Stem Cells in Bone and Bone Marrow after Contamination with Bone-seeking Radionuclides

Summary

The effects of time, mass and oxidation state on plutonium gastrointestinal absorption and tooth adsorption were studied during and after chronic ingestion of plutonium-238 (IV) or (VI) (1·55–15·60 kBq/ml) in 6·5 mm bicarbonate medium by fed rats via drinking water for 8 days to 3 months. Animals were killed during the ingestion to follow the kinetics of whole-body storage and clearance of plutonium. At 1·55 kBq/ml the amount of plutonium retained in the skeleton increased continuously during the 85 days of ingestion and reached a plateau thereafter. This plutonium retention was therefore dependent on the total mass administered but not proportional to this mass, as the fraction of administered plutonium retained decreased during the first 22 days of ingestion and then stabilized. This is reflected by the gastrointestinal transfer (f₁), which had risen to (3·80 ± 0·82) × 10^?5 on Day 3 of ingestion and then decreased to a stabilized value of (1·07 ± 0·06) × 10^?5 from Day 30 to the end of the ingestion period. In the liver, the amount of plutonium retained reached a plateau, which lasted from Day 30 to the end of ingestion. The kidneys and spleen were also found to be retention sites. By Day 3 of ingestion, for a mass ingested of 5 × 10^?7 g/kg of body mass, the maximum mean value of f₁ we found was smaller than the 10^?4 recommended by ICRP Report 30. The oxidation state had no effect on f₁. Large plutonium deposition was observed on the teeth. For both oxidation states (IV) and (VI), about 0·10% of the administered dose was deposited on the teeth after 3 days of ingestion, whatever the plutonium concentration administered. However, whereas the amount of plutonium (IV) deposited did not change throughout the ingestion period, tooth deposition of plutonium (VI) decreased.     

DNA Strand Breaks from Tritium Decay: A Local Effect for Cytosine-6-3H

Summary

Twelve-week-old female (C3H × 101)F₁ mice were injected intravenously with an ultrafiltered solution of ²³⁹Pu in 1 per cent trisodium citrate, and mated to uninjected PCT males. The plutonium content was examined radiochemically and autoradiographically in placentae and foetuses on the 12th and 18th days of gestation, and in neonates during the 24 hours after birth and also at 18 days post-natally.

Plutonium was distributed in most tissues of the late foetus and the suckling as it is in adult mice. However, on both the 12th and 18th days of gestation the concentration in the yolk-sac splanchnopleure was much higher than in any other foetal tissue. The amount of ²³⁹Pu in 18-day-old sucklings was between two and seven times as great as in 1-day-old neonates because of ingestion of milk from the lactating dams. In the first litter following administration of the radionuclide to the dam, about 0·02 per cent of the plutonium injected was transferred to an individual offspring by the time of birth, and a further 0·08 per cent by the time of weaning.     

The Effect of Dietary Phosphorus on the Metabolism of Calcium and Strontium in the Rat

Summary

An investigation has been made of the way in which changes in dietary phosphorus within the physiological range influence the comparative metabolism of calcium and strontium in the rat. Radioactive calcium and strontium were used as tracers.

The absorption of both calcium and strontium after oral administration was dependent on the phosphorus level in the diet, but not to the same degree. The skeletal ratio ⁸⁵Sr/⁴⁷Ca decreased by some 40 per cent as the dietary phosphorus was increased from 0·5 to 1·3 g per cent.

After intraperitoneal injection of radioactive calcium and strontium, the skeletal retention of ⁸⁵Sr was about 25 per cent more on a diet containing 1·3 per cent phosphorus than on a diet containing 0·5 per cent phosphorus. Skeletal retention of calcium varied little.

It is concluded that renal discrimination against strontium as well as intestinal absorption of calcium and strontium were affected by the phosphorus content of the diet, but in opposite directions.     

The Kinetics of Repair in Mouse Lung after Fractionated Irradiation

Summary

The kinetics of repair of sublethal damage in mouse lung was studied after fractionated doses of ¹³⁷Cs γ-rays. A wide range of doses per fraction (1·7–12 Gy) was given with interfraction intervals ranging from 0·5 to 24 h. The data were analysed by a direct method of analysis using the incomplete repair model. The half-time of repair (T_1/2) was 0·76 h for the pneumonitis phase of damage (up to 8 months) and 0·65 h for the later phase of damage up to 12 months. The rate of repair was dependent on fraction size for both phases of lung damage and was faster after large dose fractions than after small fractions. The T_1/2 was 0·6 h (95 per cent c.1. 0·53, 0·69) for doses per fraction greater than 5 Gy and 0·83 h (95 per cent c.1. 0·76, 0·92) for doses per fraction of 2 Gy. Repair was nearly complete by 6 h, at least for the pneumonitis phase of damage. To the extent that extrapolation of these data to humans may be valid, these results imply that treatments with multiple fractions per day that involve the lung will not be limited by the necessity for interfraction intervals much longer than 6 h.     

Rescue of Marker Phenotypes: Effects of BUdR Sensitization,Hypoxia and High LET

Summary

The survival curve of colony-forming ability of Chinese hamster wg3h cells has been compared with the dose—response curve for the expression of an active thymidine kinase (TK) gene from these cells. The TK⁺ phenotype was measured by hybrid colony formation after fusion of wg3h (TK⁺) cells with Chinese hamster A23 (TK^?) cells. The TK⁺ survival data fitted a multi-target curve up to 3 krad of ¹³⁷Cs irradiation, when a highly resistant fraction of hybrid colonies was seen at about 1 per cent survival. The D₀ of TK⁺ survival for the multi-target region was 3·1–4·0 times greater than that of wg3h survival, even when the D₀ for cell survival varied between 136 and 545 rad by 14 MeV neutrons and hypoxia respectively. This parallel modification of cell and TK⁺ sensitivities suggests that the lesions causing cell inactivation are of the same type as those that cause marker inactivation. Using 14 MeV neutron data the approximate target size for TK inactivation was calculated to be 0·54–0·91 per cent of the DNA content of the cell (or about one-fifth to one-tenth of a chromosome). The data support the idea that marker inactivation results primarily from damage occurring outside the marker gene. BUdR labelling of wg3h cells before irradiation caused slight toxicity (30 per cent reduction in plating efficiency) and a twofold increase in cell sensitivity. However, the sensitivity of the TK⁺ phenotype increases by only 30 per cent. The increased cell sensitivity thus appeared to result from synergism between increased sensitivity of DNA to strand breakage and metabolic toxicity, the latter being largely overcome by fusion with normal cells.     

The Renal Excretion of Strontium and Calcium in Dogs

Renal-excretion and plasma-clearance rates of ⁸⁵Sr and ⁴⁵Ca were determined in trained female dogs 18 to 24 hours after the simultaneous intravenous or subcutaneous administration of these two isotopes. Tubular reabsorption of ⁴⁵Ca in the normal dog was nearly complete, being more than 99 per cent of that filtered at the glomeruli. The reabsorption of ⁸⁵Sr was consistently less than that of calcium, averaging less than 98 per cent. Clearance measurements, using endogenous creatinine for determining the glomerular filtration rate, indicated a more rapid clearance from the plasma of the strontium isotope (0·91 ml/min for ⁸⁵Sr and 0·17 ml/min for ⁴⁵Ca). As a result of these differences, the excretion of ⁸⁵Sr was 5 to 6 times more rapid than that of ⁴⁵Ca.

Drastic changes in plasma-calcium levels, as achieved by thyropara-thyroidectomy and parathyroid-extract injection, did not appreciably alter the ratio of the excretion rates despite marked changes in the absolute values. The infusion of sodium citrate caused large increases in the urinary excretion of both strontium and calcium and countered the renal discrimination process. The excretion of both ions was also significantly increased during alkalosis brought about by the infusion of sodium bicarbonate. Acidosis was without effect.     

Metabolic Behaviour of 137Cs-137mBa in the Lactating Goat

Summary

Investigations on the metabolism of ¹³⁷Cs in the lactating goat demonstrated that: (a) the biological half-time for removal of ¹³⁷Cs from the body was about 2–3 days, (b) at steady state, about 7 per cent of the daily dose of ingested ¹³⁷Cs was secreted daily in milk, about 40 per cent in the urine and about 40 per cent in the faeces, (c) the ¹³⁷Cs/K ratio in milk was 1·4 based on a dietary ¹³⁷Cs/K ratio of unity, (d) the mammary gland did not differentiate between ¹³⁷Cs and K, (e) the organ of highest concentration was kidney, with values 1·5 times that of muscle and other soft tissues, (f) the ^137mBa daughter metabolically dissociated from its ¹³⁷Cs parent in vivo so that the gamma activity of the whole blood of the living animal was 5·5 times that as usually determined. These and other findings are discussed and reference made to differences in ¹³⁷Cs metabolism between ruminants and non-ruminants.     

Overview of Radiation-induced Skin Cancer in Humans

Summary

There are about a dozen studies of the incidence of skin cancer among irradiated populations with known skin doses that are available for estimating the risk of radiation-induced skin cancer. It is of note that they provide no evidence for a dose threshold and are compatible with a linear dose–response relationship, at least for ultraviolet radiation exposed skin. The studies also provide varying amounts of evidence concerning a number of other important issues in assessing skin cancer risk: types of skin cancer induced by ionizing radiation, the appropriateness of relative risk vs absolute risk models, combined effects of ionizing and UV radiations, and variations in sensitivity to skin cancer induction among demographic and genetic subgroups. Little epidemiological information is available on several factors, such as the RBE for high-LET radiation, the effects of dose protraction or fractionation, or variations in risk by age at irradiation. A reasonable estimate of skin cancer lethality was 0·2 per cent when weighted for the relative proportions of squamous cell and basal cell skin cancers. Average risk estimates of radiation-induced skin cancer incidence were: absolute risk (AR) of 8·5 × 10^?4 person-year-Sv and excess relative risk (RR) of 52 per cent/Sv. Lifetime skin cancer risk was calculated by life-table methods for males from exposures spread out over ages 20–60 years. The estimates for excess skin cancer incidence were 2 per cent and 11 per cent per Sv under the AR and RR models, respectively, while the corresponding mortality risks were 4 × 10^?5 and 2 × 10^?4 per Sv.     

Distribution of 239Pu in the Skeleton of the Tree Shrew (Tupaia Belangeri) between 15 and 50 Months after Injection

Summary

The macroscopic and microscopic distribution of intramuscularly injected, essentially monomeric, ²³⁹Pu was studied in the skeleton of the adult tree shrew (Tupaia belangeri). Data for the period between 15 and 50 months after injection are presented and compared with the data from earlier time points. Between 83 and 500 days after injection the nuclide content and the wet weight of the skeleton decreased to a constant level at about 55 per cent of the maximum values. The microscopic distribution has been analysed in distal femora, proximal humerus, proximal tibia and lumbar vertebra over the whole observation time; additionally at some selected time points proximal femur, femur shaft, distal humerus and distal tibia were analysed. The initial endosteal surface activity ranged from 3·8 to 5·3 Bq/cm² and decreased to a minimum at about 1000 days after injection and increased thereafter. A similar behaviour was found for the dose rate near bone surfaces which was initially about 0·075 Gy/day on endosteal surfaces. In the deep bone and the deep marrow the dose rate was negligible, about 0·008 Gy/day and 0·001 Gy/day, respectively. The average cumulative dose 1500 days after injection was about 67 Gy on the endosteum, six times greater than the cumulative dose calculated from the mean concentration of plutonium in the whole skeleton. All values are normalized to an injected activity of 37 kBq/kg body weight. The tupaia data are discussed in relation to the available data from monkeys, dogs and rats.     

High Radiosensitivity of the MOLT-4 Leukaemic Cell Line

Summary

Radiation survival of MOLT-4, a leukaemic T-lymphocyte cell line, was measured by counting colonies formed in 0·8 per cent methyl cellulose. The survival curve was a simple exponential and showed the cells to be radiation sensitive, with D₀ = 0·49 ± 0·02 Gy and extrapolation number n = 0·92 ± 0·09. No increase in survival as measured by colony-forming ability or trypan blue dye exclusion was seen when the dose was split into two fractions, separated by a 5 h incubation period. Electron microscopy and trypan blue dye exclusion showed that 5 h after exposure to high doses, MOLT-4 cells began to die and displayed condensed, marginated chromatin and cellular vesticulation.     

Long Term Retention of 237Np in Rats

Summary

This interim report summarizes the results of observations during the first year after a single injection of ²³⁷Np nitrate (0·2 or 1·0 mg/kg body weight) into adult female rats and further preliminary data obtained with young animals. The retention of ²³⁷Np was followed by whole body counting and serial sacrifice of groups of animals. The retention data could be fitted to three-component exponential equations which show no major differences between the two ²³⁷Np dose levels. The half-times and extrapolated initial fractions calculated from the first two exponential terms indicate that one fraction, representing about 40 per cent of the injected ²³⁷Np was excreted within the first 5 days and an additional 15 per cent within the first 5 months, while the rest was excreted with a half-time of about 3.5 years. This final long term component is assumed to indicate the rate of loss of ²³⁷Np from the skeletal compartment. In young animals both whole-body and skeletal retention of ²³⁷Np during the first 5 months of observation was about 50 per cent higher than in the adults. Several soft tissue tumours, mostly mammary tumours, have appeared to approximately the same extent in both control and ²³⁷Np treated adult rats but no osteosarcomas were detected up to 15 months after injection of ²³⁷Np.     

Relationship between Field Strength and Abnormal Development in Chick Embryos Exposed to 50 Hz Magnetic Fields

Summary

Chick embryos were exposed to sinusoidally oscillating 50 Hz magnetic fields during their first 2 days of development. In the first series of experiments magnetic field strengths of 0·1, 0·3, 1 and 10 A/m were used. The percentage of abnormal embryos (%AE) was 16 per cent in the sham-exposed control group. %AE was increased at 1 A/m (29 per cent) and 10 A/m (32 per cent), but not at 0·1 A/m (16 per cent) or 0·3 A/m (14 per cent). In the second series of experiments field strengths of 0·4, 0·6, 0·9 and 1·35 A/m were used. %AE was 17 per cent in the control group, 10 per cent at 0·4 A/m, 19 per cent at 0·6 A/m, 17 per cent at 0·9 A/m and 36 per cent at 1·35 A/m. Only the 1·35 A/m group was significantly different from the controls. The results of this study suggest that exposure of chick embryos to a 50 Hz magnetic field causes abnormal development, and that no abnormalities are induced below a threshold between 0·9 and 1 A/m.     

Reactions of OH Radicals with Poly(U) in Deoxygenated Solutions: Sites of OH Radical Attack and the Kinetics of Base Release

Summary

Pulse radiolysis of N₂O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2·9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t_½ < 4 min at 0°C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50°C the half lives are 83 min and 26 h, respectively (E_a (fast) = 68 kJ mol^?1, E_a (slow) = 89 kJ mol^?1, A (fast) = 1·5 × 10⁷ s^?1, A (slow) = 1·9 × 10⁹ s^?1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.     

The Incorporation of Plutonium by the Embryo and Fetus of Rats and Guinea-pigs

Summary

The transfer of ²³⁸Pu from the maternal circulation to the developing embryo and fetus was studied in rats and guinea-pigs to provide data for the development of dosimetric models for the human embryo and fetus. Following administration at different stages of gestation, measurements were made after 3 days in the rat and 7 days in guinea-pigs. The amount transferred was greater after administration at later stages of gestation, up to a maximum of about 0·8–0·9% of the injected activity per fetoplacental unit (FPU) and about 0·2% per fetus in both species. In advanced gestation the yolk sac, the initial site of haemopoietic stem cells, contained up to 80% of the total activity in the FPU in rats, compared with about 25% for the guinea-pig; retention in placental trophoblast was greater in the guinea-pig. The concentrations of ²³⁸Pu in the yolk sac were generally about two to three orders of magnitude greater than fetal concentrations and of the same order as in maternal liver and bone. In both species, concentrations in the embryo and fetus were greatest after injection early in gestation, reached their lowest around mid-organogenesis and increased again in later gestation. The fetus:mother whole-body concentration ratios in late gestation were about 0·1 and 0·05 in rats and guinea-pigs, respectively. Measurements were also made of the ²³⁸Pu retention in neonates at birth. In guinea-pigs the liver accounted for about 6–9% of retained activity; similar values for femora indicated skeletal retention of about 60–80%. For administration at each stage of gestation, and particularly at early stages, transfer of ²³⁸Pu to the fetus continued throughout gestation but concentrations decreased due to fetal growth.     

Inactivation of Urease by Nitrogen Mustard and a Comparison with Inactivation by X-rays

Urease is inactivated by small amounts of nitrogen mustard (Di-2-(chloroethyl) methylamine: HN2) in unbuffered solution at ph 7·0, the inhibition or inactivation being complete when 10^?3 M nitrogen mustard is used. With 10^?5 M solutions of HN2, 12 per cent inhibition was observed. The inhibition is ph-dependent, the percentage inactivation with 10^?3 M HN2 being 100 at ph 6·5 and 0 at ph 5·2. Additions of many different substances protected the urease. Buffer salts protected, citrate and phosphate being most effective; for example, urease was completely protected against inactivation by 10^?3 M nitrogen mustard by the addition of 5 × 10^?3 M, or stronger, solutions of citrate buffer. Proteins are effective protectors, but nucleic acids are not. All the amino acids tested protect, but cysteine and histidine are much more effective than most others investigated. Blocking of urease —SH groups is also an effective protective measure.

The inactivation is considered likely to be a result of the combination of nitrogen mustard with histidine residues and with —SH groups of the urease.     

Measurement of the G-value for 1·5 keV X-rays

Summary

Although there are several theoretical predictions of the dependence of the G-value on X-ray energy, measurements have not been made below ≈ 7 keV. Using a ferrous sulfate solution modified by the addition of benzoic acid, we have measured the relative G-values for Al_k characteristic X-rays (1·5 keV), ²³⁸Pu α-particles (3·7 MeV), ⁶⁰Co (1·17 MeV) and ¹³⁷Cs (0·66 MeV) γ-rays. This modified ferrous sulfate solution gave a 4-fold increase in sensitivity relative to the conventional solution, making measurements with the Al_k X-rays feasible. The relative ferrous-to-ferric conversions as a function of dose were similar for the two γ-ray energies, yielding G-values of 1·62 and 1·59 µmol J^?1 for the ⁶⁰Co and ¹³⁷Cs radiations, respectively. The α-particle G-value was 0·52 µmol J^?1, or 31 per cent of that for the ⁶⁰Co γ-rays, in good agreement with previous measurements. The Al_k X-rays had a G-value of 0·92 µmol J^?1 or 57 per cent of that of the ⁶⁰Co radiation. This G-value for the 1·5 keV X-rays is within 20 per cent of the values predicted by current theories, and theoretical values are within the error range of our measurement. The consistency between the experimental value reported here and theoretical G-values for ultrasoft X-rays should be valuable for models of radiation action on biological systems.     

Alpha-particles Induce Preneoplastic Transformation of Rat Tracheal Epithelial Cells in Culture

Summary

To characterize the potential role of high-l.e.t. radiation in respiratory carcinogenesis, the cytotoxic and transforming potency of 5·5 MeV α-particles from electroplated sources of ²³⁸Pu were determined using primary cultures of rat tracheal epithelial cells. The α-particle response was compared to the effects of 280 kVp X-rays and of the direct-acting carcinogen N-methyl-N'-nitro-N-nitrosoguanidine. Increasing the α-particle dose caused an exponential decrease in survival with a D₃₇ of 1·6 Gy. X-rays also caused a dose-dependent decrease in survival (D₃₇ = 3·6 Gy) but the survival curve had a significant shoulder. The RBE for cell killing by α-particles versus X-rays varied with dose, and ranged between 4 and 1·5 for α doses in the range 0·2–4 Gy. At equally toxic doses (relative survival 0·18–0·2), all three agents induced similar frequencies of preneoplastic transformation. For preneoplastic transformation induced by doses of α- and X-radiations giving 80 per cent toxicity, an α RBE of 2·4 was derived. The similar RBEs for cell killing and for preneoplastic transformation suggest an association between the type or degree of radiation-induced damage responsible for both cell killing and cell transformation.     

Partial Additivity of the Mitotic Delays Induced by Separate γ-irradiation of Sea-urchin Eggs and Sperm

Summary

Natural cell-mediated cytotoxicity (NCMC) by lymphocytes from Japanese atomic bomb survivors now living in the United States was measured. Seventy-one individuals were exposed to an estimated ‘0·00’ Gy (‘0 rads’) (S₀ group) and 58 to greater than ‘0·00 Gy’ (S₊ group) at the time of the bomb. Of this 58, 51 (88 per cent) received less than 0·50 Gy and 30 (52 per cent) received less than 0·10 Gy. NCMC was measured against ⁵¹Cr-labelled K562 target cells. Activity by lymphocytes from S₊ group donors was significantly greater than that for the S₀ group (p = 0·028 by the stratified Wilcoxon rank-sum test). This difference between the S₊ and S₀ populations was detected 35 years after exposure to the bomb. It is therefore feasible and important to examine appropriate biologic parameters to elucidate the effects of low doses of radiation in humans.     

Age-dependent Variation of Zinc-65 Metabolism in LACA Mice

Summary

Mice were gavaged with zinc-65 solution, 8·6–19·3 kBq per mouse, and the whole-body retention and organ content of zinc-65 were measured at different times after administration. The age-dependence of the fractional absorption of zinc-65 from the gastrointesinal tract (f₁), the endogenous faecal excretion fraction of zinc-65 (EFEF), tissue distribution and whole-body retention were determined. The f₁ values obtained were 0·86 ± 0·15, 0·64 ± 0·11, 0·52 ± 0·07 and 0·39 ± 0·02 in suckling, adolescent, young adult and older mice, respectively. The EFEF values determined were 0·083 ± 0·008, 0·099 ± 0·004, 0·122 ± 0·018 and 0·144 ± 0·005 of intraperitoneally injected zinc-65 in suckling, adolescent, young adult and older mice at administration. Zinc-65 mainly distributed in the liver, muscle, lung, kidneys and bone. In some tissues, there was an inverse relationship between the relative content of gavaged zinc-65 and the animal's age at administration. The whole-body biological half-lives of zinc-65 increased with animal age. The influence of the age-dependent variation of zinc-65 metabolism on internal dose and on radiation protection is discussed.     

Lung Tumour Induction in Mice after X-rays and Neutrons

Summary

Dose–response curves were determined for pulmonary adenomas and adenocarcinomas in mice after single acute doses of 200 kVp X-rays and cyclotron neutrons (E¯ = 7·5 MeV). A serial-killing experiment established that the radiation induction of chromosome aberrations. The validity of the concept of PLD neous cancers. The dose versus incidence (I) of tumours in male and female mice for X-ray doses between 0·25 and 7·5 Gy is ‘bell-shaped’ and best fitted with a purely quadratic induction and exponential inactivation terms, i.e. I = A + BD²e^?αD. In contrast, the tumour dose–response after 0·1–4·0 Gy of neutrons is best fitted by I = A + BD e^?αD and is steeply linear ≤ 1 Gy, peaks between 1 and 3 Gy and sharply declines at 4·0 Gy. The data for the female mice ≤ 1 Gy neutrons are best fitted to the square root of the dose.

A major objective of the experiments was to derive neutron RBE values. Because of the differences between the X-ray (quadratic) and neutron (linear) curves, the RBE_n will vary inversely with decreasing X-ray dose. The RBE values at 1 Gy of X-rays derived from the B coefficients in the above equations are 7·4 ± 3·2 (male and female); 8·6 ± 3·6 (female) and 4·7 ± 1·8 (male). These are high values and imply even higher values at the doses of interest to radiation protection. If, however, one restricts the analysis to the initial, induction side of the response (≤ 1 Gy neutrons, ≤ 3 Gy X-rays) then good linear fits are obtainable for both radiations and indicate neutron RBE values of 7·4 ± 2·3 for female mice and 4·5 ± 1·8 for males, and these are independent of dose level.