THYROID HORMONE‐DEPENDENT INTERCONVERSION OF MYOCARDIAL α‐ AND β‐ADRENOCEPTORS IN THE RAT

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Abstract
1 The effects of thyroid state on the properties of adrenoceptors mediating inotropic and chronotropic responses of the rat heart were assessed on the basis of the relative potencies of α- and β-adrenoceptor agonists, the effects of α- and β-adrenoceptor antagonists and the tissue uptake of [3H]-phenoxybenzamine ([3H]-PB). 2 In isolated, electrically driven left atria the ratio of the inotropic potencies of isoprenaline and phenylephrine and the inhibitory potency of propranolol (40nM-4 μm) were significantly reduced after thyroidectomy and were moderately increased after thyroxine treatment of control rats. 3 Block of inotropic responses to noradrenaline and to phenylephrine by PB (7.3nM-7.3 μm) and the tissue uptake of [3H]-PB were significantly greater in preparations from thyroidectomized than in those from control or from thyroxine treated rats. α-Adrenoceptor inhibition by phentolamine (0.26-2.6 μm) also increased after thyroidectomy, and phentolamine effectively protected α-adrenoceptors from block by and binding of [3H]-PB. 4 The β1-receptor antagonist H 93/26 (0.1 μm) significantly potentiated α-adrenoceptor blockade by PB in hypothyroid but not in control preparations. 5 In spontaneously beating right atria the chronotropic potency of agonists and the effects of antagonists were altered in the same way as were inotropic responses and the slope of the agonist concentration-response curves were significantly reduced after thyroidectomy. Effects of agonists and antagonists were not significantly influenced by thyroxine treatment. 6 Changes in the effects and tissue uptake of sympathomimetic drugs observed after thyroidectomy were reversed to or beyond control levels by thyroid hormone treatment of thyroidectomized animals. 7 The results presented are interpreted as indicating a thyroid hormone-dependent interconversion of myocardial α- and β-adrenoceptors. It is suggested that this interconversion is similar to that observed earlier in frog hearts at different temperatures, and that both effects may reflect an allosteric transition between two forms of a single basic structure.