Rad GTPase deletion increases L-type calcium channel current leading to increased cardiac contraction.

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. ventricular myocytes from adult rad(-/-) mice were isolated and evaluated by patch-clamp recordings for i(ca,l) and action potentials,ca(2+) 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 β-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 ca(2+) 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 β-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. this study demonstrates that the depletion of rad gtpase is equivalent to sympathomimetic β-adrenergic receptor,without stimulating cardiac hypertrophy. thus,targeting rad gtpase is a novel potential therapeutic target for ca(2+)-homeostasis-driven positive inotropic support of the heart.