Rad-deletion Phenocopies Tonic Sympathetic Stimulation of the Heart

Sympathetic stimulation modulates L-type calcium channel (LTCC) gating to contribute to increased systolic heart function. Rad is a monomeric G-protein that interacts with LTCC. Genetic deletion of Rad (Rad−/−) renders LTCC in a sympathomimetic state. The study goal was to use a clin- ically inspired pharmacological stress echocardiography test, including analysis of global strain, to determine whetherRad−/ − confers tonic positive inotropic heart function. Sarcomere dynamics and strain showed partial parallel isoproterenol (ISO) responsiveness for wild-type (WT) and for Rad−/−. Rad−/− basal inotropy was elevated compared to WT but was less responsiveness to ISO. Rad protein levels were lower in human patients with end-stage non-ischemic heart failure. These results show that Rad reduction provides a stable ino- tropic response rooted in sarcomere level function. Thus, re- duced Rad levels in heart failure patients may be a compen- satory response to need for increased output in the setting of HF. Rad deletion suggests a future therapeutic direction for inotropic support.