RBE of α-Particles for Delayed Production of Colonies by Irradiated Yeast Cells.

The effect of delayed colony appearance by irradiated cells exemplifying genetic instability is confirmed to be more expressed for diploid wild-type yeast cells characterized by a sigmoidal shape of the survival curve and capable of recovery from radiation damage in contrast to isogenic haploid strain incapable of diploid-specific recovery and characterized by the exponential form of the survival curve. The dependence of the delayed appearance of colonies by diploid yeast cells on the dose of ionizing radiation shows more pronounced manifestation after the action of α-particles (RBE = 4.2 ± 0.3) than after irradiation with y-rays. This effect may be associated with the greater efficiency of densely ionizing radiation to produce lethal radiation damage and accompanying sublesions responsible for the delay in the formation of colonies by the cells surviving after irradiation. It is shown that the dependence of the delayed colony appearance by diploid yeast cells on their survival was substantially the same after exposure to sparsely and densely ionizing radiation. Since exposure to ionizing radiation of different quality induced equally effective number of lethal damage and the same survival rates, these data indicate that the identical number of the accompanying sublesions responsible for the delayed colony appearance are produced by irradiated cells.

[INCREASE IN CELL RADIOSENSITIVITY AFTER INHIBITION OF CELL ABILITY TO RECOVER FROM POTENTIALLY LETHAL RADIATION DAMAGE].

The paper presents new experimental results demonstrating the recovery of irradiated cells on a nutrient medium, which usually occurs at a delay of cell division. Using the previously proposed method, the contribution of such a recovery in radiation sensitivity of diploid yeast cells of different strains after exposure to ionizing and ultraviolet radiation has been estimated. It is shown that the actual increase in cell radiosensitivity significantly exceeds the expected one in the case of inhibition of the recovery from potentially lethal damage. These data indicate that inhibition of cell ability to recover from potentially lethal radiation damage by exposing the irradiated cells in non-nutritive medium in many cases serve as an indicator of suppression of the overall ability of cells to repair. Experimental data indicating that it is not a universal rule are presented.

[The manifestation of synergy during simultaneous thermochemical action on yeast cells].

In order to obtain new fundamental knowledge, patterns of manifestation of synergy have been studied after simultaneous combined action of hyperthermia (47.5-60 degrees C) with anti-tumor agents (cyclophosphamide, cisplatin) on the survival of yeast cells. To calculate the efficiency of the synergistic interaction, the dependence of cell survival on the duration of exposure at separate and simultaneous action of chemical agents and hyperthermia was used. We have found that there is a certain temperature range within which there is a synergistic enhancement of anticancer drugs and high temperature effects. Any deviation from the optimum values of the temperature results in a decrease in synergy. The possible mechanism of the revealed patterns is discussed.

[Cisplatin influence on: the radiosensitivity and recovery of yeast cells].

The effect of the simultaneous combined action of ionizing radiation and cisplatin on the radiosensitivity and liquid holding recovery (LHR) of diploid yeast cells was studied. It was shown that regardless of the cisplatin concentration (0; 0.002; 0.01; 0.02 g/ml) the radiosensitivity of cells was increased by 1.3 times. The ability of a cell to the LHR was progressively decreased with the increasing cisplatin concentration up to the complete inhibition. It was shown that the LHR of yeast cells after a combined action of ionizing radiation and chemical agents is mainly inhibited due to formation of a greater proportion of irreversible damage. The constant of recovery, characterizing the probability of recovery per a unit of time, was independent on cisplatine concentration.

[Qantitative description of mammalian cell recovery after combined exposure to ionizing radiation with chemical agents].

The parameters of recovery of mammalian cells after exposure to ionizing radiation combined with chemical agents were calculated based on a mathematical model of post-radiation recovery. The data presented, in contrast to the previously published results, indicate that the inhibition of the recovery may be due either to the damage or disruption of the process of recovery or the increase in the part of irreversible damages from which the cells are incapable to recover; or both of these processes can be realized simultaneously. It is concluded that the combination of ionizing radiation with chemical agents that inhibit the recovery processes through the formation of irreversible damages or affect the probability of recovery can be a perspective way in terms of finding the most effective means to increase radiosensitivity.