Dissociation of Cardiac Inotropic and Adenylate Cyclase Activating Adrenoceptors

1 At higher temperatures, near the physiological range for mammals and nonhibernating frogs, the adrenoceptors for both inotropic responses to adrenaline and noradrenaline and for cyclic 3',5'-adenosine monophosphate (cyclic AMP) production in rat and frog isolated heart preparations, had typical beta characteristics. Phenoxybenzamine potentiated the inotropic response and the accumulation of cyclic AMP; conversely, propranolol inhibited the two responses.2 When the ambient temperature was reduced, the adrenoceptors mediating cyclic AMP production changed very little; they were blocked as effectively as at the higher temperature by propranolol and were not blocked by phenoxybenzamine. However, the adrenoceptors mediating the inotropic response were markedly changed by the decrease in temperature; phenoxybenzamine now inhibited this response and the inhibitory activity of propranolol was reduced about tenfold.3 These results indicate that the adrenoceptors that mediate cardiac inotropic responses at physiological temperatures are distinct from those that mediate the production of cyclic AMP, and that the activation of adenylate cyclase and the accumulation of cyclic AMP are probably not intermediate steps in cardiac inotropic responses to catecholamines.