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Regulation of adenylate cyclase coupled beta adrenergic receptor binding sites by beta adrenergic catecholamines in vitro



Regulation of adenylate cyclase coupled beta adrenergic receptor binding sites by beta adrenergic catecholamines in vitro



Molecular Pharmacology 12(3): 409-419



Tolerance to catecholamines was studied using a frog erythrocyte model system in vitro. Incubation of cells with (-)-isoproterenol resulted in up to a 58% decline (P < 0.001) in maximum catecholamine-stimulated adenylate cyclase activity in membrane preparations. Desensitization was characterized by a decrease in Vmax with no change in hormone affinity. Enzyme activity in the unstimulated state and after stimulation with NaF and prostaglandin E1 (PGE1) was unaltered. The number of functional .beta. adrenergic receptors, measured by binding of the .beta. adrenergic antagonist (-)-[3H]dihydroalprenolol, showed a parallel decline of up to 47% (P < 0.001) with no change in receptor affinity. The dose-response curve for desensitization of enzyme activity and decreased receptor binding was the same as for (-)-isoproterenol stimulation of adenylate cyclase activity. The time course for the desensitization of adenylate cyclase and decrease in receptor number was much longer than that required for receptor binding or cyclic 3',5'-AMP (cAMP) production, requiring 2-3 h for completion. The ability of .beta. adrenergic agonists to cause a decrease in enzyme activity and in receptor number was directly related to their affinity for the .beta. adrenergic receptor. The potency series was isoproterenol > epinephrine > norepinephrine. The .beta. antagonist propranolol, while having no effect of its own, blocked the desensitization caused by isoproterenol. In contrast, the .alpha. adrenergic antagonist phentolamine did not interfere with the phenomenon. The desensitized state was reversible. If isoproterenol was removed from desensitized cells by washing, and cells were further incubated with propranolol, receptor binding and enzyme activity returned to nearly normal levels. Incubation of cells with PGE1 produced densensitization to subsequent prostaglandin stimulation but had no effect on catecholamine sensitivity or .beta. receptor binding. Dibutyryl cAMP did not cause a decrease in binding or enzyme activity. Catecholamines, through .beta. adrenergic receptor interactions, play a role in regulation of the number of functioning receptors at the cell surface. This may be a major mechanism for the induction of catecholamine tolerance.

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Accession: 006286347

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PMID: 934056


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