Pharmacological experiments demonstrate that toad (Bufo marinus) atrial beta-adrenoceptors are not identical with mammalian beta 2- or beta 1-adrenoceptors.

Chronotropic responses to sympathomimetic amines of isolated atrial preparations from toads (Bufo marinus) were mediated by beta-adrenoceptors since isoprenaline was more potent than adrenaline and noradrenaline, and propranolol was a potent antagonist (pA2, adrenaline as agonist = 9.33). The beta-adrenoceptors had some of the characteristics of mammalian beta 2-adrenoceptors in that (i) adrenaline was more potent than noradrenaline and (ii) the pA2 values of two selection beta-adrenoceptor antagonists, atenolol (pA2 = 5.84) and alpha-methylpropranolol (pA2 = 8.42), were close to the values reported on beta 2-adrenoceptors in mammalian tissues. However, the relative potencies of adrenaline, isoprenaline, noradrenaline, rimiterol, salbutamol and fenoterol (1 : 45.8 : 0.07 : 3.3 : 1.05 : 0.32) did not correspond to the relative potencies reported for these agonists on mammalian tissues which contain predominantly beta 2-adrenoceptors. Also the pA2 value for the beta 2-selective antagonist, ICI 118,551 (7.89, adrenaline as agonist) was lower than its reported pA2 on beta 2-adrenoceptors in mammalian tissues. There was no evidence that the response was mediated by both beta 1- and beta 2-adrenoceptors since Schild plots for ICI 118,551 using three agonists of differing selectivity (adrenaline, rimiterol and noradrenaline) were superimposed. It is concluded that, although toad atrial beta-adrenoceptors have several characteristics in common with beta 2-adrenoceptors in mammalian tissues, these amphibian beta-adrenoceptors are not identical with mammalian beta 2-adrenoceptors.