THE INFLUENCE OF HYPERTHYROIDISM ON VASOCONSTRICTOR AND VASODILATOR RESPONSES IN ISOLATED CORONARY AND RENAL RESISTANCE ARTERIES

The influence of hyperthyroidism on the functional vascular responsive ness of isolated coronary and renal resistance vessels was investigate d. Hyperthyroidism was established by feeding rats for 1 and 4 weeks w ith 5 mg/kg L-thyroxine (T-4)-containing rat chow. Preparations of eit her coronary or renal resistance vessels were mounted in an isometric wire myograph. Subsequently, concentration-effect curves were determin ed for the effects of 5-hydroxytryptamine (5-HT), 9,11-dideoxy-11 alph a,9 alpha-epoxymethanoprostaglandin F-2 alpha (U46619) and isoproteren ol in coronary vessels, and for those of methoxamine, U46619 and isopr oterenol in renal vessel preparations. Our results indicate that hyper thyroidism does not induce major changes in the sensitivity of both co ronary and renal resistance vessels towards 5-HT, U46619 and methoxami ne. A clearly sensitizing influence of acute hyperthyroidism (1 week o f T-4 treatment) was found for isoproterenol-induced relaxant response s, whereas hyperthyroidism for 4 weeks did not influence the responses mediated by isoproterenol in coronary resistance arteries. Furthermor e, the isoproterenol-induced relaxation in renal arteries was not infl uenced by the chronic hyperthyroid state of the animal. The present re sults indicate that in acute hyperthyroidism beta-adrenoceptormediated vasodilation is increased. However, in chronic hyperthyroidism change s in responsiveness to vasoconstrictor or vasodilator agents of corona ry and renal resistance arteries appear not to play a major role. The influence of hyperthyroidism on the functional response of resistance arteries appears to be both tissue and time dependent.