Mechanisms of the vasorelaxant effect of 1, 5-dihydroxy-2, 3-dimethoxy-xanthone, an active metabolite of 1-hydroxy-2, 3, 5-trimethoxy-xanthone isolated from a Tibetan herb, Halenia elliptica, on rat coronary artery.
1, 5-Dihydroxy-2, 3-dimethoxy-xanthone (HM-5) is one of the naturally-occurring xanthones of a Tibetan medicinal herb Halenia elliptica. Recently, it has been shown that HM-5 is one of the phase I metabolites of 1-hydroxy-2, 3, 5-trimethoxy-xanthone (HM-1), the major active component of H. elliptica with potent vasorelaxant actions. This study investigated the vasorelaxant effect of HM-5 and its mechanism(s). HM-5 (0.35-21.9 microM) produced a concentration-dependent relaxation in rat coronary artery rings pre-contracted with 1 microM 5-hydroxytryptamine (5-HT), with an EC(50) of 4.40+/-1.08 microM. Unlike HM-1, the effect of HM-5 was endothelial-independent such that removal of the endothelium did not affect its vasodilator potency. Nitric oxide synthase (NOS) inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME, 100 microM), the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-alpha] quinoxalin-1-one (ODQ, 10 microM) did not affect the vasodilatory effects of HM-5, thus confirming the non-involvement of endothelium related mechanisms. In endothelium-denuded coronary artery rings, the vasorelaxant effect of HM-5 was inhibited by a potassium channel blocker, TEA (10 mM), and 4-aminopyridine (4-AP, a K(v) blocker; 1 mM) but not by other K+ channel blockers such as iberiotoxin (100 nM), barium chloride (100 microM) and glibenclamide (10 microM). The involvement of Ca2+ channel was studied in artery rings pre-incubated with Ca2+-free buffer (intact endothelium or endothelium-denuded) and primed with 1 microM 5-HT or 60 mM KCl prior to the addition of CaCl2 to elicit contraction. In the 5-HT-primed preparations, HM-5 (34.7 microM) significantly inhibited the CaCl(2)-induced vasoconstriction (89.9% inhibition in intact endothelium artery rings; 83.3% inhibition in endothelium-denuded rings). In the KCl-primed preparations, HM-5 (34.7 microM) produced a 34% inhibition in endothelium-denuded rings. The same concentration of HM-5 inhibited (by 62.3%) the contractile response to 10 microM phorbol 12, 13-diacetate (PDA), a protein kinase C activator, in Ca2+-free solutions. Taken together, this study showed that the mechanisms of the vasorelaxant effects of HM-5 were distinctly different from those of its parent drug HM-1. The vasorelaxant effect of HM-5 was mediated through opening of potassium channel (4-AP) and altering intracellular calcium by partial inhibition of Ca2+ influx through L-type voltage-operated Ca2+ channels and intracellular Ca2+ stores.
School of Chinese Medicine, The Chinese University of Hong Kong, Hong Kong, China.
Yan
Y
Wang
Jian-Gong
JG
Shi
Mu-Zou
MZ
Wang
Chun-Tao
CT
Che
John H K
JH
Yeung
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism / Q000502:physiology; D003331:Coronary Vessels / Q000187:drug effects* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D029965:Gentianaceae / Q000737:chemistry*; D066298:In Vitro Techniques; D008297:Male; D038861:Medicine, Tibetan Traditional; D015221:Potassium Channels / Q000378:metabolism / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D014664:Vasodilation / Q000187:drug effects*; D044004:Xanthones / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology*
C527148:1,5-dihydroxy-2,3-dimethoxyxanthone; C478004:1-hydroxy-2,3,5-trimethoxyxanthone; D015220:Calcium Channels; D015221:Potassium Channels; D014662:Vasoconstrictor Agents; D044004:Xanthones; D002118:Calcium