The biosynthesis of sarcoplasmic reticulum.

Muscle differentiation provides a slow-motion picture of the assembly of highly specialized sarcoplasmic reticulum endowed with Ca2+ transport activity from its constituents. During development of chicken embryo pectoralis muscle, the sarcoplasmic reticulum evolves from the rough endoplasmic reticulum of myoblasts by insertion of Ca2+ transport ATPase molecules synthesized on membrane-bound polysomes into the phospholipid-rich endoplasmic reticulum membrane. The process continues until the Ca2+ ATPase content of the membrane approaches physical saturation. The rate of synthesis of Ca2+ ATPase sharply increases after fusion of myoblasts into multinucleated myotubes and the accumulation of sarcoplasmic reticulum and myofibrillar proteins follows a roughly similar time course. The regulation of Ca2+ ATPase synthesis during development involves myogenic as well as neurogenic mechanisms. There are indications that changes in intracellular free Ca2+ concentration may play a role in this regulation.