Background— Excessive production of nitric oxide (NO) by the inducible isoform of NO synthase (iNOS) is critically involved in endotoxin (ET)-induced hypotension. Tumor necrosis factor-α (TNF-α) plays an important role in induction of iNOS. Because activated protein C (APC), a physiological anticoagulant, inhibits TNF-α production, it might prevent hypotension by inhibiting excessive production of NO. In this study, we examined this possibility using a rat model of septic shock.

Methods and Results— Intravenous administration of APC prevented both ET-induced hypotension and the increases in plasma levels of NO₂⁻/NO₃⁻. The hypotension was also inhibited when APC was administered 30 minutes after ET administration. 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-α and expression of TNF-α mRNA in animals given ET. Neither DEGR-F.Xa, a selective inhibitor of thrombin generation, nor DIP-APC, an active site-blocked APC, showed any effect on these ET-induced changes. Both inhibition of TNF-α production by leukocytopenia and treatment with anti-rat TNF-α antibody produced effects similar to those induced by APC. Aminoguanidine, a selective inhibitor of iNOS, inhibited both the hypotension and the increases in plasma levels of NO₂⁻/NO₃⁻ in this animal model.

Conclusions— These observations strongly suggest that APC inhibits iNOS induction by decreasing TNF-α production, leading to the prevention of ET-induced hypotension. Furthermore, such effects of APC were not dependent on its anticoagulant effects but rather on its serine protease activity.