Inducible nitric oxide synthase plays a critical role in resolving intestinal inflammation.

Overproduction of nitric oxide by inducible nitric oxide synthase (iNOS) has been proposed as a pathogenic factor in colitis. The objective of this study was to examine the role of iNOS using iNOS-deficient mice in experimental colitis. Colitis was induced by intrarectal instillation of 3% acetic acid and assessed for neutrophilic infiltration and intestinal injury over 7 days. iNOS messenger RNA expression was also measured. At 24 hours, acetic acid induced a mild colitis in wild-type mice. An increase in neutrophil infiltration and tissue edema was also observed. In the iNOS-deficient mice, a twofold increase in macroscopic damage was observed. Neutrophil infiltration and tissue edema were similar to those in wild-type animals at this time point. Although inflammation in wild-type mice had resolved by 7 days, a sevenfold increase in damage score and elevated myeloperoxidase level were still evident in iNOS-deficient mice. A striking increase in the message for iNOS was observed in inflamed wild-type mice at 24 hours and was still present at 72 hours. No message was found in iNOS-deficient mice. Induction of iNOS seems to be a critical protective response to injury in intestinal inflammation possibly by reducing leukocytic infiltration.