This study aimed to investigate the relaxant effect of chroman compound HEF-04 on isolated vascular smooth muscle （VSM） and the possible underlying mechanisms involved. Isolated rabbit thoracic aorta was used as the in vitro model and the relaxant effects of HEF-04 on endothelium-intact （＋EC） and endothelium-denuded （EC） thoracic aortic rings were compared. Potassium channel blockers, guanylate cyclase inhibitors, and COX-inhibitors were used to explore associations between the relaxant effects of HEF-04 with potassium channels, NO, and prostaglandin-like substances and endothelial hyperpolafizing factor （EDHF）, respectively. The results indicated that HEF-04 （1 × 10^-5 mol/L to 3× 10^-3 mol/L） had no significant impact on basal vascular tension, but could relax the contraction of vascular smooth muscle caused by high-K＋ solution in a dose-dependent manner. Indomethacin （5.6× 10^-6 mol/L） had no effect on vasorelaxation effect of HEF-04. In contrast, the vasorelaxant effect of HEF-04 was enhanced by methylene blue, which was significantly inhibited by calcium-dependent potassium channel blocker TEA. The vasorelaxant effect of HEF-04 on ＋EC thoracic aortic rings was significantly stronger than that on -EC thoracic aortic rings. The endothelium dependent relaxant effect of HEF-04 on VSM might be attributed to the interaction of HEF-04 with vascular relaxing factors or the increased release of EDHF.
