Consequences of prolonged inhalation of ozone on F344/N rats: collaborative studies. Part IX: Changes in the tracheobronchial epithelium, pulmonary acinus, and lung antioxidant enzyme activity.

The effect of ozone on the respiratory system is not confined to a single region or a specific cell type. Ozone-induced injury can occur at all levels of the respiratory system. However, the effects of this oxidant gas throughout the tracheobronchial tree and the lung parenchyma can be highly variable. The doses of ozone delivered to the various regions may also be different, and these differences may have a significant effect on the extent of injury. To examine the effects of chronic exposure to ozone on the lungs, we used a systematic sampling approach to perform morphometric, histochemical, and enzymatic analyses of selected airway generations and pulmonary acini arising from short and long airway paths of the tracheobronchial tree. The objectives of this study were to define compositional, cytochemical, and architectural changes that occur in epithelial cells of the airways and major tissue components of the pulmonary acini after 20 months of exposure to 0.0, 0.12, 0.5, or 1.0 parts per million (ppm)* ozone in male and female F344/N rats. We found in the trachea and bronchi significant alterations in stored secretory product following exposure to ozone, but no changes in epithelial thickness or the volume density of nonciliated cells. The volume density of nonciliated cells was significantly increased in terminal bronchioles arising from a long airway path (caudal region) of the left lung. The predominant change within the pulmonary acini was the extension of bronchiolar epithelium beyond the bronchiole-alveolar duct junction into alveoli. This change was concentration-dependent and site-specific, with ventilatory units arising from a short path (cranial region) of the left lung in male rats being most affected. The antioxidant enzymes superoxide dismutase, glutathione peroxidase, and glutathione S-transferase were significantly elevated in the distal bronchiole to central acinus following 20 months of exposure to 0.5 or 1.0 ppm ozone. Changes in antioxidant enzyme levels were more variable in other airway generations. We conclude that the effects of long-term (20-month) exposure to ozone are dose-dependent and site-specific along the tracheobronchial tree and pulmonary acini of the lungs. With the tissue sampling strategies used in this study, for the first time microdosimetric relations between ozone concentrations and biological changes in precisely delineated regions of the lungs can be defined along the entire lower respiratory tract.