Independent characterization of thymidine transport and subsequent metabolism in Hymenolepis diminuta--I. Comparison of short- and long-term transport kinetics

1. Initial transport kinetics of thymidine in the rat tapeworm, Hymenolepis diminuta have been characterized independently of subsequent metabolic activity, using short term (5s) incubation times. 2. Thymidine uptake by 10 day old worms using long-term (2 min) incubation times underrepresents actual transport capabilities by allowing subsequent phosphorylation to act as a rate limiting step in the coupled transport-metabolism pathway. 3. Apparent Kt values for 6 and 10 day old H. diminuta thymidine transport at 5s incubation times (0.073 nM and 0.091 mM) are close in value. Distinctions previously observed at 2 min incubation times may actually represent metabolic (and not transport) differences.     

Human lymphocytes exposed to low doses of ionizing radiations become refractory to high doses of radiation as well as to chemical mutagens that induce double-strand breaks in DNA

Human lymphocytes exposed to low doses of ionizing radiation from incorporated tritiated thymidine or from X-rays become less susceptible to the induction of chromatid breaks by high doses of X-rays. This response can be induced by 0.01 Gy (1 rad) of X-rays, and has been attributed to the induction of a repair mechanism that causes the restitution of X-ray-induced chromosome breaks. Because the major lesions responsible for the induction of chromosome breakage are double-strand breaks in DNA, attempts have been made to see if the repair mechanism can affect various types of clastogenic lesions induced in DNA by chemical mutagens and carcinogens. When cells exposed to 0.01 Gy of X-rays or to low doses of tritiated thymidine were subsequently challenged with high doses of tritiated thymidine or bleomycin, which can induce double-strand breaks in DNA, or mitomycin C, which can induce cross-links in DNA, approximately half as many chromatid breaks were induced as expected. When, on the other hand, the cells were challenged with the alkylating agent methyl methanesulfonate (MMS), which can produce single-strand breaks in DNA, approximately twice as much damage was found as was induced by MMS alone. The results indicate that prior exposure to 0.01 Gy of X-rays reduces the number of chromosome breaks induced by double-strand breaks, and perhaps even by cross-links, in DNA, but has the opposite effect on breaks induced by the alkylating agent MMS. The results also show that the induced repair mechanism is different from that observed in the adaptive response that follows exposure to low doses of alkylating agents.     

Variability of the adaptive response to ionizing radiations in humans

Human lymphocytes exposed to low doses of ionizing radiations from incorporated tritiated thymidine ([3H]dThd) or from X-rays become less susceptible to the induction of chromatid aberrations by high doses of X-rays. This indicates that low doses of ionizing radiation can produce an effect similar to the adaptive response observed with alkylating agents in prokaryotes, animal and plant cells. To determine whether there is individual variability in the adaptive response to ionizing radiations we exposed human lymphocytes from 18 different healthy donors to 'adapting' doses of [3H]dThd (0.01 microCi/ml) or X-rays (0.01 Gy) and subsequently to a 'challenge' treatment of 0.75 Gy of X-rays delivered 2 h before fixation. Four of the 18 donors did not show an adaptive response; in some cases in these individuals a synergistic response of increased, rather than decreased, damage was found. Two of these 4 donors showed no adaptive response in 3 subsequent experiments separated by 4-month intervals. This suggests that the human population exhibits a heterogeneity in the adaptive response to ionizing radiations which might be, at least in part, genetically determined.     

Direct stimulatory action of blood serum, vitamin D3, and its hydroxy-analogs on calcium transport in the small intestine of chicks in vitro

A study was made of the effect of blood serum, vitamin D3 and its hydroxy-analogs (25-hydroxyvitamin D4. 1 alpha-hydroxyvitamin D3) on Ca2+ transport across the wall of the noninverted small sac of D-avitaminosis chicken during incubation in vitro. It was shown that blood serum from chickens fed vitamin D3 in different doses (50--20 000 IU) and at varying time (1--72 h) before sacrifice produced a marked stimulating action on the cation transport 10 min after administration into the intestinal cavity as compared with the effect produced by the serum from D-avitaminosis chickens. Administration into the intestine of vitamin D3 or its hydroxy-analogs in physiological doses (6.25--25.0 ng) also significantly stimulated Ca+ transport over 10 min of incubation.     

Uptake of thymidine into isolated rat hepatocytes. Evidence for two transport systems

Thymidine transport was studied in isolated rat hepatocytes. In these cells no phosphorylation of the substrate by thymidine kinase occurred subsequent to transport. Results from studies of the concentration-dependent uptake of thymidine indicated two transport systems with about 80-fold differences in their kinetic constants. These systems were denoted as high affinity [Km = 5.3 micron, V = 0.47 pmol/(10(6) cells X s)] and low affinity systems [Km = 480 micron, V = 37.6 pmol/(10(6) cells X s)]. From intracellular to extracellular distribution ratios of [3H]thymidine it could be concluded that the uptake by the high affinity system was a concentrative process while the transport by the low affinity system was non-concentrative. The uptake of [3H]-thymidine by the high affinity system could only be inhibited by unlabeled thymidine. In contrast, all other nucleosides tested (uridine, 2'-deoxycytidine, and 2'-deoxyguanosine) were equally effective in inhibiting the low affinity system competitively. The results would suggest that in hepatocytes lacking phosphorylation by thymidine kinase, thymidine is taken up by a high and a low affinity system working in tandem. The high affinity system seems to be an active transport process with narrow substrate specificity. Thymidine uptake by the low affinity system is a facilitated diffusion process. This system is considered to be a common transport route for nucleosides of different structures.     

Herpes Simplex Virus Type 2 Functions Expressed During Stimulation of Human Cell DNA Synthesis

Experiments were designed to identify herpes simplex virus type 2 (HSV-2)-specific functions expressed during stimulation of human embryo fibroblast DNA synthesis. Cultures were partially arrested in DNA synthesis by pretreatment with 5-fluorouracil and maintenance in low-serum (0.2%) medium during virus infection. Results showed that continuous [methyl-(3)H]thymidine uptake into cellular DNA was ninefold greater in HSV-2-infected than in mock-infected cultures measured after 24 h of incubation at 42 degrees C. Shifting mock-infected cultures from low- to high-serum (10%) medium also caused some stimulation, but [methyl-(3)H]thymidine uptake was only twofold greater than in cells maintained with low serum. Plating efficiencies of both HSV-2-infected and mock-infected cells at 42 degrees C were essentially the same and ranged from 37 to 76% between zero time and 72 h of incubation. De novo RNA and protein syntheses were continuously required for HSV-2 stimulation of cellular DNA synthesis. HSV-2 infection markedly enhanced transport, phosphorylation, and rate of incorporation of [methyl-(3)H]thymidine into cellular DNA, starting at 3 h and reaching a maximum by 12 h; after 12 h, these processes gradually declined to low levels. In mock-infected cells these processes remained at low levels throughout the observation period. Pretreatment of cells with interferon or addition of arabinofuranosylthymine at the time of virus infection inhibited stimulation caused by HSV-2. 5-Bromodeoxyuridine density-labeled experiments revealed that HSV-2 stimulates predominantly semiconservative DNA replication and some DNA repair. Stimulation of [methyl-(3)H]thymidine into cellular DNA correlated with detection of virus-specific thymidine kinase activity. In conclusion, HSV-2 stimulation of cellular DNA synthesis appeared to involve at least four virus-specific functions: induction of thymidine transport, HSV-2 thymidine kinase activity, semiconservative replication, and repair of cellular DNA.     

3'-azido-3'-deoxythymidine. An unusual nucleoside analogue that permeates the membrane of human erythrocytes and lymphocytes by nonfacilitated diffusion

The demonstrated in vitro and in vivo activity of 3'-azido-3'-deoxythymidine (N3dThd) against the infectivity and the cytopathic effect of human immunodeficiency virus has prompted an investigation of the mechanism by which this nucleoside analogue permeates the cell membrane. As with the transport of thymidine, the influx of N3dThd into human erythrocytes and lymphocytes was nonconcentrative during short incubation times (less than 5 min) which did not allow significant metabolism of this nucleoside. However, in contrast with thymidine transport, the initial velocity of N3dThd influx was strictly a linear function of nucleoside concentration (0.5-10 mM), without evidence of saturability; insensitive to micromolar concentrations of potent inhibitors of nucleoside transport (dipyridamole, 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine, and dilazep); insensitive to a 1000-fold excess of other nucleosides (thymidine, uridine, 2-chloroadenosine); and relatively insensitive to temperature, with Q10 values (37-27 degrees C) of 1.4 and 2.7 for N3dThd and thymidine, respectively, determined in erythrocytes. Although the above results indicate that N3dThd permeates the cell membrane chiefly by nonfacilitated diffusion and not via the nucleoside transporter, millimolar concentrations of this nucleoside analogue were observed to inhibit both zero-trans influx of thymidine and efflux of thymidine from [3H]thymidine-loaded erythrocytes. The partition coefficients (1-octanol:0.1 M sodium phosphate, pH 7.0) of N3dThd and thymidine were determined to be 1.26 and 0.064, respectively. The unusual ability of N3dThd to diffuse across cell membranes independently of the nucleoside transport system may be attributed to the considerable lipophilicity imparted to this molecule by the replacement of the 3'-hydroxyl group of thymidine with an azido moiety.     

Residual nitrobenzylthioinosine-resistant nucleoside transport in a transport mutant (AE1) of S49 murine T-lymphoma cells.

The uptake of various nucleosides by S49 mouse T-lymphoma cells and that by a single-step nucleoside transport-defective mutant thereof (AE1) were compared. Residual nucleoside entry into AE1 cells occurred via two routes, nonmediated permeation and saturable, non-concentrative transport with broad substrate specificity and a Michaelis-Menten constant approximating that for thymidine transport in wild-type cells. However, in contrast to nucleoside transport in wild-type cells, residual nucleoside transport in AE1 cells was resistant to inhibition by nitrobenzylthioinosine. In its properties the latter resembled nitrobenzylthioinosine-resistant nucleoside transport observed in other types of mammalian cells. It amounted to less than 1% of the total nucleoside transport activity of wild-type S49 cells. The results indicate that nitrobenzylthioinosine-resistant and -sensitive nucleoside transports are genetically distinguishable. In wild-type cells, the salvage of thymidine, when present at concentrations higher than 1 to 10 microM, was limited by phosphorylation, because of the saturation of thymidine kinase. In AE1 cells, entry into the cells mainly limited thymidine salvage, but at high thymidine concentrations the combined entry via residual transport and nonmediated permeation was sufficiently rapid to support intracellular thymidine phosphorylation at rates comparable to those observed in wild-type cells.     

Gas exchange and chlorophyll a fluorescence measurements as proxies of X-ray resistance in Phaseolus vulgaris L.

Phaseolus vulgaris L. plants were irradiated with different doses (0.3, 10, 50 and 100 Gy) of X-rays in order to obtain a reference curve of response to ionizing radiations for this species. Growth analysis, gas exchange and chlorophyll a fluorescence measurements were performed to estimate the radio-resistance of bean plants. Specifically, there was a negative influence of X-rays on the net photosynthesis rate at 50 and 100 Gy, already on the day of irradiation. Experimental data showed a recovery over time in the gas exchange while the theoretical maximum photochemical efficiency of the photosystem II (Fv/Fm) was fairly constant throughout the period of measurements (20 days) and for all the experimental conditions. On the other hand, the quantum yield of PSII linear electron transport (Φ_PSII) and non-photochemical quenching (NPQ) were deeply influenced over time by X-ray dose, suggesting a decrease in the functionality of the photosynthetic apparatus at the highest radiation doses. The growth was affected only at the highest doses of radiation with a significant and severe reduction of leaf expansion and number of leaves per plant. Despite the arrest in growth, X-ray exposure seems to trigger an increased photochemical activity probably signifying that P. vulgaris plants have a fairly elevated resistance to this kind of ionizing radiation. Our current results will provide a complete analysis of the photosystem II (PSII) response of P. vulgaris to different doses (0.3, 10, 50 and 100 Gy) of X-rays, providing sound references for both space-oriented and radioecology questions.

     

Dissociation by cytochalasin B of movement, DNA synthesis and transport in 3T3 cells.

Cytochalasin B was used as a tool to study the inter-relationships between cell movement, the reinitiated DNA synthesis and the enhanced transport of specific small molecules stimulated by serum in quiescent 3T3 cells. Cytochalasin at concentrations of less than 1 mug/ml inhibits serum-stimulated movement within the monolayer and migration into a wound. Even at ten times this concentration there is little effect on the increase in DNA in the culture, indicating that movement away from neighboring cells is not required for the initiation of DNA synthesis. While DNA synthesis is not inhibited by concentrations of cytochalasin up to 10 mug/ml, the increased thymidine transport which is associated with the onset of the S phase of the cell cycle is inhibited and DNA synthesis cannot be measured by the labelling of nuclei with radioactive thymidine. Cytochalasin has a differential effect on the early transport changes produced by serum addition. Glucose transport is inhibited by low concentrations of the drug (less than 1 mug/ml) while the enhanced uptake of phosphate and uridine is unaffected by a 10-fold increase in concentration. Although the doses of cytochalasin required for 50% inhibition of hexose uptake and of cell movement are the same, no causal relationship between sugar transport and locomotion can be demonstrated. Cytochalasin affects membrane functions in at least two different ways. The drug inhibits the uptake of glucose directly but affects only the S-phase associated increase in thymidine transport.     

Inhibition of thymidine uptake in asynchronous HeLa cells by nitrobenzylthioinosine

Nitrobenzylthioinosine (NBMPR), an inhibitor of nucleoside transport by human erythrocytes, was found to be a potent inhibitor of thymidine uptake by asynchronous monolayer cultures of HeLa cells. Rates of thymidine uptake by the cultures at 20 °C were constant between 10 and 40 sec after thymidine addition and were assayed during this interval; TTP was the principal metabolite of thymidine and the thymidine phosphates accumulated at constant rates which extrapolated through time zero. The lack of an effect of NBMPR on thymidine kinase activity, or on the relative proportions of thymidine metabolites in cell extracts, indicated that NBMPR inhibited thymidine transport. When mediated entry (transport) was eliminated by 2 μM NBMPR, a significant diffusional component of thymidine entry was apparent. The mediated component of thymidine uptake exhibited Michaelis-Menten kinetics and apparent K_m and V_max values of 0.5 μM and 10–21 pmoles/min/10⁶ cells were obtained. When NBMPR-treated cells were transferred to NBMPR-free medium, inhibition of thymidine uptake persisted, suggesting that NBMPR was firmly bound to the transport inhibitory sites.     

Facilitated detection of chromosome break and repair at low levels of ionizing radiation by addition of wortmannin to G1-type PCC fusion incubation

We have measured rejoining kinetics of chromosome breaks using a modified cell fusion-based premature chromosome condensation (PCC) technique in confluent cultures of normal human fibroblasts irradiated at low doses of X-rays. In order to enhance the sensitivity of the fusion-based PCC assay, we added a DNA double strand break (DSB) repair inhibitor wortmannin during the incubation period for PCC/fusion process resulting in a significantly higher yield of G1-type chromosome breaks. The initial number of chromosome breaks (without repair) gave a linear dose response with about 10 breaks per cell per Gy which is about two times higher than the value with the conventional G1-type PCC method. The chromosome rejoining kinetics at 0.5 and 2.0 Gy X-rays reveal a bi-phasic curve with both a fast and a slow component. The fast component (0-30 min) is nearly identical for both doses, but the slow component for 2 Gy kinetics is much slower than that for 0.5 Gy, indicating that the process occurring during this period may be crucial for the ultimate fate of irradiated cells. The chromosome rejoining kinetics obtained here is similar to that of other methods of detecting DNA DSB repair such as the gammaH2AX assay. Our chromosome repair assay is useful for evaluating the accuracy of other assays measuring DNA DSB repair at doses equal or less than 0.5 Gy of ionizing radiation.     

Homologous recombination is involved in the repair response of mammalian cells to low doses of tritium

Radioactive compounds incorporated in tissues can have biological effects resulting from energy deposition in subcellular compartments. We addressed the genetic consequences of [(3)H] or [(14)C]thymidine incorporation into mammalian DNA. Low doses of [(3)H]thymidine in CHO cells led to enhanced sensitivity compared with [(14)C]thymidine. Compared with wild-type cells, homologous recombination (HR)-deficient cells were more sensitive to lower doses of [(3)H]thymidine but not to any dose of [(14)C]thymidine. XRCC4-defective cells, however, were sensitive to both low and high doses of [(3)H] and [(14)C]thymidine, suggesting introduction of DNA double-strand breaks, which were confirmed by gamma-H2AX focus formation. While gamma rays induced measurable HR only at toxic doses, sublethal levels of [(3)H] or [(14)C]thymidine strongly induced HR. The level of stimulation was in an inverse relationship to the emitted energies. The RAD51 gene conversion pathway was involved, because [(3)H]thymidine induced RAD51 foci, and [(3)H]thymidine-induced HR was abrogated by expression of dominant negative RAD51. In conclusion, both HR and non-homologous end-joining pathways were involved after labeled nucleotide incorporation (low doses); genetic effects were negatively correlated with the energy emitted but were positively correlated with the energy deposited in the nucleus, suggesting that low-energy beta-particle emitters, at non-toxic doses, may induce genomic instability.     

Bovine thyrotropin inhibits DNA synthesis inversely with stimulation of cyclic AMP production in cultured porcine thyroid follicles

The effects of bovine thyrotropin (TSH) on DNA synthesis and cyclic AMP production were studied in porcine thyroid follicles using suspension culture. During the early 72 hours incubation, the time-dependent uptake of [3H]thymidine by the follicles was observed. In the presence of 10 mU/ml TSH, the uptake of [3H] thymidine was significantly depressed at 72 hours incubation. TSH inhibition of [3H] thymidine incorporation was related to its concentration and the 50% inhibition was observed by using 1.0 mU/ml TSH. Under the same conditions, cyclic AMP production was stimulated by TSH and the stimulation was observed to be related to TSH concentration. In these experiments, the incubation time was 30 min. Dibutyryl cyclic AMP, an analogue of cyclic AMP, inhibited the [3H] thymidine uptake at 72 hours incubation. From these results, it is suggested that TSH inhibits DNA synthesis, and that the inhibition may be mediated by cyclic AMP that is produced by TSH stimulation.     

DNA crosslinks and DNA replication in mouse FM3A cells after treatment with 8-methoxypsoralen plus near-ultraviolet radiation

The rate of DNA-chain elongation was studied in mouse FM3A cells after treatment with 8-methoxypsoralen plus near-ultraviolet radiation using the minimal doses (1 μg/ml 8-methoxypsoralen plus 1–2.5 kJ/m² of near-ultraviolet radiation) which inhibited cell-cycle progression or DNA replication. A rapid decrease in incorporation of [³H]thymidine and recovery to some extent during incubation after treatment have been reported (Hyodo, M., Fujita, H., Suzuki, K., Yoshino, K., Matsuo, I. and Ohkido, M. (1982) Mutat. Res. 94, 199–211). The results of the present study showed that the rate was not changed suggesting that the decrease in [³H]thymidine incorporation was not due to the rate of DNA-chain elongation, but was due to change in the frequency of initiation of replication. Formation of DNA crosslinks was then studied by the sedimentation of pre-labeled DNA in an alkaline sucrose gradient. The results showed that, at these doses of 8-methoxypsoralen plus near-ultraviolet radiation, approx. 2–7 crosslinks were formed per 10⁹ Da. It was also suggested that some of the DNA crosslinks might be repaired during the prolonged incubation, but unrepaired crosslinks were still present after 24 h incubation.     

Thymocyte apoptosis in response to low-dose radiation

Thymocyte apoptosis was assessed by counting apoptotic bodies with flow cytometry (FCM) and measuring DNA fragmentation with fluorescence spectrophotometry (FSP). J-shaped dose-response curves were obtained after both whole-body irradiation (WBI) of mice and in vitro irradiation of EL4 cells with doses ranging from 0.025 to 4 Gy X-rays. There was a significant reduction of apoptosis rate to below control level with doses within 0.2 Gy, and a dose-dependent increase in apoptosis with doses above 0.5 Gy. When thymocytes were cultured 24 h after WBI with 75 mGy X-rays in complete RPMI 1640 medium, a reduction in apoptosis was observed in the course of incubation for 72 h, and the presence of Con A in the medium accentuated this reduction in a dose- and time-dependent manner. The implications of these observations and the possible molecular mechanisms for future studies are proposed.     

Evaluation of spermatogenic response of mice to the induction of mutations by combined treatment with X rays and antineoplastic drugs

Combined treatment with low doses of X-rays plus cyclophosphamide (0.25 Gy+25 mg/kg body weight) or X-rays plus mitomycin C (0.25 Gy+1.75 mg/kg body weight) did not induce significant dominant lethal effects in any stage of spermatogenesis when a parameter representing pre- and postimplantation loss, such as the decrease of live implants per female, was applied. After combined exposure to high doses of X-rays plus cyclophosphamide (1.00 Gy+100 mg/kg body weight) an increase of dominant lethal mutations (DLMs) was observed in differentiating spermatogonia, spermatids, and spermatozoa with the same parameter. Combined treatment with high doses of X-rays plus mitomycin C (1.00 Gy+5.25 mg/kg body weight) produced DLMs in differentiating spermatogonia and late spermatocytes. A calculation of enhanced risk was applied to the data of DLMs from the combined treatment regimen and was based on the proportion of dead implants (postimplantation loss only). Enhanced risk could be shown not only after high but also after low combined exposure to X-rays plus cyclophosphamide and X-rays plus mitomycin C. With low doses this enhanced risk was observed in spermatids for X-rays plus cyclophosphamide and in differentiating spermatogonia to early spermatocytes for X-rays plus mitomycin C.     

Incorporation of P and C into Photosynthetic Products of Ankistrodesmus braunii as Affected by X-Rays

The incorporation of ³²P and ¹⁴C into organic compounds by Ankistrodesmus is strongly inhibited by X-rays. In the same phosphorylated compounds ³²P-incorporation apparently is more severely inhibited by X-rays than the ¹⁴C-labelling. The ³²P-incorporation into organic compounds is more strongly inhibited than ³²P-labelling of inorganic phosphate in the cell. The inhibition of ³²P-incorporation into a number of compounds is strikingly uniform. It is concluded that the inhibition of ³²P-incorporation and of ¹⁴C-incorporation into phosphorylated compounds in vivo is due to an uncoupling by X-rays of photophosphorylation as in vitro. The difference in X-ray sensitivity of ¹⁴C- and ³²P-incorporation into one organic phosphorous compound is attributed to a dual action of X-rays on ³²P-incorporation in organic compounds (both via the uncoupling of photophosphorylation) and only a single effect on ¹⁴C-incorporation and ³²P-labelling of inorganic phosphate. The effect of X-rays on ¹⁴C-incorporation into organic compounds included inhibition in most cases but also stimulation as in the case of glycolic acid. These differences may be due to interference in the intercellular regulations following the application of X-rays. The inhibition of ¹⁴C-incorporation in many cases exhibits different behaviour at low (<200 krad) and high doses. These changes are discussed on the assumption that at the lower doses X-rays cause uncoupling of photophosphorylation and at the higher doses an additional inhibition of electron transport.     

DNA semi-conservative synthesis in normal and Fanconi anemia fibroblasts following treatment with 8-methoxypsoralen and near ultraviolet light or with X-rays

Summary The effect of treatment with 8-methoxypsoralen (8-MOP) plus near-UV radiation (UVA) or with X-rays on the rate of DNA semi-conservative synthesis of fibroblasts from 10 Fanconi anemia (FA), two heterozygous, and three normal cell lines was studied. Following treatments with either X-rays or low doses of 8-MOP plus UVA leading to a majority of monoadducts over cross-links per genome, the FA and hetcrozygous cell lines were indistinguishable from normals: the transient inhibition of semi-conservative DNA synthesis was followed by the recovery of a nomral rate of synthesis. In contrast treatment with higher (but not saturating) doses of 8-MOP plus UVA allowed us to distinguish two classes among the FA cell lines. One class demonstrated a pattern of recovery similar to that of heterozygous and normal cell lines. This indicates that in such cell lines, the predominant lesion in this condition, the cross-links, do not arrest DNA synthesis and are likely to be normally repaired. Another class of FA cell lines did not show a recovery of a normal rate of DNA synthesis even after prolonged post-treatment incubation and although the proportion of cells in S phase was similar to that of the strains of the first category. This indicates that in such cell lines the repair of cross-links is inhibited at some step which is not necessarily the incision one.     

Adaptive response of human lymphocytes for the repair of radon-induced chromosomal damage

In human lymphocytes low doses of X-rays can decrease the number of chromatid deletions induced by subsequent high doses of sparsely ionizing X-rays. Because of the concern with the carcinogenic effects of low doses of -particles from radon in homes, experiments were carried out to see if low doses of X-rays could also decrease the yield of chromosomal aberrations induced by subsequent exposure to radon. Human peripheral blood lymphocytes were irradiated with low doses of X-rays (2 cGy) at 48 h of culture, exposed to radon at 72 h of culture, and analyzed for the presence of chromatid aberrations at subsequent intervals. The frequency of chromatid aberrations induced by radon alone increased with time after exposure, indicating exaggerated differences in the stage sensitivity of cell cycle stages to high-LET radiation. Furthermore, the numbers of aberrations per cell did not follow a Poisson distribution but were over dispersed, as might be expected since high-LET radiations have a high relative biological effectiveness compared with low-LET radiations. Nevertheless, lymphocytes exposed to 2 cGy of X-rays before radon exposure contained approximately one-half the number of chromatid deletions compared with lymphocytes treated with radon alone and analzed at the same time. Thus, the putative chromosomal repair mechanism induced by low doses of sparsely ionizing radiation is also effective in reducing chromosomal aberrations induced by radon, which hitherto had been thought to be relatively independent of repair processes.