Quantitative relationship between β-adrenergic receptor number and physiologic responses as studied with a long-lasting β-adrenergic antagonist

Abstract

The aminobenzyl analog of propranolol, 1-(p-amino-.alpha.,.alpha.-dimethylphenethylamino)-3-(l-naphthoxy)-2-propranol, was synthesized and found to be a potent .beta.-adrenergic blocking agent. The .beta.-adrenergic receptors of cultured rat C6 glioma cells (2B clone) as assessed by [125I]iodohydroxybenzylpindolol binding were decreased 50 and 95% after pretreatment with 8 nM and 1 .mu.M aminobenzylpropranolol, respectively. Unlike propranolol, aminobenzylpropranolol displayed a prolonged blockade of receptors that was maintained during several hours of washing. [125I]Iodohydroxybenzylpindolol saturation binding experiments in cells exposed to aminobenzylpropranolol and subsequently washed indicated that the compound effectively diminished receptor number with no change in the affinity of the remaining receptors for iodohydroxybenzylpindolol. Aminobenzylpropranolol inhibited catecholamine-stimulated intracellular cyclic[c]AMP accumulation; with increasing blockade, isoproterenol dose-response curves became progressively shifted to the right but the maximal response was unaltered. Aminobenzylpropranolol inhibited the .beta.-adrenergic contractile response in atria isolated from rats and guinea pigs. Treatment with 0.1 and 10 .mu.M aminobenzylpropranolol produced decreases of 0.5 and 2 orders of magnitude in the contractile potency of isoproterenol. As in glioma cells, aminobenzylpropranolol failed to decrease the maximal response to isoproterenol. The effects of aminobenzylpropranolol persisted during extensive washing of atria (up to 17 h). Repeated exposures to isoproterenol at concentrations sufficient to produce maximal tension development also failed to alleviate the blockade. The inotropic potency of histamine in guinea pig atria was not affected by aminobenzylpropranolol. Apparently catecholamines are capable of eliciting full biological responses in glioma cells and isolated atria even though the great majority of .beta.-adrenergic receptors are persistently blocked.