Additional evidence against universal modulation of beta-adrenoceptor responses by excessive thyroxine.

1 The effect of prolonged, excessive thyroxine on beta-adrenoceptor-mediated relaxation of guinea-pig trachea was studied. 2 Thyroxine did not significantly affect the potency of the beta 1-adrenoceptor agonist, noradrenaline, or the beta 2-adrenoceptor agonist, terbutaline. 3 Thyroxine did not significantly affect the apparent KB values of the selective beta 1-adrenoceptor antagonist, practolol, or the selective beta 2-adrenoceptor antagonist, butoxamine. 4 Thyroxine did not significantly affect the maximum response to noradrenaline. The maximum response to terbutaline in tissues from the thyroxine-treated animals was only slightly lower than the maximum response in tissues from paired control animals. 5 These results suggest that excessive thyroxine does not significantly affect the beta-adrenoceptor-mediated relaxation of guinea-pig trachea and that the reported modulation of beta-adrenoceptor-mediated responses in other tissues is specific for the given tissue rather than common to all beta-adrenoceptor systems.