AIM To study why
Astragalus membranaceus (AM) c an be used as a hypotensive, the vasomotor effect of AM on rat thoracic
aorta ring s and the underlying mechanism were investigated. METHODS AM 0.01- 100 g·L -1 was cumulatively added into organ bath. Isometric tension of e ndothelium -intact or denuded thoracic aorta rings in basal tension,
preconstricted by KCl or phenylephrine (PE), respectively, was recorded. RESULTS Cumulat ive administration of AM 0.01-100 g·L -1 did not affect the vasomotion of aortic rings with endothelium either in basal tension or preconstricted by KC l. Exposure of endothelium intact rings precontricted by PE to AM at low concent ration (0.1-3.0 g·L -1) induced a significant concentration dependent rel axation, which was inhibited by preincubation with Nω-nitro-L-arginine methyl ester or methylthioninium chloride. But at the high concentration (10-100 g·L -1), AM transiently potentiated the PE evok ed contr action that can be reversed by phosphoramidone. Moreover, a weak vasorelaxant re sponse to AM in endothelium-denuded rings precontracted by PE was observed. CONCLUSION AM has endothelium-dependent diphasic effects of relaxat ion and contraction on aorta rings. The effect of relaxation may be mediated by the NO-guanylyl cyclase pathway, while the contration may be related to end othelin release.