Survival and recovery of yeast cells after combined treatment with ionizing radiation and heat.

Cell survival, recovery kinetics and inactivation forms after successive and simultaneous treatments with gamma rays (60Co) and high temperatures were studied in diploid yeast cells capable of recovery. Both the extent and the rate of the recovery were shown to be greatly decreased with increase in the duration of heat treatment (60 degrees C) followed by radiation and with increase in exposure temperature after simultaneous treatment with heat and radiation. A quantitative approach describing the recovery process was used to estimate the probability of recovery per unit time and the irreversible component of damage after the combined treatment with heat and radiation. It was shown that the probability of recovery was independent of the conditions of the treatment with heat and radiation, while the irreversible component gradually increased as a function of the duration of heat treatment (60 degrees C) after sequential treatment with heat and radiation and as a function of the exposure temperature after simultaneous treatment with heat and radiation. The increase in the irreversible component was accompanied by an increase in cell death without postirradiation division. It is concluded on this basis that the synergistic interaction of ionizing radiation and hyperthermia in yeast cells is not related to the impairment of the recovery capacity itself and that it may be attributed to an increased yield of irreversible damage.