent outward currents sensitive to SK inhibitor apamin. SR Ca(2+) release induced by rapid application of 10 mM caffeine evoked repolarizing SK currents, whereas complete depletion of SR Ca(2+) content eliminated SK currents in response to depolarizations, despite intact Ca(2+) influx through L-type Ca(2+) channels. Furthermore, voltage-clamp experiments showed that SK channels can be activated by global spontaneous SR Ca(2+) release events Ca(2+) waves (SCWs). Current-clamp experiments revealed that SK overexpression reduces the amplitude of delayed afterdepolarizations (DADs) resulting from SCWs and shortens action potential duration. Immunolocalization studies showed that overexpressed SK channels are distributed both at external sarcolemmal membranes and along the Z-lines, resembling the distribution of endogenous SK channels. In summary, SR Ca(2+) release is both necessary and sufficient for the activation of SK channels in rat ventricular myocytes. SK currents contribute to repolarization during action potentials and attenuate DADs driven by SCWs. Thus SK upregulation in HF may have an anti-arrhythmic effect by reducing triggered activity.

Ca²⁺-Activated K⁺ Channels as Therapeutic Targets for Myocardial and Vascular Protection

“…61 We recently showed that SKCa channels overexpressed in rat ventricular myocytes are responsive to spontaneous Ca 2+ waves and can effectively diminish the amplitude of pro-arrhythmic afterdepolarizations. 29 In line with our findings, single-cell electrophysiology studies of ventricular myocytes from a chronic canine model of HF demonstrated that SKCa inhibition effectively evokes AP instability. 59 Underscoring a protective role of SKCa channels in diseased hearts, blockade of SKCa channels in rabbit failing hearts was shown to enhance predisposition to Ca 2+ -dependent triggers exacerbating arrhythmic potential.…”

“…28 Importantly, in cultured rat ventricular myocytes overexpressing SK2 we recently demonstrated that Ca 2+ release from the sarcoplasmic reticulum (SR) Ca 2+ channels' ryanodine receptors (RyR2 s) is not only necessary but also sufficient to evoke hyperpolarizing ISK. 29 In parallel, Mu et al showed that RyR2-mediated SR Ca 2+ release is at least in part responsible for activation of atrial plasmalemmal SKs. 30 Colocalization of SKCa channels with voltage-dependent Ca 2+ channels points to their involvement in repolarization of the action potential.…”

SK channels and ventricular arrhythmias in heart failure

“…In cardiomyocytes, trans-sarcolemmal Ca 2+ influx through L-type Ca 2+ channels, Ca 2+ release from sarcoplasmic reticulum (SR), or combination of both, regulates the gating of SK channels (Lu et al, 2007; Terentyev et al, 2013). SK channels are coupled to L-type Ca 2+ channels, and Ca 2+ influx through L-type Ca 2+ channels directly activates SK channels (Lu et al, 2007; Maingret et al, 2008).…”

Antiarrhythmic Mechanisms of SK Channel Inhibition in the Rat Atrium

“…9 SK channels have prominent impact on atrial electrophysiology, and some reports also describe functional ventricular importance under pathophysiological conditions, such as heart failure. [10][11][12][13][14] Moreover, genomewide association studies have provided evidence for common variants in the KCNN3 gene being associated with risk of atrial fibrillation (AF). [15][16][17] We and others have previously shown SK channel inhibition to be antiarrhythmic in various ex vivo and in vivo animal models of experimental AF.…”