INHIBITION OF OZONE-INDUCED NITRIC-OXIDE SYNTHASE EXPRESSION IN THE LUNG BY ENDOTOXIN

Inhalation of the pulmonary irritant ozone is associated with an accum ulation of macrophages in the lung. These cells, along with type II ep ithelial cells, are activated to release increased quantities of hydro gen peroxide and nitric oxide, two reactive mediators that have been i mplicated in tissue injury. In the present studies we determined wheth er pretreatment of rats with bacterially derived endotoxin, which modu lates oxidant levels in tissues, could abrogate the effects of ozone o n lung injury and nitric oxide production. Acute exposure of rats to o zone (2 parts per million, 3 h) resulted in nitric oxide production in the lung as measured by electron paramagnetic resonance spin trapping . This was correlated with expression of inducible nitric oxide syntha se (iNOS) mRNA in the lung as determined by in situ hybridization. Par ticularly high levels of iNOS were evident in alveolar macrophages and type II cells. Alveolar macrophages isolated from ozone-treated rats also expressed increased iNOS mRNA and protein as measured by Northern and Western blotting, respectively, and produced more nitric oxide co mpared with cells from air-exposed animals. Treatment of rats with end otoxin (5 mg/kg, intravenously), 30 min prior to ozone, was found to a brogate ozone-induced increases in iNOS mRNA and protein expression, a s well as nitric oxide production by alveolar macrophages. This was as sociated with a reduction in ozone-induced tissue injury as determined by levels of lung lavage fluid protein. Ozone inhalation also resulte d in a reduction in intracellular glutathione in alveolar macrophages, an effect that was blocked by endotoxin administration. Taken togethe r, these data provide evidence that the protective effects of endotoxi n against ozone-induced injury are mediated, at least in part, by alte rations in levels of lung oxidants and antioxidants.