REOXYGENATION ASSOCIATED RADIOSENSITIZATION AFTER CHRONIC HYPOXIA - EFFECT OF TEMPERATURE AND OXYGEN-TENSION

A431 human squamous carcinoma cells reoxygenated after 12 hr hypoxia were found to be more radiosensitive than aerobic cells not previously exposed to hypoxia. Experiments were carried out to investigate the effect of temperature and oxygen tension during the hypoxia and the reoxygenation periods on the sensitization effect. Incubation at 0-degrees-C during 12 hr of hypoxia inhibited the sensitization seen after reoxygenation. During the reoxygenation period, reduced temperature did not modify the levels of enhancement but suppressed the decay of the sensitization. If oxygen tension during 12 hr of hypoxia was < 10 ppm, maximal enhancement by reoxygenation was seen, but there was no enhancement if the oxygen concentration was greater than 1%. In cells subjected to < 10 ppm oxygen for 12 hr, the level of sensitization by reoxygenation in 2 to 20% oxygen concentration was similar. The results indicate that radiosensitization by reoxygenation may require metabolic change(s) of cells during both the hypoxia and reoxygenation periods. The sensitization may also involve oxygen dependent species of reactions.