Alanine or pyruvate is required for the development of myotubes from myoblasts and cortisol satisfies this requirement.

Development of myotubes from chick embryo breast muscle myoblasts has been shown to occur in the combined presence of insulin, and both the low and high molecular weight components (LMW, HMW) of chick embryo extract. We report here that alanine, or pyruvate, will replace the LMW component with serine enhancing this effect. In addition, extracts of embryonic kidneys were the most active of five embryonic tissues tested in replacing the LMW component, which led to the finding that cortisol is active in promoting both the development of myotubes and the expression of creatine phosphokinase activity, a muscle-specific indicator of myogenesis. Since it was observed that myoblasts are highly glycolytic, it may be that pyruvate is limiting, suggesting that some aspect of oxidative metabolism is also limiting in these cells.