We previously showed that 0.23 M NaCl was able to fix slowly repairing potentially lethal damage (PLD), and that this slowly repairing PLD is distinct from rapidly repairing PLD that is sensitive to 0.5 M NaCl (Ikebuchi et al., Radiat. Res. 141, 19-27, 1995). In the present study, the effect of 0.23 M NaCl on repair of sublethal damage (SLD) was examined in cells of three rodent cell lines with normal radiosensitivity (Chinese hamster V79, BALB/c 3T3, RD13B2) and two radiosensitive lines derived from severe combined immunodeficient (scid) mice. Repair of SLD was detected as an increase in survival when the radiation dose was fractionated with an interval of incubation between the two doses. Repair of SLD occurred in V79 and BALB/c 3T3 cells but did not occur in the two scid cell lines which were defective in repair of double-strand breaks (DSBs), demonstrating that repair of DSBs is involved in repair of SLD. This was confirmed by the observation that repair of SLD occurred in RD13B2 cells, the scid line which had regained the ability to repair DSBs. When the V79 and BALB/c 3T3 cells were treated with 0.23 M NaCl during the interval between the split doses, repair of SLD was completely inhibited. On the other hand, repair of SLD occurred when the cells were incubated in culture medium between the treatment with 0.23 M NaCl and the second dose. From these observations, it is concluded that the inhibition of repair of SLD by 0.23 M NaCl is reversible, which is in contrast to the irreversible inhibition of repair of PLD by 0.23 M NaCl found previously. In addition, the fact that scid cells that were shown to have the ability to repair PLD that is sensitive to 0.23 M NaCl had little capacity to repair SLD suggests that there may be different processes involved in the two types of cellular repair.

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