Ca2+ Release Via Insp(3)Rs Enhances Ryr Recruitment During Ca2+ Transients By Increasing Dyadic [Ca2+] In Cardiomyocytes

Excitation-contraction coupling (ECC) relies on temporally synchronized sarcoplasmic reticulum (SR) Ca2+ release via ryanodine receptors (RyRs) at dyadic membrane compartments. Neurohormones, such as endothelin-1 (ET-1), that act via G alpha(q)-associated G protein-coupled receptors (GPCRs) modulate Ca2+ dynamics during ECC and induce SR Ca2+ release events involving Ca2+ release via inositol 1,4,5-trisphosphate (InsP(3)) receptors (InsP(3)Rs). How the relatively modest Ca2+ release via InsP(3)Rs elicits this action is not resolved. Here, we investigated whether the actions of InsP3Rs on Ca2+ handling during ECC were mediated by a direct influence on dyadic Ca2+ levels and whether this mechanism contributes to the effects of ET-1. Using a dyad-targeted genetically encoded Ca2+ reporter, we found that InsP(3)R activation augmented dyadic Ca2+ fluxes during Ca2+ transients and increased Ca2+ sparks. RyRs were required for these effects. These data provide the first direct demonstration of GPCR and InsP(3) effects on dyadic Ca2+, and support the notion that Ca2+ release via InsP(3)Rs influences Ca2+ transients during ECC by facilitating the activation and recruitment of proximal RyRs. We propose that this mechanism contributes to neurohormonal modulation of cardiac function.