ENDOTHELIUM-DEPENDENT AND ENDOTHELIUM-INDEPENDENT VASODILATION IN HYPERTHYROID AND HYPOTHYROID RATS

The effects of hyper- and hypothyroidism on the vasorelaxing Endotheli um responses to acetylcholine (ACh), sodium nitroprusside (NP), and Ca Cl2 were investigated in aortic strips and isolated perfused kidneys. The renal vascular reactivity to ACh and NP was increased in hyperthyr oid rats, whereas the concentration-response curve to ACh in hypothyro id rats was flattened. In the renal vasculature from hypothyroid rats, NP produced a dual response: vasoconstriction at low doses and vasodi lation at medium to high doses. Aortic strips from hyperthyroid rats s howed an increased response to ACh without significant differences bet ween hypothyroid and control groups. Aortic strips from all three expe rimental groups showed a similar relaxing response to CaCl2. These res ults indicate that: (1) the raised arterial pressure of hyperthyroid r ats is not associated with a reduced endothelium-dependent and calcium -induced vasodilation, and (2) the changes in responsiveness to vasodi lators in resistance vessels from hyper- and hypothyroid rats may play a role in the increased and decreased peripheral vascular resistances , respectively, previously reported in such animals.