Effects of thyroid state on adrenoceptor properties

Abstract

RECENT studies with frog hearts have shown that the adrenoceptors that mediate inotropic responses are altered qualitatively by ambient temperature^1–3. The characteristics of these receptors changed from β to α when the temperature of the isolated hearts was reduced through a critical range of 17°–22° C, which suggested that α and β adrenoceptors represent allosteric conformations of the same structure³. Although a change in adenoceptor characteristics qualitatively similar to that found in the frog heart seems to occur in the mammalian myocardium^1,4,5, the physiological significance of a temperature-induced change is obviously limited in homeothermic species. But, several observations indicate that conditions which promote α-adrenoceptor properties, including cold^1–3, iodoacetate or fluoroacetate⁶ or dinitrophenol¹, inactivity of skeletal muscle⁷ and increased vagal influence on the myocardium⁸, are associated with a decrease in metabolic activity. Therefore, it seemed possible that hypothyroidism would produce similar changes in receptor properties. We report here that the characteristics of adrenoceptors mediating inotropic responses in hypothyroid rats are similar to those in normal hearts at low temperatures.