Electronmicroscopic morphometry was performed on lung of 1, 3, 12 and21 months-old rats exposed to 0.1, 0.5, 3 and 10 ppm nitrogen dioxide (NO
2) continuously for one month. The rats used in this experiment were all supplied at one time from one colony and kept uder a barrier system until exposure.
Effects of aging on the responses of lungs to NO2 were studied by comparingthe dose-effect reaction patterns among the age groups.
A trend of dose-dependent increase of arithmetic mean thickness of air-blood barrier was found in all age groups examined. The responses of lung to NO
2 ex-posure showedage-related differences. Based on the morphometric index, the response declines from 1 to 12 months, but increases again in 21-months-old rats.
The compartmental components of alveolar wall tissue such as type I epithelial cells, type II epithelial cells, interstitial cells, interstitial matrix and capillary en-dothelium appeared to have various degrees of response due to both age at onset of exposure and NO
2 concentration, resulting in the appearance of varying stages in impairment or repair. Accordingly, the response of each compartmental com-ponent of lung to the concentrations of NO
2 did not always exhibit a simple dose-dependent increase or decrease but sometimes indicated a multiphasic reaction pattern.
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