INSULIN AND INSULIN-LIKE GROWTH-FACTOR-I RESPONSIVENESS AND SIGNALINGMECHANISMS IN C2C12 SATELLITE CELLS - EFFECT OF DIFFERENTIATION AND FUSION

In proliferating C2C12 myoblasts, serum and physiological concentratio ns of insulin and IGF-I stimulated protein synthesis and RNA accretion . After fusion, the multinucleated myotubes remained responsive to ser um but not to insulin or IGF-I, even though both insulin and type-1 IG F receptor mRNAs increased in abundance. Protein synthetic responses t o insulin and IGF-I in myoblasts were not inhibited by dexamethasone, ibuprofen or Ro-31-8220, thus phospholipase A,, cyclo-oxygenase and pr otein kinase C did not appear to be involved in the signalling mechani sms. Neither apparently were polyphosphoinositide-specific phospholipa se C or phospholipase D since neither hormone increased inositol phosp hate, phosphatidic acid, choline or phosphatidylbutanol production. On ly the phosphatidylinositol-3-kinase inhibitor, wortmannin, and the 70 kDa S6-kinase inhibitor, rapamycin, wholly or partially blocked the e ffects of insulin and IGF-I on protein synthesis. 2-deoxyglucose uptak e remained responsive to insulin and IGF-I after fusion and was also i nhibited by wortmannin. The results suggest that the loss of responsiv eness after fusion is not due to loss of receptors, but to the uncoupl ing of a post-receptor pathway, occurring after the divergence of the glucose transport and protein synthesis signalling systems, and that, if wortmannin acts at a single site, this is prior to that point of di vergence.