Biosynthetic changes in myosin heavy subunit during myogenesis in culture

In primary culture of chick embryo muscle cells myosin synthesis is detected in mononucleated cells and increased at the onset of fusion with a maximal increment of 20-fold per plate in differentiated myotube. The possibility that the myosin synthetized by duplicating myoblast could be different from that present in post-mitotic myoblast and myotube was evaluated by investigating the regulation of its synthesis and the turnover of the molecule. Following Actinomycin D treatment (0.05 microgram/ml, 8 h), myosin synthesis is partially affected (about 50% inhibition) in pre-fusion myoblast while the synthesis is more sensitive to the drug at the onset of fusion (80% inhibition). With the progress of the differentiative stage the half-life of the molecule increases from 30 h in duplicating myoblasts to 200 h in fibers. The half-life of myosin synthetized by duplicating myoblasts in the explanted embryonic muscle, is 12 h. These data show different features of myosin heavy chains related to specific stages of differentiation and suggest the possibility that modulative changes of the molecule could induce its functional maturation during myogenesis.