Increased nitric oxide synthesis during the development of endotoxin tolerance.

The role of nitric oxide (NO) synthesis was investigated in endotoxin (LPS) tolerance induced in rats by intraperitoneal injection of a sublethal dose of Salmonella enteritidis LPS (100 micrograms/kg intraperitoneally). Peritoneal macrophages were harvested 6 and 24 h after LPS injection and stimulated in vitro with LPS. LPS significantly stimulated arachidonic acid metabolism, as assessed by 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) levels, and NO production, as assessed by nitrite, in macrophages collected from control rats. In macrophages from tolerant rats LPS-stimulated 6-keto-PGF1 alpha production was significantly reduced, while nitrite production was increased compared to control macrophages (p < .001). In in vivo mortality studies, rats that were pretreated 24 h earlier with sublethal LPS were resistant to the lethal effect of a subsequent dose of LPS (15 mg/kg intravenously) in comparison to control rats (p < .001). NG-Nitro-L-arginine-methyl ester, an inhibitor of NO synthase, decreased mean survival time in control rats and abrogated the resistance to the lethal effect of LPS in tolerant rats. In contrast, molsidomine, a NO donor, improved survival in control rats but did not modify the resistance to the lethal dose of LPS in tolerant rats. The results suggest that sustained NO synthesis may be a beneficial mechanism for the induction of LPS tolerance.