The Influence of Variable Oxygen Concentration on the Response of Cells to Heat or X Irradiation

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Abstract
The influence of O2 concentration on the lethal response of cells exposed to 43.degree. C hyperthermia was determined and compared to the response of cells exposed to radiation under equivalent culturing and environmental conditions. Chinese hamster ovary (CHO) cells were heated or irradiated 0.5 h after induction of hypoxia and then reoxygenated following treatment. The medium was equilibrated with a gas-phase O2 concentration ranging from 10-20 ppm to 95% O2 with a balance of 5% CO2 and N2. The resultant medium pH continued at 7.3-7.4. The O2 enhancement ratio (OER) for heat or radiation was determined at the 1% survival level from least-squares fit of survival curves. For radiation, a classic Alper and Howard-Flanders curve was observed; sensitization was apparent at an O2 concentration as low as 0.05% with a half-maximum sensitization occurring at 0.5%. A maximum OER of 3.1 .+-. 0.2 was observed in the 20-95% O2 concentration range. The OER for heat was 1.0 .+-. 0.1 irrespective of the gas-phase O2 concentration. The lethal effects of heat apparently are not influenced by the O2 concentration at the time of treatment in CHO cells exposed to 43.degree. C hyperthermia. Implications for treatment of cancer are discussed.