BACKGROUND: Nitric oxide(NO) is an endogenously produced free radical that plays an important role in regulating vascular tone, inhibition of platelet aggregation and white blood cell adhesion to endothelial cells, and host defense against infection. The highly reactive nature of NO with oxygen radicals suggests that it may either promote or reduce oxidant-induced cell injury in several biological pathways. Oxidant injury and interactions between pulmonary vascular endothelium and leukocytes are important in the pathogenesis of acute lung injury, including acute respiratory distress syndrome(ARDS). In ARDS, therapeutic administration of NO is a clinical condition providing exogenous NO in oxidant-induced endothelial injury. The role of exogenous NO from NO donor or the suppression of endogenous NO production was evaluated in oxidant-induced endothelial injury. Method : The oxidant injury in cultured rat lung microvascular endothelial cells(RLMVC) was induced by hydrogen peroxide generated from glucose oxidate(GO). Cell injury was evaluated by 51chromium(51Cr) release technique. NO donor, such as S-nitroso-N-acetylpenicillamine(SNAP) or sodium nitroprusside(SNP), was added to the endothelial cells as a source of exogenous NO. Endogenous production of NO was suppressed with N-monomethyl-L-arginine(L-NMMA) which is an NO synthase inhibiter. L-NMMA was also used in increased endogenous NO production induced by combined stimulation with interferon-gamma(INF-gamma), tumor necrosis factor-alpha(TNF-alpha), and lipopolysaccharide(LPS). NO generation from NO donor or from the endothelial cells was evaluated by measuring nitrite concentration. Result : 51Cr release was 8.7+/-0.5% in GO 5 mU/ml, 14.4+/-2.9% in GO 10 mU/ml, 32.3+/-2.9% in GO 15 mU/ml, 55.5+/-0.3% in GO 20 mU/ml and 67.8+/-0.9% in GO 30 mU/ml ; it was significantly increased in GO 15 mU/ml or higher concentrations when compared with 9.6+/-0.7% in control(p<0.05 ; n= 6). L-NMMA(0.5mM) did not affect the 51Cr release by GO. Nitrite concentration was increased to 3.9+/-0.3 microMin culture media of RLMVC treated with INF-gamma(500 U/ml), TNF-alpha(150 U/ml) and LPS(1 microgram /ml) for 24 hours ; it was significantly suppressed by the addition of L-NMMA. The presence of L-NMMA did not affect 51Cr release induced by GO in RLMVC pretreated with INF-gamma, TNF-alpha and LPS. The increase of 51Cr release with GO(20 mU/ml) was prevented completely by adding 100 microM SNAP. But the add of SNP, potassium ferrocyanate or potassium ferricyanate did not protect the oxidant injury. Nitrite accumulation was 23+/-1.0 microM from 100 microM SNAP at 4 hours in phenol red free Hanks' balanced salt solution. But nitrite was not detectable from SNP upto 1 mM. The presence of SNAP did not affect the time dependent generation of hydrogen peroxide by GO in phenol red free Hanks' balanced salt solution. CONCLUSION: Hydrogen peroxide generated by GO causes oxidant injury in RLMVC. Exogenous NO from NO donor prevents oxidant injury, and the protective effect may be related to the ability to release NO. These results suggest that the exogenous NO may be protective on oxidant injury 13 the endothelium.