INSULIN, BUT NOT CONTRACTION, ACTIVATES AKT PKB IN ISOLATED RAT SKELETAL-MUSCLE/

Insulin and muscle contraction potently stimulate glucose uptake in ma mmalian skeletal muscle. Studies in muscle and adipose tissue have sho wn that insulin induces its receptor-dependent phosphorylation of insu lin receptor substrates 1 and 2, which leads to activation of polyphos phatidylinositol (PI) 3'-kinase. In contrast, muscle contraction stimu lates glucose transport via a mechanism that is independent of insulin , but the two pathways may converge downstream at the level of stimula tion of GLUT4 translocation. In the present study, we have examined th e role of Akt, an insulin-activated serine threonine kinase that has p reviously been shown to increase glucose transport in adipocytes. Eith er insulin or in vitro muscle contraction significantly elevated gluco se transport in isolated rat epitrochlearis and soleus muscles. Howeve r, Akt kinase activity was significantly stimulated by insulin and not contraction. Moreover, wortmannin, an inhibitor of PI 3'-kinase, comp letely blocked the insulin-stimulated increase in Akt activity and glu cose transport but did not alter either of these parameters in contrac ting muscles. The increases in Akt activity were paralleled by a decre ase in the electrophoretic mobility of Akt, indicative of phosphorylat ion of Akt by an upstream kinase. These changes in Akt mobility appear ed to be at least partially because of phosphorylation of Akt on serin e 473. Mutative down-stream target of Akt, p70 S6 kinase, showed simil ar changes in mobility in response to insulin but not contraction. The se data support the view that Akt is a downstream target of PI 3'-kina se and is involved in the signaling pathways involved in insulin but n ot contraction stimulation of glucose transport in skeletal muscle. Th ese data provide further evidence that two distinct pathways exist for the stimulation of glucose transport in mammalian skeletal muscle.