Agonist-induced increase in apparent beta-adrenergic receptor size.

The properties of digitonin-solubilized beta-adrenergic receptors from frog erythrocyte membranes were studied by gel exclusion chromatography on AcA 34 Ultragel. beta-Adrenergic receptor binding activity in these membranes can be identified by both an agonist ligand, [(3)H]hydroxybenzylisoproterenol, and the antagonist ligands, [(3)H]dihydroalprenolol and (125)I-labeled hydroxybenzylpindolol. Occupancy of the beta-adrenergic receptors with the [(3)H]hydroxybenzylisoproterenol agonist prior to their solubilization from the membrane leads to an increase in apparent receptor size. Alterations in the molecular size of the receptor cannot be mimicked by occupancy of the binding site with the antagonist ligands. Exposure of frog erythrocyte membranes to [(3)H]hydroxybenzylisoproterenol agonist in the presence of 10 muM Gpp(NH)(p), a guanyl nucleotide analog that exerts multiple regulatory effects on the catecholamine-sensitive adenylate cyclase [ATP pyrophosphate-lyase (cyclizing); EC 4.6.1.1] system, results in the elution of the [(3)H]hydroxybenzylisoproterenol radioligand in both the region characteristic of the agonist-receptor complex and the region characteristic of the antagonist-receptor complex. The precise molecular interactions responsible for the agonist-induced increase in apparent beta-adrenergic receptor size are still unresolved. However, the low concentrations of agonist that are capable of altering apparent receptor size and the sensitivity of this effect to guanyl nucleotides suggest that these phenomena may be intimately involved in eliciting the physiological effects of beta-adrenergic catecholamines at the molecular level.