Protection from O2 toxicity by preexposure to hypoxia: lung antioxidant enzyme role

Adult rats preexposed to 10% O2 for 3 days had marked tolerance to hyperoxia-induced lung damage and lethality. The survival of preexposed vs. nonpreexposed rats at 72 h of hyperoxic exposure was 62/62 vs. 7/47 (15%), P less than 0.0001; and after 7 days in 96-98% O2, the comparative survival was 31/33 (94%) vs. 1/20 (5%), P less than 0.0005. Hypoxic exposure produced significant elevations in rat lung superoxide dismutase, catalase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase activities. In contrast, in adult mice and hamsters, no increased lung antioxidant enzyme levels were produced by preexposure to hypoxia and no significant tolerance to high O2 was realized. (Lethal time50 values for hypoxia-preexposed and nonpreexposed mice, 5.2 and 4.4 days, respectively; and for hamsters, 6.4 and 6.1 days, respectively.) Thus the protective effect of hypoxic preexposure is correlated with adaptive changes in lung antioxidant enzyme activity. Evidence in the literature suggests that superoxide anion (O-2) and H2O2 production may increase under hypoxic conditions. Increased cellular concentrations of their normal substrates could stimulate antioxidant enzyme rises during the preexposure period in hypoxia.