A Change in the Oxygen Effect Throughout the Cell-cycle of Human Cells of the Line NHIK 3025 Cultivated in Vitro

Summary

NHIK 3025 cells were synchronized by repeated mitotic selection. The S-phase was determined by ³H-thymidine incorporation and scintillation counting.

By comparing the age–response curves of aerobic cells irradiated with 500 rad with those of extremely hypoxic (< 4 p.p.m. O₂) cells irradiated with 1500 rad, it was found that the sensitizing effect of oxygen was not constant throughout the cycle. It was significantly higher in S, G2 and mitosis than in G1.

No significant sensitizing effect of 120 p.p.m. O₂ (compared with < 4 p.p.m. O₂) was found on cells in G1 when the cells were irradiated with 1500 rad. In S, G2 and mitosis, however, the sensitizing effect of oxygen at 120 p.p.m. is considered to be significant. Experiments performed with cells irradiated with 2000 rad in contact with either < 4 p.p.m. O₂ or 80 p.p.m. O₂ showed the same trend, little sensitizing effect in G1 and higher in S, G2 and mitosis.

Dose–response curves for cells in mid-G1 and mid-S under aerobic and extremely hypoxic conditions were well fitted by the formula S = exp (− αD − βD²). From the dose–response curves it was concluded that the change in the sensitizing effect of oxygen throughout the cell-cycle only appeared for low doses (in the dose region where α dominates). The sensitizing effect of oxygen on cells in mid-G1 was found to be increasing with increasing dose.