Mechanisms involved in the vasodilator effect induced by diosgenin in rat superior mesenteric artery

The aim of this study was to investigate the vasorelaxant effect induced by diosgenin in superior mesenteric rings. In rings pre-contracted with phenylephrine (10 microM), diosgenin caused concentration-dependent relaxations [EC(50) = (3.3 +/- 1.2) x 10(- 4)M, E(max) = 94.2 +/- 2.6 %]. Vascular relaxation induced by diosgenin was significantly inhibited after removal of the endothelium (E(max) = 46 +/- 8.8%, p < 0.001) or after pre-treatment of the rings with N-nitro-L-arginine methyl esther (l-NAME) 100 or 300 microM (E(max) = 35.3 +/- 4%; 28.1 +/- 3.3%, respectively, p < 0.001), atropine 1 microM (E(max) = 24.6 +/- 3.4%, p < 0.001), hydroxocobalamin 30 microM (E(max) = 54.0 +/- 9.6%, p < 0.001), 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) 10 microM (E(max) = 46.0 +/- 8.0%, p < 0.001) or indomethacin 1 microM (E(max) = 22.6 +/- 11.8%, p < 0.001). Vasorelaxation evoked by diosgenin was significantly inhibited after pre-treatment of preparations with both selective and non-selective inhibitors of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels, iberiotoxin 100 nM or tetraethylammonium (TEA) 1mM, respectively (E(max) = 62.5 +/- 9.1%; 65.7 +/- 1.1%, p < 0.001). Conversely, in endothelium-denuded vessels, none of BK(Ca) channel blockers modified the relaxant effect induced by diosgenin. In mesenteric endothelial cells loaded with FURA-2 diosgenin was able to increase intracellular calcium concentrations, which were significantly decreased by atropine 1 microM. In addition, in isolated mesenteric rings, diosgenin induced marked increase in nitric oxide (NO) levels, which was completely abolished after functional endothelium removal. The results obtained here demonstrated that diosgenin-induced relaxation appears to involve endothelial muscarinic receptor activation with increase in intracellular calcium concentrations and consequent release of endothelium-derived relaxing factors (EDRFs), mainly NO and cyclooxygenase derivatives, which activate BK(Ca) channels. Nevertheless, further studies are necessary to clearly elucidate residual endothelium-independent relaxation induced by diosgenin.