Coupling of SK channels, L-type Ca2+ channels, and ryanodine receptors in cardiomyocytes.

Small-conductance Ca2+-activated K+ (SK) channels regulate the excitability of cardiomyocytes by integrating intracellular Ca2+ and membrane potentials on a beat-to-beat basis. The inextricable interplay between activation of SK channels and Ca2+ dynamics suggests the pathology of one begets another. Yet, the exact mechanistic underpinning for the activation of cardiac SK channels remains unaddressed. Here, we investigated the intracellular Ca2+ microdomains necessary for SK channel activation. SK currents coupled with Ca2+ influx via L-type Ca2+ channels (LTCCs) continued to be elicited after application of caffeine, ryanodine or thapsigargin to deplete SR Ca2+ store, suggesting that LTCCs provide the immediate Ca2+ microdomain for the activation of SK channels in cardiomyocytes. Super-resolution imaging of SK2, Cav1.2 Ca2+ channel, and ryanodine receptor 2 (RyR2) was performed to quantify the nearest neighbor distances (NND) and localized the three molecules within hundreds of nanometers. The distribution of NND between SK2 and RyR2 as well as SK2 and Cav1.2 was bimodal, suggesting a spatial relationship between the channels. The activation mechanism revealed by our study paved the way for the understanding of the roles of SK channels on the feedback mechanism to regulate the activities of LTCCs and RyR2 to influence local and global Ca2+ signaling.