Hormones that activate receptor tyrosine kinases have been shown to regulate G protein-coupled receptors and herein we investigate the ability of insulin-like growth factor-I to regulate the ₁-adrenergic receptor. Treating Chinese hamster ovary cells in culture with insulin-like growth factor-I is shown to functionally antagonize the ability of expressed ₁ -adrenergic receptors to accumulate intracellular cyclic AMP in response to stimulation by the -adrenergic agonist isoproterenol. The attenuation of ₁-adrenergic action was accompanied by internalization of ₁-adrenergic receptors in response to insulin-like growth factor-I. Inhibiting either phosphatidylinositol 3-kinase or the serine/threonine protein kinase Akt blocks the ability of insulin-like growth factor-I to antagonize and to internalize ₁-adrenergic receptors. Mutation of one potential Akt substrate site Ser412Ala, but not another Ser312Ala, of the ₁-adrenergic receptor abolishes the ability of insulin-like growth factor-I to functionally antagonize and to sequester the ₁-adrenergic receptor. We also tested the ability of insulin-like growth factor-I to regulate ₁-adrenergic receptors and their signaling in adult canine cardiac myocytes. Insulin-like growth factor-I attenuates the ability of ₁ -adrenergic receptors to accumulate intracellular cyclic AMP in response to isoproterenol and promotes internalization of ₁-adrenergic receptors in these cardiac myocytes.