Nitric oxide synthase isoenzyme activities in a premature piglet model of necrotizing enterocolitis: effects of nitrergic manipulation.

Nitric oxide (NO) plays a major role in gut mucosal protection and motility. Having demonstrated the protective effects of intravenous L-arginine (L-arg) and the NO donor, sodium nitroprusside (SNP), in an in-vivo premature piglet intraluminal model of necrotizing enterocolitis (NEC) that incorporates both mucosal damage and intestinal dysmotility, we measured the effects on NO synthase (NOS) isoenzyme activities during i.v. manipulation of the nitrergic system in the NEC-injured gut. In newborn premature Yorkshire piglets, NEC was induced in four groups by intraluminal injection of acidified casein solution in closed test loops of bowel separated by normal saline-injected control loops. Group 1 (n = 4) underwent no further treatment. Group 2 (n = 4) received concomitant continuous i.v. L-arg, a NO substrate. Group 3 (n = 6) received concomitant continuous i.v. SNP, a NO donor. Group 4 (n = 5) received concomitant continuous i.v. N-omega-nitro-L-arginine-methyl-ester (L-NAME), a non-selective NO inhibitor. Control and test gut specimens were harvested after 3 h. NO synthase activity in frozen gut segments was assessed using the (14)C-L-arg to (14)C-L-citrulline conversion assay. Total NOS (TNOS), constitutive NOS (cNOS), and inducible NOS (iNOS) activities were compared. The mean and standard error were calculated for each specimen. Group means were used to compare test and control gut enzyme activities in the different treatment groups. One-way analysis of variance and the Bonferroni post test were used to compare differences among groups. A P value of less than 0.05 was considered significant. In the L-NAME group, cNOS activity was lower than in the untreated NEC group. The SNP group had higher iNOS and TNOS activities than the L-arg group; cNOS was also higher in test and control loops in the SNP versus both L-arg and L-NAME groups. However, in L-arg control loops, cNOS activity was greater than in the L-NAME group. SNP and L-arg treatment of NEC did not significantly modify NOS isoenzyme activities. Thus, in this premature piglet 3-h model of NEC, i.v. L-NAME significantly decreases cNOS activity and correlates with our previously published histopathologic findings confirming the protective role of cNOS-derived NO in NEC-injured gut mucosa. In order to further elucidate the mechanisms involved in the mucosal protection afforded by i.v. L-arg and SNP in this NEC model, studies of a longer duration have been undertaken.