X rays have been shown to induce two forms of potentially lethal damage (PLD), "fast-repairing" PLD and "slowly repairing" PLD, whose repair is completed in 1 h and 4-6 h, respectively. In this study three modes of treatment with hypertonic solutions containing different NaCl concentrations for different durations (0.5 M for 30 min, 0.225 M for 4 h, 0.16 M for 16 h) were examined to determine which form of PLD is expressed under each condition. These three modes of treatment enhanced the cell-killing action of X rays on actively growing V79 cells due to fixation of PLD. The kinetics of recovery from PLD was assessed by delayed treatments with hypertonic solutions. Cells exposed to one of the three treatments (see above) had completed recovery times of 1, 4 and 8 h, respectively, suggesting the possibility that these three modes of treatment cause the expression of different forms of PLD. As has been reported, treatment with 0.5 M NaCl for 30 min expressed fast-repairing PLD. The independence of the PLD expressed by 0.225 M NaCl for 4 h from fast-repairing PLD expressed after 0.5 M NaCl for 30 min was shown by combined treatment with the two modes, which reduced survival to the level that would be reached if the two modes acted independently. The data on the recovery time and on the inhibition by 0.225 M NaCl of recovery from slowly repairing PLD in plateauphase cells indicated that the PLD expressed after 0.225 M NaCl for 4 h may be related to slowly repairing PLD. The combined treatment of 0.16 M NaCl for 16 h with 0.225 M NaCl for 4 h indicated independent action, albeit incomplete, of the PLD expressed after 0.16 M NaCl for 16 h from slowly repairing PLD. We propose for the first time that "very slowly repairing" PLD is expressed after 0.16 M NaCl for 16 h in exponentially growing cells and that therefore three forms of PLD are expressed by hypertonic treatments after X irradiation.

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