Antiinflammatory properties of inducible nitric oxide synthase in acute hyperoxic lung injury.

The objective of this study was to determine whether endogenous nitric oxide (NO), specifically the inducible NO synthase isoform (iNOS: NOS II), reduces or amplifies lung injury in mice breathing at a high oxygen tension. Previous studies have shown that exogenous (inhaled) NO protects against hyperoxia-induced lung injury, and that endogenous NO derived from iNOS inhibits leukocyte recruitment and protects against lung injury induced by lipopolysaccharide. In the present study, hyperoxia (> 98% O(2) for 72 h) induced acute lung injury in both wild-type and iNOS-deficient mice as determined by elevated albumin and lactate dehydrogenase levels in bronchoalveolar lavage fluid (BALF) and by increased extravascular lung water. Lung injury was greater in iNOS-deficient mice than in wild-type mice and was associated with an increased number of polymorphonuclear leukocytes in BALF. iNOS messenger RNA expression levels increased in the lungs of wild-type hyperoxic mice. Nitrotyrosine, a marker of reactive NO species, was expressed in both wild-type and iNOS-deficient mice in hyperoxia, indicating an iNOS-independent pathway for protein nitration. We conclude that iNOS is capable of reducing pulmonary leukocyte accumulation and lung injury. The data indicate that iNOS induction serves as a protective mechanism to minimize the effects of acute exposure to hyperoxia.