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The influence of hyperthermia on the survival of irradiated yeast cells of different species has been studied. The experiments reported in the paper have shown: (1) simultaneous action of ionizing radiation and high temperatures appeared to increase the radiation response by a factor of approximately 2.7 for diploid and only by a factor of 1.5 for haploid cells of wild-type; (2) the combined action of high temperature and ionizing radiation had no synergistic effect for rad51 mutant diploid yeast cells; (3) heating before or after irradiation did not alter the radiation response of yeast cells; (4) enhancement of yeast cell sensitivity by simultaneous action of hyperthermia and 239Pu-alpha-particles was negligible; (5) the magnitude and the rate of liquid holding recovery is lowered with increasing of irradiation temperature. On this basis, it was concluded that possible mechanism for thermal sensitization of yeast cells may involve the reduced capacity of cells to recover damages resulted from the combined action of both modalities.

A simple theoretical model is proposed for estimating the differential contribution of ionization and excitation to the lethal effect of ionizing radiation. Numerical results were obtained on the basis of published experimental data on the ability of bacterial cells Escherichia coli to undergo photoreactivation of radiation-induced damage. It was shown that inactivation by excitation may be highly significant for UV-hypersensitive cells capable of photoreactivation; inactivation by excitation increased with the energy of ionizing radiation and the volume of irradiated suspensions. The data are in qualitative agreement with the assumption of a possible contribution of the UV-component of Cerenkov radiation to the formation of excitations responsible for the lethal effect and the phenomenon of photoreactivation after ionizing radiation. Some predictions from the model are discussed.

This communication reports the observation of synergistic relationships between ultrasound and gamma-irradiation of stationary phase cultures of Saccharomyces cerevisiae of different strains. The gamma-ray dose was applied before or after the sound. The extent of synergism depended upon the sequence of application; it was smaller for (US + gamma-ray)-exposure in comparison with gamma-ray +US)-treatment. The combined action of both modalities had smaller or no synergistic effect for mutant(rad51) yeast cells incapable of recovery. On this basis, it was concluded that possible mechanisms for ultrasound radiosensitization of yeast cells may involve the reduced capacity of cells to recover damages resulted from the combined action and/or the enhanced expression of lethal damage.

Experimental data on photoreactivation of damage induced by ionizing radiation in yeast cells are presented. The value of photoreactivation was found to be the highest for the following conditions predicted by us as optimum ones: large volume of irradiated suspension, hypoxia and high energy sparsely ionizing radiation. A comparison of data for yeast and bacterial cells shows that Cerenkov emission from ionizing radiation may produce photoreactivated pyrimidine dimers in both prokaryotic and eukaryotic cell systems.

The radioprotective efficiency of cysteamine and cysteine has been studied on haploid and diploid, Saccharomyces cerevisiae, wild-type and various X-ray repair deficient rad mutants. The correlation between the radioprotecting action of cysteamine and cell repair capacity was demonstrated for diploid yeasts; such a correlation was not expressed for wild-type and rad mutant haploid yeast cells. It was concluded that the radioprotective action may involve cellular recovery processes, which may be mediated by a recombination-like mechanism, for which the diploid state is required. Liquid holding recovery was shown not to participate in radioprotection, judged by the absence of the influence of cysteine on the delay of the first postradiation budding as well as by the additive action of cysteine and liquid holding recovery.