In this study, six major xanthones, isolated and identified from Halenia elliptica
were investigated for their vasodilatory actions in isolated rat coronary artery.
The xanthones, including 1-hydroxy-2,3,5-trimethoxy-xanthone (HM-1), 1-hydroxy-2,3,4,7-tetramethoxy-xanthone
(HM-2), 1-hydroxy-2,3,4,5-tetramethoxy-xanthone (HM-3), 1,7-dihydroxy-2,3,4,5-tetramethoxy-xanthone
(HM-4), 1,5-dihydroxy-2,3-dimethoxy-xanthone (HM-5) and 1,7-dihydroxy-2,3-dimethoxy-xanthone
(HM-7) caused vasodilation in the coronary artery pre-contracted with 1 microM 5-hydroxytryptamine
(5-HT), with EC(50) values ranging from 1.4+/-0.1 microM (HM-1) to 6.6+/-1.4 microM
(HM-2). The EC(50) values of the other xanthones were between those of HM-1 and HM-2.
Removal of endothelium of the coronary artery led to decreases in the vasorelaxant
effects of HM-1, HM-7 but not HM-2, HM-3, HM-4 and HM-5. Our results showed that xanthones
isolated from Halenia elliptica are vasoactive substances which exhibit either endothelium-dependent
or endothelium-independent mechanisms in rat coronary artery. The potency and mechanism(s)
of the vasorelaxant effects of these xanthones may be relevant to the structure-activity
differences in the level and the position of the substituent groups with the primary
xanthone structure.