Long-term exposure to ozone increases acute pulmonary centriacinar injury by 1-nitronaphthalene: II. Quantitative histopathology

Long-term exposure to the oxidant air pollutant ozone (O₃) is associated with tolerance to the acute effects of oxidant injury. To test whether this resistance to acute injury extends to bioactivated pulmonary toxicants, male Sprague-Dawley rats were exposed to filtered air (FA) or 0.8 ppm O₃ (8 h/day) for 90 days and administered 1-nitronaphthalene i.p. at doses of 0, 50, or 100 mg/kg. 1-Nitronaphthalene is a pulmonary cytotoxicant requiring metabolic activation. High-resolution histopathology, transmission electron microscopy, and morphometry revealed significantly greater 1-nitronaphthalene toxicity in the central acinar region of O₃- compared with FA-exposed rats. At 100 mg/kg, injury to terminal bronchioles in O₃-exposed rats involved denudation of 86% of the basement membrane; 78% of the cells remaining on the epithelium were necrotic. This is compared with denudation of 4% of the basement membrane of FA-exposed rats administered 100 mg/kg 1-nitronaphthalene; only 25% of the cells remaining on the epithelium were necrotic. The key difference between FA- and O₃-exposed rats treated with 1-nitronaphthalene was the heightened severity of ciliated cell toxicity in O₃-exposed animals. This is despite the fact that long-term exposure to ozone produces tolerance to oxidant stress in the epithelium of the central acinus. No differences in the susceptibility of intrapulmonary airways or trachea to 1-nitronaphthalene were observed between filtered air- and ozone-exposed rats. This study demonstrates a siteselective synergy between the copollutants ozone and 1-nitronaphthalene in the production of acute lung injury.