Mechanism of endothelium-dependent vasodilation induced by a proanthocyanidin-rich fraction from Ouratea semiserrata

The vasodilator effects of Ouratea semiserrata stem hydroethanolic extract (OSE) and its ethyl acetate fraction (OSR) were evaluated in endothelium-intact aortic rings. OSR produced a more potent vasodilatation (IC50 = 3.5 +/- 0.8 mug/ml) than OSE (IC50 > 30 mug/ml). OSR also presented a higher content of total proanthocyanidins (21.8 +/- 1.5%) in comparison to OSE (6.5 +/- 0.4%), suggesting that compounds of this class play a role in the vasorelaxing activity. The vasodilatation mechanism of OSR was further investigated. In endothelium-intact aortic rings, its vasorelaxing effect was completely abolished by L-NAME (300 muM), a nitric oxide (NO) synthase inhibitor, but not by a muscarinic antagonist (atropine, 1 muM) nor by a cyclo-oxygenase inhibitor (indomethacin, 10 MM). The OSR vasodilator effect was completely abolished in endothelium-denuded vessels. Furthermore, OSR did not change the vasodilatation produced by SIN-1, an NO donor, in endothelium-denuded vessels. These findings led us to conclude that OSR, a proanthocyanidin rich fraction of O. semiserrata, induces vasodilatation by a mechanism dependent on endothelium-derived factors, likely NO.