Age-dependence of responses to acute ozone exposure in rats.

Previous work from this laboratory demonstrated that neonatal rats and postweanling rabbits are more sensitive to ozone-induced stimulation of pulmonary arachidonic acid (AA) metabolism than are young adults (Fundam. Appl. Toxicol. 15, 779.) In the study reported here, we have extended our initial investigation to include the influence of animal age on temporal aspects of pulmonary AA metabolism and several other responses to brief exposures to 1 ppm ozone. Rats of discrete ages ranging from 13 days to 16 weeks were exposed to 1 ppm ozone or to air for 2, 4, or 6 hr. Immediately following exposure the lungs were lavaged with six consecutive volumes of phosphate-buffered saline and the acellular fluid from the first lavage volume recovered was analyzed for its content of prostaglandin E2 (PGE2), protein, and lactate dehydrogenase. Leukocytes recovered by lavage were quantitated and characterized by viability and percentage of polymorphonuclear (PMN) cells. Several lines of evidence verified that PGE2 was produced by the lung as a consequence of ozone exposure and that its concentration in the fluid from the first lavage was a reasonably good index of pumonary AA metabolism to prostanoids. We also demonstrated that the lavage process itself stimulates the lung, resulting in increased AA metabolism to prostanoids that were recovered in the second and following lavage volumes. The time course of PGE2 production by the ozone-exposed lung varied considerably with animal age. Neonatal rats 13 days of age were the most sensitive to ozone stimulation. At 2 hr of exposure, PGE2 concentration in the first lung lavage of these animals peaked at values approximately two orders of magnitude above controls and then decreased sharply with continued exposure. Adults and older neonates (18 days of age) were much less responsive to 2-hr exposures; however, continued exposure of these rats for up to 6 hr resulted in increasing PGE2 concentration in the first lung lavage. Other responses showed various degrees of age dependence. The percentage of lavaged leukocytes that were nonviable (i.e., trypan blue-positive) showed a strong inverse correlation with animal age. In 13-day-old rats that were exposed for 6 hr, the percentage of dead leukocytes reached nearly 50%. In addition, sheets or clumps of dead cells that were judged to be epithelial cells were lavaged from these animals. Conversely, 16-week-old adult males exposed to ozone for 6 hr showed little evidence of damage to cells of the respiratory tract. This order of age-dependent sensitivity to ozone-induced cellular damage is opposite that reported by some other investigators. Other responses to ozone showed minimal or no evidence of age dependence. Ozone exposure decreased the number of leukocytes lavaged from the lung and increased the concentration of protein in the first lung lavage, but neither variable showed any evidence of age dependence. PMN influx into the airspaces following ozone exposure (estimated from lavaged cells) was weakly age-dependent, with neonates demonstrating slightly greater influx than adults. However, since the time of sampling of all age groups was prior to the time of expected maximum response for both PMN influx and protein extravasation into the airspaces, the observed age-dependent patterns of these two response are not definitive.