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

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Abstract
Injection of frogs with .beta.-adrenergic catecholamines produced a selective desensitization (loss of responsiveness) of the erythrocyte membrane adenylate cyclase to subsequent stimulation in vitro by isoproterenol. Basal, prostaglandin E1 and fluoride-sensitive enzyme activities were unaffected. A 77% (P < 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 < 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 alone 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. The chronic occupancy of .beta.-adrenergic receptors by .beta.-adrenergic agonists (but not antagonists) probably decreases the number of functional .beta.-adrenergic receptor binding sites and lowers the responsiveness of adenylate cyclase to catecholamine stimulation. The lack of effect of cycloheximide on these regulatory effects suggests that inactivation and subsequent reactivation of the receptors, rather than changes in receptor turnover, are involved.