Are Changes in Breathing Pattern on Exposure to Ozone Related to Changes in Pulmonary Surfactant?

Abstract

Rats respond to the inhalation of ozone with changes in breathing pattern during the exposure and the development of a pulmonary inflammatory response 24–48 h postexposure. We report experiments designed to investigate the relationships between changes in breathing pattern and the composition and surface tension-reducing properties of pulmonary surfactant immediately after the exposure. A total of 64 male Fischer 344 rats were exposed to 0.8 ppm O₃ for 4 h in 4 replicate exposures with matched purified air control exposures and 8 rats per exposure group. Those exposed to O₃ developed the rapid-shallow breathing pattern characteristic of oxidant pulmonary irritation during exposure. The rats were sacrificed immediately following the exposure, and pulmonary surfactant was isolated from samples of bronchoalveolar lavage fluid pooled from groups of eight rats. After esterification, the fatty acid methyl ester composition was measured using GC-MS. in the ozone-exposed animals, the decreases in unsaturated species (linoleic, oleic, palmitoleic, and an unidentified fatty acid) relative to the major saturated component, palmitic acid, were highly statistically significant, while stearic acid showed no significant change. Total protein in the lavage fluids of the exposed animals was not elevated, indicating that sacrifice and analysis were performed early in the O₃ injury-inflammation response sequence, and suggesting that the fatty acid changes in the pulmonary surfactant may be due in part to a direct reaction with inhaled O₃. The group mean change in breath frequency (as percent of matched purified air control values) was significantly correlated with the percent change in lnoleic acid fraction among replicate exposures, suggestive of a possible relationship between ozone-induced changes in pulmonary surfactant and changes in breathing patterns. There was no significant change in the surface pressure-area isotherms of monolayers of pulmonary surfactant upon ozone exposure. However, comparison to isotherms from an in vitro exposure of a synthetic mixture of saturated and unsaturated phospholipids suggests that changes due to the observed change in the fatty acid composition in the in vivo experiments may be too small to be observed. Furthermore, the pulmonary surfactant isolation procedure was specifically designed to recover the undamaged surfactant and may discriminate against products of reaction with ozone; hence the isotherms may not necessarily reflect the actual changes during the exposure. Further experiments to elucidate the interaction of inhaled O₃ with pulmonary surfactant and its relationship to changes in breathing pattern are discussed.