Certain β-blockers can decrease β-adrenergic receptor number: I. Acute reduction in receptor number by tertatolol and bopindolol

Abstract

We have previously reported that a potent new β-blocker, tertatolol, when given at therapeutic doses to healthy volunteers, rapidly reduced the number of human mononuclear leukocyte β-receptors. In the present study, the mechanism of receptor regulation by β-antagonists incubated with target cells in vitro was investigated. Two different cell types (human mononuclear leukocytes and S49 murine lymphoma cells) were used, and β-adrenergic receptors were measured using either the hydrophilic ligand 3H-CGP 12177 (specific for surface receptors) or lipophilic 125I-pindolol (which measures total receptors). In a comparison between β-blockers, tertatolol and bopindolol, but not propranolol and pindolol, were found to rapidly (1 hour at 37°C) reduce the number of β-adrenergic receptors. This was paralleled by a reduction in isoproterenol-stimulated cyclic AMP accumulation. The reduction in receptors was the same whether surface or total receptors were measured; thus, it was not due to receptor sequestration. This effect was not caused by partial agonist activity (bopindolol is a weak partial agonist); in parallel experiments, tertatolol and bopindolol, but not pindolol (potent partial agonist) and isoproterenol (full agonist), reduced β-adrenergic receptors. Finally, this effect was not due to irreversible binding: the receptor reduction induced by the irreversible blocker bromo-acetyl-alprenolol-methane (BAAM) was stable for several hours, while the effect of tertatolol and bopindolol was slowly reversed over the same time course. We suggest that tertatolol and bopindolol have two effects on β-adrenergic receptors: they bind competitively, and then they modify the receptors so that they are no longer available for binding by ligands or catecholamines. We propose that the reduction in β-receptors produced by antagonists is an additional mechanism for β-blockade therapy.