PRODUCTION OF NITRIC-OXIDE AND PEROXYNITRITE IN THE LUNG DURING ACUTEENDOTOXEMIA

Nitric oxide is a short-lived cytotoxic mediator that has been implica ted in the pathogenesis of endotoxin-induced tissue injury and septic shock. In the present studies we determined whether this mediator is p roduced in the lung during acute endotoxemia. We found that intravenou s injection of rats with bacterially derived lipopolysaccharide (LPS), a condition that induces acute endotoxemia, caused a time-dependent i ncrease in inducible nitric oxide synthase (iNOS) mRNA expression in t he lung, which reached a maximum after 24 h. This was correlated with nitric oxide production in the lung as measured by electron paramagnet ic spin trapping, which was detectable within 6 h. Alveolar macrophage s (AMs) and interstitial macrophages (IMs) isolated from rats 6-12 h a fter induction of acute endotoxemia were also found to exhibit increas ed nitric oxide production in response to in vitro stimulation with in terferon-gamma (IFN-gamma) and LPS measured by nitrite accumulation in the culture medium. The effects of acute endotoxemia on nitric oxide production by these cells were, however, transient and returned to con trol levels by 24 h in AMs and 36 h in IMs. Interestingly, although ni trite accumulation in the culture medium of IMs isolated 48 h after in duction of acute endotoxemia and stimulated with low concentrations of IFN-gamma and LPS was reduced, when compared with cells from control animals, these cells, as well as AMs, continued to express high levels of iNOS protein and mRNA. This was correlated with increased peroxyni trite production by the cells. Peroxynitrite has been shown to act as a nitrating agent and can generate nitrotyrosine residues in proteins. Using a specific antibody and immunohistochemistry, we found evidence of nitrotyrosine residues in sections of lungs 48 h after treatment o f rats with endotoxin. These data suggest that nitric oxide produced b y IMs and AMs can react with superoxide anion to form peroxynitrite. T aken together, the present studies demonstrate that AMs and IMs are ac tivated following acute endotoxemia to produce reactive nitrogen inter mediates and that both cell types contribute to inflammatory responses in the lung.