Inducible Nitric Oxide Synthase Contributes to Ventilator-induced Lung Injury

Rationale: Inducible nitric oxide synthase (iNOS) has been implicated in the development of acute lung injury. Recent studies indicate a role for mechanical stress in iNOS and endothelial NOS (eNOS) regulation. Objectives: This study investigated changes in lung NOS expression and activity in a mouse model of ventilator-induced lung injury. Methods: C57BL/6J (wild-type [WT]) and iNOS-deficient (iNOS−/−) mice received spontaneous ventilation (control) or mechanical ventilation (MV; VT of 7 and 20 ml/kg) for 2 hours, after which NOS gene expression and activity were determined and pulmonary capillary leakage assessed by the Evans blue albumin assay. Results: iNOS mRNA and protein expression was absent in iNOS−/− mice, minimal in WT control mice, but significantly upregulated in response to 2 hours of MV. In contrast, eNOS protein was decreased in WT mice, and nonsignificantly increased in iNOS−/− mice, as compared with control animals. iNOS and eNOS activities followed similar patterns in WT and iNOS−/− mice. MV caused acute lung injury as suggested by cell infiltration and nitrotyrosine accumulation in the lung, and a significant increase in bronchoalveolar lavage cell count in WT mice, findings that were reduced in iNOS−/− mice. Finally, Evans blue albumin accumulation in lungs of WT mice was significant (50 vs. 15% increase in iNOS−/− mice compared with control animals) in response to MV and was prevented by treatment of the animals with the iNOS inhibitor aminoguanidine. Conclusion: Taken together, our results indicate that iNOS gene expression and activity are significantly upregulated and contribute to lung edema in ventilator-induced lung injury.