Adrenoreceptor‐mediated Responses in the Isolated Portal Vein of the Hypothyroid Rat

Abstract

1 The effect of hypothyroidism, induced by methimazole, upon the response of the isolated portal vein of the rat to adrenoceptor agonists and antagonists, to angiotensin II and to papaverine has been investigated. 2 The positions and maxima of log doses-response curves, to the vasoconstrictor agonists, noradrenaline and angiotensin II, and to the vasodilator agonists isoprenaline and papaverine, were unaffected by methimazole treatment. 3 The alpha-adrenoreceptor antagonist phentolamine competitively inhibited the effects of noradrenaline to a similar extent in preparations from control and hypothyroid animals. The competitive antagonism of isoprenaline by the beta-adrenoreceptor antagonist propranolol was similarly unaffected by hypothyroidism. 4 These results taken together indicate that hypothyroidism is without significant effect upon either the properties of postjunctional alpha- and beta-adrenoreceptors in the rat portal vein, or upon the reactivity of this blood vessel to the vasoactive agonists studied.

The effects of altered thyroid state upon responses mediated by atrial muscarinic receptors in the rat

“…Thyroxine administration caused elevation of circulating T3 and T4 levels, and increases in both left and right atrial weights (Table 1). Methimazole administration produced the opposite effects (Table 1) and in addition, a marked decrease in the rate of gain of body weight, as found previously (Yoong et al, 1982). The administration of thyroxine to methimazole-treated rats prevented the decline in circulating T4 and T3 levels.…”

“…(Hershman & van Middlesworth, 1962;Chopra, 1977;Aizawa & Yamada, 1981). Thus we were able, by co-administering methimazole and thyroxine, to establish an additional control group of animals which were chemically and physiologically euthyroid (Yoong et al, 1982;. We have discussed elsewhere the advantages of the use of either methimazole or propylthiouracil over thyroidectomy .…”

“…In contrast, the affinity of the right and left atrial muscarinic receptors, for the competitive muscarinic antagonist, atropine, remain unchanged in altered thyroid state. In this study, as in three related studies from this laboratory (Yoong et al, 1982; in which the effects of altered thyroid state upon atrial and vascular adrenoceptor-mediated responses were examined, we have used methimazole to induce the hypothyroid state. Methimazole is water soluble and has a longer biological half life than propylthiouracil (Pittman, Beschi & Smitherman, 1971).…”

“…A line of best fit and a pA2 value was obtained from this plot for each preparation. For further analysis, results from all experiments with one tissue from each experimental group were pooled, to allow estimation of pA2 values and slopes, together with 95 % confidence limits, as described previously (Yoong, Story, Ishac, Pennefather & Handberg, 1982).…”

The circadian variation of blood pressure and heart rate in patients with hyperthyroidism.

“…Therefore, T3 level may be closely related to the diurnal variation of SBP through its effect on the heart (Klein and Levey 1984; Feldman et al 1986). On the other hand, it has been reported that the vascular responsiveness mediated by a-receptors is unaffected by the thyroid state (Yooung et al 1982), which may maintain a nocturnal fall in DBP in patients with hyperthyroidism.…”