On spare beta-adrenoceptors of inotropic effect of catecholamines in kitten ventricle.

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