Nitric oxide decreases oxidant-mediated hepatocyte injury.
Nitric oxide (NO) is a readily diffusible, short-lived free radical with a multitude of organ-specific regulatory functions. Within the hepatocyte, NO production is associated with inhibition of mitochondrial electron transport enzyme activity, activation of soluble guanylyl cyclase, and inhibition of glyceraldehyde-3-phosphate dehydrogenase. However, while NO can regulate a number of hepatocyte functions, it is unknown whether NO production is hepatoprotective or hepatotoxic. Using isolated rat hepatocytes in primary short-term culture, we investigated the role of cytokine-mediated NO production in toxin-induced hepatocyte injury. In a model of acetaminophen (AM) hepatotoxicity, inhibition of cytokine-mediated NO production potentiated AM injury. In the presence of an inhibitor of NO synthesis, NG-monomethyl-L-arginine (L-NMMA), hepatocyte release of aspartate aminotransferase was increased twofold in the presence of 4.0 and 8.0 mM AM (P < 0.01). In addition, in the presence of AM, cytokine-mediated NO production was increased by 75% over baseline (P < 0.01). Maximum NO synthesis occurred at an AM concentration of 2 mM. A potential mechanism for the hepatoprotective effect of NO centers on its role in glutathione (GSH) homeostasis. In the presence of increasing concentrations of AM, hepatocyte GSH stores decreased in parallel in both control and cytokine-stimulated hepatocytes (ANOVA, P < 0.01). When cytokine-stimulated hepatocytes were exposed to 50 microM L-NMMA, NO release was ablated, while glutathione levels decreased by threefold in comparison to controls (P < 0.01). In the presence of increasing concentrations of AM, cytokine-treated cells exposed to 50 microM L-NMMA exhibited significant decremental decreases in GSH levels (P < 0.05). These data suggest that inhibition of cytokine-mediated NO production potentiates AM hepatoxicity by modulation of hepatocyte glutathione stores.
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