Insulin receptor substrate-1 (IRS-1) and Shc protein have the same binding site at the insulin receptor and compete in their association with the phosphorylated receptor. The present study demonstrates that a decrease in the level of muscle insulin receptor phosphorylation induced by chronic growth hormone (GH) treatment or acute epinephrine infusion is accompanied by a reduction in the level of IRS-1 phosphorylation and in the association with phosphatidylinositol 3-kinase. In contrast, no change is observed in insulin-stimulated Shc tyrosine phosphorylation, or in the association of this substrate with Grb2. These data suggest that a reduction in insulin receptor phosphorylation may affect post-receptor processes differentially by preserving the phosphorylation of some substrates and pathways, but not of others.421191-6