Developmental changes in cardiac sarcoplasmic reticulum in sheep.

Physiologic studies suggest that the myocardium from fetal and newborn sheep functions at a higher contractile state with decreased contractile reserve when compared to the myocardium of adult sheep. To investigate the role of Ca2+ transport by the sarcoplasmic reticulum (SR) in this phenomenon, we studied functional properties and protein composition of cardiac SR vesicles isolated from fetal and maternal sheep. Active accumulation of Ca2+ and the density of the Ca2+ pump protein were decreased 60% (p less than 0.01) in fetal SR vesicles; however Ca2+-dependent ATPase activity was decreased only 30% (p less than 0.01). This decreased difference in Ca2+-dependent ATPase activities was accounted for by the higher turnover number measured for the Ca2+ pump of fetal SR vesicles (1.6-fold increased, p less than 0.01). Ryanodine, an alkaloid which blocks Ca2+ efflux from cardiac SR vesicles, stimulated Ca2+ uptake more effectively in fetal SR vesicles, suggesting that these vesicles had a higher passive Ca2+ permeability during conditions of active Ca2+ transport. Protein compositional studies showed that the content of phospholamban was decreased in fetal SR vesicles and was correlated with the decrease in the density of Ca2+ pumps. In contrast, the content of calsequestrin and the density of [3H]nitrendipine-binding sites were increased approximately 2-fold in fetal SR vesicles. These functional and compositional differences between SR vesicles isolated from fetal and maternal sheep may indicate that there is relatively more junctional SR in fetal hearts. Since the SR regulates muscle contraction by modulating intracellular Ca2+ concentration, it is possible that developmental alterations in cardiac SR may contribute to the decreased myocardial contractile reserve noted in fetal sheep.