ROLE OF NITRIC-OXIDE IN ACETAMINOPHEN-INDUCED HEPATOTOXICITY IN THE RAT

Acetaminophen is a mild analgesic and antipyretic agent known to cause centrilobular hepatic necrosis at toxic doses. Although this may be d ue to a direct interaction of reactive acetaminophen metabolites with hepatocyte proteins, recent studies have suggested that cytotoxic medi ators produced by parenchymal and nonparenchymal cells also contribute to the pathophysiological process. Nitric oxide is a highly reactive oxidant produced in the liver in response to inflammatory mediators. I n the present studies we evaluated the role of nitric oxide in the pat hophysiology of acetaminophen-induced liver injury. Treatment of male Long Evans Hooded rats with acetaminophen (1 g/kg) resulted in damage to centrilobular regions of the liver and increases in serum transamin ase levels, which were evident within 6 hours of treatment of the anim als and reached a maximum at 24 hours. This was correlated with expres sion of inducible nitric oxide synthase (iNOS) protein in these region s. Hepatocytes isolated from both control and acetaminophen-treated ra ts were found to readily synthesize nitric oxide in response to inflam matory stimuli. Cells isolated from acetaminophen-treated rats produce d more nitric oxide than cells from control animals. Production of nit ric oxide by cells from both control and acetaminophen-treated rats wa s blocked by aminoguanidine, a relatively specific inhibitor of iNOS. Arginine uptake and metabolism studies revealed that the inhibitory ef fects of aminoguanidine were due predominantly to inhibition of iNOS e nzyme activity. Pretreatment of rats with aminoguanidine was found to prevent acetaminophen-induced hepatic necrosis and increases in serum transaminase levels. This was associated with reduced nitric oxide pro duction by hepatocytes. Inhibition of toxicity was not due to alterati ons in acetaminophen metabolism since aminoguanidine had no effect on hepatocyte cytochrome P4502E1 protein expression or N-acetyl-p-benzoqu inone-imine formation. Taken together, these data demonstrate that nit ric oxide is an important mediator of acetaminophen-induced hepatotoxi city.