Neuronal nitric oxide synthase (NOS) regulates the expression of inducible NOS in rat small intestine via modulation of nuclear factor kappa B.

We previously reported that neuronal nitric oxide synthase (nNOS) is the predominant NOS in the intestine. Inducible NOS (iNOS), an enzyme involved in the inflammatory response, is regulated by cytokines via the transcriptional factor NF-kappaB. We examined a new mechanism of intestinal iNOS regulation with respect to the role of nNOS and its effect on NF-kappaB. Young Sprague-Dawley rats were treated for 4 days with 1) saline, 2) 7-nitroindazole (7-NI, specific nNOS inhibitor), 3) 7-NI + pyrrolidine dithiocarbamate (PDTC, NF-kappaB inhibitor), or 4) PDTC. Intestinal iNOS mRNA, NF-kappaB activity, and the tissue content of the regulatory IkappaBalpha were examined. We found that 7-NI-treated animals had higher intestinal NF-kappaB (p50-p65) activity, lower IkappaBalpha content, and increased intestinal iNOS mRNA, iNOS protein, and iNOS activity compared with controls. All of these changes were abolished when PDTC was given together with 7-NI. PDTC alone had no effect. 7-NI induces a delayed increase in intestinal myeloperoxidase activity (after elevation in NF-kappaB and iNOS), which could be abrogated by PDTC. We conclude that in normal rat small intestine, nNOS suppresses the gene expression of iNOS through NF-kappaB down-regulation and that nNOS suppression leads to IkappaBalpha degradation, NF-kappaB activation, and iNOS expression.