Potentially Lethal Damage Repair Research Articles

In vitro radiation sensitivity of mouse lung fibroblasts isolated by flow cytometry

Recently, we have isolated two major fibroblast cells (Thy-1+, Thy-1-) from mouse LAF1 lung tissue using the anti-Thy-1 antibody expression and fluorescence activated cell sorter. To examine the possibility that x- or gamma-ray-induced pulmonary fibrosis at the late stage of injury could arise from radioresistant cell subpopulations, the radiation sensitivities of Thy-1+ and Thy-1- cells were evaluated by the colony forming assay. Cell survival curves, repair of potentially lethal damage (PLD) and sublethal damage (SLD), and cell-age response curves were obtained after Cs-137 gamma-ray irradiation. The cell survival curves measured after 0-10 Gy gamma-ray showed that Thy-1+ cells were slightly more radioresistant than Thy-1- cells. The D0, n, alpha, and beta values measured from the survival curves also confirmed this observation. After a single dose of 10 Gy, a small amount of PLD repair was observed in Thy-1- cells, while no PLD repair was found in Thy-1+ cells. Although the initial cell survival level of Thy-1- cells was lower, the final survival levels of Thy-1+ and Thy-1- cells became identical at 8 h after irradiation due to the PLD repair. After split-dose irradiation of 4 Gy followed by 4 Gy, a similar extent and rate of SLD repair was found in Thy-1+ and Thy-1- cells. Cell-age response curves were obtained from irradiated G0/G1, S, and G2M cells separated by centrifugal elutriation and irradiated with 8 Gy gamma-ray. The results indicated that Thy-1+ and Thy-1- cells had a similar S resistant, and G1, G2M-sensitive radiation cell-age response curve. This study suggests that the selection of radioresistant lung fibroblast may not be responsible for the development of lung fibrosis in irradiated LAF1 mouse.

Effect of a pure anti‐estrogen, ICI 182,780, on the in vitro radiosensitivity of MCF‐7 breast carcinoma cells

AbstractUse of the anti‐estrogen tamoxifen (TAM) has been demonstrated to contribute a survival advantage in postmenopausal women with early stage or advanced breast cancer. Although conflicting results exist in the literature, some reports have suggested that TAM may exert potential deleterious effects on radiation sensitivity and response of breast cancers to radiotherapy (RT). A new non‐steroidal anti‐estrogen, ICI 182,780, which, unlike TAM, is without any estrogenic properties, has been used in European clinical trials in place of TAM. This study reports the effects of ICI 182,780 on radiation sensitivity and potentially lethal damage repair (PLDR) in estrogen‐receptor positive (ER+) MCF‐7 human breast carcinoma cells irradiated in vitro. At a concentration of 10−7 M, ICI 182,780 can inhibit the growth‐stimulatory effect of 10−10M 17β‐estradiol (E2). Growth‐arrested MCF‐7 cells were divided into four treatment groups: estradiol‐deprived (−E2) and estradiol‐stimulated (+E2) cultures incubated in the presence or absence of ICI 182,780 (10−7 M). Five days later each group was exposed to γ‐irradiation and cell survival was measured by clonogenic assay in complete medium containing E2. In one series of experiments, the cells were harvested immediately after irradiation, while in a second series, the cells were held at 37°C for an additional 24 hr prior to harvesting. No statistically significant survival difference was detected between groups plated immediately after irradiation or 24 hr later, although a trend toward improved survival for the delayed plating groups was routinely observed. ICI 182,780 did not significantly alter radiation sensitivity of MCF‐7 cells regardless of their hormonal status (+/−E2), nor did it change the sensitivity of cells harvested immediately or allowed 24 hr to express PLDR. These results suggest that ICI 182,780 may be used as an adjuvant in the treatment of breast cancer without altering the radiation sensitivity of breast cancer cells to concurrent RT. © 1995 Wiley‐Liss, Inc.

Molecular mechanism of potentially lethal damage repair. I. Enhanced fidelity of DNA double-strand break rejoining under conditions allowing potentially lethal damage repair.

This study contributes to the elucidation of the molecular mechanism underlying potentially lethal damage (PLD) repair. Repair of DNA double-strand breaks (dsbs) is involved in PLD repair in yeast, i.e. in the enhanced survival of cells due to post-irradiation treatment under non-growth conditions before plating cells on nutrient agar (growth conditions). However, dsbs are rejoined when cells are kept either in non-growth or growth medium. One possibility to explain the enhanced survival of cells after post-irradiation treatment in non-growth medium might be an enhanced fidelity of dsb rejoining under non-growth relative to growth conditions. We have addressed this problem by using a plasmid-mediated assay. Into one of the two selectable plasmid markers a single dsb was introduced by a restriction enzyme. The cut plasmid was transfected into an appropriate yeast mutant. Transformants that had correctly rejoined the dsb were selected on the basis of restoration of the function of the cut gene. The yeast mutant was allowed to rejoin the cut plasmid under either non-growth or growth conditions. The results show that the fidelity of dsb rejoining is higher in cells kept under non-growth relative to growth conditions.

Inhibitors of poly (ADP-Ribose) synthesis inhibit the two types of repair of potentially lethal damage

Purpose : The purpose of this study was to examine whether 3-amino-benzamide (3ABA), an inhibitor of poly (ADP-ribose) synthesis, inhibits the two types of potentially lethal damage (PLD) repair, termed slow and fast. Methods and Materials : The fast-type PLD repair was measured by the decrease in survival of V79 Chinese hamster cells by postirradiation treatment with 3ABA. The slow-type PLD repair was measured by the increase in survival by posttreatment with conditioned medium (CM), which became conditioned by growing a crowed culture of cells and supports the slow-type PLD repair. Results : Up to 1 mM, 3-ABA inhibited the slow type repair; at doses of 2 mM and above, it inhibited the fast type of PLD repair. Conclusion : There are quantitative differences in cellular effects of 3ABA dependent on concentration. Poly (ADP-ribose) appears to play an important role in the PLD repairs and has little effect on the repair of sublethal damages.

Repair of potentially lethal radiation damage in human squamous carcinoma cells after chronic hypoxia

Purpose : To examine the repair of radiation induced potentially lethal damage in A431 and CaSki cells after chronic hypoxia. Methods and Materials : Cells in exponential phase are subjected to hypoxia (< 10 ppm oxygen) for up to 12 h and then are allowed to reoxygenate in air for up to 4 h. Cells are then irradiated with γ rays. Cell survivals are measured by clonogenic assay immediately and at different times after irradiation. Results : Compared to aerobic controls, an increase in the level of potentially lethal damage repair (PLDR) is demonstrated in A431 cells reoxygenated for 10 min after > 4 h of hypoxia. The repair returned to aerobic control level by 3 h of reoxygenation. PLDR of A431 cells reached maximum at about 9 h after irradiation in cells reoxygenated for 10 min after hypoxia. However, the repair is maximum at 6 h in cells reoxygenated for 3 h after hypoxia and in aerobic cells not previously exposed to hypoxia. Reoxygenation after chronic hypoxia did not affect the PLDR capacity and repair kinetics of CaSki cells. Conclusion : The results suggest that radiosensitization by reoxygenation after chronic hypoxia is not related to inhibition of PLDR.

A Benchmark of Cell Survival Models Using Survival Curves for Human Cells after Completion of Repair of Potentially Lethal Damage

Six models of radiation action (the linear-quadratic model, the multitarget model with initial slope, the repair-misrepair model, the lethal-potentially lethal model, the cybernetic model, the saturable repair model) were tested for their goodness of fit to survival curves for human cells. Fifty-three survival curves for human cells irradiated in plateau phase and after completion of repair of potentially lethal damage (PLD) provided the experimental basis for the tests. Three criteria were considered. The capacity to describe the survival data was estimated, using the error left unexplained by the model. A validation of models was achieved by consideration of the mean residual squared errors. The ability of the parameters to characterize survival curves was investigated, studying their variation within and among curves. The models were not equivalent, whatever the test. The saturable repair model and the multitarget with initial slope model gave the most accurate description of survival data. The linear-quadratic model had the most reliable parameters, so that comparisons of the cell survival curves could be made advantageously. The cybernetic model and the lethal-potentially lethal model were found inappropriate for the analysis of survival curves for human cells after completion of PLD repair.

Influence of an autocrine multitherapy resistance factor on radiation responses of melanoma cells

An autocrine multitherapy resistance factor (MTRF) produced by a radioresistant subclone of S91 mouse melanoma (S91/I3) causes an increase in radioresistance of a radiosensitive subclone (S91/amel). MTRF has no effect on the survival of S91/I3, which is already relatively resistant to gamma-irradiation. In this study, we examined the effect of MTRF in the form of S91/I3 conditioned medium or as S91/I3 heavily-irradiated cells (I3-HRCells) on cellular responses of S91/amel cells after exposure to gamma-rays. Target S91/amel cells retained more than half of their ability to respond to rescue by MTRF on day 4 after exposure to 3 Gy. Continuous presence of MTRF during colony formation was necessary for maximum plating efficiency. Although the extent of double strand DNA breakage and repair was the same in S91/amel and S91/I3, split-dose recovery experiments with MTRF revealed previously undetected repair of sublethal damage in S91/amel cells. MTRF did not alter the extent of potentially lethal damage repair (PLDR) in S91/I3 or S91/amel. S91/amel cells were more responsive to MTRF if they had been harvested from confluent dishes, while S91/I3 cells produced a more effective factor if they had been harvested in exponential phase. These findings demonstrate that MTRF has unique properties. It does not appear to be involved in genome repair since it does not alter the extent of PLDR and it is effective when added to cells after complete split-dose recovery has occurred.

Stereotactic radiosurgery: Relative biological effect of continuous vs. intermittent radiation exposures

AbstractLinear accelerator‐based stereotactic radiosurgery is increasingly used in place of gamma knife equipment. Conforming radiation isodoses to complex volumes requires multiple isocenters and arcs per isocenter and may result in a protracted and intermittent radiation exposure. The experiments in this study were designed to quantitate the difference between continuous and intermittent radiation schedules which are typical for gamma knife and linear accelerator‐based radiosurgery, respectively. Two radiobiologically well‐characterized cell lines, V79 Chinese hamster and U‐87MG human malignant glioma cells, were irradiated in vitro to 12 Gy delivered as a continuous 5 min exposure and an intermittent exposure lasting maximally 112 min. The effect of continuous vs. intermittent irradiation to a dose of 31.5 Gy was also measured in vivo using FSaII fibrosarcomas, implanted into C3Hf/Sed mice, in a tumor growth assay. The in vitro experiments demonstrated a significant reduction in biological radiation effect reflected in 2‐ to 3‐fold increases in cell survival with intermittent vs. continuous irradiation schemes. Similar effects were seen with the FSaII tumors. The reduced effect of the intermittent irradiation schemes is most likely due to repair processes after radiation exposure, such as Potentially Lethal Damage Repair (PLDR) and Sublethal Damage Repair (SLDR), which can occur during 5–15 min intervals between radiation increments of intermittent stereotactic radiosurgery treatments. © 1995 Wiley‐Liss, Inc.

Thermal radiosensitization in two pairs of CHO wild-type and radiation-sensitive mutant cell lines.

Two pairs of Chinese hamster ovary cell lines, each comprising a parental normal-responding and a radiation-sensitive mutant, were tested for their X-radiation sensitivity, hyperthermia sensitivity and thermal radiosensitization. The relative radiation responses were similar to those already published in the literature, indicating that the mutant cell lines, V-3 and 5-11, were extremely sensitive to radiation while their parental strains (AA8-4 and K1) exhibited normal sensitivity. The hyperthermia response of V3 was the same as that of the parental cell line AA8-4; the thermal sensitivity of 5-11 was greater than that of its parental line K1 but this difference decreased at higher temperatures (greater than 43 degrees C). These data indicate that the hyperthermia response and its targets are probably not related to the mutation-causing radiation sensitivity in the two pairs of parent-mutant cell lines. When heat and radiation were combined, the thermal enhancement of radiation sensitivity for different heat treatments was the same or larger in the mutant than in the parental cell lines. The mutants are known to be deficient in potentially lethal damage repair (PLDR), sublethal damage repair (SLDR) and repair of double-stranded DNA breaks; however, these deficiencies did not impair thermal radiosensitization, indicating that thermal radiosensitization may not be through these pathways in these particular mutants. In addition, the parental cell lines exhibited a significant sequence dependence of thermal radiosensitization (i.e. whether heat was applied before or after irradiation) at low survival levels (10(-4); this sequence dependence was very small to absent in the mutant cell lines at these survival levels.

Radiosensitizing and repair-inhibiting properties of dipyridamole

Radioresistance and postirradiation repair of potentially lethal damage (PLD repair) are important factors underlying failure to control local disease in cancer. Dipyridamole (DP) is known as a modifier of the action of cytotoxic drugs. We therefore investigated DP as a potential radiosensitizer and inhibitor of PLD repair in X-irradiated Chinese hamster ovary (CHO) cells in vitro. Exposure to the drug alone resulted in a slight reduction of the clonogenic capacity of the cells. Preincubation for 18 h with 10 and 20 μM DP in cells subcultured at low density, led to a significant radiosensitization. In confluent density-inhibited cultures, preincubation alone as well as pre- and postincubation with 20 μM DP resulted in a significant inhibition of PLD repair. Dipyridamole and related compounds may thus be useful tools for modifying and investigating the response of mammalian cells to radiation.

An inhibitor of potentially lethal damage (PLD) repair reduces the frequency of γ-ray-induced mutations in cultured Chinese hamster V79 cells

Cordycepin (3′-deoxyadenosine, 3′-dA) is an RNA antimetabolite and a radiosensitizer in cultured mammalian cells. In the present paper, the effects of 3′-dA on γ-ray-induced lethality and 6-thioguanine (6TG)-resistant mutations in cultured Chinese hamster V79 cells were examined. 3′-dA had the effect of sensitizing the lethality induced by γ-rays. The potentially lethal damage (PLD) repair produced by post-incubation of cells in Hanks' solution after γ-irradiation was almost completely suppressed by 5 × 10 −5 M 3′-dA. When cells were irradiated with 10 Gy γ-rays and incubated with 3′-dA for 5 h, the frequency of 6TG-resistant mutations induced by γ-rays decreased to one-sixth of that of irradiated cells incubated without 3′-dA. The decrease in the frequency of γ-ray-induced mutations was dependent on the length of incubation time with 3′-dA. It is suggested that the inhibition of PLD repair by 3′-dA may be that of error-prone repair.

Mechanism of Radiosensitization by Halogenated Pyrimidines: Effect of BrdU on Repair of DNA Breaks, Interphase Chromatin Breaks, and Potentially Lethal Damage in Plateau-Phase CHO Cells

There is evidence suggesting that radiosensitization induced in mammalian cells by substitution in the DNA of thymidine with BrdU has a component that relies on inhibition of repair and/or fixation of radiation damage. Here, experiments designed to study the mechanism of this phenomenon are described. The effect of BrdU incorporation into DNA was studied on cellular repair capability, rejoining of interphase chromosome breaks, as well as induction and rejoining of DNA double- and single-stranded breaks (DSBs and SSBs) in plateau-phase CHO cells exposed to X rays. Repair of potentially lethal damage (PLD), as measured by delayed plating of plateau-phase cells, was used to assay cellular repair capacity. Rejoining of interphase chromosome breaks was assayed by means of premature chromosome condensation (PCC); induction and rejoining of DNA DSBs were assayed by pulsed-field gel electrophoresis and induction and rejoining of DNA SSBs by DNA unwinding. A decrease was observed in the rate of repair of PLD in cells grown in the presence of BrdU, the magnitude of which depended upon the degree of thymidine replacement. The relative increase in survival caused by PLD repair was larger in cells substituted with BrdU and led to a partial loss of the radiosensitizing effect compared to cells tested immediately after irradiation. A decrease was also observed in the rate of rejoining of interphase chromosome breaks as well as in the rate of rejoining of the slow component of DNA DSBs in cells substituted with BrdU. The time constants measured for the rejoining of the slow component of DNA DSBs and of interphase chromosome breaks were similar both in the presence and in the absence of BrdU, suggesting a correlation between this subset of DNA lesions and interphase chromosome breaks. It is proposed that a larger proportion of radiation-induced potentially lethal lesions becomes lethal in cells grown in the presence of BrdU. Potentially lethal lesions are fixed via interaction with processes associated with cell cycle progression in cells plated immediately after irradiation, but can be partly repaired in cells kept in the plateau-phase. It is hypothesized that fixation of PLD is caused by alterations in chromatin conformation that occur during normal progression of cells throughout the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Leptomycin B-Induced Fixation of X-Ray-Related Potentially Lethal Damage

Effects of an antifungal antibiotic, Leptomycin B (LMB), on X-ray cell killing were studied using cultured mammalian cells (BHK21). LMB at concentrations from 0.05 to 2 ng/ml inhibited cell proliferation but enhanced cell killing without significant cytotoxic effect when added to cell cultures pre- and postirradiation. The degree of enhanced cell killing decreased as the interval between irradiation and LMB treatment was prolonged. Enhancement of cell killing was probably due to fixation of potentially lethal damage (PLD) to LMB. Radioresistant S-phase cells were more sensitive to LMB-induced PLD fixation than G1-phase cells. Furthermore, a preliminary study showed that LMB not only inhibited the repair of PLD which was induced by conditioned medium but also inhibited splitdose repair of sublethal damage. Although the target molecule of LMB has not been identified, we suggest that LMB inhibits repair processes by altering the structure of the nuclear scaffold, which is supposed to be a radiosensitive site.

Intrinsic radiosensitivity and PLD repair in osteosarcoma cell lines.

The response to radiation of seven osteosarcoma cell lines was analysed by in vitro colony-forming assay and compared with that of eight human fibroblast strains. The values of D0, the surviving fraction after 2 Gy (S2Gy), and the mean inactivation dose (D) of osteosarcoma cells in log-phase culture were significantly higher than those of fibroblast strains (p less than 0.01). PLD (potentially lethal damage) repair of osteosarcoma cells evaluated in the plateau phase of growth showed great variation for enhancement of survival, although all of the values were maximized within 12 h after irradiation. In the osteosarcoma, intrinsic radiosensitivity in vitro reflected the clinical response to radiation. However, the capacity for PLD repair might not be a good indicator for predicting the results of radiation therapy.

Sublethal damage, potentially lethal damage, and chromosomal aberrations in mammalian cells exposed to ionizing radiations

Sublethal and potentially lethal damage are abstract terms that originated and were defined without reference to molecular and subcellular entities in which such radiation damage was registered. The establishment of a cause-and-effect relationship between chromosome fragment loss and cell killing by ionizing radiations, along with a substantial body of knowledge in radiation cytogenetics, allows a definition of these terms in a context where hypotheses become testable and progress toward a better understanding of these phenomena is more likely. Accordingly, the simplest hypothesis which best fits the observations is as follows. Most aberrations are exchange types requiring an interaction to form the exchange between two broken regions of a chromosome or chromosomes, that is, a break-pair. Very few single breaks fail to rejoin or restitute, so the vast majority are sublethal. Any such sublethal break may become a potentially lethal break-pair if another sublethal break occurs within some range where it is possible for the two to interact. The proportion of break-pairs in which a mis-repair event results in a lethal acentric fragment-producing exchange, can be altered depending on treatment conditions. Such conditions change the balance between “PLD repair” and “PLD fixation.” Studies on the control of radiosensitivity have focused on differences in repair processes, but large differences in radiation response may just as well occur with identical repair processes in operation but with different conditions of fixation.

Mechanism of Radiosensitization by Halogenated Pyrimidines: Bromodeoxyuridine and β-Arabinofuranosyladenine Affect Similar Subsets of Radiation-Induced Potentially Lethal Lesions in Plateau-Phase Chinese Hamster Ovary Cells

Chinese hamster ovary (CHO) cells were grown to plateau phase in the presence of various amounts of bromodeoxyuridine (BrdU) and treated after irradiation with beta-arabinofuranosyladenine (ara-A), an inhibitor of DNA and potentially lethal damage (PLD) repair, in order to investigate the importance of repair reactions in general and of PLD repair, in particular, on the mechanism of radiosensitization by halogenated pyrimidines. The degree of BrdU-mediated radiosensitization observed in ara-A-treated cells was compared to that of cells incubated after irradiation in the absence of ara-A. A substantial reduction in BrdU-mediated radiosensitization was observed in cells treated with ara-A at concentrations that, when given alone, produced maximum potentiation in cell killing (500-1500 microM). The residual BrdU-mediated radiosensitization observed at high levels of thymidine replacement could be explained by a BrdU-mediated increase in DNA and chromosome damage induction per gray. These findings are similar to those reported previously for a repair-deficient mutant of CHO cells, the xrs-5 cell line, and consistent with the hypothesis that BrdU-mediated radiosensitization has two distinct components, one that derives from an increase in damage induction per gray, and a second one that derives from an effect of BrdU on the repair of radiation-induced damage. It is proposed that the reduction in BrdU-mediated radiosensitization observed in ara-A-treated cells is the result of ara-A-mediated expression of radiation damage, the repair of which would have been otherwise modulated by BrdU. Since ara-A is known to act by fixing a form of radiation-induced PLD (alpha-PLD), we further propose that BrdU acts by fixing alpha-PLD. A synergistic effect in the potentiation of cell killing was observed between ara-A and BrdU when ara-A was given at concentrations below 100 microM. This result suggests that a benefit may be expected in the clinic from the combined application of halogenated pyrimidines with repair inhibitors, if administered at a carefully screened range of concentrations.

The Influence of Cell-Cell Contact on Radiosensitivity of Human Squamous Carcinoma Cells

Previous reports have indicated that for a number of cell lines, radiation resistance was greater when cells were grown as small spheroids than when grown as monolayer cultures. Experiments were conducted to study the increased radioresistance relative to recoveries from potentially lethal and sublethal damage (PLD and SLD, respectively) in two human squamous carcinoma cell lines, A431 and CaSki. Exponential-phase monolayer cultures and small spheroids 30-70 microns in diameter were used. No significant difference in the distribution of cells in the stages of the cell cycle was seen between the two cultures. To study PLD repair, cells were irradiated and then allowed to repair, all at 37 degrees C, for 6 h. In split-dose experiments, the two doses were given 6 h apart. After irradiation, the surviving fraction of cells was measured by clonogenic assay. Cells from CaSki spheroids (D0 = 1.25 Gy, Dq = 3.08 Gy) were more radioresistant than cells from the monolayer culture (D0 = 0.92 Gy, Dq = 2.8 Gy). No difference in radiation response was seen between the spheroid and monolayer cultures of A431 cells. The radioresistance of CaSki cells, which was maximum in intact small spheroids and immediately after disaggregation of spheroids, decayed completely by 15 h at 37 degrees C. The decay process was inhibited if cells were incubated at 0 degrees C. Relative PLD and SLD repair capacities for both cell lines were greater for monolayer cultures than for small spheroids. However, no correlation was found between the two cell lines in terms of repair capacity and cellular radiosensitivity.

The effect of lonidamine (LND) on radiation and thermal responses of human and rodent cell lines

Rodent and human cells were tested for response to Lonidamine (LND) (1-(2,4 dichlorobenzyl) 1-indazol-3carboxylic acid) combined with radiation or hyperthermia. Lonidamine exposure before, during, and after irradiation caused varying degrees of inhibition of potentially lethal damage (PLD) repair which was cell line dependent. In human glioma, melanoma, squamous cell carcinoma, and fibroblasts, LND exposure did not inhibit or only partially inhibited repair of potentially lethal damage. LND up to 100 μg/ml produced only a low level of toxicity in these cells and only slightly inhibited glucose consumption at the maximum concentration. In human glioma cells, LND treatment alone did not inhibit PLD repair, but when combined with hyperthermia treatment at moderate levels easily achievable in the clinic, there was complete inhibition of potentially lethal damage repair. These data suggest that LND effectiveness is cell type dependent. Combinations of LND, hyperthermia, and radiation may be effective in cancer therapy especially in tumors such as glioma in which repair of potentially lethal damage may be extensive.

Effect of Caffeine on the Expression of a Major X-Ray-Induced Protein in Human Tumor Cells

We have examined the effect of caffeine on the concomitant processes of the repair of potentially lethal damage (PLD) and the synthesis of X-ray-induced proteins in the human malignant melanoma cell line, Ul-Mel. Caffeine administered at a dose of 5mM after X radiation not only inhibited PLD repair but also markedly reduced the level of XIP269, a major X-ray-induced protein whose expression has been shown to correlate with the capacity to repair PLD. The expression of the vast majority of other cellular proteins, including seven other X-ray-induced proteins, remained unchanged following caffeine treatment. A possible role for XIP269 in cell cycle delay following DNA damage by X irradiation is discussed.

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.