Induction of nitric oxide synthesis and its reactions in cultured human and rat hepatocytes stimulated with cytokines plus LPS.

We have examined the time course of appearance of mRNA for nitric oxide synthase (NOS), intracellular nonheme iron-nitrosyl complexes (NHFeNO, detected by EPR spectroscopy), and rates of medium appearance of NO2- + NO3- in cultured rat and human hepatocytes stimulated with a combination of cytokines (TNF-alpha, IFN-gamma, IL-1 beta) and LPS. In both cells types, NOS mRNA precedes NHFeNO formation which in turn precedes maximum rates of NO2- + NO3- (NOx) formation. This profile occurs earlier in human hepatocytes than rat hepatocytes and the appearance of NOS mRNA is also more transient. These results indicate that (1) NOS is stable intracellularly (peak NOx production occurs substantially after peak mRNA levels), (2) intracellular iron is an early target (preceding maximum NOx production) for NO in both human and rat hepatocytes, and (3) decline in NHFeNO in the face of maximum NOx production indicates the presence of a "repair" or "removal" mechanism for these intracellular iron-nitrosyl complexes.