Rad GTPase Deletion Increases L-type Calcium Channel Current Leading to Increased Cardiac Contraction

Background-—The small GTPase Rad is a negative regulator of voltage-dependent L-type calcium channel current (ICaL); however, the effects of Rad ablation on cardiomyocyte function are unknown. The objective of this study is to test the hypothesis that Rad-depletion causes positive inotropic effects without inducing cardiac hypertrophy. Methods and Results-—Ventricular myocytes from adult Rad/ mice were isolated and evaluated by patch-clamp recordings for ICa,L and action potentials, Ca 2+ transients, and sarcomere shortening. Maximum ICaL is elevated in Rad / (maximal conductance 0.350.04 picoSiemens/picoFarad (pS/pF) wild-type; 0.610.14 pS/pF Rad/), decay kinetics are faster, and ICa,L activates at lower voltages (activation midpoint 7.20.6 wild-type; 11.70.9 Rad/) mimicking effects of b-adrenergic receptor stimulation. Diastolic and twitch calcium are elevated in Rad/ (F340/380: 1.03 diastolic and 0.35 twitch for wild-type; 1.47 diastolic and 0.736 twitch for Rad/) and sarcomere shortening is enhanced (4.31 % wild-type; 14.13 % Rad/) at lower pacing frequencies. Consequentially, frequency-dependence of Ca2+ transients is less in Rad/, and the frequency dependence of relaxation is also blunted. In isolated working hearts, similar results were obtained; chiefly, +dP/dt was elevated at baseline and developed pressure was relatively nonresponsive to acute b-adrenergic receptor stimulation. In single cells, at subphysiological frequencies, nonstimulated calmodulin-dependent protein kinase–sensitive calcium release is observed. Remarkably, Rad