NO, Nitric Oxide Synthase and Redox Equilibrium in an Experimental Hypovolaemic and Septic Shock

Following the administration of Escherichia coli lipopolysaccharide (LPS) i.p. (2 mg/100 g b.w.), or hypovolaemic shock induced by a controlled haemorray, nitric oxide (NO) production was measured as nitrosyl haemoglobin complex (NO-Hb) in the blood of male Wistar rats. Highly reactive free radical intermediates were determined using electron paramagnetic resonance spectroscopy-spin trapping, plasma peroxides were concurrently determined using the D-ROM test. At the onset of shock, a massive increase in NO-Hb was observed, associated with the presence of reactive free radicals, without any change in plasma peroxide concentration. NO blood levels increase up to 6 hours following LPS, while reactive free radical intermediates levelled off at 3 hours treatment. Treatment with the antioxidant n-acetyl-cysteine (NAC), a well known donor of sulfhydryl groups, resulted in the diminution of the NO-HB adduct, while plasma reactive free radical intermediates resulted unaffected. NAC administration also prevented the formation of hepatic Dinitrosyl-iron complexes, indicating an effective inhibition of NO synthesis. The co-presence of mainly oxygen centred reactive free radicals and NO will yield peroxynitrite, whose formation is determined by the chemical environment, and by the ratio of superoxide versus NO. Peroxynitrite is thought to be responsible for toxic oxidative reactions. Treatment affecting redox equilibrium causes a diminution of NO level, acting by inhibiting NO synthases and by the formation of nitroso-thiols. These two different pathways will probably give rise to contrasting effects: NO inhibition will prevent peroxynitrite formation, while nitroso-thiols will prolong NO availability for reaction with superoxide. A possible mechanism of protection exerted by redox equilibria manipulation, has to take in account the two different mechanisms of action.

  
 
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