Localization of increased collagen in ferret lung tissue after chronic exposure to nitrogen dioxide

Chronic exposure of experimental animals to moderate levels of nitrogen dioxide (NO2) leads to increased collagen deposition in the lung parenchyma. To localize this increased collagen, lung sections from ferrets exposed to 0.5 or 10 ppm of NO2 4 h daily for 8 or 15 weeks were stained with Sirius red under conditions in which the stain is specific for collagen. Stained areas of respiratory bronchiolar submucosa were compared between NO2-exposed and air-exposed control animals using computer-aided image analysis. The stained areas, relative to the total respiratory epithelium, were increased in the sections from NO2-exposed ferrets as compared to air-exposed controls after 8 or 15 weeks. Removal from exposure after 8 weeks and allowing a 7-week recovery did not result in a decrease in stained areas. In the 10 ppm group, but not in the 0.5 ppm group, the increase was statistically significant. Measurement of total collagen in lung tissue by chemical means showed no differences among the various treatment groups. This study showed that increases in collagen deposition can be readily localized in order to identify the site(s) of potentially adverse effects of air pollutants. It is suggested that the use of Sirius red staining may offer a sensitive method for detection of early fibrotic changes in the lung.