The effect of X irradiation on the progression of mouse 10T1/2 cells released from density-inhibited cultures.
When mouse C3H 10T1/2 cells were allowed to reach the density-inhibited plateau phase of growth, 0-3% of the cells were in the S phase as determined by pulse-labeling indices and about 90% of cells were in the G1 phase as determined by flow microfluorometry (FMF). Density-inhibited cultures were exposed to moderate doses of X ray: immediately after X irradiation the cultures were trypsinized and the cells were subcultured at low density in fresh medium. FMF profiles and pulse and continuous-labeling indices were studied. It was determined by pulse-labeling indices that the cell population which was released from the plateau-phase cultures remained in the G1 phase longer than the duration of the normal G1 phase in exponentially growing cells. When plateau-phase cultures were irradiated with 400 rad before subculture, the cells moved into the S phase approximately 4 hr later than did nonirradiated cells. Despite the G1/S delay, both irradiated and nonirradiated cell populations showed a similar rate of progression through the S phase. As determined by cumulative labeling indices, the fraction of cells which remained in G1 up to 120 hr post-subculture (G1 block) increased linearly with X-ray doses. There was no measurable X-ray-induced G1 delay or G1 block in cells irradiated after subculture at the G1/S border during semi-synchronous growth.