Regulation of Adenylate Cyclase Coupled β-Adrenergic Receptors by β-Adrenergic Catecholamines

Injection of frogs with beta-adrenergic catecholamines produced a selective desensitization (loss of responsiveness) of the erythrocyte membrane adenylate cylase to subsequent stimulation in vitro by isoproterenol. Basal, prostaglandin E1- and fluoride-sensitive enzyme activities were unaffected. A 77% (p less than 0.001) decline in isoproterenol-responsive enzyme activity in the cells from the treated animals was observed with no change in the Km for isoproterenol stimulation of the enzyme (concentration causing 1/2 maximal enzyme activation). The decrease in catecholamine-sensitive adenylate cyclase was accompanied by a parallel 68% (p less than 0.001) fall in the apparent number of beta-adrenergic receptors in the erythrocyte membranes, assessed by (-) (3H)alprenolol binding studies. There was no change in the affinity of the receptor binding sites. The catecholamine-induced desensitization and fall in the beta-adrenergic receptor number were both concentration and time-dependent and displayed beta-adrenergic specificity. Isoproterenol was more potent in desensitizing cells and in lowering the receptor number than was norepinephrine. The beta-adrenergic antagonist propranolol, but not the alpha-adrenergic antagonist phentolamine, blocked the desensitizing effects of isoproterenol. Propranolol itself, however, did not cause desensitization. Cells became resensitized to the stimulatory effects of catecholamines, in association with a return in beta-receptor number, when propranolol was injected into previously desensitized animals. The changes in receptor number in membranes from desensitized and resensitized animals were also reflected in soluble receptor preparations. The protein synthesis inhibitor cycloheximide did not affect either desensitization, resensitization, or the changes in receptor number which accompanied the changes in adenylate cyclase sensitivity to catecholamines. These findings suggest that the chronic occupancy of beta-adrenergic receptors by beta-adrenergic agonists (but not antagonists) decreases the number of functional beta-adrenergic receptor binding sites and, hence, lowers the responsiveness of adenylate cylase to catecholamine stimulation. The lack of effort of cycloheximide on these regulatory effects suggests that "inactivation" and subsequent "reactivation" of the receptors, rather than changes in receptor turnover, are involved.