Catecholamine binding to the beta-adrenergic receptor.

The adenylate cyclase-coupled beta-adrenergic receptors of frog erythrocyte membranes have been identified by direct radioligand binding techniques using the potent catecholamine agonist (+/-)[3H]hydroxybenzylisproterenol (2-[3, 4-dihydroxyphenyl]-2-hydroxy-1', 1'-dimethyl-2'-[4-hydroxyphenyl]-diethylamine). The successful experimental conditions included the use of (i) high concentrations of catechol and ascorbic acid to suppress nonreceptor binding, (ii) a very potent radiolabeled catecholamine (10 times more potent than isoproterenol), and (iii) membranes rich in binding sites for beta-adrenergic receptors. Thus, previous problems in accomplishing successful catecholamine binding to the beta-receptors have been overcome. The binding sites identified with (+/-)[3H]hydroxybenzylisoproterenol in the erythrocyte membranes have all the characteristics expected of true beta-adrenergic receptors. These include rapidity of binding, saturability, specificity for beta-agonists and antagonists, and stereospecificity [(-)isomers more potent than (+)isomers]. Physiologically inactive compounds containing a catechol moiety do not compete for occupancy of these binding sites. Dissociation of the radiolabeled agonist from the receptors is slow and incomplete in the absence of guanine nucleotides. In the presence of nucleotide, however, dissociation is rapid and complete. beta-Adrenergic agonists and antagonists compete for the (+/-)[3H]hydroxybenzylisoproterenol binding sites in a fashion parallel to their competition for the receptors, as previously delineated with the beta-adrenergic antagonist (-)[3H]dihydroalprenolol.