The relationship between the inhibition of repair of radiation-induced DNA damage and the inhibition of recovery from radiation-induced potentially lethal damage (PLD) by hypertonic treatment was compared in 9L/Ro rat brain tumor cells. Fed plateau phase cultures were gamma-irradiated with 1500 rad and then immediately treated for 20 min with a 37 degree C isotonic (0.15 M) or hypertonic (0.50 M) salt solution. The kinetics of repair of radiation-induced DNA damage as assayed using alkaline filter elution were compared to those of recovery from radiation-induced PLD as assayed by colony formation. Hypertonic treatment of unirradiated cells produced neither DNA damage nor cell kill. Post-irradiation hypertonic treatment inhibited both DNA repair and PLD recovery, while post-irradiation isotonic treatment inhibited neither phenomenon. However, by 2 h after irradiation, the amount of DNA damage remaining after a 20 min hypertonic treatment was equivalent to that remaining after a 20 min isotonic treatment. In contrast, cell survival after hypertonic treatment remained 2 logs lower than after isotonic treatment even at times up to 24 h. These results suggest that the repair of radiation-induced DNA damage per se is not causally related to recovery from radiation-induced PLD. However, the data are consistent with the time of DNA repair as an important parameter in determining cell survival and, therefore, tend to support the hypothesis that imbalances in sets of competing biochemical or metabolic processes determine survival rather than the presence of a single class of unrepaired DNA lesions.

DNA from unirradiated and irradiated cultured 9L rat brain tumor cells was held for varying times in low ionic strength solutions at pH 11.0, 12.3, or 12.9. The effect of this exposure to alkali on the DNA size distribution was determined by comparing the DNA filter elution profiles obtained experimentally with those theoretically predicted for monodispersed and random distributions. At pH 12.3 or 12.9, DNA from cells irradiated with 300 rad eluted with first-order kinetics corresponding to a random DNA size distribution. The median size of the distribution decreased if the irradiated DNA was exposed to pH 12.3 for 24 h. At pH 12.3 or 12.9, DNA from unirradiated cells eluted initially with complex kinetics that later became linear (18-21 h for pH 12.3 or 13-15 h for pH 12.9), characteristic of a monodispersed DNA size distribution. Holding either unirradiated or irradiated DNA at pH 11.0, below the critical unwinding pH, produced no effect on the elution profiles. Analysis of these filter elution data indicated that after sufficient exposure to pH 12.3 or 12.9, undamaged DNA molecules from mammalian cells elute as a single-stranded monodispersed size distribution of approximately 1 X 10(10) daltons. While the possibility cannot be completely eliminated that this monodispersed size represents an upper limit determined by physical forces, these results, in conjunction with those obtained using other techniques, lend credence to the existence of a nonrandom higher-order structure in mammalian chromosomal DNA.