"Single-Hit" Potentially Lethal Damage: Evidence of Its Repair in Mammalian Cells

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Abstract
Following mid to large doses of X-rays, or of fission spectrum neutrons, the repair of potentially lethal damage in V79 Chinese hamster [lung] cells can be inhibited by anisotonic phosphate-buffered saline or by medium containing 90% D2O. The foregoing post-treatments do not affect the viability of unirradiated cells. Using single synchronized cells irradiated in late S-phase, the most resistant phase of the cell cycle, repair of potentially lethal damage in the single-hit, initially exponential, or small-dose part of the survival curve was examined. The use of synchronized cells avoids misinterpretations due to population heterogeneity. The slope of the small-dose, exponential region of the neutron survival curve is much steeper than that of the X-ray survival curve. Even so, it is demonstrated with post-treatments consisting of hypertonic phosphate-buffered saline, medium containing D2O, or medium containing caffeine that in the small-dose region cells ordinarily repair potentially lethal, single-hit neutron damage. Sensitive cells, i.e., cells not able to repair potentially lethal damage expressible by hypertonic buffer appear not able to repair single-hit damage.