Tolerance and cross-tolerance using NO2 and O2 II. Pulmonary morphology and morphometry

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Abstract
Rats exposed to 85% O2 for 5 days are tolerant to exposures to 100% O2 and partially cross-tolerant to exposures to 75 ppm NO2. Rats exposed to 25 ppm NO2 for 6 h/day on 5 successive days are tolerant to exposures to 75 ppm NO2, but are not cross-tolerant to 100% O2 exposures. The pathological changes occurring in the lungs of these animals were studied by light microscope and scanning electron microscopy, and were quantitated by using stereologic techniques and EM. Exposure to 85% O2 for 5 days caused a diffuse lesion throughout the gas exchange portion of the lung with an 91% increase in total tissue and a 32% decrease in the capillary blood volume. Proliferation of alveolar type II cells and interstitial cells was demonstrated, and the morphometric estimate of diffusion capacity was decreased to 63% of the control value. Exposure to 25 ppm NO2 for 6 h/day for 5 days caused changes in the bronchial and bronchiolar epithelium and focal changes involving the alveolar epithelium and the interstitium in the proximal portions of each acinus. The distal portions of each acinus were normal. The differences in the distribution of the O2 and the NO2 lesions may be explained by the diffusion characteristics of a reactive gas in low concentrations (NO2) and a less reactive gas in high concentrations (O2). The relationships of tolerance and cross-tolerance between these 2 gases may be due to the specific distribution of each lesion and to specific differences in biochemical adaptations that occur.

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