Mechanisms underlying the relaxant effect of galetin 3,6-dimethyl ether, from Piptadenia stipulacea (Benth.) Ducke, on Guinea-Pig Trachea

Abstract

Galetin 3,6-dimethyl ether (FGAL), a flavonoid from the aerial parts of Piptadenia stipulacea (Benth.) Ducke, was found to exert a relaxant effect on carbachol (CCh)-pre-contracted Guinea-pig trachea. Based on cumulative concentration-response curves to CCh, FGAL antagonized muscarinic receptors pseudo-irreversibly and noncompetitively, since it inhibited and shifted these curves towards higher concentrations in a nonparallel manner. In addition, FGAL was more potent in relaxing contractions induced by 18 mM as compared to 60 mM KCl (pD2 = 5.50±0.36 and 4.80±0.07, respectively), indicating the participation of K+ channels. In the presence of 10 mM tetraethylammonium (TEA+) chloride, a nonselective K+ channel blocker, the relaxant potency of FGAL was reduced (from pD2 = 5.12±0.07 to 4.87±0.02). Among several selective blockers of K+ channel subtypes, only apamin, an SKCa (small-conductance Ca2+-activated K+ channels) blocker, attenuated the relaxant potency of FGAL (pD2 = 4.85±0.06), suggesting SKCa activation. FGAL was equipotent in relaxing trachea contracted by 60 mM KCl (pD2 =4.80±0.07) or 10-6 M CCh (pD2 = 5.02±0.07), suggesting CaV (voltage-gated calcium channel), but not ROCs (receptor-operated calcium channels) participation. Furthermore, aminophylline-induced relaxation (pD2 = 4.12±0.06) was potentiated around 4-fold (pD2 = 4.80±0.44) in the presence of FGAL. Moreover, forskolininduced relaxation (pD2 = 6.51±0.06) was potentiated around 2.5-fold (pD2 = 6.90±0.05) by FGAL. Conversely, sodium nitroprusside-induced relaxation was unaffected, indicating that the AC=cAMP=PKA pathway, but not the NO pathway, may be modulated by the flavonoid. These results suggest that, in Guinea-pig trachea, FGAL induces relaxation by pseudo-irreversible noncompetitive antagonism on muscarinic receptors, modulation of K+ and Ca2+ channels, as well as activation of the AC=cAMP=PKA pathway.