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

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Abstract
1 The effect of prolonged, excessive thyroxine on β-adrenoceptor-mediated relaxation of guinea-pig trachea was studied. 2 Thyroxine did not significantly affect the potency of the β1-adrenoceptor agonist, noradrenaline, or the β2-adrenoceptor agonist, terbutaline. 3 Thyroxine did not significantly affect the apparent KB values of the selective β1-adrenoceptor antagonist, practolol, or the selective β2-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 β-adrenoceptor-mediated relaxation of guinea-pig trachea and that the reported modulation of β-adrenoceptor-mediated responses in other tissues is specific for the given tissue rather than common to all β-adrenoceptor systems.
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