AIRWAY EPITHELIAL LABELING INDEX AS AN INDICATOR OF OZONE-INDUCED LUNG INJURY

Rats were implanted subcutaneously with a bromodeoxyuridine-filled min ipump and then were exposed to ozone delivered at a low dose rate (0.4 ppm during 12 h per night) or at a high dose rate (0.8 ppm during 6 h per night). Three and 7 days after pump implantation the cumulative l abeling indices were measured in the alveolar zone and in the airways. Greater alveolar labeling indices were observed 7 days after implanta tion of the minipumps than after 3 days in all groups, but no ozone-re lated changes were found in the alveoli-of rats in either experimental group at either time. After 3 or 7 days, the labeling index in the la rge intrapulmonary airways and in the terminal bronchioli of the rats exposed to the higher dose rate (0.8 ppm) was increased. In rats expos ed to the lower dose rate (0.4 ppm) the labeling index was significant ly elevated in the terminal bronchioli after 3 days and in both the te rminal bronchioli and large intrapulmonary airways after 7 days. In th e terminal bronchioli the extent of cell proliferation appeared to be defined by dose rate rather than by cumulative exposure. It is conclud ed that measurement of the airway labeling index is a sensitive indica tor of the response of the rat lung to acute exposure to ozone.