Ozone Adaptation in Rats after Chronic Exposure to a Simulated Urban Profile of Ozone(1)

WIESTER, M. J.; TEPPER, J. S.; DOERFLER, D. L.; COSTA, D. L.

Studies in both humans and rats have indicated that certain pulmonary responses induced by exposure to an acute provocative concentration of ozone (O(3)) will eventually attenuate if the exposure is repeated on a daily basis. This phenomenon is commonly referred to as O(3) adaptation. Whether or not a “state” of adaptation develops due to long-term low level O(3) exposure is unknown. Two human studies have reported adaptation in subjects living in Los Angeles during periods when ambient O(3) concentrations have been relatively high. At present, however, we are not aware of comparable information from rats. This study assessed O(3) adaptation in rats following chronic (12 or 18 months) exposure and after a 4-month recovery period. A chronic exposure pattern, similar to that found in an urban area during the summer (0.06 ppm O(3) for 13 hr/day, 7 days/week; Monday–Friday, peak to 0.25 ppm O(3), over 9 hr), was used. To assess whether adaptation had occurred and/or persisted, awake rats were challenged with high provocative concentrations of O(3) for up to 2 hr. During a challenge, rats were monitored for typical O(3)-induced alterations in spontaneous breathing parameters (e.g., increase in breathing frequency and decrease in tidal volume). Adaptation was defined as attenuation of breathing response during the challenge in rats chronically exposed to O(3) as compared to that in “control” rats (chronically exposed to air). Adaptation was found in the rats within 8 hr following the chronic O(3) exposure but not after the 4-month recovery period. Spontaneous breathing parameters that were significantly attenuated in the chronically exposed rats were breathing frequency, tidal volume, inspiratory and expiratory times, and maximum expiratory flow. We conclude that rats demonstrated adaptation to O(3) after long-term exposure to an urban-type O(3) profile and that the adaptation was not seen 4 months postexpo-sure. These results suggest that exposure to environmental O(3) in Los Angeles air may have been responsible for the adaptation found in residential subjects.