Sarcoplasmic reticulum membrane and heart development.

Intracellular Ca2+ concentrations in cardiac cells are dependent on trans-sarcolemmal Ca2+ fluxes and the ability of sarcoplasmic reticulum to release and take up Ca2+. Ca2+ accumulation by sarcoplasmic reticulum membranes causes muscle to relax, whereas Ca2+ release from sarcoplasmic reticulum initiates contraction. Ca2+ transport by the sarcoplasmic is mediated by a Ca2+-dependent ATPase enzyme. Ca2+ release from sarcoplasmic reticulum may be mediated by a ligant-gated Ca2+ channel. The physiological role of sarcoplasmic reticulum in developing muscle is not well established. In this report we investigated the composition and function of sarcoplasmic reticulum membranes during cardiac myogenesis. Phospholamban, a major phosphoprotein in mature sarcoplasmic reticulum membranes was present during early stages of cardiac myogenesis. The embryonic form of phospholamban was phosphorylated by cAMP-dependent protein kinase but not in the presence of Ca2+ and calmodulin. Ca2+ uptake and Ca2+-dependent ATPase activity were low in fetal sarcoplasmic reticulum compared to adult control membranes, although the apparent affinities of the enzyme for Ca2+ were similar. Sarcoplasmic reticulum vesicles used in these studies had very low levels of plasma membrane and mitochondrial contamination. The amounts of both 110-kDa Ca2+-ATPase and 55-kDa calsequestrin in the sarcoplasmic reticulum membrane were lower in fetal sarcoplasmic reticulum vesicles compared to mature membranes. Ca2+-ATPase and calsequestrin were identified in the isolated sarcoplasmic reticulum vesicles using specific antibodies produced against these membrane proteins. Age-related differences in Ca2+ transport properties of cardiac sarcoplasmic reticulum and in the amount of Ca2+-ATPase and calsequestrin may explain alterations in the regulation of intracellular Ca2+ concentrations in fetal heart muscle. This may relate to the developmental changes observed in myocardial function.