The effect of hypothermic treatment on the repair or expression of X-ray damage measured in split dose experiments on L5178Y-S cells.

The nature of the post-irradiation lesions and processes leading to cellular reproductive death or survival were investigated in mouse lymphoblastic leukemia L5178Y-S (LY-S) cells. Post-(x-)irradiation incubation at 25 degrees C protects LY-S cells against the fixation of biologically expressed damage which takes place at 37 degrees C. An optimal condition for the repair of damage, assayed in split-dose experiments as split-dose recovery (SDR), is 1 h at 37 degrees C followed by 4 h holding at 25 degrees C prior to the second half of a split dose, or 5 h holding at 25 degrees C without a 37 degrees C incubation during the interval between doses. Longer incubations at 37 degrees C resulted in progressively decreased survivals. Postirradiation inhibition of DNA synthesis at 37 degrees C was observed only during the first 30 min; thereafter, 3H-dThdR incorporation was higher than in unirradiated controls. The excess synthesis effect was removed by shifting irradiated cells to 25 degrees C holding. The inhibition observed at 25 degrees C was reversed by shifting to 37 degrees C. Thus the degree of postirradiation DNA synthesis is inversely related to SDR. DNA filter elution shows complete strand break repair by 20 min at 37 degrees C, and by 3 h at 25 degrees C; DNA double-strand break (DSB) repair plateaus at 80% (37 degrees C) and 60% (25 degrees C) after 90 min. An inverse correlation was found between total strand break repair rate, as assayed by filter elution methods, and cell survival. This work was supported by a grant from The Mathers Charitable Foundation.