SOURCES OF ARGININE FOR INDUCED NITRIC-OXIDE SYNTHESIS IN THE ISOLATED-PERFUSED LIVER

Hepatocytes can be stimulated to express high levels of inducible nitr ic oxide synthase (iNOS), which utilizes arginine for nitric oxide (NO ) synthesis. Hepatocytes also synthesize and catabolize arginine, an i ntermediate in the urea cycle, raising the possibility that the urea p athway may provide substrate for hepatic NO synthesis. To identify the sources of arginine for iNOS, we measured the release of NO2- + NO3- and urea in isolated rat livers perfused in a recirculation model with a Krebs-Henseleit-bicarbonate buffer containing either no added amino acid, arginine, or precursors for urea synthesis. To induce iNOS expr ession, rats were injected with killed Corynebacterium parvum (C. parv um) or with endotoxin. In livers from C. parvum- and endotoxin-treated rats, we found that 1) an intracellular source of arginine exists tha t provides substrate to iNOS; 2) additional exogenous arginine increas es NO synthesis, demonstrating that endogenous arginine is insufficien t for maximal NO synthesis; and 3) an increase in the rate of endogeno us arginine synthesis within the urea cycle is inefficient in increasi ng NO synthesis, demonstrating the independence of the two pathways in the liver.