PREDICTION OF THE INITIAL SLOPE OF THE ACUTE CLONOGENIC SURVIVAL-CURVE BY THE POSTIRRADIATION BEHAVIOR OF CYTOCHALASIN-INDUCED POLYKARYONS

We have investigated the behaviour of 11 lines of cultured cells in a survival assay whose endpoint is the ability of cells to become polypl oid when incubated in the presence of cytochalasin B (CB). Single cell s were induced by CB to form polykaryons after irradiation, and, by an alogy with the colony-forming assay, the survival of polykaryon-formin g units (PFUs) was defined as the fraction of cells able to achieve a given DNA content (at least 16C in most experiments). There was a radi ation dose-dependent reduction in PFU survival, which, following the a ppearance of cells containing at least 16C DNA, was not markedly depen dent upon the sampling time. In all cases, PFUs appeared to be more ra dioresistant than clonogens, especially at high dose. In 9/11 lines th e PFU dose-response curves were exponential, while in two there was a pronounced curvature (quadratic parameter). There was a highly signifi cant positive correlation between PFU response and the corresponding c lonogenic initial slope. We suggest that in polykaryons the opportunit y for the mechanical loss of DNA fragments may be reduced because the cells do not divide, and therefore that DNA damage resulting from chro mosome aberrations may be less important in PFUs than in clonogens. Co nsequently, PFUs may express lesions not directly associated with mech anical gene loss. This assay may yield an alternative estimate for the clonogenic initial slope, and could be of use where colony-forming as says fail due to incomplete cell monodispersion.