Oxygen free radicals : mediators of vascular tone

In vivo and in vitro studies on numerous types of blood vessels obtained from a variety of vascular beds and species have demonstrated that oxygen free radicals (OFRs) can evoke both vasodilation and vasoconstriction. Specific OFRs have been shown to elicit different and often times opposite effects on vascular smooth muscle. Therefore, this thesis attempts to define the vascular actions and mechanism of oxygen free radicals (OFRs) [superoxide anion (O2•-), hydrogen peroxide (HO2) and hydroxyl radical (•OH)] on isolated rabbit aorta. This thesis will examine the role of •OH in Ach- and nitroglycerin (NTG)-induced relaxation of isolated rabbit aorta. Superoxide anions generated by xanthine (X) plus xanthine oxidase (XO) produced concentration-dependent contractions of isolated rabbit aorta. The contractile response to O2•- was completely abolished in preparations denuded of endothellum or pretreated with superoxide dismutase (SOD), a scavenger of O2•-. The contractile response was reduced by indomethacin (I), a cyclooxygenase inhibitor. These results suggest that O2•- mediated by vasoconstrictor arachidonic acid metabolites. Hydrogen peroxide generated by glucose and glucose oxidase produced contraction (low concentrations) and relaxation followed by contraction (high concentrations) in isolated rabbit aorta. The contractile response was abolished in the presence of catalase, a scavenger of H2O2 however the relaxant effect was exaggerated. Indomethacin markedly reduced the H2O2-induced contraction. Relaxation was completely prevented by de-endothelialization or pretreatment with NG-monomethyl-L-arginine (LNMMA), an inhibitor of nitric oxide synthetase. These results suggest that H2O2 in large concentrations produces a biphasic response, relaxation followed by contraction. Relaxation is endothelium dependent and is mediated by endothelium-derived relaxing factor (EDRF), nitric oxide (NO). The contractile response is endothelium independent and is mediated by vasoconstrictor arachidonic acid metabolites of smooth muscle. Hydroxyl radicals generated by dihydroxyfumarate (DHF), ferric chloride (FeCl3) and adenosine diphosphate (ADP) (DHF/FeCl3-ADP) produced concentration dependent relaxations of NE-precontracted rabbit aorta. Mannitol (Ml) completely inhibited •OH-induced relaxation. Relaxation was markedly reduced in aortic rings mechanically denuded of endothelium. The relaxant effect was reduced by an inhibitor of NO synthesis (LNMMA), by an inhibitor of guanylate cyclase (methylene blue), by an inhibitor of cyclooxygenase (indomethacin) and by an inhibitor of an ATP-sensitive K+ channel blocker (glyburide). These results indicate that •OH produces relaxation that is endothelium-dependent and partially mediated by an endothelium-derived relaxing factor (NO), vasodilatory arachidonic acid metabolites and an ATP-sensitive K+ channel. We hypothesized that Ach-induced vascular relaxation is mediated by •OH derived from the interaction of NO and O2•-. To test this hypothesis we investigated the effect of Ach and NTG on NE-precontracted isolated rabbit aortic preparations in the absence or presence of scavengers of O2•- (SOD), and •OH (dimethylthiourea (DMTU) or mannitol or Garlicin). The OFR scavengers (SOD, dimethylthiourea, mannitol, garlicin and histidine) alone or the combination of SOD and DMTU markedly reduced Ach- or NTG-induced relaxation. However, the combination of histidine, (a 1O2 scavenger) SOD and DMTU completely abolished Ach-induced relaxation