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 (I-CaL); 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 I-Ca,I-L and action potentials, Ca2+ transients, and sarcomere shortening. Maximum I-CaL is elevated in Rad(-/-) (maximal conductance 0.35 +/- 0.04 picoSiemens/picoFarad (pS/pF) wild-type; 0.61 +/- 0.14 pS/pF Rad(-/-)), decay kinetics are faster, and I-Ca,(L) activates at lower voltages (activation midpoint -7.2 +/- 0.6 wild-type; -11.7 +/- 0.9 Rad(-/-)) mimicking effects of beta-adrenergic receptor stimulation. Diastolic and twitch calcium are elevated in Rad(-/-) (F-340/380: 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 beta-adrenergic receptor stimulation. In single cells, at subphysiological frequencies, nonstimulated calmodulin-dependent protein kinase-sensitive calcium release is observed. Remarkably, Rad(-/-) hearts did not show hypertrophic growth despite elevated levels of diastolic calcium. Conclusions-This study demonstrates that the depletion of Rad GTPase is equivalent to sympathomimetic beta-adrenergic receptor, without stimulating cardiac hypertrophy. Thus, targeting Rad GTPase is a novel potential therapeutic target for Ca2+-homeostasisdriven positive inotropic support of the heart.