Sarcoplasmic Reticulum Ca2+ATPase and Cell Contraction in Developing Rabbit Heart

The purpose of the present study was to determine whether age-related changes in the expression and function of the cardiac isoform of the sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) play a role in SR Ca2+release and cell contraction. SERCA2a protein levels and subcellular localization were compared between fetal, neonatal, juvenile and adult New Zealand White rabbits. Studies of SERCA function in isolated myocytes were performed in situ by examining the rate of reloading of the SR Ca2+stores following caffeine-induced depletion. We found that significant quantities of SERCA2a were present early in immature heart and that SERCA2a expression reached adult levels within 15–30 days after birth. Furthermore, SERCA2a protein is present as a series of transverse striations within the cell as early as 1 day of age. In contrast to previous studies of SERCA in vitro, the SERCA protein function in situ was found to be comparable between neonatal and adult myocytes in maintaining SR Ca2+stores. These results indicate that the paucity of SR Ca2+release in immature ventricular cardiac myocytes is not the result of immaturity in SERCA2a expression.