The radiation responses of synchronous L5178Y S/S cells and their significance for radiobiological theory

Abstract

The X-ray responses of the very radiosensitive S/S variant of the L5178Y murine leukaemic lymphoblast have been studied at different phases of the cell cycle. Throughout G 1 and the first half of S, survival was constant; then it rose to a maximum 8 h after mid-G 1 before falling back to the preceding (plateau) level. Although the variation in chromosomal aberration frequency through the cycle mirrored the survival response, the death of only one S/S cell in three was attributable to chromosomal aberrations. Since normally radiosensitive cells exhibit a second peak of survival in G 1 , the response of the S/S variant permitted an experimental precision not available previously for examination of the survival curves of a mammalian cell line and the extant theories concerning them. In the plateau region of cycle response, the survival curve was linear at all doses; in the peak region, however, the survival curve was linear at low doses, then it curved downward until it became linear again at high doses. Throughout the plateau region of cycle response, the survival of the S/S cells was not altered by dose fractionation, but it was increased by post-irradiation hypothermia; in the peak region of cycle response, the converse was true. That behaviour can be explained if, at all phases of the cell cycle, the survival curve of the S/S variant is determined by metabolism, presumably DNA repair, rather than by patterns of primary radiochemical damage.