SMALL DOSES OF HIGH-LINEAR ENERGY-TRANSFER RADIATION INCREASE THE RADIORESISTANCE OF CHINESE-HAMSTER V79 CELLS TO SUBSEQUENT X-IRRADIATION

Chinese hamster V79 cells show a complex X-ray survival response which is characterized by hypersensitivity followed by increased resistance as the dose increases to 1 Gy. This hypersensitivity can be eliminate d by pretreating cells with X rays or hydrogen peroxide. Accordingly, the protective effect that results from the priming treatments could b e considered analogous to the ''adaptive response'' induced by low-lin ear energy transfer (LET) radiation and some chemical agents in human lymphocytes. Indeed, no hyper-radiosensitive response after single tre atments in V79 cells or adaptive response in human lymphocytes has bee n reported after exposure to high-LET radiation. To investigate this f urther, we measured the survival after X irradiation of V79-379A cells previously irradiated with small priming doses of high-LET radiation. After a 0.2-Gy priming dose of neutrons followed by a l-Gy 250 kVp X- ray dose given 4 h later, survival was 1.08 +/- 0.04 compared to 0.73 +/- 0.03 when the doses were given concurrently, Increases in survival were also observed from 0.80 +/- 0.03 to 0.96 +/- 0.05 after a 0.2-Gy priming treatment with 250 kVp X rays and from 0.78 +/- 0.03 to 0.84 +/- 0.03 with a priming dose of Bragg-peak negative pi mesons. The res ults indicate that a protective effect, as measured by an increase in radioresistance, is induced by high-LET neutrons, as well as by Bragg- peak pi mesons and X rays, and that a threshold level of damage is req uired for adaptation to occur. (C) 1996 bg Radiation Research Society