Consequences of prolonged inhalation of ozone on F344/N rats: collaborative studies. Part XIII. A comparison of changes in the tracheobronchial epithelium and pulmonary acinus in male rats at 3 and 20 months.

A limitation of the NTP/HEI Collaborative Ozone Project conducted with F344/N rats at the Battelle Pacific North-west Laboratories in Richland, WA (1991-1993) was that the study used only one time point (20 months) to examine the chronic effects of exposure to ozone. Issues the design of that study could not address were (1) the status of cellular differentiation at earlier time points during the course of ozone exposure; (2) whether changes that appeared to be compensatory after 20 months of exposure were due to ozone, or were aspects of the natural aging process in rats; (3) the inability to define adequately which effects were related specifically to the prolonged duration of exposure; and (4) how and what changes brought about by the natural aging process may have overridden or confounded a clear definition of the effects of exposure to ozone at ambient concentrations (e.g., 0.12 parts per million [ppm]), which are of most concern with long-term exposure to this pollutant. The present study examined the effects of a 3-month exposure to ozone under conditions identical to those of the 20-month NTP/HEI Collaborative Ozone Project. In our facilities at the University of California, Davis, we exposed 42 male F344/N rats to either filtered air or 0.12 or 1.0 ppm ozone. After 3 months of exposure to 1.0 ppm ozone, changes in the distribution of superoxide dismutase (SOD) in the copper-zinc (Cu-Zn) form were shown by a pattern of reduced staining in terminal bronchioles and the centriacinar region; and the manganese (Mn) form of SOD was elevated within the centriacinar region. Further analysis by transmission electron microscopy and immunogold labeling confirmed that Mn SOD was elevated within epithelial type II cells immediately distal to the bronchiole-alveolar duct, junction (BADJ). The trachea, three major bronchi, and a short-length and long-length airway path relative to the trachea were examined by morphometric techniques. The pulmonary acini arising from each of these two paths were also examined morphometrically as a function of distance into the alveolar duct. Cellular changes occurring in each of these anatomical regions after 3 months of exposure were analyzed and compared to the changes noted after the 20-month ozone exposures. We found significant increases in the volume density of nonciliated epithelial cells lining the trachea and caudal bronchi as well as in the proximal and terminal bronchioles of the cranial region at a concentration of 1.0 ppm ozone after both 3 and 20 months of exposure. Remodeling of the centriacinar region, particularly within the cranial region of the lungs after exposure to 1.0 ppm ozone, was statistically significant at both 3 and 20 months. No statistically significant effects were noted following exposure to 0.12 ppm ozone for either 3 or 20 months. An important finding was that age did not influence the effect of ozone on the lungs of rats. We conclude that long-term exposure to ozone, rather than the effects of aging, lead to significant alterations of epithelial cell populations lining the airways and centriacinar region of the lung. Marked cellular changes were noted after exposure to 1.0 ppm ozone, but not to 0.12 ppm.