Ventilatory responses to ozone are reduced in immature rats

Abstract

During ozone (O3) exposure, adult rats decrease their minute ventilation (V̇E). To determine whether such changes are also observed in immature animals, Sprague-Dawley rats, aged 2, 4, 6, 8, or 12 wk, were exposed to O3 (2 ppm) in nose-only-exposure plethysmographs. Baseline VE normalized for body weight decreased with age from 2.1 ± 0.1 ml · min-1 · g-1 in 2-wk-old rats to 0.72 ± 0.03 ml · min-1 · g-1 in 12-wk-old rats, consistent with the higher metabolic rates of younger animals. In adult (8- and 12-wk-old) rats, O3 caused 40-50% decreases in V̇E that occurred primarily as the result of a decrease in tidal volume. In 6-wk-old rats, O3- induced changes in V̇E were significantly, less, and in 2- and 4-wk-old rats, no significant changes in V̇E were observed during O3 exposure. The increased baseline V̇E and the smaller decrements in V̇E induced by O3 in the immature rats imply that their delivered dose of O3 is much higher than in adult rats. To determine whether these differences in O3 dose influence the extent of injury, we measured bronchoalveolar lavage protein concentrations. The magnitude of the changes in bronchoalveolar lavage induced by O3 was significantly greater in 2- than in 8-wk-old rats (267 ± 47 vs. 165 ± 22%, respectively, P < 0.05). O3 exposure also caused a significant increase in PGE2 in 2-wk-old but not in adult rats. The results indicate that the ventilatory response to O3 is absent in 2-wk-old rats and that lack of this response, in conjunction with a greater specific ventilation, leads to greater lung injury.