Developmental Changes in the Composition and Function of Sarcoplasmic Reticulum

Abstract

Abstract Developmental changes in the protein, phospholipid, cholesterol, and fatty acid composition of skeletal and cardiac microsomes were correlated with Ca2+ transport activity in chicken embryos and during the early postnatal period. In skeletal muscle microsomes isolated from leg, superficial pectoralis, or deep pectoralis muscles, the rapid increase in the rate and extent of calcium accumulation around the time of hatching was accompanied by increase in total and Ca2+-sensitive ATPase activity, the concentration of phosphoprotein intermediate and the amount of transport ATPase enzyme determined by gel electrophoresis. The increased enzyme content of the membrane is reflected in its increased density and a marked decrease in phospholipid to protein ratio with development. The concentration of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin, and phosphatidylinositol changed relatively little during the first 50 days of development but the appearance of calcium transport activity was accompanied by a marked decrease in the palmitate and increase in the linoleate content of membrane phospholipids and a decrease in cholesterol concentration. These changes were clearly apparent in purified sarcoplasmic reticulum membranes isolated by calcium loading or in less pure membrane fractions obtained by differential or sucrose gradient centrifugation. Corresponding but much less pronounced changes in the enzymatic activity of cardiac microsomes were observed during development. Although the observations are consistent with nearly all possible mechanisms of membrane assembly they may suggest that the phospholipid-rich membranes of embryonic muscle are gradually converted during development into calcium-transporting structures by stepwise insertion of the ATPase enzyme and other components of the transport system.