Previous observations have shown that cells cultured in standard growth medium (100%) demonstrated similarly enhanced survival when incubated postirradiation either in non-growth-promoting conditioned medium or in growth-promoting 40% growth medium (Reddy and Lange, Radiat. Res. 119, 338-347, 1989). From these results, it was suggested that nutrient dilution altered radiosensitivity by a mechanism independent of progression of cells through the cell cycle. In this study, we have examined the effects on radiosensitivity of incubation in 40% growth medium prior to irradiation on both log- and plateau-phase Chinese hamster V79 cells and the effects on the distribution of cells in the cell cycle of incubation in 40% or 100% growth medium before and after irradiation. Radioresistance increased by a factor of 1.5-1.6 compared to 100% growth medium for both logphase and plateau-phase cells cultured in 40% growth medium prior to X irradiation and incubated in either 40% growth medium or conditioned medium after X irradiation. The cell cycle distributions of log-phase cells in 100% and 40% growth medium before irradiation were identical. The change in cell cycle distribution induced by 10 Gy did not differ among log-phase cells incubated for 3 h postirradiation in 100% growth medium, 40% growth medium or conditioned medium. These results, in addition to supporting our previous conclusions, demonstrate that culturing prior to irradiation in 40% growth medium alone increases cell survival and that incubation in 40% growth medium before and after irradiation maximizes the survival of V79 cells.

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