Relative effects of the somatomedins, multiplication-stimulating activity, and growth hormone on myoblasts and myotubes in culture.

Responses of muscle cells in culture to somatomedins A and B (SM-A and SM-B), Temin's multiplication-stimulating activity (MSA), and a number of GH preparations from various species have been determined at both the myoblast and myotube stages of differentiation. Proliferation and amino acid (AIB) uptake by myoblasts as well as AIB uptake by myotubes were equally responsive to SM-A and MSA, demonstrating that MSA serves as a useful surrogate for SM-A in studies on muscle cells. SM-B exhibited no activity in either system, and it did not enhance the actions of SM-A. A number of GH preparations were inactive in muscle cell cultures from various sources, but bovine GH stimulated AIB uptake in myotubes. Both the magnitude of the response and the sensitivity of myotubes to bovine GH increased with time after differentiation of the cells in vitro, but conditions could not be found to demonstrate an effect of GH at or near physiological levels of the hormone. On the basis of our observations and published studies on the effects of GH on isolated muscles, we suggest that the early growth of muscle is stimulated primarily by the somatomedins, and that a direct response to GH develops relatively late after the differentiation of skeletal muscle.