Endotoxin‐induced nitric oxide synthesis in the perfused rat liver: Effects of L ‐arginine and ammonium chloride

We used the single‐pass–perfused rat liver model to study short‐term regulation of endotoxin‐inducible nitric oxide synthesis by following the release of nitrite and nitrate, the oxidation products of nitric oxide, into the effluent perfusate. In endotoxin‐pretreated livers, the basal nitrite + nitrate release was 5.3 ± 1.2 nmol·gm liver −1 ·min −1 . Nitrite and nitrate release was stimulated by L‐arginine in a dose‐dependent and saturable fashion. Maximal nitrite + nitrate release with 1 mmol/L L‐arginine infused to the influent perfusate was 10.2 ± 1.1 nmol·gm liver −1 ·min −1 , with a half‐maximal effect at 53 μmol/L L‐arginine. In the absence of molecular oxygen, nitric oxide synthesis was inhibited. Ammonium chloride infusion also stimulated nitrite and nitrate release to a maximal rate of 9.2 ± 0.8 nmol·gm liver −1 min −1 with half‐maximal effects at 60 μmol/L ammonium chloride. Ammonium chloride‐stimulated nitrite and nitrate release was abolished when urea synthesis was inhibited by bicarbonate‐free liver perfusion. Citrulline and ornithine (200 μmol/L each) were without effect on nitrite and nitrate release. L‐Nitroarginine methyl ester inhibited both, L‐arglnine‐and ammonium chloride‐induced nitrite and nitrate release. Stimulation of nitric oxide synthesis by L‐arginine and ammonium chloride addition (1 mmol/L each) was accompanied by a threefold‐to‐fourfold increase of cyclic GMP release into the effluent perfusate. In livers of endotoxin‐pretreated rats the urea production from L‐arginine was higher than that in untreated livers, suggesting induction of an L‐arginine transport system in hepatocytes by endotoxin. The regulation of hepatic nitric oxide production by physiological concentrations of L‐arginine and ammonia in the portal vein may be of importance in cirrhosis. (Hepatology 1994;19:641–647).