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 an activity are significantly upregulated and contribute to lung edema in ventilator-induced lung injury.