Contraction inhibits insulin-stimulated insulin receptor substrate-1/2-associated phosphoinositide 3-kinase activity, but not protein kinase B activation or glucose uptake, in rat muscle

The initial stages of insulin-stimulated glucose uptake are thought to invo lve tyrosine phosphorylation of insulin receptor substrates (IRSs), which r ecruit and activate phosphoinositide 3-kinase (PI 3-kinase), leading to the activation of protein kinase B (PKB) and other downstream effecters. In co ntrast, contraction stimulates glucose uptake via a PI 3-kinase-independent mechanism. The combined effects of insulin and contraction on glucose upta ke are additive. However, it has been reported that contraction causes a de crease in insulin-stimulated IRS-1-associated PI 3-kinase activity. To inve stigate this paradox, we have examined the effects of contraction on insuli n-stimulated glucose uptake and proximal insulin-signalling events in isola ted rat epitrochlearis muscle. Stimulation by insulin or contraction produc ed a 3-fold increase in glucose uptake, with the effects of simultaneous tr eatment by insulin and contraction being additive. Wortmannin completely bl ocked the additive effect of insulin in contracting skeletal muscle, indica ting that this is a PI 3-kinase-dependent effect. Insulin-stimulated recrui tment of PI 3-kinase to IRS-1 was unaffected by contraction; however, insul in produced no discernible increase in PI 3-kinase activity in IRS-1 or IRS -2 immunocomplexes in contracting skeletal muscle. Consistent with this, co ntraction inhibited insulin-stimulated p70(S6K) activation. In contrast, in sulin-stimulated activation of PKB was unaffected by contraction. Thus, in contracting skeletal muscle, insulin stimulates glucose uptake and activate s PKB, but not p70(S6K), by a PI 3-kinase-dependent mechanism that is indep endent of changes in IRS-1- and IRS-2-associated PI 3-kinase activity.