Potentially lethal damage (PLD) and sublethal damage (SLD) modification in L5178Y-S (LY-S) and L5178Y-R (LY-R) cells was investigated for postirradiation holding in either plateau phase or log phase at either 25 or 37°C. Incubation in both plateau and log phases increased PLD repair (PLDR) at 25°C but increased PLD fixation (PLDF) at 37°C in LY-S cells, with the opposite result (PLDR at 37°C, PLDF at 25°C) in LY-R cells. Conditioned medium (CM from plateau-phase cells) had only a slight effect on the radiosensitivity of log-phase LY-S cells. CM was highly toxic to log-phase LY-R cells even without radiation. Postirradiation incubation of log-phase cells at 25°C increased PLDF between 5.5 and 18 h in LY-R cells, whereas PLDR was completed by 5.5 h in LY-S cells. Flow cytometric data show that the LY-R vs LY-S plateau-phase PLD results are not caused by a differential cell cycle distribution. Although all cell cycle stages are found in the plateau-phase cells, G1 is enriched and cells in all stages must be slowly cycling or stationary. Split doses decrease survival of LY-S cells at 37°C but maintain survival at the single acute total dose level at 25°C (no SLD repair (SLDR)). SLDR is observed at both 25 and 37°C in LY-R cells. The results for both the split dose and delayed plating experiments are best understood in terms of competition between PLD repair and fixation with their relative contributions depending on cellular metabolism, which can be altered by changes in temperature or medium constituents. The opposite PLD results obtained for LY-S and LY-R cells suggest that radiosensitivity in these cells is determined by two different mechanisms.

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