Type I nitric oxide synthase (NOS) is the predominant NOS in rat small intestine. Regulation by platelet-activating factor.

Constitutive nitric oxide synthase (cNOS) may play an important protective role in the intestine, since our previous study has shown that the degree of bowel injury induced by platelet-activating factor (PAF), a potent inflammatory mediator, is inversely related to the cNOS content of the intestine. This study aims to examine the composition of the cNOS system in rat small intestine, and its regulation by PAF. We found that an approximately 120 kDa NOS I (neuronal NOS) is the predominant NOS in rat intestine, as evidenced by the following: (a) immunoblotting with specific antibodies detected a NOS I of approximately 120 kDa, but little NOS III; (b) the Ca(2+)-dependent, constitutive NOS (cNOS) activity of the rat intestine was removed by immunoprecipitation with the anti-NOS I, but not anti-NOS II or anti-NOS III antibodies; (c) RT-PCR revealed constitutive expression of NOS I in the intestinal tissue, but only a minute amount of NOS III. Immunofluorescent staining with anti-NOS I located NOS in the Auerbach plexus and nerve fibers in the muscle layer. We also found that this 120 kDa NOS I is rapidly (within 1 h) down-regulated in response to PAF administration. The protein level, enzyme activity as well as mRNA of nNOS were all decreased in the intestine.