Abstract 279: Loss Of Rad GTPase Produces A Sympathomimetic Cardiac Phenotype Leading To Calcium Overload And Arrhythmia.

Background: Rad is a small GTPase that regulates inward calcium current in excitable cells. In human heart failure, calcium dysregulation is accompanied by lowered levels of Rad expression, and dampened responsiveness to beta-adrenergic stimulation. However, the effects of Rad ablation on calcium cycling in the heart have yet to be examined. Hypothesis: Loss of Rad emulates beta-adrenergic stimulation in cardiac myocytes, elevating calcium levels, and promoting after depolarizations. Methods: The role of Rad was evaluated using knockout mice (RadKO)at several levels of biological organization. Inward calcium current, action potentials, fractional shortening, and calcium transients were measured in isolated ventricular myocytes. Functional parameters were recorded in the isolated working heart, and telemetry was used to monitor ECGs in intact freely-roaming mice. Results: Isolated ventricular myoctyes from Rad null mice display increased ICaL density, greater inward current at low voltages, and after depolarizations at low frequencies. Isolated cells exhibit increased diastolic calcium levels and increased dynamic calcium changes in transient amplitude during pacing. Further, RadKO cardiomyocytes develop a significantly greater number of spontaneous calcium transients than wildtype cohorts. Ventricular myocytes and intact working hearts from RadKO mice fail to respond to beta-adrenergic stimulation at the level of channel activation, calcium transient kinetics, and exhibit a significantly dampened change in +dP/dt in the isolated heart. Consistent with these effects, PKA substrates are phosphorylated at baseline in RadKO cardiomyocytes, suggesting tonic PKA activation. Conclusion: These data suggest the new provocative hypothesis that beta-adrenergic-mediated changes in excitation, calcium handling, and heart contraction involve relief of Rad-dependent negative regulation of channel function.