Effects of peptide anabolic hormones on growth of myoblasts in culture.

The roles of growth hormone and somatomedin in stimulating muscle cell proliferation were investigated in a series of experiments on myoblast growth in culture. At levels 100 times normal circulating concentrations, neither growth hormone nor insulin stimulated growth of rat muscle cells in serum-free medium. In contrast, Temin's Multiplication Stimulating Activity (MSA), a close biological and chemical analog which served as a somatomedin surrogate in this study, was active at concentrations corresponding to the reported circulating levels of the somatomedins; MSA gave consistent stimulation of myoblast proliferation, protein accumulation, and [3H]thymidine incorporation by rat myoblasts and by Yaffe's L6 myogenic cell line. (The unreliability of [3H]thymidine in corporation as a quantitative assay for mitogenic activity is illustrated.) Addition of insulin at physiological concentrations and at the very high levels often used to stimulate cell growth did not enhance the effects of MSA on myoblast proliferation; effects were barely additive. We interpret these results to support earlier indications from studies on diaphragm muscle that the somatomedins are physiologically important mediators of the growth-promoting actions of growth hormone in muscle.