| Abstract: | The influence of the Ca2+ ionophores, ionomycin and A23187 upon the incorporation of [35S]methionine into proteins of cultured chicken pectoralis muscle was studied during differentiation of myoblasts into multinucleated myotubes. Fusion was reversibly arrested by growing cells in low-calcium media from the time of plating. Exposure of normal and fusion blocked cultures to 10-6-10-5 M ionomycin or A23187 for 2-6 h on the second to fourth day of growth, resulted in a selective increase in the incorporation of [35S]methionine into two proteins of about 100 000 and 80 000 dalton. When 10-5 M ionomycin or A23187 were added to older cultures, all large myotubes contracted and detached from the plate. Only the adhering myoblasts and small myotubes incorporated [35s[methionine into the muscle proteins and showed increased incorporation of label into 100 000 and 80 000 proteins. After ionophore pulse, the adhering cells retained the ability to differentiate and accumulate myosin. The effect of Ca2+ ionophores upon the rate of protein synthesis is presumably related to increased influx of extracellular Ca2+ with a rise in the Ca2+ concentration of the cytoplasm. We conclude that Ca2+ sensitive mechanisms may regulate the synthesis of a select group of muscle proteins. |
