The phosphoinositide 3-kinase (PI3K) inhibitors, LY294002 (LY) and wortmannin (WM), are widely used to examine the role of PI3K in growth factor signaling. These compounds inhibit the kinase action of PI3K, thus preventing the accumulation of PI(3,4,5)P3 and PI(3,4)P2 (PIs) and subsequent phosphorylation and activation of the downstream effectors of PI3K, Akt and p70S6K. The efficacy of these inhibitors has been demonstrated by measuring cellular levels of PIs or the kinase activity of immunoprecipitated PI3K. However, their effects on activation of Akt and p70S6K, more widely used markers of PI3K activation, has not been formally tested. We have examined the effects of LY and WM on phosphorylation of Akt and p70S6K by insulin-like growth factor-I, insulin, and platelet-derived growth factor in skeletal muscle cells. LY is much less effective in blocking the phosphorylation of Akt than p70S6K; at concentrations which completely inhibit phosphorylation of p70S6K, phosphorylation of Akt is only partially in...