Simulated microgravity enhances vasoconstrictor responsiveness of rat basilar artery.

Recently, hypertrophy and increased myogenic tone of brain vessels have been observed in rats after simulated microgravity. It is expected that simulated microgravity may also induce hyperreactivity of brain vessels. To test this hypothesis, Sprague-Dawley rats were subjected to a 4-wk tail-suspended hindlimb unloading (TS) to simulate the cardiovascular deconditioning effect of microgravity. After 4 wk, the vasoreactivity of isolated basilar arterial rings from TS rats to both receptor- and non-receptor-mediated vasoconstrictors, such as KCl, arginine vasopressin, or 5-hydroxytryptamine (5-HT), and vasodilators such as ACh, thrombin, adenosine, or sodium nitroprusside were examined and compared with those from simultaneous control (Cn) rats. In the first part of this study, it was found that the maximal isometric contractile responsiveness evoked by vasoconstrictors such as KCl, arginine vasopressin, or 5-HT was enhanced in basilar arterial rings from TS rats, whereas vasodilatory responsiveness to vasodilators showed no significant difference between TS and Cn rats. In the second part of this study, it was found that removal of the endothelium had no effects on the contractile responsiveness to 5-HT in basilar arterial rings from TS rats but enhanced markedly the responsiveness in basilar arterial rings from Cn rats to an extent comparable with that of TS rats. Application of tetraethylammonium also had no effects on the contractile response to 5-HT in basilar arterial rings from TS but significantly increased the responsiveness of basilar arterial rings from Cn rats with endothelium intact. These results showed that 4-wk simulated microgravity enhanced the vascular contractile responsiveness of basilar arterial rings to both receptor- and non-receptor-mediated vasoconstrictors, and the enhancement of 5-HT-induced contraction in TS rat basilar arteries was due to an impairment of endothelium-dependent mechanism. These results suggest that endothelium-derived hyperpolarizing factors are responsible for this endothelium-dependent attenuating modulatory mechanism in contractile responsiveness of rat basilar arteries to 5-HT.