Hypothermic enhancement of the lethal effect of 3.5 Gy of 220-kV X rays in the absence of caffeine as well as in its presence (4 mM) was examined at temperatures between 10 and 34°C in monolayer cultures in the G1 phase of the cell cycle. Correction has been made for the toxicity of low temperatures, and of caffeine at low temperatures, by concomitantly measuring cell killing in unirradiated cells. In the absence of caffeine, incubation of irradiated cells for up to 34 h at temperatures in the range 15 to 30°C (or possibly 34°C) enhances killing compared to that observed at 38°C; the amount of enhancement is about the same throughout this range, but is nil at 10°C. The enhanced killing induced by caffeine at 38°C decreases as the temperature is lowered to 15°C; there is no enhancement at 10°C. Less killing is manifested in the range 15 to 25°C in the presence of caffeine than in its absence. Recovery (loss of sensitivity to caffeine) and fixation of potentially lethal damage were studied in$\text{late-}{\rm S}/{\rm G}_{2}\text{-phase}$ cells at reduced temperatures by delaying treatment with caffeine for increasing times after irradiation. As the temperature is progressively lowered to 20°C, less recovery is manifested after 5 h of incubation; no recovery is detected in the range 10 to 20°C. Despite extensive recovery at 34°C, no fixation is observed at that (or any lower) temperature in G2-phase cells: the cells are able to recover essentially fully when returned to 38°C. In addition, responses of unirradiated control series to incubation at low temperatures appear to differ from those reported by others for longer treatment times of different cell systems.

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