Relaxing effects of Ligstrum purpurascens extract and purified acteoside in rat aortic rings

The present study describes the effects of an extract obtained from the leaves of Ligstrum purpurascens and acteoside purified from the extract on the contractile response to various agonists in rat isolated aortic rings. L. purpurascens extract relaxed 9,11-dideoxy-11α,9α-epoxymethanoprostaglandin F_2α (U46619)-preconstricted rings in a concentration-dependent manner (IC₅₀:0.14 ± 0.01 mg/ml with endothelium and 0.16 ± 0.01 mg/ml without endothelium). The extract also reduced contraction induced by 35 mM K⁺ or by 1 μM phorbol 12,13-diacetate (PDA) in endothelium-intact rings. The extract (0.1-0.3 mg/ml) reduced the concentration-response to U46619 in normal Krebs solution or to CaCl₂ in 35 mM K⁺-containing solution. Acteoside accounts for 2.05% of total L. purpurascens extract in weight. Acteoside induced relaxation of rings preconstricted by U46619 (IC₅₀:0.22 ± 0.01 mg/ml) but it caused an increase in 35 mM K⁺-induced tone. Removal of endothelium enhanced the relaxing effect of acteoside. Besides, pretreatment with acteoside inhibited endothelium/nitric oxide-mediated relaxation induced by acetylcholine. These results indicate that acteoside is unlikely the major ingredient responsible for the vasodilator effect of L. purpurascens extract. The extract relaxed the preconstricted aortic rings probably through multiple mechanisms by acting on smooth muscle cells. The inhibitory effect on endothelial nitric oxide-mediated relaxation suggests that acteoside could also act on the endothelial cells to reduce nitric oxide release.