VARIATION IN RADIOSENSITIVITY DUE TO CELL AGE AND SPLIT-DOSE RECOVERYIN POLYKARYONS INDUCED BY CYTOCHALASIN

We have investigated the properties of an in vitro cell survival assay that uses as its endpoint the ability to form polyploid cells (polyka ryons) in the presence of cytochalasin B (CB). The criterion for survi val is that a polykaryon-forming unit (PFU) must reach the arbitrary D NA content of at least 16C. The age-dependence of PFU sensitivity to C s-137 irradiation was determined using V79-379A cells synchronized at mitosis. Cells assayed as PFUs demonstrated much less variation in rad iosensitivity with age than did clonogens, but the changes in curve sh ape were qualitatively similar. In both assays mitotic cells yielded a n exponential survival curve while that obtained at 5 h (mid-late S) h ad a marked quadratic component. Owing to the small overall variation in PFU survival with age, at doses greater than about 25 Gy the surviv ing fraction at 5 h was lower than in mitosis. In both V79-379A and He La S3 cells, PFUs demonstrated a capacity for split-dose recovery and yielded recovery ratios at 2.6 at 50 Gy in V79 and 1.5 at 20 Gy in HeL a. Since these ratios were much lower than in clonogens at the same do se, we suggest that this is consistent with an association that we hav e previously demonstrated between PFU response and the clonogenic init ial slope. In an attempt to clarify the DNA lesions to which PFUs may be sensitive, we determined PFU response following exposure to 254-nm UV irradiation. In contrast with ionizing radiation, PFU response to U V was very similar to that of clonogens. This suggests that following UV exposure the absence of cytokinesis in polykaryons may confer less protection than in the case of ionizing radiation, possibly due to fun damental differences in the spectrum of DNA lesions produced.