Chelating agents and cadmium toxicity: problems and prospects.

Symptoms and signs in humans after excessive exposure to cadmium usually involve the gastrointestinal tract after single oral intake, the lung after acute inhalation, and the kidney after long-term exposure. These organs are usually considered to be the "critical" organs, i.e., the organs most sensitive at a certain type of exposure. The type of Cd-related damage that is most common in humans is probably the renal toxicity after long-term exposure. Most animal experiments, including the most recently published ones, have involved study of gross toxicity and tissue distribution after injection of cadmium in acute experiments. The observations in the older literature that the influence of chelating agents on Cd distribution and excretion is confined to the early period after acute Cd exposure has been confirmed and extended, and the relationship to the time course of metallothionein synthesis has been demonstrated. Although the injection experiments concerning cadmium distribution, particularly those employing repeated exposure, may furnish information that can form a basis for speculation about long-term toxicity to the kidney, there is a lack of direct studies in animals of possible beneficial effects of chelating agents on renal toxicity of cadmium after prolonged exposure. Among the few studies reported, either an increased renal toxicity or a lack of influence on renal toxicity has been observed. The problems in finding a treatment scheme that can be beneficial for the renal toxicity resulting from long-term cadmium exposure thus remain; however, the prospects of finding such schemes in the future seem favorable in view of some of the recent observations.

Metabolic model for cadmium in man.

A metabolic model for cadmium has been formulated in terms of a flow scheme for cadmium among eight body compartments. The mathematical description of the flow of cadmium between compartments consits of a number of differential equations, and the accumulation of cadmium may thus be calculated. Coeffcients for the flow of cadmium were estimated from empirical data both from animals and man. The modelling serves two main purposes: it provides a means of using present knowledge about metabolism in order to calculate expected accumulation in critical organs and other tissues and body fluids at certain intake levels and it makes it possible to define deficiencies in our present knowledge about metabolism. Data recently collected, partly as a result of considerations related to the model, have confirmed the assumptions of a very long biological half-time in other tissues and the small excretion of cadmium via bile.

Fundamental aspects of dose--response relationships and their extrapolation for noncarcinogenic effects of metals.

Fundamental differences in dose--response relationships between "stochastic" and "nonstochastic" effects of chemicals are identified and discussed. The difficulties in extrapolating into the low-dose region of dose--response curves are pointed out. In some instances of nonstochastic effects, observations concerning interindividual variability in biological half-time and threshold body burden for symptoms may be used for such extrapolation. An example based on data from the literature concerning effects of methyl-mercury on the nervous system is given. The confidence intervals of the extrapolated risk-values are computed and discussed in relation to assumptions concerning the mathematical model to be used in the extrapolation process.

Metal interactions in carcinogenesis: enhancement, inhibition.

Metals constitute a fundamentally important part of the total human environment. Since human exposure often involves complex mixtures of metal compounds and, possibly, organic compounds which may be carcinogenic per se, interactions between these compounds may add significantly to human cancer risk. Our present knowledge about these kinds of interactions is very limited. The best investigated area is benzo(a)pyrene (BP)-metal oxide particle interactions in respiratory carcinogenesis in the hamster. Metal oxide particles were also shown to modify the carcinogenic effect of nitrosamines. Several reports describe experiments in which selenium compounds exerted a generally anticarcinogenic and antimutagenic activity. Inorganic arsenic compounds, which are accepted to be carcinogenic in man, have so far been negative in animal experiments except for one recent suggested report. Several authors have, however, suggested that these compounds may act as cocarcinogens due to their inhibition of DNA repair, although animal experiments to demonstrate a cocarcinogenic effect of arsenic compounds have been negative so far, except for one preliminary report. The concentration of zinc in the diet seemed to influence both transplanted tumor growth and the carcinogenicity of several organic compounds, and the possibility of a correlation between dietary zinc and certain cancer forms in man has been suggested. Protection against development of Leydigiomas usually induced by cadmium injection was afforded by simultaneous injection of zinc salts. Nickel carcinogenesis has been reported to be antagonized by manganese, and synergism between Ni and organic carcinogens, e.g. BP, has been demonstrated. There is no firm evidence that lead may be a cocarcinogen, although some limited experimental evidence is available. Oxidizing agents have been demonstrated to increase, and reducing agents to antagonize, the mutagenic effect of chromium compounds in vitro. The content of carcinogenic and other metals in asbestos has been suggested to modify the carcinogenic properties of asbestos. Since much of the information available at present is suggestive, further research on these interactions as well as other possible interactions in metal carcinogenesis is needed. Studies should be made both in well defined in vitro systems and in relevant animal models.

Distribution of metallothionein-bound cadmium and cadmium chloride in mice: preliminary studies.

Metallothionein from livers of mice was isolated by gel chromatography and isoelectric focusing. One of two forms thus obtained contained 32 percent cysteine. This form, labeled in vitro with 109Cd, was injected intravenously in mice, and the distribution of 109Cd was studied. Animals killed after 4 hrs had over 80 percent of the injected dose in the kidneys. Protein obtained after gel chromatography, containing both forms of cadmium-binding protein, was also labeled in vitro with 109Cd and injected intravenously. Animals killed 4 hrs after injection had 50 percent of the injected dose in the kidneys. Whole-body measurements and wholebody autoradiography demonstrated that approximately 40-60 percent of the injected dose had been excreted in urine. The results show a selective accumulation of metallothionein-bound cadmium in the kidney and indicate possible differences in distribution and excretion of cadmium depending on binding to different forms of low molecular weight cadmium-binding proteins.

Subcellular targets of cadmium nephrotoxicity: cadmium binding to renal membrane proteins in animals with or without protective metallothionein synthesis.

Nephrotoxic effects of cadmium exposure are well established in humans and experimental animals. An early manifestation of such toxicity is calciuria a few hours after injection of CdMT in rats. Protection against calciuria and other adverse effects such as proteinuria (occurring later) is offered by pretreatment with Cd, which effectively induces metallothionein synthesis. In the present experiment, one group of animals was given pretreatment with CdCl2 to induce metallothionein synthesis. The comparison group was left without pretreatment. The distribution of Cd from a normally nephrotoxic dose of 109CdMT was studied by gel chromatography in subcellular fractions of kidney cortex in both groups. In the pretreated animals, 109Cd in the plasma membrane and microsome fractions of renal cortical cells was mainly bound to metallothionein and other low molecular weight proteins at 4 hr. In nonpretreated animals the major part of 109Cd was bound to high molecular weight proteins. These findings indicate that membrane proteins may be important targets for Cd when inducing nephrotoxicity and that sequestering of Cd by metallothionein (and other low molecular weight proteins) may be a mechanism of protection.

Impact of effects of acid precipitation on toxicity of metals.

Acid precipitation may increase human exposure to several potentially toxic metals by increasing metal concentrations in major pathways to man, particularly food and water, and in some instances by enhancing the conversion of metal species to more toxic forms. Human exposures to methylmercury are almost entirely by way of consumption of fish and seafood. In some countries, intakes by this route may approach the levels that can give rise to adverse health effects for population groups with a high consumption of these food items. A possible increase in methylmercury concentrations in fish from lakes affected by acid precipitation may thus be of concern to selected population groups. Human exposures to lead reach levels that are near those associated with adverse health effects in certain sensitive segments of the general population in several countries. The possibility exists that increased exposures to lead may be caused by acid precipitation through a mobilization of lead from soils into crops. A route of exposure to lead that may possibly be influenced by acid precipitation is an increased deterioration of surface materials containing lead and a subsequent ingestion by small children. A similar situation with regard to uptake from food exists for cadmium (at least in some countries). Human metal exposures via drinking water may be increased by acid precipitation. Decreasing pH increases corrosiveness of water enhancing the mobilization of metal salts from soil; metallic compounds may be mobilized from minerals, which may eventually reach drinking water. Also, the dissolution of metals (Pb, Cd, Cu) from piping systems for drinking water by soft acidic waters of high corrosivity may increase metal concentrations in drinking water. Exposures have occasionally reached concentrations which are in the range where adverse health effects may be expected in otherwise healthy persons. Dissolution from piping systems can be prevented by neutralizing the water before distribution. Increased aluminum concentrations in water is a result mainly of the occurrence of Al in acidified natural waters and the use of Al chemicals in drinking water purification. If such water is used for dialysis in patients with chronic renal failure, it may give rise to cases of dialysis dementia and other disorders. A possible influence on health of persons with normal renal function (e.g., causing Alzheimer's disease) is uncertain and requires further investigation.(ABSTRACT TRUNCATED AT 400 WORDS)

Placental transfer of cadmium in rats: influence of dose and gestational age.

Placental transfer rates of cadmium were investigated in rats in relation to dose (0.1, 0.4, and 1.6 mg Cd/kg) and the gestational age (12, 15, and 20 days) when rats were treated. Pregnant rats were injected intravenously with a single dose of 109CdCl2 (approximately 20 muCi/animal), and animals were sacrificed after 24 hr. 109Cd concentrations were measured in the fetus, placenta, maternal liver, and blood. Cadmium crossed the placenta at all doses and at all gestational ages tested. However, higher percentages of administered cadmium accumulated in the fetus with increasing dose and increasing gestational age. For example, after pregnant rats were injected with low, middle, and high doses of Cd on day 12 of gestation, fetuses accumulated 0.0001, 0.0028, and 0.0095 per cent of the injected dose, respectively. Percentages of administered Cd detected in placental tissue did not change consistently with dose but Cd levels did increase with gestational age. Placental to maternal blood Cd concentration ratios increased with gestational age but not with dose. Maternal liver to fetal liver concentration ratios were 295, 137, and 27 for low, middle and high doses, respectively, 24 hr after pregnant rats were treated on day 20 of gestation. These results are discussed in relation to placental damage, metallothionein inducibility, and fetotoxicity.

Characterization studies on the cadmium-binding proteins from two species of New Zealand oysters.

Two different types of New Zealand oysters--Ostrea lutaria (OL) and Crassostrea glomerata (CG)--contained different concentrations of zinc, copper, and cadmium. OL oysters had 5.3 micrograms Cd/g, 3.4 micrograms Cu/g, 100 micrograms Zn/g; CG oysters had 1.4 micrograms Cd/g and 936 micrograms Zn/g. Both kinds of oysters were shown by gel filtration (G-75) to contain cadmium and zinc in fractions corresponding to a high molecular weight protein (corresponding to the size of albumin or larger) which was heat labile. OL oysters contained cadmium in fractions corresponding to a molecular weight of approximately 6500. The cadmium-binding protein in these fractions was heat-stable. This protein contained no detectable amounts of zinc and was not present in the CG oysters. Further purification by gel filtration (G-50) was performed to obtain a purer protein fraction. Isoelectric focusing of the protein obtained by G-50 filtration showed one main fraction of protein with a pI approximately 5.9 at approximately 13 degrees C. CG oysters contained cadmium and zinc in a polypeptide with low molecular weight (MW 1000). The cadmium-binding oyster proteins are minimally reactive in a competitive binding radioimmunoassay in comparison to the reactivity of a typical vertebrate metallothionein; the proteins may be metallothioneins, but, if so, they do not exhibit the principal determinants characteristic of vertebrate metallothioneins.

Long-term carcinogenicity study in Syrian golden hamster of particulate emissions from coal- and oil-fired power plants.

Male Syrian golden hamsters were given 15 weekly intratracheal instillations with suspensions of coal fly ash or oil fly ash. Controls were instilled with saline containing gelatine (0.5 g/100 mL) or to check particle effects with suspensions of hematite (Fe2O3). The common weekly dose was 4.5 mg/hamster. In addition, one subgroup of hamsters was treated with oil fly ash at a weekly dose of 3.0 mg/hamster and another with coal fly ash at a weekly dose of 6.0 mg/hamster. Other groups of hamsters were treated with suspensions of benzo[a]pyrene (BaP) or with suspensions on coal fly ash, oil fly ash, or Fe2O3 coated with BaP. The mass median aerodynamic diameters of the coal and oil fly ashes were 4.4 microns and 28 microns, respectively. Hamsters treated with oil fly ash showed a higher frequency of bronchiolar-alveolar hyperplasia than hamsters in the other treatment groups. Squamous dysplasia and squamous metaplasia were most frequent in animals treated with suspensions of BaP or BaP-coated particles. The earliest appearance of a tumor, the highest incidence of tumors, and the highest incidence of malignant tumors were observed in hamsters treated with oil fly ash coated with BaP. Squamous cell carcinoma and adenosquamous carcinoma were the most frequent malignant tumors. No malignant tumors and only few benign tumors were observed in hamsters instilled with suspensions of fly ash not coated with BaP. The present study gives no indication that coal fly ash could create more serious health problems than oil fly ash.

Differential calcium transport disturbances in renal membrane vesicles after cadmium-metallothionein injection in rats.

Cadmium-metallothionein (CdMT) induced calciuria may result from disturbed calcium (Ca) transport through the renal tubular epithelium. The present study aimed at defining time of onset and the degree of disturbed calcium transport. Kidneys were obtained from rats at 4, 12 and 24 h after a single injection of CdMT (dose 0.4 mg Cd/kg b.w.), and compared to saline injected controls. Rapid-filtration 45Ca-assays were performed on basolateral and luminal membrane vesicles, isolated from kidney cortex using a sequential ultracentrifugation procedure. Luminal 45Ca uptake was increased at 4 h and then declined to about 80% of controls, suggesting an early phase perturbation of Ca absorption. Basolateral 45Ca uptake was reduced to less than 50% of controls, starting already at 4 h while 45Ca binding was reduced at 8 h. This may reflect an inhibited basolateral Ca pump mechanism after the binding step. Since the Ca pump normally expels Ca from the cell, an accumulation of intracellular calcium was indicated. Metal analysis verified a four-fold increase of Ca in kidney cortex at 24 h. This suggests that Cd impact on tubular cells involves disturbances on cellular absorption as well as expulsion of Ca.

Nephrotoxic impact of multiple short-interval cadmium-metallothionein injections in the rat.

The cadmium-metallothionein (CdMT) injection model was used to examine whether multiple short-interval injections of CdMT, instead of a single dose, could better reproduce the features of chronic exposure to inorganic cadmium. Male Wistar rats were given an initial CdMT dose and four subsequent doses subcutaneously at 2-h intervals. A control group, given saline, was compared with a low dose group (0.2 + 4 x 0.1 mg Cd/kg b.w.) and high dose group (0.4 + 4 x 0.1 mg Cd/kg b.w.). Nephrotoxic effects were seen at the high dose. A marked proteinuria began 6-12 h after the first injection and extended to day 9. A progressive, unreversed calciuria appeared at 6 h and reached its maximum at day 13. This was a marked increase in duration compared with the transient peaks of proteinuria and calciuria observed in previous single dose studies. The unreversed calciuria and the marked proteinuria are suggestive of residual tubular damage, which may be irreversible. In conclusion, the model with multiple short-interval CdMT injections more closely reproduces the situation in long-term exposure to inorganic cadmium, compared to the single dose models previously employed.

Modulation of calciuria by cadmium pretreatment in rats with cadmium-metallothionein-induced nephrotoxicity.

One group of male Wistar rats (Group B) was pretreated by a daily subcutaneous injection with CdCl2 during 5 days with increasing doses (0.5, 1, 1, 2 and 2 mg Cd/kg). Another group of rats (Group A) was daily given normal saline subcutaneously for 5 days. On the second day after the last injection, a single s.c. injection of 109Cd-metallothionein (CdMT, 0.4 mg Cd/kg) was given to each animal in both groups. Urinary calcium, protein, metallothionein (MT), N-acetyl-beta-D-glucosaminidase (NAG) and gamma glutamyltransferase (gamma-GT) were measured. In Group A, calciuria, proteinuria, metallothioneinuria and enzymuria was induced by CdMT. Calciuria reached a peak during 0-6 h after the administration of CdMT, thus appearing earlier than other effects. Enzymuria was displayed at 6-12 h for gamma-GT and 12-24 h for NAG. A prominent increase of proteinuria appeared at 24-48 h after the challenge of CdMT. In Group B, no significant increase of urinary calcium, protein, or NAG was observed after the CdMT injection and urinary gamma-GT was only slightly elevated, thus demonstrating the protective action of pretreatment. This study demonstrates for the first time that calciuria, one of the signs of cadmium nephrotoxicity, can be prevented by cadmium pretreatment. Urinary MT increased slightly during the 4-5 days of CdCl2 pretreatment. This is in accordance with previous observations that cadmium pretreatment induces new synthesis of MT which is likely to constitute the background for the resistance to the CdMT challenge to the kidney.

Cadmium-metallothionein nephrotoxicity in the rat: transient calcuria and proteinuria.

After a s.c. injection of 0.4 mg Cd/kg as cadmium-metallothionein (CdMT) in rats, a marked increase in urinary protein concentration appeared at 16-40 h. There was a peak of urinary Cd content during the first 4 h after the treatment. Urinary Ca was increased at 8 h after the CdMT injection and returned to normal level at 32 h. Luminal and basolateral renal membrane vesicles were isolated from both control group and CdMT (0.4 mg Cd/kg) group at 24 h after the injection. Calcium uptake and binding of both fractions were decreased in the group treated with CdMT. Cd, Zn and MT concentrations in the kidney cortex were increased, but Ca concentration was not significantly changed. Since injected CdMT is probably only partly reabsorbed by tubular cells at the dose level of 0.4 mg Cd/kg as CdMT, excessive plasma CdMT is rapidly excreted in urine, explaining the increased Cd excretion during the first few hours observed in the present experiment. Decreased Ca binding in the luminal membranes as observed in vitro could be one of the mechanisms of production of calcuria if occurring in vivo. Another possible explanation of calcuria is that Cd ions released from CdMT into the cytoplasm of the tubular cell, may exert ionic interference with Ca transport across the luminal membranes and produce decreased Ca reabsorption. It is known that a disturbance of Ca metabolism could influence the membrane stability and such a change may contribute to explaining the proteinuria characteristic of CdMT nephrotoxicity. The reversibility of the proteinuria observed after a single dose of CdMT may be related to the induction of metallothionein synthesis in the renal cells.

The susceptibility of spontaneously diabetic mice to cadmium-metallothionein nephrotoxicity.

Cadmium metallothionein (CdMT) was injected subcutaneously into obese hyperglycaemic Umeå ob/ob mice or their lean litter mates (normal mice) at doses of 0, 0.1 and 0.4 mg Cd/kg. Proteinuria and calciuria were induced in both types of mice, but in the ob/ob mice this condition developed at a lower dose of CdMT (0.1 mg Cd/kg) than in the normal mice (0.4 mg Cd/kg). These results show, therefore, that Umeå ob/ob mice are particularly susceptible to CdMT-induced nephrotoxicity. The mechanism underlying this phenomenon needs to be further investigated. After the administration of CdMT, a dose-related increase in glycosuria was observed in both types of mice, in spite of decreased levels of serum insulin and glucose. It is suggested that such glycosuria induced by CdMT could be one of the signs of cadmium nephrotoxicity. The results of the present study thus indicate that metabolic changes like those in diabetes may increase susceptibility to cadmium-induced renal tubular damage.

Cadmium carcinogenesis and its relationship to other health effects in humans.

Several different adverse health effects can be caused by cadmium exposure to humans and animals. In environmental and occupational health it is important to identify effects that occur at relatively low exposures (ie., the critical effects that are crucial for preventive action). In long-term human exposures to cadmium, effects of cadmium on the kidney have been considered to be critical effects and quantitative risk assessments of these effects have been performed on the basis of both risk modeling and direct epidemiologic observation. However, experimental and epidemiologic studies are providing increasing evidence that cadmium is carcinogenic, and this effect, which is considered to be stochastic in character, can be considered to be the critical effect. A quantitative evaluation of the cancer risk is currently difficult to make, but the preventive action against such effects is usually to limit the exposure as much as possible.

Lung cancer in smelter workers--interactions of metals as indicated by tissue levels.

The concentrations of the elements antimony, arsenic, cadmium, chromium, cobalt, lanthanum, lead, selenium, and zinc were determined in lung tissue of 85 decreased smelter workers by neutron activation analysis and atomic absorption spectrophotometry. The concentrations of all these elements, except zinc, were significantly higher among the workers as compared with rural referents. Workers who died from lung cancer (N = 7) had the lowest lung selenium content relative to other metals, both compared with workers with other diseases and with rural (N = 15) and urban (N = 10) referents. The low lung tissue levels may have influenced the development of lung cancer. The highest lung cadmium concentrations were observed in the lung cancer group, in which, however, smokers and ex-smokers were over-represented. The observations make it likely that the excess lung cancer risk in this smelter environment is multifactorial in character, involving interactions between both carcinogenic and anticarcinogenic factors.

Antimony in lung, liver and kidney tissue from deceased smelter workers.

Tissue concentrations of antimony in lung, liver, and kidney tissue from a group of deceased smelter workers from northern Sweden have been compared with those of a group of persons without occupational exposure from a nearby area. Neutron activation analysis was used to determine the antimony concentration of lung tissue from exposed workers; these concentrations were 12-fold higher than those of referents (p less than 0.001). For lung tissue there was no tendency towards decreased antimony concentrations with time (up to 20 a) after the cessation of exposure, and this result indicates a long biological half-time. The highest values were found for workers who had worked for many years at the roasters and in the arsenic and selenium departments. There was no significant difference between the antimony concentration of the lung tissue from workers who had died of lung cancer and those of persons who died of other malignancies, cardiovascular disease, or other causes. This finding does not however rule out the possibility of a role for antimony in the etiology of lung cancer among smelter workers since multiple factors may have been operating. The antimony concentration of the liver tissue and the kidney cortex did not differ from the corresponding values of the reference group; this finding indicates either a short biological half-time or insignificance for the systemic distribution of antimony.

Reproductive and developmental toxicity of metals.

This paper discusses metal exposure in the male, the nonpregnant female, and the maternal-offspring unit. In the first two situations, the primary targets are the gonads. In the mother-offspring unit, consideration must be given to effects on the fertilized ovum, the growth of the embryo, and, finally, to the fetal and perinatal stages. The central nervous system may be especially vulnerable during development. The placenta also undergoes development, and either the placenta or the fetus may be the primary target. In humans, certain metals may cause abortion or other effects on the conceptus. Effects may also be produced by metal exposure both in utero and in the suckling infant. For example, methylmercury gives rise to a range of effects on the central nervous system at doses lower than those producing damage to the mature nervous system. Effects of lead and arsenic are associated mainly with postnatal exposures during infancy and early childhood, but there is reason to believe from animal experiments that some effects may occur from prenatal exposures to certain metal compounds.

Lung retention of antimony and arsenic in hamsters after the intratracheal instillation of industrial dust.

Airborne factory dust (with a volume median diameter of 5.0 micron and a geometric standard deviation of 2.1 micron) from a Swedish copper smeltery contained antimony (Sb) (1.6 weight %) and arsenic (As) (19 weight %). The dust was neutron activated and intratracheally instilled in hamsters. In vivo measurements of lung clearance were undertaken of the radionuclides 76As, 122Sb, and 124Sb. Comparison was made with pure substances of antimony trioxide (Sb2O3) and arsenic trioxide (As2O3). Two phases were recognized in the clearance curves. The approximate half-time for the initial phase was 13 h for As2O3, 20 h for arsenic dust, about 40 h for Sb2O3, and about 30 h for antimony dust. The second phase had an approximate half-time of 20-40 d for Sb2O3 and antimony dust. Because of the short physical half-life of 76As, the second phase of the lung clearance was not possible to follow for AS2O3 and arsenic dust. The observed differences in clearance were primarily related to the solubility of the dust particles in saline, while particle size seemed to be less important in this instillation experiment. The low solubility of antimony in factory dust combined with a long biological half-time may be of importance in explaining the observed lung accumulation of antimony in exposed workers. The greater solubility and shorter biological half-time restricted the lung retention of arsenic.