Modification of recovery from radiation damage by hyperthermia in yeast

Recovery from radiation damage was studied in the diploid yeast Saccharomyces cerevisiae BZ 34 subjected to pre-irradiation, post-irradiation or simultaneous hyperthermic treatment. Pre-irradiation heat treatment at 51 degrees C for 20 m min sensitized the cells to lethal damage; however, these cells could efficiently recover from potentially lethal damage (PLD) on liquid holding. In contrast, post-irradiation hyperthermia (51 degrees C for 20 min or 52 degrees C for 15 min) partially inhibited recovery from PLD. For treatments at 40 and 45 degrees C the inhibition of recovery was restricted to the duration of heat treatment. As soon as the temperature of the cells was reduced to 30 degrees C for 48 h, they showed a significant recovery from PLD. The kinetics of the liquid holding recovery of cells exposed to 600 and 900 Gy of radiation followed by heat treatment at 51 degrees C, 20 min or 52 degrees C, 15 min was studied. Post-irradiation heat treatment inhibits PLD repair (PLDR) during the first 3 h. Subsequent liquid holding for longer durations results in recovery from PLD, reaching a maximum of nearly 50% of that obtained in cells exposed to radiation alone. When the cells were held for 0-48 h between radiation and heat treatment, they showed recovery from radiation damage such that sensitization by subsequent heat treatment was reduced significantly over the 48 h interval. The inhibition of PLDR by hyperthermia also decreases progressively with an increase in the time interval between the two treatments. When both heat and radiation treatments were carried out simultaneously by irradiating the cells at 48 degrees C, the inhibition of PLDR was partial. The recovery factor, calculated as the ratio of survival after recovery to that on immediate plating (without PLDR), was 2.6 +/- 0.9 for simultaneous heat and radiation treatment compared to 11.9 +/- 2.7 for radiation alone.