Cell Renewal in the Lungs of Rats Exposed to Low Levels of NO₂

Abstract

Effects of continuous exposure to low concentrations of NO₂ on cell proliferation in bronchiolar and alveolar tissues were determined. Young rats were exposed continuously for as long as 360 days, removed at intervals, and killed. Dividing cells were labeled with tritiated thymidine. Animals exposed to 17 ppm NO₂ showed an Increased labeling index in terminal bronchioles and alveoli after two to three days of exposure; the index returned to control levels by five days. Similar results were found in animals exposed to 2 ppm NO₂, but the increase was not as great. This resulted in epithelial hyperplasia of terminal bronchioles and increased cellularity of alveoli associated with the distal ends of the terminal bronchioles. In peripheral alveoli, there was an increase in turnover rate of type 2 alveolar cells. All of these changes occurred within three days of continuous exposure. After this time, the areas of cellularity did not increase In size and the labeling indexes returned to normal for the remainder of the study.