VASCULAR REACTIVITY TO VASOCONSTRICTORS IN AORTA AND RENAL VASCULATURE OF HYPERTHYROID AND HYPOTHYROID RATS

Vascular reactivity to vasoconstrictors in relation to altered thyroid function was studied in two preparations: aortic strips and the isola ted perfused kidney. To assess whether the possible alterations in vas cular reactivity were restricted to a specific agonist or whether they involved the contractile system, receptor-mediated and nonspecific sm ooth muscle stimulants were used. Male Wistar rats were divided into t hree groups: control, hyperthyroid and hypothyroid rats. Aortic strips from hypothyroid rats were less sensitive to phenylephrine and KCl wh en the data were expressed in absolute values or as percentages of the maximum responses. Sensitivity and reactivity in strips from hyperthy roid rats were similar to those observed in control strips. Renal vasc ulature obtained from hypothyroid rats also showed a markedly reduced sensitivity to phenylephrine, with normal maximal responses. The respo nse to vasopressin at 3.10(-11) mol/l was also decreased, as was the r eactivity to barium chloride. In contrast, renal vasculature of hypert hyroid rats showed markedly enhanced reactivity to all agonists: the c oncentration-response curves were characterized by a similar threshold and a greater maximal response. These results demonstrate that hypoth yroidsm is accompanied by a marked decrease in sensitivity to vasocons trictors in large arteries as well as in resistance vessels. This decr ease may be secondary to a generalized alteration in the contractile s ystem of vascular smooth muscle cells and may play a role in the decre ased blood pressure in these animals. In contrast, isolated perfused k idneys of hyperthyroid rats showed increased vascular reactivity to va soconstrictors, which may play a role in the maintenance of elevated b lood pressure in these animals.