Differences in myocardial alpha- and beta-adrenergic receptor numbers in different species

In the present study, we tested the hypotheses that 1) different species have different myocardial adrenergic receptor numbers and 2) selected "slow-channel" calcium antagonists compete with alpha-adrenergic antagonists for binding to varying degrees in different species. The data obtained in the present study demonstrate that there is a markedly decreased number of alpha 1-adrenergic and increased number of beta-adrenergic receptors in canine compared with rabbit and rat myocardium. The differences in adrenergic receptor numbers exist without major differences in alpha 1-adrenergic receptor affinity in the species studied. There was no significant difference in left ventricular or plasma catecholamine content between the rat and dog. Selected slow-channel calcium antagonists compete for alpha 1-adrenergic receptor binding in rabbit, rat, and canine myocardium. However, only in rabbit myocardium does verapamil antagonize alpha 1-adrenergic receptor binding at moderate concentrations, whereas verapamil in canine and rat myocardium and 1) 600 in all three species antagonize alpha 1-adrenergic receptor binding only at relatively high concentrations. Nifedipine, a dihydropyridine-type slow-channel calcium antagonist, had no effect on prazosin binding to rat, rabbit, and dog myocardial membranes.