Background.

Although inhibition of endothelial nitric oxide synthase (eNOS) has been reported to aggravate hepatic ischemia-reperfusion (I/R) injury, the role of inducible nitric oxide synthase (iNOS) has been still unknown. We investigated the role of NO produced by iNOS, and evaluated the effect of an iNOS inhibitor on prolonged warm I/R injury in the pig liver.

Methods.

Pigs were subjected to 120 min of hepatic warm I/R under the extracorporeal circulation. We investigated the time course of changes in serum and hepatic microdialysate NO 2−+ NO 3−(NOx) and the cellular distribution of eNOS and iNOS by immunohistochemistry, including a double-immunofluorescence technique in combination with confocal laser scanning microscopy. The effect of iNOS inhibitor was also investigated.

Results.

Hepatic I/R induced new nitric oxide production in serum and hepatic microdialysate NOx after reperfusion and severe hepatic damage in the centrilobular region where nitrotyrosine was strongly expressed. Diffuse eNOS expression in sinusoidal endothelium did not differ before and after reperfusion. In contrast, strong iNOS expression in Kupffer cells and neutrophils appeared strongly in the centrilobular region after reperfusion. Pigs with intraportal administration of N G-nitro-L-arginine (10 mg/kg) died during the period of ischemia or early in the period of reperfusion with a high mortality rate (80.0%). Intraportal administration of aminoguanidine hemisulfate (10 mg/kg) significantly suppressed nitric oxide production and serum aspartate aminotransferase after reperfusion, inhibited nitrotyrosine expression, and attenuated hepatic damage.

Conclusions.

These results indicate that hepatic I/R injury is triggered by centrilobular iNOS expression; and attenuated by inhibition of iNOS.