Background and objectives: toxic liver injury results in nitrooxidative stress. Melatonin is a potent free radical scavenger, an inducible nitric oxide synthase (iNOS) inhibitor and an activator of antioxidant enzymes. The aim of this study was to investigate the hepatoprotective effect of exogenous melatonin on animals with acute toxic hepatitis. Material and methods: 36 healthy Sprague-Dawley male rats were split into three equal groups and given carbon tetrachloride (CCl₄), 2 g/kg (CCl₄ group) or the same dose of CCl₄ and melatonin, 10 mg/kg (CCl₄/melatonin group) or saline (control group). The effect of melatonin on prooxidant and antioxidant system indexes, NO and NOS levels in serum and liver, data of mitochondrial chain functions and cytolysis in liver were evaluated in all three groups. Results: melatonin significantly decreased activities of AST, ALT, ceruloplasmine and thiobarbituric acid reactive substance (TBARS) in serum. Catalase activity was lowered in serum but not in the liver. Hepatic TBARS, lipid hydroperoxides and glutathione concentrations were decreased, while superoxide dismutase, mitochondrial cytochrome oxidase and succinate dehydrogenase activities increased. Melatonin inhibited synthesis of stable NO metabolites in serum: NO₂-by 37.9%; NO₃-by 29.2%. There was no significant difference in content NO₂-in the liver, but concentration of NO₃-increased by 32.6%. Melatonin significantly reduced iNOS concentrations both in serum (59.7%) and liver (57.8%) but did not affect endothelial isoform enzyme activities neither in serum, nor in liver. The histopathological liver lesions observed in the CCl₄/melatonin group were less severe than those seen in the CCl₄ group. Conclusions: we demonstrated an ameliorating effect of melatonin on prooxidants and antioxidants, NO-NOS systems balance, mitochondrial function and histopathological lesions in the liver in rats with CCl₄-induced hepatitis.

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