The histidine-rich Ca2+ binding protein (HRC) is a high capacity Ca2+ binding protein in the sarcoplasmic reticulum (SR). Because HRC appears to interact directly with triadin, HRC may play a role in the regulation of Ca2+ 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 Ca2+ release from the SR were increased significantly in the HRC overexpressing cardiomyocytes. Consistently, the Ca2+ 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)/Ca2+ release channel were slightly reduced or not significantly altered in the HRC overexpressing cardiomyoctcs. We suggest that HRC is involved in the regulation of releasable Ca2-content into the SR. (C) 2002 Elsevier Science (USA). All rights reserved.