Effects of 60% oxygen inhalation on the survival and antioxidant enzyme activities of young and old rats.

Under 60% oxygen, both the 50% and maximum survival times of old rats were markedly shortened, and the maximum survival time of young rats did not change although the 50% survival time was shortened. In addition, the mean body weight of the old rats decreased rapidly, while that of the young rats increased very slowly after the small decrease. In lungs of the young and old rats, the activities of catalase and Mn superoxide dismutase (SOD) were increased, while those of CuZn SOD and glutathione peroxidase remained unchanged. In the liver of the young rats, the activities of Mn SOD and glutathione peroxidase were increased. In the lungs of the old rats, the messenger RNA (mRNA) levels of these antioxidant enzymes were markedly increased. The oxygen-dependent mRNA induction did not correspond to the augmentations of the activities of antioxidant enzymes. The protein levels and activity of CuZn SOD did not changed by 60% oxygen inhalation although the mRNA level was increased to 4.7-fold at 2 weeks of oxygen exposure. Translational efficiency of antioxidant enzymes in old rats might be reduced under oxidative stress. These results indicate that old rats are less tolerant to the oxidative stress of 60% oxygen than young rats because antioxidant enzyme activities are less induced due to low translational efficiency, and suggest that the activities of antioxidant enzymes, not only in the lung but also in the liver, may contribute to the tolerance to oxidative stress.