On spare ?-adrenoceptors for inotropic effects of catecholamines in kitten ventricle

Abstract

β-Adrenoceptors of ventricular membranes of kitten heart were labelled reversibly with (-)-³H-propranolol (binding constant 6 nM). Bound (-)-³H-propranolol was displaced with β-adrenoceptor blocking agents and catecholamines. Displacement constants K _D(M) of the blockers were of the same of magnitude as their constants K _B obtained from the antagonism of inotropic effects of catecholamines in papillary muscels. K _D's of catecholamines were, however, 2 orders of magnitude larger than concentrations causing half maximal inotropic effects, indicating large spare β-adrenoceptor capacity.