Effects of hyperthyroidism on vascular contractile and relaxation responses

Previous research has shown that skeletal muscle blood flow, at rest and during muscular contractions, is elevated in the hyperthyroid state. We hypothesized that reduced vascular contractile and enhanced endothelium-dependent relaxation responses contribute to these observations. To test these hypotheses, male rats were administered triiodothyronine (Hyper, n = 27; 300 micrograms/kg) for 6-12 wk. Compared with euthyroid control rats (Eut, n = 27), Hyper exhibited left ventricular hypertrophy (Eut, 2.01 +/- 0.04 mg/g body wt; Hyper, 2.70 +/- 0.06; P < 0.0005) and greater oxidative enzyme activity in several skeletal muscles (all P < 0.0005). Vascular rings, 2-3 mm in axial length, were prepared from abdominal aortas, and responses to vasoactive agents were determined in vitro. Compared with Eut, vascular rings with intact endothelium from Hyper exhibited reductions in contractile responses to norepinephrine (NE) across a range of NE concentrations (P < 0.05). Maximal tension developed in response to NE was reduced approximately 30% in hyperthyroidism (Eut, 3.8 +/- 0.2 g; Hyper, 2.6 +/- 0.4; P < 0.01). Contractile responses to NE were not different between Eut and Hyper in rings denuded of endothelium. Maximal vasorelaxation responses to acetylcholine (ACh), after precontraction with NE (10(-7) M), were enhanced in the hyperthyroid state (Eut, 65.1 +/- 4.8%; Hyper, 84.0 +/- 7.1; P < 0.05). Enhanced vasorelaxation to ACh was also observed when precontraction was induced by prostaglandin F2 alpha. These findings indicate that vascular contractile and relaxation responses are altered in male hyperthyroid rats.