Induction of nitric oxide synthase and microvascular injury in the rat jejunum provoked by indomethacin.

1. The role of nitric oxide (NO) formed by the inducible isoform of NO synthase (NOS) in the generation of indomethacin-induced intestinal microvascular leakage was investigated in the rat. 2. Indomethacin (10 mg kg-1, s.c.) provoked an elevation of vascular leakage of radiolabelled human serum albumin in the jejunum over 48 h, commencing 18 h after its administration. This was associated with the induction of a calcium-independent NOS, as assessed by the conversion of radiolabelled L-arginine to citrulline. 3. Pretreatment with the glucocorticoid, dexamethasone (1 mg kg-1 day-1, s.c.) inhibited the induction of NOS and reduced jejunal microvascular leakage, determined 24 and 48 h after indomethacin. 4. Administration of the broad-spectrum antibiotic, ampicillin (800 mg kg-1 day-1, p.o.) likewise inhibited both the induction of NOS and the plasma leakage observed 24 and 48 h after indomethacin. 5. Ampicillin pretreatment did not, however, inhibit the induction of NOS, determined 5 h following endotoxin (3 mg kg-1 i.v.) challenge. Furthermore, incubation with ampicillin (1 mM, 10 min) did not inhibit the activity of the calcium-independent isoform in vitro. 6. Administration of the NOS inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 2-10 mg kg-1, s.c.), at the time of the detectable expression of the inducible NOS (18 h after indomethacin), dose-dependently attenuated the plasma leakage, determined 6 later. This effect was reversed by pretreatment with L-arginine (300 mg kg-1, s.c.) 15 min before L-NAME. 7. These findings suggest that induction of a calcium-independent NOS following indomethacin administration involves gut bacteria and leads to microvascular injury in the rat jejunum.