beta-adrenergic receptors in rat liver: effects of adrenalectomy

The response of rat liver adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] to catecholamines is enhanced after adrenalectomy. To investigate this phenomenon, we developed an in vitro assay for beta-adrenergic receptors of plasma membranes derived from livers of control and adrenalectomized rats, using [125I]iodohydroxybenzylpindolol (IHYP), a potent beta-adrenergic receptor antagonist. Binding of IHYP reached equilibrium within 30 min and dissociation occurred with a half-time of approximately 60 min. The l-isomers of isoproterenol and propranolol were at least 50 times more potent as inhibitors of IHYP binding than were the corresponding d-isomers. Adrenalectomy did not affect the rates of association or dissociation of IHYP or the dissociation constants of several ligands that are active at beta-adrenergic receptors. The number of binding sites for IHYP was determined in homogenates and in purified membranes of livers from control and adrenalectomized rats. The number of sites increased 3- to 5-fold after adrenalectomy. A similar increase in hormone stimulation of adenylate cyclase was observed. These changes were reversed by the administration of cortisone. The increase in the number of binding sites for IHYP may be a compensatory response to the impairments in gluconeogenesis and glycogenolysis which occur after adrenalectomy.