ALTERATION IN THE HYPOXIC FRACTION OF QUIESCENT CELL-POPULATIONS BY HYPERTHERMIA AT MILD TEMPERATURES

We investigated oxygenation of quiescent (Q) tumour cells in vivo by m ild heat treatment. C3H/He mice bearing SCC VII tumours received BrdU continuously for 5 days via implanted mini-osmotic pumps, to label all proliferating (P) cells. The tumours were then irradiated after treat ment, and were excised, minced and trypsinized. The tumour cell suspen sions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus (MN) frequency in cells without BrdU la belling was determined using immunofluorescence staining for BrdU. Thi s MN frequency was then used to calculate the surviving fraction of un labelled cells from the regression line for the relationship between t he MN frequency and the surviving fraction of total (P + Q) tumour cel ls. Thus, a cell survival curve could be determined for the cells not labelled with BrdU, which can be regarded as the Q cells in a tumour f or all practical purposes. The MN frequency in total tumour cell popul ation was determined from the irradiated tumours that were not pretrea ted with BrdU. Assays performed immediately after irradiation of both normally aerated and hypoxic tumours showed that Q cells contained hig her hypoxic fractions than the total tumour cell population. Mild heat treatment (40.0 degrees C, 60 min) before irradiation decreased the h ypoxic fraction, even when it was combined with nicotinamide administr ation. In contrast, mild heating did not decrease the hypoxic fraction when the mice were placed in a circulating carbogen (95% O-2/5% CO2) chamber. Therefore, mild heat treatment was thought to preferentially oxygenate the chronically hypoxic fraction.