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

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 Ca²⁺ 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 Ca²⁺ imaging experiments in SK2-overexpressing cells demonstrated that depolarizations resulted in Ca²⁺-dependent outward currents sensitive to SK inhibitor apamin. SR Ca²⁺ release induced by rapid application of 10 mM caffeine evoked repolarizing SK currents, whereas complete depletion of SR Ca²⁺ content eliminated SK currents in response to depolarizations, despite intact Ca²⁺ influx through L-type Ca²⁺ channels. Furthermore, voltage-clamp experiments showed that SK channels can be activated by global spontaneous SR Ca²⁺ release events Ca²⁺ 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 Ca²⁺ 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.