Regrowth and Radiation Sensitivity of Quiescent Cells Isolated from EMT6/Ro-fed Plateau Monolayers

Abstract

A quiescent [denoted as Q(G₀/G₁)] subpopulation was isolated from EMT6/Ro-fed plateau monolayers by centrifugal elutriation. The median Coulter volume of these cells was significantly smaller than that of the original population from which they were elutriated. Using two-step acridine orange staining and dual parameter flow cytometric analysis, over 95% of quiescent cells were found to have G₁ DNA content, and 80% of the cells had a decreased RNA content as compared to rapidly proliferating exponential G₁ cells. After labeling for 24 hr (two doubling times) with [³H]thymidine, less than 2% of the quiescent cells incorporated [³H]thymidine as measured by autoradiography. The colony-forming efficiency of these cells was not significantly different from that of exponential cells. When such Q(G₀/G₁) cells were replated in fresh medium at a lower density, there was a lag time of 30 hr before any increase in cell number was detected, after which the cell-doubling rate matched that of exponential culture. Results obtained from the radiation dose-response curves showed that quiescent (G₀/G₁) cells were more radiosensitive than exponential G₁ or unseparated fed plateau cells.