Changes in vascular reactivity induced by acute hyperthyroidism in isolated rat aortae

Hyperthyroidism was induced by subcutaneous injections of l-thyroxine (T₄) (500 mg/kg/day) for 3 days in order to study whether adrenergic and muscarinic receptor-mediated vascular responses alter at an early stage of the disease. T₄ treatment was sufficient to induce a significant degree of thyroid weight loss, tachycardia, cardiac hypertrophy, and an elevation in serum T₄ levels. The tension of aortic ring preparations isolated from rats was measured isometrically to investigate the influence of acute hyperthyroidism. The contractions induced by norepinephrine (NE) were significantly suppressed in aortic rings from rats treated with T₄ compared with control rats. N^G-nitro-l-arginine (l-NOARG), an inhibitor of nitric oxide synthase (NOS), significantly enhanced NE-induced contraction in aortic rings from both control and T₄-treated rats, and the enhancement was greater in rats treated with T₄ than control rats. The relaxations induced by either acetylcholine (ACh) or sodium nitroprusside (SNP) were also significantly enhanced by T₄ treatment. l-NOARG abolished the relaxation induced by ACh in aortic rings from both control and T₄-treated rats. l-NOARG shifted SNP-induced relaxation curves of aortic rings from those of control rats to the left, but not with rats treated with T₄. T₄ treatment showed no influence on the amount of endothelial NOS (eNOS) protein. These results suggest that vascular responses alter at an early stage of hyperthyroidism and that it may be due to a modification in the NO system which is independent from the amount of eNOS protein.