In Vitro Vascular Effects of Eucommia ulmoides Oliv: A Plausible Scientific Basis for its Use in the Elimination of Blood Stagnation in Traditional Chinese Medicine (Kanpo)

The vascular effects of three extract preparations from 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 μM phenylephrine (PE), but the methanol extract of the leaf (M) had no effect. Vessels precontracted with KCl and de-endothelialized vessels pre-contracted 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 μM 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. In smaller muscular artery, like rat mesenteric artery, the action of Eucommia extract also involves endothelium derived hyperpolarizing factor in addition to NO. Endothelium-dependent relaxation elicited by Eucommia extract also occur in dog carotid artery. This vasodilatory action of E. ulmoides Oliv. provides a pharmacological basis for its well-documented antihypertensive action.