Morphological basis of tolerance to ozone.

The purpose of this research was to study Type 1 epithelial cells in the ozone (O3)-tolerant lung epithelium. Rats were made tolerant by exposure to 0.5 ppm O3 for 2 days and allowed to recover in air. Reexposure to a lethal concentration of O3 (6 ppm) at 3, 7, and 15 days of recovery revealed that tolerance was present at 3 days but almost absent at 7 and 15 days of recovery. Using Type 2 cell proliferation as a means of quantitating Type 1 cell injury, it was observed that when the preexposed rats were reexposed to 0.5 ppm at 3, 7, and 15 days, very little Type 1 cell injury occurred at 3 days. However, at 7 and 15 days the amount of Type 1 cell injury was the same as that associated with the original exposure. To determine whether there was any change in the alveolar epithelial cell populations between the periods of tolerance (3 days) and its decline (7 and 15 days), the percentage of tritiated thymidine [( 3H]TdR-labeled Type 1 and 2 cells at these times were determined. There was a significant decrease in [3H]TdR-labeled Type 1 and 2 cells between the third and fifteenth days of recovery as excess cells were sloughed off and the tissue returned to normal. Using electron microscopic morphometry, Type 1 and 2 cells were then studied during the decline of tolerance. No change was found in the morphology of Type 2 cells; however, the morphology of Type 1 cells revealed a 58% decrease in surface area and a 25% increase in the arithmetic mean thickness when tolerance was present at 3 days. As tolerance declined (7 and 15 days), Type 1 cell morphology returned to normal. It was concluded that tolerance exists when the surface area of a cell exposed to a particular concentration of ozone is small enough so that the existing antioxidant mechanism contained within that cell volume can protect it from damage.