Endotoxin reverses the decreased tolerance of rats to greater than 95% O2 after preexposure to lower O2.

Bacterial endotoxin has a marked protective effect against pulmonary O2 toxicity in rats placed directly in atmospheres of greater than 95% O2. To determine whether endotoxin treatment during exposure to relatively low levels of hyperoxia would protect rats from the accelerated O2 toxicity that normally occurs when these rats are transferred to greater than 95% O2, we gave endotoxin or saline 1) during exposure to 40% O2 (5 days), or 2) during exposure to 40%-60%-85% O2 (2 days at each level). Saline-treated rats showed significantly decreased tolerance on transfer to greater than 95% O2 [LT50 = 47.5 h (exposure 1) and 48.5 h (exposure 2)] compared with normal nonpreexposed rats (LT50 = 66 h). In contrast, endotoxin-treated rats showed a marked tolerance on transfer to greater than 95% O2 [% of rats surviving 72 h = 14/16 (88%) endotoxin-treated vs. 2/16 (13%) saline-treated]. The endotoxin-treated rats, unlike the saline-treated rats, showed significant elevations in lung superoxide dismutase, catalase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase levels after the O2 preexposure periods; this may account for their significantly improved tolerance when challenged with greater than 95% O2 exposure.