Regulation of beta-adrenergic receptors by guanyl-5'-yl imidodiphosphate and other purine nucleotides.

Abstract

Guanyl-5'-yl imidodiphosphate (Gpp(NH)p), GTP, and other purine nucleotides selectively decrease the binding affinity of the beta-adrenergic receptors of frog erythrocyte membranes for beta-adrenergic agonists but not antagonists. Shifts in binding affinity were assessed by determining the ability of unlabeled ligands to compete with (-)-[3H]dihydroalprenolol for the membrane-bound receptors. The magnitude of the"right" shift in the binding displacement curve for any of 13 ligands tested was directly related to the intrinsic activity (maximal stimulatory capacity) of that agent for stimulation of the frog erythrocyte membrane adenylate cyclase. Thus, Gpp(NH)p-induced shifts in binding affinity were greatest for full agonists such as isoproterenol, intermediate for partial agonists such as soterenol, and no shifts were observed for antagonists such as propranolol. Shifts in binding affinity were observed only in preparations where agonist binding to the receptors leads to "coupling" of the receptors with adenylate cyclase. In solubilized preparations where the beta-adrenergic receptors and adenylate cyclase are functionally "uncoupled", Gpp(NH)p did not cause right shifts in agonist receptor binding displacement curves. In particulate preparations the Km of Gpp(NH)p for stimulation of adenylate cyclase was identical with that for its effect on beta-adrenergic agonist binding affinity, 1 to 2 muM. Moreover, the ability of several other nucleotides to cause shifts in receptor binding affinity directly paralleled their previously determined affinities for the nucleotide regulatory sites on adenylate cyclase. Gpp(NH)p also shifted agonist dose-response curves for stimulation of adenylate cyclase, but to the left. As with the effects on the receptor binding curves, the effects of Gpp(NH)p on the "apparent affinities" of agonists for enzyme stimulation were directly related to their intrinsic activities. Gpp(NH)p also markedly increased the intrinsic activity of partial agonists. These results appear to indicate that conformational alterations in adenylate cyclase caused by occupation of nucleotide regulatory sites by Gpp(NH)p are capable of inducing alterations in the beta-adrenergic receptors. These receptor alterations are induced only when the receptors are "coupled" to the enzyme by virtue of agonist binding. The nucleotide-altered conformation of the beta-adrenergic receptors is characterized by decreased binding affinity for agonist but increased functional efficacy in stimulating the enzyme.