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In the present study, vasorelaxant effect of the extract of seeds of Oenothera odorata (SOO) and its possible mechanism responsible for this effect were examined in vascular tissues isolated from rats. Changes in vascular tension, 3',5'-cyclic monophosphate (cGMP) levels were measured in thoracic aorta rings from rats. Methanol extract of seeds of Oenothera odorata relaxed endothelium-intact thoracic aorta in a dose-dependent manner. A dose-dependent vascular relaxation was also revealed by treatment of ethylacetate, n-butanol, and H2O (aqua extract of seeds of Oenothera odorata, ASOO) extracts partitioned from methanol, but not by hexane extract. However, the vascular relaxation induced by ASOO were abolished by removal of endothelium of aortic tissues. Pretreatment of the endothelium-intact vascular tissues with NG-nitro-L-arginine methyl ester (L-NAME) or ¹H-[1,2,4]-oxadiazole-[4,3-α]-quinoxalin-1- one (ODQ) significantly inhibited vascular relaxation induced by ASOO. Moreover, incubation of endothelium-intact aortic rings with ASOO increased the production of cGMP. However, ASOO-induced increases in cGMP production were blocked by pretreatment with L-NAME or ODQ. The vasorelaxant effect of ASOO was attenuated by tetraethylammonium (TEA), 4-aminopyridine, and glibenclamide attenuated. On the other hand, the ASOO-induced vasorelaxation was not blocked by verapamil, and diltiazem. Taken together, the present study demonstrates that ASOO dilate vascular smooth muscle via endothelium-dependent NO-cGMP signaling pathway, which may be closely related with the function of K+ channels.
