as a novel regulator of excitationcontraction coupling and beta-adrenergic signaling in heart. Circ Res

Rationale: Rad (Ras associated with diabetes) GTPase, a monomeric small G protein, binds to Cav subunit of the L-type Ca2 channel (LCC) and thereby regulates LCC trafficking and activity. Emerging evidence suggests that Rad is an important player in cardiac arrhythmogenesis and hypertrophic remodeling. However, whether and how Rad involves in the regulation of excitation–contraction (EC) coupling is unknown. Objective: This study aimed to investigate possible role of Rad in cardiac EC coupling and -adrenergic receptor (AR) inotropic mechanism. Methods and Results: Adenoviral overexpression of Rad by 3-fold in rat cardiomyocytes suppressed LCC current (ICa), [Ca2]i transients, and contractility by 60%, 42%, and 38%, respectively, whereas the “gain ” function of EC coupling was significantly increased, due perhaps to reduced “redundancy ” of LCC in triggering sarcoplasmic reticulum release. Conversely, 70 % Rad knockdown by RNA interference increased ICa (50%), [Ca2]i transients (52%) and contractility (58%) without altering EC coupling efficiency; and the dominant negative mutant RadS105N exerted a similar effect on ICa. Rad upregulation caused depolarizing shift of LCC activation and hastened time-dependent LCC inactivation; Rad downregulation, however, failed to alter these attributes. The Na/Ca2 exchange activity, sarcoplasmic reticulum Ca2 content, properties of Ca2 sparks and propensity for Ca2 waves all remained unperturbed regardless of Rad manipulation. Rad overexpression, bu