Evidence for differences among the sectors of potentially lethal damage expressed by hypertonic treatment in plateau-phase V79 cells after exposure to neutrons or gamma rays: the importance of distinction between alpha and beta-PLD forms.

Expotentially growing and plateau-phase V79 cells were exposed to various doses of neutrons and plated either immediately or after treatment in hypertonic medium (250-500 mM NaCl) to express radiation-induced potentially lethal damage (PLD). Postirradiation treatment of exponentially growing cells in hypertonic medium (500 mM) resulted in a decrease in both Dq and D0, whereas postirradiation treatment of plateau-phase cells in hypertonic medium (in the range between 200 to 1,500 mM) resulted mainly in a reduction of Dq. This difference in response between exponentially growing and plateau-phase cells may reflect differences in the chromatin structure in cells at various stages of the cell cycle, affecting fixation of radiation-induced damage. Exposure of plateau-phase cells to gamma rays, on the other hand, resulted in a treatment time and salt concentration-dependent decrease in Dq along with a decrease in D0. Repair of neutron-induced, hypertonic treatment-sensitive PLD, measured by delaying treatment for various periods after irradiation, was found to proceed with a t1/2 of about 1 h. This is similar to the repair kinetics obtained by delaying treatment of plateau-phase cells with 150 microM beta-D-arabinofuranosyladenine (araA) after exposure to gamma rays or neutrons and contrasts the repair kinetics observed after exposure of cells to gamma rays. In this case, hypertonic treatment was found to affect a form of PLD repaired with a t1/2 of 10-15 min (beta-PLD) and araA, a different form of PLD, repaired with a t1/2 of about 1 h (alpha-PLD). Based on these results it is hypothesized that the sector of lesions affected by hypertonic treatment and araA coincides after exposure to neutrons (effect on alpha-PLD) but only partly overlaps after exposure to gamma rays (due to the effect on beta-PLD of hypertonic treatment). The results presented, together with previously published observations, suggest a differential induction and/or fixation by hypertonic medium of the alpha- and beta-PLD forms as the LET of the radiation increases. Furthermore, they indicate that direct comparison of the effects of a postirradiation treatment, as well as of the repair kinetics obtained by its delayed application after exposure to radiations of various LET, should be made with caution.