Insulin acts as a somatomedin analog in stimulating myoblast growth in serum-free medium.

A serum-free medium that supports the proliferation of myoblasts (but not of fibroblasts) has been developed recently in this laboratory. It is composed of 10(-6) M insulin, 10(-7) M dexamethasone, and 10(-5) M fetuin, and is designated medium MM-1. The latter two components gave optimal stimulation at or near "physiological" concentrations, but insulin was required at levels far in excess of those found in serum. Accordingly, we have now investigated the possibility that insulin acts as a weak analog of the somatomedins, as has been suggested in other systems. We found that maximal growth rates were observed when 10(-6) M insulin was replaced by 0.5 to 1.0 microgram/ml multiplication stimulating activity (MSA), indicating that insulin serves a somatomedinlike function of MM-1. We also investigated the possibility that a contaminant of fetuin is responsible for its action in MM-1 but found no evidence to support this suggestion. We conclude that MM-1 is suitable for the study of muscle cell growth and differentiation under rather well-defined conditions, and that insulin probably is serving as a somatomedin analog in this medium.