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

Studies in both humans and rats have indicated that certainpulmonary responses induced by exposure to an acute provocativeconcentration of ozone (O₃) will eventually attenuate if theexposure is repeated on a daily basis. This phenomenon is commonlyreferred to as O₃ adaptation. Whether or not a "state" of adaptationdevelops due to long-term low level O₃ exposure is unknown.Two human studies have reported adaptation in subjects livingin Los Angeles during periods when ambient O₃ concentrationshave been relatively high. At present, however, we are not awareof comparable information from rats. This study assessed O₃adaptation in rats following chronic (12 or 18 months) exposureand after a 4-month recovery period. A chronic exposure pattern,similar to that found in an urban area during the summer (0.06ppm O₃ for 13 hr/day, 7 days/week; Monday–Friday, peakto 0.25 ppm O₃, over 9 hr), was used. To assess whether adaptationhad occurred and/or persisted, awake rats were challenged withhigh provocative concentrations of O₃ for up to 2 hr. Duringa challenge, rats were monitored for typical O₃-induced alterationsin spontaneous breathing parameters (e.g., increase in breathingfrequency and decrease in tidal volume). Adaptation was definedas attenuation of breathing response during the challenge inrats chronically exposed to O₃ as compared to that in "control"rats (chronically exposed to air). Adaptation was found in therats within 8 hr following the chronic O₃ exposure but not afterthe 4-month recovery period. Spontaneous breathing parametersthat were significantly attenuated in the chronically exposedrats were breathing frequency, tidal volume, inspiratory andexpiratory times, and maximum expiratory flow. We conclude thatrats demonstrated adaptation to O₃ after long-term exposureto an urban-type O₃ profile and that the adaptation was notseen 4 months postexpo-sure. These results suggest that exposureto environmental O₃ in Los Angeles air may have been responsiblefor the adaptation found in residential subjects.