Activated protein C prevents endotoxin-induced hypotension in rats by inhibiting excessive production of nitric oxide

Background-Excessive production of nitric oxide (NO) by the inducible isofo rm of NO synthase (iNOS) is critically involved in endotoxin (ET)-induced h ypotension. Tumor necrosis factor-alpha (TNF-alpha) plays an important role in induction of iNOS. Because activated protein C (APC), a physiological a nticoagulant, inhibits TN-F-alpha production, it might prevent hypotension by inhibiting excessive production of NO. In this study, we examined this p ossibility using a rat model of septic shook. Methods and Results-Intravenous administration of APC prevented both ET-ind uced hypotension and the increases in plasma levels of NO2-/NO3-. The hypot ension was also inhibited when APC was administered 30 minutes after ET adm inistration. APC inhibited the increases in lung levels of iNOS activity by inhibiting expression of iNOS mRNA in animals given ET. APC significantly inhibited the increases in lung tissue levels of TNF-alpha and expression o f TNF-alpha mRNA in animals given ET. Neither DEGR-F.Xa, a selective inhibi tor of thrombin generation, nor DIP-APC, an active site-blocked APC, showed any effect on these ET-induced changes. Both inhibition of TNF-alpha produ ction by leukocytopenia and treatment with anti-rat TNF-alpha antibody prod uced effects similar to those induced by APC. Aminoguanidine, a selective i nhibitor of iNOS, inhibited both the hypotension and the increases in plasm a levels of NO2-/NO3- in this animal model. Conclusions-These observations strongly suggest that APC inhibits iNOS indu ction by decreasing TNF-alpha production, leading to the prevention of ET-i nduced hypotension. Furthermore, such effects of APC were not dependent on its anticoagulant effects but rather on its serine. protease activity.