Abstract
Small conductance calcium-activated potassium (SK) channels are upregulated in ventricular myocytes from human patients and animal models of heart failure (HF). However, their activation mechanism and function in ventricular myocytes remain elusive. We overexpressed SK2 channels in adult rat ventricular myocytes using adenovirus gene transfer to test the hypotheses that activation of SK channels in ventricular myocytes requires calcium release from sarcoplasmic reticulum (SR), and that upregulation of SK currents contributes to reducing triggered activity. Simultaneous voltage clamp and confocal calcium imaging experiments in SK2-overexpressing cells demonstrated that depolarizing voltage steps resulted in transient outward currents sensitive to the specific SK channel inhibitor apamin. SR calcium release induced by rapid application of 10 mM caffeine evoked repolarizing SK currents, whereas complete exhaustion of SR calcium stores eliminated SK currents in response to depolarizing voltage steps, despite intact calcium influx through L-type calcium channels. Furthermore, apamin-sensitive SK currents were activated by pro-arrhythmic global spontaneous SR calcium release events (calcium waves, SCWs). Current-clamp experiments demonstrated that SK overexpression reduced the amplitude of delayed afterdepolarizations (DADs) resulting from SCWs and shortened action potential duration (APD). Immuno-localization studies revealed that overexpressed SK channels were distributed both at external sarcolemmal membranes and along the Z-lines, resembling the distribution of endogenous SK channels. In summary, SR calcium 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 shortening APD and reducing triggered activity.
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