Increased Ca2+ storage capacity in the sarcoplasmic reticulum by overexpression of HRC (histidine-rich Ca2+ binding protein)

The histidine-rich Ca(2+) binding protein (HRC) is a high capacity Ca(2+) binding protein in the sarcoplasmic reticulum (SR). Because HRC appears to interact directly with triadin, HRC may play a role in the regulation of Ca(2+) release during excitation-contraction coupling. In this study, we examined the physiological effects of HRC overexpression in rat neonatal cardiomyocytes. Both caffeine-induced and depolarization-induced Ca(2+) release from the SR were increased significantly in the HRC overexpressing cardiomyocytes. Consistently, the Ca(2+) content, normally depleted from the SR in the presence of cyclopiazonic acid (CPA), remained elevated in these cells. In contrast, the density and the ryanodine-binding kinetics of the ryanodine receptor (RyR)/Ca(2+) release channel were slightly reduced or not significantly altered in the HRC overexpressing cardiomyocytes. We suggest that HRC is involved in the regulation of releasable Ca(2+) content into the SR.