Abstract

Rats and guinea pigs were exposed to 0.8mg ozone (O 3)/m 3 (∼0.4 ppm) for 12 hr during the daytime, 12 hr during the nightime, or continuously to investigate circadian variation in O 3-induced pulmonary toxicity during single and repeated O 3 exposures. Biomarkers in bronchoalveolar lavage (BAL) fluid and lung tissue were measured as indicators of biochemical and inflammatory responses. Nighttime O 3 exposure of rats resulted in larger increases of protein, albumin, and inflammatory cells in BAL fluid compared to those after daytime O 3 exposure and this daytime-nighttime difference was statistically significant ( p < 0.05). Single daytime or nighttime O 3 exposure of guinea pigs resulted in comparable increases of BAL fluid proteins and inflammatory cells without a daytime-nighttime difference. Nighttime and continuous O 3 exposure of rats for 3 days resulted in comparable increases in lung antioxidant enzyme activities, both of which differed statistically from effects from daytime O 3 exposures ( p < 0.05). Continuous O 3 exposure of guinea pigs for 3 days caused, in general, statistically larger increases in lung tissue parameters compared to nighttime O 3 exposures ( p < 0.05). These results suggest that the extent of O 3-induced acute pulmonary biochemical and inflammatory responses is directly related to the level of physical and respiratory activity. For rats, effects from continuous O 3 exposure appear to be controlled by the nighttime, physically active period. In guinea pigs, the comparable responses following daytime or nighttime O 3 exposure seem in accordance with their random behavioral daily activity pattern. This study supports the view that physical activity-related increases in inhaled dose significantly enhance the pulmonary O 3 responses.

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