MODIFICATION OF RADIATION RESPONSE OF CHO CELLS BY METHYL-ISOBUTYL XANTHINE .2. INCREASE IN EXTRAPOLATION NUMBER

CHO cells exposed for several hours prior to irradiation to 0.2 mM 1-methyl-isobutyl xanthine (MIX) show a more than twofold increase in postirradiation survival over a wide dose range. Increase in radioresistance was shown to be independent of cell-cycle perturbation produced by MIX but was concomitant with MIX-induced morphological conversion of cells from an epithelial form to an elongated, fibroblast-like form. MIX added early in G1 protected cells irradiated subsequently in G1 and to a lesser extent in S. No enhancement of postirradiation survival was seen, however, if addition of MIX was delayed until after the commencement of S. Although MIX-treated cells appeared to accumulate more sublethal damage than did controls, the capacity to repair this damage during the interval between split doses was no greater in MIX-treated cells than in controls.