Endothelium-dependent vasorelaxant effects of the aqueous extracts of the Eucommia ulmoides Oliv. leaf and bark: implications on their antihypertensive action

The vascular effects of three extract preparations from the Chinese medicinal herb, Eucommia ulmoides Oliv., which is historically an active ingredient commonly used in antihypertensive herbal prescriptions in China, were investigated with isometric contraction using isolated rat aortic and dog carotid rings. Both aqueous extracts isolated from eucommia leaf (L) and bark (B) concentration dependently caused endothelium-dependent relaxation in vessels precontracted with 1 microM phenylephrine (PE), but the methanol extract of the leaf (M) had no effect. Vessels precontracted with KCl and de-endothelialized vessels precontracted with PE were not affected by B or L. The endothelium-dependent relaxation evoked by B and L was either abolished or substantially inhibited by NG-nitro-L-arginine methyl ester (L-NAME) and methylene blue (MB), indicating the involvement of the nitric oxide (NO) synthase pathway in the vasorelaxant action of B and L. The relaxation to the aqueous extract of eucommia bark was not inhibited with 1 microM atropine, but was inhibited by 3-5 mM tetraethylammonium (TEA) and 3 mM 4-aminopyridine. This suggests that the endothelium-dependent, NO-mediated relaxation evoked by the aqueous eucommia extracts was not mediated via the activation of endothelium muscarinic receptors and may involve the activation of K+ -channels. Results in this study have provided the first evidence on the in vitro vasorelaxant action of E. ulmoides Oliv. that forms the pharmacological basis for its well-documented antihypertensive action.