Summary.-Chronically hypoxic cells were 5 times more resistant to Adriamycin (ADR) than exponentially growing oxic cells. On reoxygenation, resistance decreased slowly to reach the ADR sensitivity of oxic cells after 24 h. With increasing pH, ADR efficiency increased more in oxic than in chronically hypoxic cells. With increasing cell density, ADR efficiency decreased linearly.The differences in ADR efficiency under the various conditions were accompanied by differences in intracellular ADR uptake. Chronically hypoxic cells incorporated 1.6 times less than oxic cells; the incorporation rate at pH 6-5 was half that at pH 7-4; and at a cell density of 5 x 105/bottle the intracellular uptake was 6 times that at 5 x 106/bottle.The observed differences in uptake of ADR were not, however, sufficient to explain the differences in cytotoxicity.

No abstract

The duration of DNA synthesis of a diploid cell line of Chinese hamster fibroblasts was determined in a comparative study by the FLM technique, and also by a new technique for measuring the rate of DNA synthesis of individual cells. These methods produced comparable results when applied during exponential growth of the cells. The rate of DNA synthesis was measured by means of quantitative autoradiography following a short‐term incubation of the cells with 5 × 10‐6 M FUdR and 10‐5 M 14C‐TdR. The choice of the medium for this purpose did not seem to be critical. The autoradiographic silver grains over cells and 14C‐standard sources are counted by microphotometry using incident light bright‐field. The direct measurements of DNA synthesis rate are ‘compartment’ statistics which have been converted into ‘flux’ parameters for comparison with the FLM method and applicability in cell‐kinetic calculations. Frequency distributions of the rate of DNA synthesis of individual cells thus obtained may resemble normal distributions quite closely. They result from several factors: differences in the rate of synthesis in different parts of the S‐phase, the density distribution of cells within the S‐phase, the variation in the time of DNA synthesis among individual cells, and the experimental error. In the case of a pronounced partial synchronization as probably has been present in one experiment performed in the lag phase, an incorrect time of DNA synthesis may result from the rate values. Due to the variation in DNA synthesis rate in different parts of the S‐phase it is not possible to determine the duration of DNA synthesis of an individual cell. However, the mean values of DNA synthesis time are reliable. The new method will be preferentially applied for determining the duration of DNA synthesis of human cells in as far as difficulties are encountered with the classical methods. In addition, it may be used to advantage for studying cells which make up low percentages in mixed populations. It finally permits a safer morphological classification of the cells under study than is possible with the classical methods.

Cisplatin-resistant mouse fibrosarcoma cells, SSK-R, were isolated after short and low-dose drug treatment of the sensitive SSK cells in vitro. These SSK-R sublines exhibit up to sevenfold cisplatin resistance and are characterized mainly by an increased metallothionein content. Loss of drug resistance after about 140-180 cell divisions in drug-free medium coincides with loss of metallothionein content. The glutathione level is the same in the sensitive and resistant sublines; inhibition of glutathione synthesis by buthionine sulphoximine enhances the sensitivity in both cells lines by a factor of 2.7. The resistant sublines are not cross-resistant to radiation; a radiation exposure followed immediately by cisplatin treatment results in an additive effect. The cellular cisplatin content is slightly reduced in SSK-R2 cells and it remains at this level also upon loss of drug sensitivity.

The clonogenic potential of the progeny of irradiated cells was tested in vitro by replating irradiated cultures after various times, allowing between five and over 25 subsequent divisions to take place after irradiation. Whereas the plating efficiency of surviving Chinese hamster cells was not decreased, in C3H10T1/2 cells a dose-dependent but slight decrease in plating efficiency was observed even after the longest follow-up period. These data do not contradict the prevalent hypothesis in radiobiology that the proliferation potential of a clonogenic cell surviving after irradiation is not significantly different from that of a non-irradiated cell.