Attenuation and recovery of pulmonary injury in rats following short-term, repeated daily exposure to ozone

Controlled human and epidemiology studies have demonstrated that during repeated exposure to ozone (O-3) attenuation of lung function responses may occur. It is yet unknown whether inflammatory and biochemical effects in lower airways of humans, as observed upon single O-3 exposure, also show a diminutive response following repeated exposure to O-3. The aim of this study was to investigate inflammatory, permeability, and histopathological responses in lungs of rats following repeated daily O-3 exposure and to study the time course of attenuation and recovery of these effects using single O-3 challenges at various postexposure times. To aid in animal-to-human extrapolation, this study and a previously reported human study (Devlin et al., 1997) were designed with similar protocols. Wistar rats were exposed for 5 consecutive days to 0.4 ppm O-3 for 12 h/night. Subsequently, the time course of postexposure recovery was determined by a single challenge of 12 h to 0.4 ppm O-3 after a 5-, 10-, 15-, or 20-day recovery period. Bronchoalveolar lavage (BAL) examination and histopathology were performed 12 h after this O-3 challenge. To quantify the magnitude of the O-3 response, results were compared with a group exposed only once for 12 h to 0.4 ppm O-3 and sacrificed simultaneously. The results demonstrate that a single exposure of 0.4 ppm O-3 causes marked permeability and inflammatory responses in lower airways of rats, as evidenced by enhanced BAL fluid levels of proteins, fibronectin, interleukin (IL)-6, and inflammatory cells. However, 5 days of exposure to 0.4 ppm O-3 for 12 h/night resulted in a complete disappearance of these responses, resulting in BAL fluid values that were not different from those observed in unexposed controls. Postexposure analyses of pulmonary response to O-3 challenges demonstrated that these attenuated responses show a gradual recovery. The data indicate that with respect to BAL fluid levels of albumin, IL-6, and number of macrophages and neutrophils, the period for lung tissue to regain its full susceptibility and responsiveness to O-3 following a 5-day preexposure period is approximately 15-20 days. Remarkably, the total protein and fibronectin responses in BAL fluid still exhibited an attenuated response to an O-3 challenge at 20 days postexposure. Morphometry (number of BrdU-Iabeled cells in terminal bronchiolar epithelium, and number of alveolar macrophages) showed that after a recovery of 5-10 days following a 5-day preexposure the response to a challenge was identical to that after a single exposure. These results suggest that complete repair from lower airway inflammation caused by short-term, repeated exposure to O-3 may take longer than previously assumed.