Cytokine stimulation of nitric oxide formation and differential regulation in hepatocytes and nonparenchymal cells of endotoxemic rats.

Some disease processes in which increased endotoxin and cytokine levels exist (e.g., sepsis and infantile diarrhea) are also associated with increased levels of blood nitrates, the stable end products of nitric oxide. Available evidence suggests that the effects of an endotoxic environment, with its attendant complex cytokine networks, on liver function are mediated in part by modulation of hepatic nitric oxide synthesis. This hypothesis was tested by means of studying nitric oxide formation and its regulation in liver parenchymal and nonparenchymal cells of rats that had been continuously infused with endotoxin for 30 hr. Hepatocytes of such rats responded to in vitro stimulation for 20 hr by single cytokines, tumor necrosis factor, interleukin-1 beta and interferon-gamma with enhanced nitric oxide formation. In combination, interferon-gamma and endotoxin had greater synergistic effect on hepatocytes than did tumor necrosis factor and endotoxin. Kupffer cells of these endotoxic rats responded to 20 hr of interferon-gamma stimulation with the same enhanced nitric oxide formation we documented previously for endotoxin. Potentiation of the effect, through combination of endotoxin and interferon-gamma, was not as marked as it was with hepatocytes. Challenge of Kupffer cells with tumor necrosis factor or interleukin-1 beta evoked no response. Hepatocytes and Kupffer cells of time-matched, saline solution-treated rats were unresponsive to endotoxin or cytokine stimulation. Small quantities of nitric oxide were produced by endothelial cells spontaneously; this production was somewhat enhanced in cells of the endotoxin-infused rats by a 20-hr in vitro endotoxin challenge. Studies with inhibitors suggest that enhanced nitric oxide formation by endotoxic hepatocytes and Kupffer cells in response to in vitro endotoxin stimulation is differentially regulated. Our findings indicate modulation of nitric oxide generation by cytokines in vitro in various liver cell types of endotoxic rats. A similar scenario may exist in vivo because of the prevailing inflammatory response to endotoxin administration.