Further Studies on the Nature of the Biphasic Radiation Survival Response of Chinese Hamster Cells V-79-753B to Molecular Oxygen

Abstract

We have previously reported that for Chinese hamster cells, V-79-753B, irradiated as monolayer cultures at room temperature in phosphate buffered saline (PBSA), the radiation survival response is biphasic with respect to oxygen concentration (1). At oxygen concentrations between 1.5 and 7.0 μM sensitization is constant, equivalent to an oxygen enhancement ratio of 1.9 compared with the full oxygen enhancement ratio of 3.1. This region of constant sensitization has been designated the “plateau”. Additionally, examination of the initial yield of single strand breaks (ssb) in the irradiated DNA of this cell line showed that the full oxygen effect was produced at an oxygen concentration of 1.5 μM, the start of the “plateau” in terms of cell survival (2). Whilst a similar biphasic survival response has been seen for bacterial spores irradiated in the presence of different concentrations of oxygen (3), there is as yet no other example for either bacterial (4) or mammalian cells (5, 6). The equation developed by Howard-Flanders and Alpher (7), which postulates that oxygen is in simple competition with target repair reactions, cannot be applied to this cell line. Furthermore, the shape of the survival response curve is not unique to molecular oxygen. When hypoxic cells are irradiated in the presence of different concentrations of radiosensitizers the radiation survival response is also biphasic as a function of the concentration of sensitizer (2). Additionally, using a rapid-mix technique sensitization by misonidazole has been shown to occur by two time resolvable components (8).