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Background & Aims: Inflammatory bowel disease is characterized by increased synthesis of nitric oxide. The aim of this study was to determine if inducible NO synthase (iNOS) was responsible for tissue injury, potentially via peroxynitrite formation, in the guinea pig model of gut inflammation., Methods: Inflammation was induced in guinea pig ileum by intraluminal administration of the hapten trinitrobenzene sulfonic acid in 50% ethanol. iNOS gene expression was assessed by reverse-transcriptase polymerase chain reaction and Western blotting, immunohistochemistry was determined by its localization, and activity was inhibited with the specific inhibitor aminoguanidine administered via the drinking water for 7 days. Nitration of tyrosines was assessed by immunohistochemistry., Results: In control animals, iNOS gene expression was minimal to absent, whereas, in hapten, inflammation-marked iNOS gene expression was evident from day 1 to 7. Nitrotyrosine and iNOS immunohistochemistry were colocalized, and positive staining was most intense in epithelia and neurons. Inhibition of NO formation prevented nitrotyrosine formation. Aminoguanidine inhibited the inflammatory response and restored morphology., Conclusions: The colocalization of tyrosine nitration with iNOS immunoreactivity suggests that iNOS may be responsible for tissue injury and the formation of NO-dependent nitrating species, potentially peroxynitrite. Inhibition of iNOS may afford a new therapeutic approach to the treatment of inflammatory bowel disease.

As nitric oxide reduces gut epithelial permeability, we designed a study to determine if chronic nitric oxide synthase inhibition predisposes the gut to inflammation. Nitric oxide synthase (NOS) inhibitors were administered in the drinking water ad libitum, for seven days: aminoguanidine (10 micrograms/ml), a selective inhibitor of the inducible form of nitric oxide synthase; and NG-nitro-L-arginine methyl ester (L-NAME, 1, 10, and 100 micrograms/ml), which inhibits both the constitutive and inducible forms. Control animals drank tap water only or water with D-NAME, the inactive enantiomer. After one week, circulating leukocyte count and tissue myeloperoxidase activity were measured. L-NAME (100 micrograms/ml), but not D-NAME or aminoguanidine, caused a twofold increase in a circulating leukocyte numbers. This increase in leukocyte numbers was time- and dose-dependent, but the differential count was unaltered. Tissue myeloperoxidase (MPO) activity as an index of granulocyte infiltration was comparable in all groups in the stomach, jejunum, colon, liver, lung, kidney, heart, and skeletal muscle. However, ileal MPO activity was elevated threefold in the L-NAME-(100 micrograms/ml) treated group (P < 0.05). Results in the D-NAME and aminoguanidine groups were similar to controls. L-NAME administration resulted in a reduction in NOS activity ([14C]citrulline formation) in the ileum but not jejunum, whereas cGMP levels were elevated in both ileum and jejunum. We conclude that chronic inhibition of the constitutive form of nitric oxide synthase predisposes the ileum to inflammation and leads to a progressive leukocytosis.

We studied the effects of seven day treatment with the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine (L-NAME), administered in the drinking water (100 micrograms/ml ad lib) of female guinea pigs. The effects of NOS inhibition were evaluated in naive animals and in guinea pigs with ileitis induced by intraluminal trinitrobenzenesulfonic acid (TNBS). After 7 days, animals were anesthetized, a sterile saline lavage injected into an ileal loop and removed after 30 min for analysis. In naive guinea pigs, L-NAME caused a marked increase in ileal myeloperoxidase activity and conversion of the mucosa from an absorptive to a secretory state. TNBS-treated guinea pigs has a similar, marked increase in granulocyte infiltration and a mucosal secretory response. However, in contrast to naive animals, L-NAME treatment was anti-inflammatory, reverting all responses to the basal state. We conclude that intestinal nitric oxide serves an anti-inflammatory role under basal conditions, whereas in the TNBS model of chronic ileitis, nitric oxide is a critical mediator of gut injury.