Hyperoxia increases oxygen radical production in rat lung homogenates

Lung damage during hyperoxia has been postulated to be due to increased rates of local organ oxygen radical production. Lung homogenate respiration was inhibited with cyanide, and residual respiration was used as an indicator of electron diversion to O₂^? and H₂O₂. Cyanide-resistant respiration in lung homogenates, supplemented with 1 mm NADH, increased linearly with oxygen tension, and accounted for 7% of total respiration in air and for 17% of total respiration when homogenates were incubated in 80% oxygen. Exposure of rats to 85% oxygen for 7 days induces tolerance to the lethal effects of 100% oxygen. Rats which previously breathed 85% oxygen for 7 days had a greater CN^?-resistant respiration than control rats. This implies that adaptation to hyperoxia does not include decreased lung tissue oxygen radical production as indicated by CN^?-resistant respiration. One possible explanation for the increased CN^?-resistant respiration in oxygen tolerant rat lungs is that they contain increased cell mass. Lung homogenates of rats exposed to 85% oxygen for 7 days also had 2.5 times greater thiobarbituric acid positive material than controls, indicating that increased lung lipid peroxidation occurs as a consequence of hyperoxia. Incubation of normal rat lung homogenates under hyperoxic conditions also acutely increased lipid peroxidation, which could be inhibited by both superoxide dismutase and catalase. This confirms that hyperoxia enhances cellular production of O₂^? and H₂O₂ and implies an essential role for both O₂^? and H₂O₂ in hyperoxic lung damage.