Somatomedins inhibit protein degradation in muscle cell

Protein degradation has been measured in cultures of L6 myotubes as the rate of release of trichloroacetic acid-soluble radioactivity after prelabeling cell protein with [3H] leucine. Insulin-like growth factor-I (IGF-1), ovine somatomedin (oSM) and insulin (I), at concentrations from 10-11M to 10-7M (5 x 10-7M-oSM) were added at the beginning of a 4-hour degradation period to determine the effects of these hormones on inducible proteolysis occurring in serum-free media. In addition the effects of fetal bovine serum, at concentrations from 1% to 30%, on protein degradation were determined in parallel experiments to relate serum inhibition of proteolysis to somatomedin actions. Results from this study indicate the apparent half maximal inhibition of proteolysis (18%, 15%, 11%) occurred at .4nM-IGF-1, .6nM-oSM and 4nM-I, respectively. Thus protein degradation was approximately 10 times more sensitive to somatomedins than insulin. The half maximal inhibition of proteolysis (15%) observed with serum occurred at 7.7%. The magnitude of the response between IFG-I and serum (37% vs. 31%) was similar. These results are consistent with the hypothesis that somatomedins are important factors in regulating growth and development of muscle