RADIATION-MODIFYING EFFECT OF OXYGEN IN SYNCHRONIZED CELLS PRETREATEDWITH ACUTE OR PROLONGED HYPOXIA

The objective of the present study Tvas to measure the sensitizing eff ect of oxygen following acute as well as protracted hypoxia in cell cu ltures of maximum homogeneity. Human cells of the established line NHI K 3025 were synchronized by the method of mitotic selection and irradi ated while in late G1 (aerobic conditions) or in the oxygen-sensitive restriction point near the G1/S border. Four different condi tions of oxygen treatment were used: (1) Cells were irradiated under aerobic co nditions 5h after mitotic selection (G1 lasts for 6.5h in these cells) . (2) Cells were irradiated under extremely hypoxic conditions (i.e. < 4 ppm O-2) following an acute (30 min) deoxygenation starting 5 h afte r mitotic selection. (3) Cells were irradiated under aerobic condition s immediately following 20h of extreme hypoxia which was started 5h fo llowing mitotic selection. (4) Cells were irradiated under extremely h ypoxic conditions after 20h of extreme hypoxia starting 5h after mitot ic selection. In addition, asynchronous cells were irradiated, either without any pretreatment, or after reoxygenation, following 20 h of ex treme hypoxia. The data indicate that the sensitizing effect of oxygen (after protracted hypoxia) is strictly dose-modifying for cells rende red hypoxic while in late G1 and arrested in the oxygen-sensitive rest riction point in late G1. There is a non-dose-modifying sensitization when synchronized control cells, irradiated under aerobic conditions, are compared with synchronized cells irradiated during acute hypoxia. In this case, however, one is comparing cells in two different biologi cal states, i.e. cells in an ordinary G1 are compared with cells arres ted in the oxygen-sensitive restriction point in G1. The non-dose-modi fying nature of the oxygen sensitization as observed in this case may, therefore, reflect biological differences between the cell cultures t hat are compared rather than differences with respect to the radiochem ical sensitization as induced by oxygen at small and large radiation d oses. The present data indicate, however, that if cancer cells in huma n rumours are best represented by a mixture of aerobic cells and chron ic hypoxic cells, oxygen may appear to exert no effect, or perhaps eve n a net protective effect, at low radiation doses.