| Abstract: | Injection of frogs with beta-adrenergic catecholamines for 1-24 hr produces marked subsensitivity of the erythrocyte membrane adenylate cyclase [ATP pyrophosphate-lyase (cyclizing); EC 4.6.1.1.] to in vitro stimulation by isoproterenol. The subsensitization is specific for catecholamine stimulation, since basal and fluoride-stimulated enzyme activity are unaffected. Maximum isoproterenol-stimulated adenylate cyclase activity declines by 75% in the isoproterenol-treated animals (P less than 0.001). The concentration of isoproterenol causing one-half maximal activation of adenylate cyclase, however, is unaltered. (-)[3H]Alprenolol, a potent competitive beta-adrenergic antagonist, was used to study directly the beta-adrenergic receptor binding sites in the erythrocyte membranes from control and subsensitized animals. A highly significant (P less than 0.005) 60% fall in the number of the beta-adrenergic receptor binding sites ("specific"(-)[3H]alprenolol binding sites) in the treated animals was found. The binding affinity of the sites was not markedly altered. These data suggest that beta-adrenergic catecholamines are able to regulate catecholamine sensitivity of tissues in vivo, by regulating the properties of the beta-adrenergic receptor binding sites. |
