Enhancement of phenylephrine-induced contraction in the isolated rat aorta with endothelium by H2O-extract from an Oriental medicinal plant Leonuri herba.

Leonuri herba (I-mu-ts'ao, the Chinese motherwort) is an ancient Chinese traditional herb. Although the pharmacological effects of extracts of Leonuri herba have been shown in platelets and uteri, the effect on the vascular system has not been determined. In the present study, we investigated the effects of extracts of Leonuri herba on the contraction of the isolated rat aorta. Although the H20-extract (0.3-3 mg/ml) by itself showed a limited effect, the extract enhanced phenylephrine-induced contraction of the aorta with endothelium, but not without endothelium. The H20-extract, like N(G)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase), significantly inhibited the relaxation induced by acetylcholine in the aorta with endothelium. The inhibitory effect of H20-extract on the relaxation decreased by co-addition with 1 mM L-arginine. The vasoconstrictive effect of H20-extract was not due to leonurine, which is a constituent in Leonuri herba and shows uterotonic activity. Intravenous injection of the H20-extract (1.5 mg/kg) to rats caused an increase in blood pressure for 5 min, like L-NAME (1.35 mg/kg). These findings suggest that there is a component(s) in Leonuri herba, which shows a vasoconstrictive activity in rat aorta in vitro and in vivo and has similar pharmacological profile to that of L-NAME.