The renal toxicity of cadmium metallothionein: morphometric and X-ray microanalytical studies

Metallothionein and Cadmium Toxicology-Historical Review and Commentary

More than one and a half centuries ago, adverse human health effects were reported after use of a cadmium-containing silver polishing agent. Long-term cadmium exposure gives rise to kidney or bone disease, reproductive toxicity and cancer in animals and humans. At present, high human exposures to cadmium occur in small-scale mining, underlining the need for preventive measures. This is particularly urgent in view of the growing demand for minerals and metals in global climate change mitigation. This review deals with a specific part of cadmium toxicology that is important for understanding when toxic effects appear and, thus, is crucial for risk assessment. The discovery of the low-molecular-weight protein metallothionein (MT) in 1957 was an important milestone because, when this protein binds cadmium, it modifies cellular cadmium toxicity. The present authors contributed evidence in the 1970s concerning cadmium binding to MT and synthesis of the protein in tissues. We showed that binding of cadmium to metallothionein in tissues prevented some toxic effects, but that metallothionein can increase the transport of cadmium to the kidneys. Special studies showed the importance of the Cd/Zn ratio in MT for expression of toxicity in the kidneys. We also developed models of cadmium toxicokinetics based on our MT-related findings. This model combined with estimates of tissue levels giving rise to toxicity, made it possible to calculate expected risks in relation to exposure. Other scientists developed these models further and international organizations have successfully used these amended models in recent publications. Our contributions in recent decades included studies in humans of MT-related biomarkers showing the importance of MT gene expression in lymphocytes and MT autoantibodies for risks of Cd-related adverse effects in cadmium-exposed population groups. In a study of the impact of zinc status on the risk of kidney dysfunction in a cadmium-exposed group, the risks were low when zinc status was good and high when zinc status was poor. The present review summarizes this evidence in a risk assessment context and calls for its application in order to improve preventive measures against adverse effects of cadmium exposures in humans and animals.

Is Urinary Cadmium a Biomarker of Long-term Exposure in Humans? A Review

Cadmium is a naturally-occurring element, and humans are exposed from cigarettes, food, and industrial sources. Following exposure, cadmium accumulates in the kidney and is slowly released into the urine, usually proportionally to the levels found in the kidneys. Cadmium levels in a single spot urine sample have been considered indicative of long-term exposure to cadmium; however, such a potentially exceptional biomarker requires careful scrutiny. In this review, we report good to excellent temporal stability of urinary cadmium (intraclass correlation coefficient 0.66-0.81) regardless of spot urine or first morning void sampling. Factors such as changes in smoking habits and diseases characterized by increased excretion of proteins may produce short-term changes in urinary cadmium levels. We recommend that epidemiologists use this powerful biomarker in prospective studies stratified by smoking status, along with thoughtful consideration of additional factors that can influence renal physiology and cadmium excretion.

Cellular mechanisms of cadmium-induced toxicity: a review

Cadmium is a widespread toxic pollutant of occupational and environmental concern because of its diverse toxic effects: extremely protracted biological half-life (approximately 20-30 years in humans), low rate of excretion from the body and storage predominantly in soft tissues (primarily, liver and kidneys). It is an extremely toxic element of continuing concern because environmental levels have risen steadily due to continued worldwide anthropogenic mobilization. Cadmium is absorbed in significant quantities from cigarette smoke, food, water and air contamination and is known to have numerous undesirable effects in both humans and animals. Cadmium has a diversity of toxic effects including nephrotoxicity, carcinogenicity, teratogenicity and endocrine and reproductive toxicities. At the cellular level, cadmium affects cell proliferation, differentiation, apoptosis and other cellular activities. Current evidence suggests that exposure to cadmium induces genomic instability through complex and multifactorial mechanisms. Most important seems to be cadmium interaction with DNA repair mechanism, generation of reactive oxygen species and induction of apoptosis. In this article, we have reviewed recent developments and findings on cadmium toxicology.

Effects of Vitamin E on Renal Dysfunction in Chronic Cadmium-Poisoned Rats

Cadmium is a highly toxic metal that can be ingested or inhaled from a variety of industrial and dietary sources. The purpose of this study was to investigate the effects of vitamin E on renal dysfunction and blood pressure changes in chronic cadmium-poisoned rats. Sprague-Dawley rats weighing 100 +/- 10 g were randomly assigned to one control group and three cadmium-poisoned groups. Cadmium groups were assigned to dietary groups according to levels of vitamin E supplementation: vitamin E-free diet (Cd-0E group), 40 mg of vitamin E/kg of diet (Cd-40E group), and 400 mg of vitamin E/kg of diet (Cd-400E group). The animals were raised for 20 weeks, and cadmium was supplied in the drinking water at 50 ppm Cd(2+). The morphological changes observed by both light and electron microscopy revealed mitochondria and tubule epithelial cell edema in the Cd-0E group, yet this was alleviated with the highest level of vitamin E supplementation (Cd-400E group). The urinary beta(2)-microglobulin levels indicated that glomerular injury was higher in the Cd-poisoned groups than in the control group, but were lowered by vitamin E supplementation. Although the glomerular filtration rate (GFR) of the Cd-0E group was significantly lower than that of the control group, the vitamin E-supplemented groups exhibited a similar GFR to the control group, suggesting that vitamin E protected the kidney from functional damage. Angiotensin converting enzyme activity, and blood pressure, and heart rate were all significantly higher in the Cd-poisoned group, but each remained nearly normal with vitamin E supplementation. Accordingly, these results indicate that vitamin E supplementation in chronic cadmium-poisoned rats normalized renal dysfunction and blood pressure regulation.

Environmental cadmium induces histopathological changes in kidneys of roe deer

Kidney samples of 224 roe deer (113 males, 111 females) aged three months to approximately nine years were collected in eastern Austria. Cadmium contents were examined and histological examinations were performed considering 11 different pathomorphological parameters. Cadmium burden was relatively high (range: 0.010-22.076 ppm) and increased with age. Females aged one to two years had higher contents than males of the same age. The culling site had no influence on cadmium load. The relation between the occurrence of histopathological changes and age, sex, origin, and cadmium concentration in 208 roe deer was tested. The frequency of vacuolic degeneration, pycnotic nuclei, caryolysis, and necrosis was related to increased cadmium levels. Increasing age correlated with lymphohistiocytic infiltration, interstitial fibrosis, and swelling of glomeruli. Pigment deposits and thickening of Bowman's capsule could be related to both cadmium and age. Furthermore, roe deer from an industrialized region showed alterations more frequently than animals from rural areas. We found no relation between morphological changes and sex. Though it remains to be established whether environmental cadmium exposure is the sole cause for the histopathological alterations observed, the results of this study indicate that chronic cadmium poisoning may be an important cofactor in the pathogenic mechanisms of renal damage in roe deer and that cadmium intoxication may be more widespread among wildlife than previously known.

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.

The susceptibility to nephrotoxicity of streptozotocin-induced diabetic rats subchronically exposed to cadmium chloride in drinking water

Streptozotocin-induced diabetic rats and normal non-diabetic (ND) rats were exposed to cadmium chloride in drinking water in doses of 0, 50 and 100 ppm for 90 days. There was a dose-related increase in urinary protein and enzymes in the diabetic group, but an increase in proteinuria only in the high exposure subgroup of the ND group. It is suggested that diabetic rats induced by streptozotocin are more susceptible to cadmium nephrotoxicity than normal (ND) rats. Metallothionein synthesis in liver was estimated to be similar in both the diabetic and non-diabetic groups after exposure to cadmium. Less excretion of cadmium in urine and greater accumulation of cadmium in kidney were observed in the diabetic group, and this may be one of the mechanisms underlying the susceptibility of diabetic animals to the effects of cadmium. Further biochemical and histological studies are required in order to explain the detailed events involved in inducing such changes in the toxicokinetics of cadmium.

Excursions of intake above ADI: case study on cadmium

High oral intake of cadmium via food or drink in a single dose by humans gives rise to vomiting, abdominal pain, and diarrhea. Concentrations of cadmium in drinks giving rise to such symptoms have been 16 mg/liter and higher corresponding to doses of 3 mg and higher. Longer term intakes of food (rice) with concentrations around 1 mg/kg corresponding to daily intakes of 600 micrograms have given rise to some less pronounced symptoms including signs of malabsorption. Reproductive and developmental effects have been observed in animal experiments at oral and other exposures. The present provisional tolerable weekly intake (PTWI) for Cd is 500 micrograms (a weekly intake of 7 micrograms/kg body wt), corresponding to a daily intake of 70 micrograms or 1 microgram per kg body wt. Recent data demonstrating renal dysfunction in humans at even lower lifelong oral exposures indicate that the PTWI needs to be lowered in the future. An estimated lowest-observed-adverse-effect level (LOAEL) for symptoms from the gastrointestinal tract in humans after intake of a single oral dose is 43 micrograms/kg body wt. If a safety factor of 3-10 is used based on LOAEL, a tolerable single dose would be 0.3-1 mg (4 to 14 micrograms/kg body wt). For longer time exposures (months-a few years) daily intakes of 200 micrograms (3 micrograms/kg body wt) may be tolerated without obvious gastrointestinal symptoms or signs. At present, there is no convincing human evidence that such doses can cause reproductive or developmental effects, but since such effects have been reported in animals, it may be advisable not to exceed a daily intake of 1 microgram/kg body wt for such potentially sensitive subsections of the population as children and women who are pregnant or lactating. Any excursions above the PTWI need to be compensated for by a corresponding period with intake below the PTWI in order for the cumulative dose to be low enough to avoid the long-term effects of cadmium on the kidney.

Urinary N-acetyl-beta-D-glucosaminidase isoenzymes as biomarker of renal dysfunction caused by cadmium in a general population

N-Acetyl-beta-D-glucosaminidase (NAG) and its isoenzymes in urine have been studied in a population group residing in a polluted area in China. The area studied was contaminated by industrial wastewater from a nearby smelter that discharged cadmium-polluted wastewater into a river used for the irrigation of rice fields. Cadmium concentrations in rice were 3.70, 0.51, and 0.07 mg/kg for the highly and moderately polluted areas and the control area, respectively. Cadmium concentrations in urine exceeded 5 microgram/liter in the majority of subjects in the most highly polluted area. There was a marked dose-dependent increase in NAG and NAG B content of urine related both to urinary cadmium and to the calculated cadmium uptake. It is concluded that urinary NAG and its isoenzymes could serve as a sensitive biomarker of renal dysfunction in cadmium-exposed populations. The mechanisms underlying the increase in NAG and its isoenzymes after cadmium exposure need to be studied further.

Cadmium-bound metallothionein induces apoptosis in rat kidneys, but not in cultured kidney LLC-PK1 cells

The ability of cadmium-bound metallothionein(Cd-MT) to induce apoptosis was investigated in vivo and in vitro. Administration of purified Cd-MT (0.15 mg MT bound Cd per kg body weight) to the rat induces DNA fragmentation, a biochemical characteristic of apoptosis in the kidney at 16 h, which was detectable by ethidium bromide staining on an agarose gel. It was still detected 24 h after administration. Induction of apoptosis by Cd-MT was specific to kidney; it was not observed in cerebrum, cerebellum, heart, lung, liver, testis, dorsolateral prostate, and ventral prostate. In contrast, addition of Cd-MT (0.01-100 microM) to the cultured porcine kidney LLC-PK1 cells failed to induce apoptosis under the condition where cadmium chloride (10 microM) did. There was no additivity of induction of apoptosis by CdCl2 (10 microM) in the presence of Cd-MT (0.01-100 microM). To examine the effect of intracellular MT on cadmium-induced apoptosis in cultured cells, new cell lines were established, which constitutively produce MT, being termed as Cd(r)-LLC-PK1 cells since Cd-MT exogenously added had much less permeability to the cultured cells. Followed by exposure of wild-type LLC-PK1 cells to 50 microM CdCl2 for 24 h, the surviving cells(Cd(r)-LLC-PK1 cells) induce MT at the level of 1.9 microg/2 x 10(6) cells. In Cd(r)-LLC-PK1 cells, 10 microM CdCl2 failed to induce apoptosis, but 60 microM CdCl2 could exert the apoptotic response, indicating that intracellular MT which was induced by CdCl2 did not facilitate CdCl2-elicited apoptosis. Furthermore, chromatin in rat kidneys was condensed by Cd-MT, but not that in LLC-PK1 cells. Thus, Cd-MT induces apoptosis in rat kidneys, but not in the cultured renal cells, suggesting that the ionic form of cadmium was required for programmed cell death.

Cadmium binding and sodium-dependent solute transport in renal brush-border membrane vesicles

Exposure to cadmium (Cd) impairs renal transport systems for glucose, amino acids, phosphate, and dicarboxylates. To investigate if these changes are directly related to a Cd binding to the renal brush-border membrane, Cd binding and the Na+-dependent uptakes of d-glucose, l-alanine, phosphate, and succinate were determined in rat renal brush-border membrane vesicles (BBMV) exposed to CdCl2. Cd uptake by BBMV showed time and concentration dependence. Changes in medium osmolality had no effect on Cd uptake, indicating that the process primarily involves binding of Cd to the membrane. Scatchard analysis indicated the presence of two types of Cd binding sites, differing in affinity and number. Increasing the medium Cd concentration from 50 to 200 microM resulted in a progressive increase in Cd binding to the membrane and decrease in Na+-dependent transport of d-glucose, l-alanine, inorganic phosphate, and succinate. In all cases, the inhibition of transport was directly proportional to the total amount of Cd binding to the membrane. These results suggest that, during chronic exposure to Cd, free Cd ions liberated in renal tubular cells may directly interact with brush-border membranes and impair Na+-dependent solute transports.

Treatment of chronic cadmium nephrotoxicity by N-acetyl cysteine

Chronic cadmium (Cd)-induced nephrotoxicity is believed to be irreversible at advanced stages and no treatment is currently available. This study examined the beneficial effect of N-acetyl cysteine (NAC) on Cd-induced nephrotoxicity. Female Sprague-Dawley rats were injected s.c. with 5 micromol CdCl2/kg per day, five times/week for up to 26 weeks. Nephrotoxicity was detected after 10 weeks by elevation in urinary lactate dehydrogenase activity and protein. NAC co-administration from week 13 prevented the progression of nephrotoxicity. In these animals, with low-level nephrotoxicity, discontinuation of Cd exposure at the end of week 22 resulted in gradual recovery over the next several weeks, without the need for treatment with NAC. On the other hand, discontinuation of NAC co-treatment at the end of week 22 resulted in quick progression of nephrotoxicity, indicating that NAC protection was short-lived. Resumption of NAC treatment and cessation of Cd exposure after 26 weeks resulted in rapid recovery from advanced nephrotoxicity. It is concluded that protection from Cd-induced nephrotoxicity is possible by continued co-administration of NAC and that recovery from advanced nephrotoxicity can also be achieved with NAC, provided that Cd exposure is stopped.

Copper metabolism in the kidney of rats administered copper and copper-metallothionein

To gain a greater understanding of the mechanism of Cu metabolism in kidneys of rats, using autofluorescence of Cu-metallothioneins (Cu-MTs) we revealed the behavior of Cu-MT in the kidneys of rats administered Cu-MT. Yellow and orange fluorescent signals of Cu-MT were observed in the cortex. By microscopic studies, Cu-MT was dominant in the proximal convolute tubular cells of the cortex. A high concentration of Cu-MT presented in the lysosome-like organelles of the proximal convolute tubular adjacent to the glomeruli. During the time course after the injection, the orange signal in lysosome-like organelles gradually converted to a yellow signal, indicating that the Cu-MT was involved in a degradation process in lysosomes by oxidation, and the MT mRNA increased in the cortex, although the immunoreactivity of MT was almost constant in the same region. These results suggested that Cu bound to the injected MT was released in lysosomes and became a new inducer of MT biosynthesis in the cortex. In conclusion, the biosynthesis and degradation of Cu-MT occur repeatedly in the proximal convolute tubular cells.

Protection against cadmium-metallothionein nephrotoxicity in streptozotocin-induced diabetic rats: role of increased metallothionein synthesis induced by streptozotocin

Protection against the development of nephrotoxicity following the administration of cadmium-metallothionein (CdMT) at a dose of 0.4 mg Cd per kg body weights was studied in streptozotocin (STZ)-induced diabetic rats. Six groups of Wistar male rats were used (Groups A and B, Groups A1 and C, and Groups A2 and D were injected intraperitoneally with STZ at doses of 0, 50 and 100 mg/kg, respectively, and then 6 days later, Groups B, C and D were injected subcutaneously with CdMT). Proteinuria, albuminuria and transferrinuria were observed after the administration of CdMT, and a dose-related decrease following the increased STZ dose was seen in Groups B, C and D. The concentrations of metallothionein (MT) and zinc (Zn) in liver and kidney were dose-dependently increased in Groups B, C and D. Induction of increased MT synthesis in liver and kidney as the result of the STZ treatment was observed in this study. In particular, a remarkable increase in liver MT concentration was induced by STZ, and transport to the kidney of MT synthesized in liver may perhaps explain the protection against cadmium nephrotoxicity in STZ-induced diabetic rats.

Physiological and toxicological changes in the skin resulting from the action and interaction of metal ions

The human environment contains more than 50 metal or metalloid elements. At least 15 are recognized as trace elements, with zinc, calcium, copper, magnesium, and iron having specific roles in skin morphogenesis and function. The present review focuses on the presumed role of metal ions in the skin, their competition for carrier proteins, and membrane receptors. Evidence presented shows that the balance of trace metal ions is critical for normal skin and repair mechanisms following injury. Xenobiotic ions can impair this balance, leading to pathological change. The skin acts as an organ of elimination of excess trace metals and xenobiotic ions from the body, but mechanisms of voidance vary for different metals. Metal ions are an important cause of allergies, and evidence is presented to show that the majority of metals or metal compounds can induce allergic changes. Except for chromium and nickel, which are among the most common human allergens, animal models have provided little information. At least cadmium, thorium, lead, chromium, nickel, beryllium, and arsenic and proven or putative carcinogens in animals or humans on the basis of cytological or epidemiological evidence. However, only arsenic exhibits a clear predilection for the skin. Other metals such as gold can induce subcutaneous sarcoma following injection, but the relevance of this observation in terms of human occupational risk is discounted.

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.

Kinetics of Cd2+ in plasma, liver and kidneys after single intravenous injection of Cd-metallothionein-II

To explore the kinetics of Cd2+ in the body, rats received a single intravenous injection of CdCl2 or Cd-saturated metallothionein-II at 0.3 mg Cd/kg body weight. Cd2+ in the two agents was biexponentially eliminated from plasma: rapidly in the first 5 min, and gradually later. Compared with CdCl2, Cd-saturated metallothionein-II showed lower Cd2+ concentrations in plasma during the first 30 min; larger values for parameters concerning distribution of Cd2+, its total body clearance and half-life time in the beta phase. Cd2+ uptake in the liver was higher with CdCl2, and, conversely, in the kidneys it was higher with Cd-saturated metallothionein-II. In Cd-saturated metallothionein-II, the renal content of Cd2+ reached a maximum (8 micrograms Cd2+/g tissue) on day 1, gradually decreasing thereafter; there was a higher area under the Cd2+ content-time curve, and a lower mean residence time of Cd2+; the kidneys showed severe necrosis and defluxion of proximal tubular cells at days 1 and 5, although there were regenerative and reversion signs on day 5. These findings suggested that, in the case of Cd-saturated metallothionein-II, Cd2+ being taken into the cells of proximal tubules was excluded predominantly due to cellular death and the resultant defluxion.

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.

Accumulation and degradation of the protein moiety of cadmium-metallothionein (CdMT) in the mouse kidney

Of major concern in Cd toxicity is its ability to produce renal damage after chronic exposure in humans and experimental animals. Renal injury affects predominantly the proximal tubules and more specifically the first segments of these tubules. Similar toxic effects to the kidneys are observed after administration of cadmium bound to metallothionein (CdMT). Therefore, CdMT was used in this study as a model to understand the mechanism(s) of Cd nephrotoxicity. It has been recently demonstrated that Cd from CdMT was preferentially taken up by the proximal convoluted tubules. Therefore, the purpose of these studies was to determine if the organic portion of the complex was also accumulated in these tubules. [35S]CdMT prepared from rat liver was administered intravenously to mice at a nonnephrotoxic dose (0.1 mg Cd/kg). The radioactivity in the kidney showed maximum level (80% of the dose) 15 min after the injection. This preferential renal uptake was also observed after administration of various doses of [35S]CdMT. In contrast to the earlier observed persistency of 109Cd in the kidney after 109CdMT administration, 35S disappeared rapidly (with a half-life of approximately 2 hr), and 24 hr after injection of [35S]CdMT, there was very little 35S left in the kidneys. These observations indicate that the protein portion of CdMT is rapidly degraded after renal uptake of CdMT and the released Cd is retained in the kidney. Within the kidney, 35S distributed mainly to the cortex. Light microscopic autoradiography showed that [35S]CdMT preferentially distributed to the proximal convoluted tubule (S1 and S2), which is the site of nephrotoxicity. Within the S1 and S2 segments, a greater distribution of 35S to the apical portion of the cells was observed after administration of both a nonnephrotoxic (0.1 mg Cd/kg) and a nephrotoxic (0.3 mg Cd/kg) dose. 109Cd administered as 109CdMT also distributed to the apical portion of the S1 and S2 cells. Therefore, both the organic (35S) and inorganic (109Cd) portions of CdMT are rapidly and efficiently taken up by the S1 and S2 cells of the proximal tubules, the site of nephrotoxicity. These observations support the concept that CdMT is readily taken up by the proximal tubular cells as a complex, and then its protein portion is rapidly degraded to release Cd that binds permanently to intracellular sites and produces nephrotoxicity.

Exogenous metallothionein and renal toxicity of cadmium and mercury in rats

The relative tissue distribution and toxicity of cadmium (Cd) and mercury (Hg) in the liver and kidneys of rats when the metals are administered as either inorganic salts or complexed with MT were studied. Male Sprague-Dawley rats were injected (i.v.) with Cd or Hg inorganic salt of chloride or in a complex of MT at a dose of 0.3 mg/kg body weight. The concentration of MT and metals in plasma and urine was monitored for 7 days, at the end of which the rats were killed. Injection of both HgCl2 and Hg-MT induced the synthesis of MT only in the kidney but not in the liver, whereas CdCl2 and Cd-MT injections induced MT synthesis in both liver and kidney, respectively. Plasma MT levels increased 3 days after CdCl2 but not after HgCl2 injection, suggesting that hepatic MT may be an important source of plasma MT under our experimental conditions. Renal toxicity was observed morphologically and by an increase in blood urea nitrogen, plasma creatinine, proteinuria in rats injected with Cd-MT and both forms of Hg. Urinary MT excretion was significantly elevated in Cd-MT injected rats compared with those injected with CdCl2. However, HgCl2 and Hg-MT injected rats showed no significant difference in urinary MT excretion. The magnitude in the renal accumulation of Hg is similar after the administration of Hg-MT or HgCl2, but our findings suggest that the site of epithelial injury may be different. Injury effects of Hg-MT localized mainly in the terminal portions of the proximal convoluted tubule and the initial portions of the proximal straight tubule whereas inorganic Hg caused necrosis in pars recta segments of the proximal tubule.

Renal cadmium deposition and injury as a result of accumulation of cadmium-metallothionein (CdMT) by the proximal convoluted tubules--A light microscopic autoradiography study with 109CdMT

Chronic, but not acute, exposure to inorganic Cd produces renal damage. However, a single injection of cadmium bound to metallothionein (CdMT) produces renal injury. It is hypothesized that an interorgan redistribution of Cd as CdMT is responsible for the chronic nephrotoxic effect of Cd. To better understand the mechanism(s) of CdMT-induced nephrotoxicity, the intrarenal distribution of 109CdMT was examined. 109CdMT isolated from rat liver was injected into mice at a nonnephrotoxic dose (0.1 mg Cd/kg, iv). The radioactivity in the kidney reached a maximum level (85% of the dose) as early as 30 min following administration and remained essentially constant for up to 7 days after injection. Within the kidney, 109Cd distributed almost entirely to the cortex. Light microscopic autoradiography of the kidney showed that, within the cortex, 109Cd distributed preferentially to the S1 and S2 segments of the proximal convoluted tubules. Within the S1 and S2 segments, the concentration of 109Cd in the basal and apical parts of the cells was similar to that after the nonnephrotoxic dose of CdMT, but after a nephrotoxic dose (0.3 mg Cd/kg) the radioactivity distributed preferentially to the apical portion of the cells. In contrast, light microscopic autoradiography studies with 109CdCl2 revealed that 109Cd was more evenly distributed throughout the proximal tubules. Moreover, after administration of a large dose of inorganic Cd (3 mg Cd/kg), a similar concentration of Cd was found in the convoluted and straight proximal tubules. These data support the hypothesis that CdMT-induced nephrotoxicity might be due, at least in part, to its preferential uptake of CdMT into the S1 and S2 segments of the proximal tubules, the site of Cd-induced nephrotoxicity.

A multivariate study of protective effects of Zn and Cu against nephrotoxicity induced by cadmium metallothionein in rats

Factorial experimental design was used to study the protective effects of Zn and Cu on cadmium-metallothionein(CdMT)-induced nephrotoxicity in male Wistar rats. In the factorial design two levels of Zn (0 and 25 mg/kg body weight), two levels of Cu (0 and 12.5 mg/kg), and two levels of CdMT (0.1 and 0.4 mg of Cd/kg) were used as varied factors. The factorial design was complemented with a center point with all three variables at an intermediate setting, i.e., Zn at 12.5 mg/kg, Cu at 6.25 mg/kg, and CdMT at 0.25 mg Cd/kg. Each of the nine combinations of settings was administered to one of nine groups with six rats in each. Zn and Cu were injected sc 24 hr prior to the injection of CdMT. The concentrations of protein and Ca in urine and Ca in renal cortex were used as effects. The relationship between the experimental design settings and the effects were modeled with multiple regression. The multiple regression analysis revealed that for the high dose of CdMT (i) the enhanced values of protein in urine caused by CdMT injection could be more efficiently reduced by Zn than by Cu, and (ii) excessive Ca in urine and renal cortex could be more efficiently reduced by Cu than by Zn. No significant synergism or antagonism between Cu and Zn was found. These models can be used to estimate the dose levels of Zn and Cu which will reduce the toxic effects of CdMT. The treatment of 20.4 mg/kg Zn, for example, will reduce the effects of 0.4 mg Cd/kg as CdMT on protein in urine, and 2.8 mg/kg Cu will reduce the Ca in urine to the levels of those caused by 0.25 mg Cd/kg (no Zn and Cu). Similarly, the effect of 0.4 mg Cd/kg on Ca level in renal cortex can be reduced to that of 0.28 mg Cd/kg as CdMT by 7.98 mg Cu/kg, which is three times as efficient as Zn. The obtained results might be of importance in understanding the mechanism of cadmium toxicity and the potential risk to the health of the population exposed to cadmium occupationally or environmentally.

Transport kinetics of glucose and alanine in renal brush-border membrane vesicles of cadmium-intoxicated rabbits

Changes in transport kinetics of D-glucose and L-alanine were studied in renal luminal membrane vesicles isolated from cadmium-intoxicated rabbits. Cadmium intoxication was induced by subcutaneous injections of CdCl2 at a dose of 2 mg Cd/kg/day for 2-3 weeks. Brush-border membrane vesicles were prepared from renal outer cortex and outer medulla by Percoll gradient centrifugation. Cadmium intoxication resulted in a marked attenuation of Na(+)-dependent transports of D-glucose and L-alanine in both outer cortical and outer medullary brush-border membrane vesicles, and this was due to reduction in Vmax and not Km. Similar results were obtained in normal vesicles directly exposed to free cadmium. These results suggest that in long-term cadmium-exposed animals free cadmium ions liberated in the proximal tubular cytoplasm may directly impair brush-border membranes thereby reducing capacity of Na(+)-dependent transport systems for glucose and amino acids.

Disturbance of sialic acid metabolism by chronic cadmium exposure and its relation to proteinuria

In workers exposed to Cd (8 years on the average), we have found a significant decrease of sialic acid in erythrocyte membranes (22.61 +/- 1.84 vs 25.80 +/- 3.01 micrograms/mg of protein in controls, p less than 0.05) and an increase of sialic acid concentration in both urine (276.7 +/- 132.3 vs 174.5 +/- 70.9 micrograms/g of creatinine, p less than 0.05) and plasma (761.8 +/- 83.5 vs 640.4 +/- 70.7 micrograms/ml, p less than 0.01). In rats exposed to Cd (100 ppm in drinking water for 5.5 months), we have observed a reduction of the sialic acid level in erythrocyte membranes (31.4 +/- 1.2 vs 33.4 +/- 1.1 micrograms/mg of protein, p less than 0.01) and glomeruli (12.5 +/- 1.3 vs 13.9 +/- 1.6 micrograms/mg of protein, p less than 0.05). These effects in Cd treated rats were accompanied by a loss of the glomerular barrier selectively as reflected by an increased urinary output of albumin and transferrin. After 10 months of Cd exposure, the albuminuria and transferrinuria were negatively correlated with the sialic acid content of glomerular membranes (r = -0.47 and -0.51, p less than 0.05), which suggests that the depletion of sialic acid is involved in the loss of glomerular barrier function induced by long term Cd exposure. In Cd-treated rats, sialidase activity was enhanced in kidney cortex and in serum but not in glomeruli.

Cadmium-metallothionein-induced kidney dysfunction increases magnesium excretion in the rat

Urinary excretion of the major minerals, calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K), as well as of protein and metallothionein, was studied following the injection of cadmium-metallothionein (CdMT) in rats. Animals were given vehicle (saline) and 0.4 and 0.8 mg Cd/kg body wt as CdMT. A marked, relatively early, and reversible increase in Mg excretion was seen. The increase was dose-related, indicating a close connection to the typical Cd-derived cellular damage in the renal tubular epithelium, including an early reversible Ca excretion and a late reversible protein excretion. The increase in Mg excretion was similar in magnitude to the one for Ca and much more prominent than that recorded for Na and K. The appearance of Mg and Ca excretion peaks at an early stage after CdMT injection makes it likely that this effect is an early event in the process of development of cellular damage and does not merely represent unspecific cellular damage giving rise to proteinuria.

Cadmium inhibition of L-alanine transport into renal brush border membrane vesicles isolated from the winter flounder (Pseudopleuronectes americanus)

Using isolated brush border membrane vesicles from the kidney of the winter flounder (Pseudopleuronectes americanus), we have studied the effect of cadmium on L-alanine transport. Pretreatment of vesicles with 0.1 mM Cd2+ resulted in inhibition of L-alanine uptake in the presence of a NaCl (but not KCl) gradient. Inhibition was due to a specific interaction with the sodium-alanine cotransport system and not a change in the driving forces for alanine transport, since Cd2+ did not affect sodium-dependent D-glucose uptake. The effect of Cd2+ on Na+-alanine cotransport showed mixed-type inhibition which is only partially reversible by EDTA. Cd2+ uptake itself was shown to be time and temperature dependent, resulting in binding to both sides of the membrane. No direct correlation was possible between inhibition of L-alanine transport and the amount of Cd2+ taken up by the membranes. Nevertheless, the striking time dependence of the effect of Cd2+ on sodium-dependent L-alanine uptake and the inability of EDTA to reverse the inhibitory action of Cd2+ suggest that Cd2+ inhibits Na+-alanine cotransport at the cytoplasmic side of the membrane.

Modulation of metal toxicity by metallothionein

Toxic properties of several metals may be modified, since they are bound to metallothionein in vivo. Such modulation is particularly well known for cadmium (Cd), whose acute effects are prevented by metallothionein induction, whereas chronic effects on the kidney are partly explained on the basis of transport of cadmium-metallothionein (CdMt) into the kidney. Although intracellular Mt synthesis is induced by Cd, offering partial protection, nephrotoxicity may occur at times when such protection is insufficient. Perturbations in renal calcium metabolism may be an important basis for membrane dysfunction leading to proteinuria.

Renal glutathione depletion and nephrotoxicity of cadmium-metallothionein in rats

Renal glutathione (GSH) concentrations were reduced approximately 80% at 4 hr after a single injection of buthionine sulfoxime (BSO) (4 mmol/kg body wt) and remained reduced for at least 16 hr in male rats. Following BSO injection, rats were injected with a nephrotoxic dose of cadmium-metallothionein (Cd-MT) (0.3 mg Cd as Cd-MT/kg body wt) and killed 1, 4, or 12 hr later. Damage to the kidney was assessed histologically and by measurement of p-aminohippuric acid (PAH) uptake into renal cortical slices. Although the renal accumulation of Cd following Cd-MT injection was significantly lower in BSO-pretreated rats as compared to nonpretreated rats, the damage to kidney was more severe. At 4 and 12 hr, both Cd-MT-induced inhibition of PAH uptake and morphological damage were significantly increased in BSO-pretreated rats. In certain experiments, the induction of renal intracellular MT synthesis by zinc pretreatment slightly decreased the renal toxicity of Cd-MT in the BSO-treated rats. The results demonstrate that although GSH depletion decreases the renal accumulation of Cd in rats injected with Cd-MT, the nephrotoxicity of Cd-MT is increased. Preinduction of MT in the kidney can only partially overcome this increase in toxicity. Therefore both GSH and intracellular MT levels can influence the renal toxicity of injected Cd-MT.

The teratogenicity of cadmium-metallothionein in the rat

A single dose in the range 0.25-1.0 mg metallothionein-bound cadmium (MT-Cd)/kg body weight, when administered parenterally to the rat between day 8 and day 14 of gestation (gd 8-gd 14), is teratogenic. In vitro, the development of the isolated rat conceptus, explanted at 8.5 days of gestation, is unaffected by the addition of 1.5 microM MT-Cd to the culture medium, whereas the same concentration of ionic Cd (as CdCl2) is lethal. The incorporation of appreciable amounts of Cd into the embryo (860 pg), placenta (970 pg) and yolk sac (65.4 ng) without toxic manifestations under the former conditions suggests that the metalloprotein is incorporated pinocytotically, but without degradation, by the conceptus in vitro. It does not follow, therefore, that MT-Cd is without embryo/foetotoxicity in the pregnant rat since, in-vivo, liberation of some of the protein-bound Cd is known to occur in the blood. At short times after injection of 0.25 mg MT-Cd/kg body weight on gd 12, however, the maximal foetal and placental contents of Cd (less than 25 pg and 2 ng, respectively) are low in comparison with those after a teratogenic dose of CdCl2 and are of the same order as those in the embryo (46 pg) and placenta (100 pg) + yolk sac (3.8 ng) of the rat conceptus, cultured in the presence of the highest no-effect concentration of CdCl2 (0.065 microM). From this evidence, therefore, it is concluded that the uptake by the conceptus in vivo of either CdMT, or of Cd liberated therefrom, is unlikely to contribute to the teratogenic response. In the pregnant, as in the non-pregnant rat, the kidney appears to be the only organ that is affected directly by the metalloprotein. All doses in the range 0.25-1.0 mg MT-Cd/kg body weight are nephrotoxic and, because of this, result in prolonged anorexia in the pregnant animal. While some of the foetal deformities that occur in the CdMT-dosed animal seem to be direct consequences of the renal dysfunction, others apparently are secondary to the maternal anorexia, since they are induced in pregnant, normal rats by appropriate reductions in food intake. In rats that are injected i.p. on gd 12 with 0.25 mg MT-Cd/kg body weight, renal uptake of Cd is slower, but the final concentration is higher than in animals that are given the same dose i.v.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular aspects, physiological function, and clinical significance of metallothioneins

Metallothioneins (MTs) are well-characterized low molecular weight, heat-stable cytosolic proteins with exceptional high content of cysteinyl sulfur and are known to bind heavy metals like cadmium (Cd), zinc (Zn), and copper (Cu). Since these proteins are induced on exposure to heavy metals, it is now accepted that they have a detoxifying role during heavy metal toxicity. It has also been suggested that the primary function of Mt is in the homeostasis of the essential metals Zn and Cu. Recently, a role MT in selenium metabolism in primates has been established. Further, MT has gained considerable importance in the clinical disorders related to trace metal metabolism.

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.

Resistance to acute nephrotoxicity induced by cadmium-metallothionein dependence on pretreatment with cadmium chloride

Three groups of rats (B-D) were given various daily doses of CdCl2 (0.5-2 mg Cd/kg) continuously or in intervals during time periods of 1-8 weeks. Another group of animals (A) were kept untreated. At the end of the period, selected subgroups of groups A-D were given a single subcutaneous injection of 109Cd-metallothionein (109CdMT) 0.05 or 0.4 mg Cd/kg ("challenge dose"). Subsequently, urinary creatinine, protein, Cd, 109Cd and MT and kidney cortex Cd, 109Cd and MT were determined. In group A (no long term pretreatment), an increased proteinuria was observed after the rats had received the lower of the challenge doses of 109CdMT, and an even greater increase after the higher challenge dose of 109CdMT. No such increase appeared in group B, C and D (repeatedly pretreated with CdCl2) at either of the challenge doses. Higher metallothionein concentrations in kidney cortex observed in the pretreated groups constitute a plausible explanation of the protective effects of pretreatment against the development of increased proteinuria after challenge dosing. It is likely that increasing Cd concentrations, gradually accumulating in the renal cortex (22-226 micrograms/g wet wt.) as a result of the pretreatment, served to induce the synthesis of metallothionein in the renal cortical cells, thus making them resistant to the challenge from 109CdMT.

Influence of cadmium-metallothionein pretreatment on tolerance of rat kidney cortical cells to cadmium toxicity in vitro and in vivo

Kidney cells were isolated from rats pretreated by daily subcutaneous doses of cadmium metallothionein (CdMT: 0.05-0.2 mg Cd/kg X 5) and from non-pretreated rats. Upon exposure to CdCl2 in vitro (0-200 micrograms Cd/ml), a concentration dependent decrease in viability was observed in the non-pretreated cells, while no such decrease occurred in the pretreated cells indicating that these cells were more resistant to the toxic action of cadmium. There was a higher in vitro uptake of Cd+2 and an increased metallothionein (MT) concentration in the pretreated cells (compared to non-pretreated cells). Subcellular distribution studies revealed that Cd was mainly recovered in the "cytosol" fraction. The higher total cadmium uptake in pretreated cells corresponded to an increase of Cd in "cytosol" and "nuclear" fractions. This observation may be explained by MT-binding of Cd in the cells and is in accordance with a possible protective effect of induced MT in the pretreated cells. In order to assess whether pretreatment-induced tolerance to cadmium toxicity--indicated by the cellular studies--could also be observed in vivo, some whole animal experiments were also performed. A dose-related proteinuria was observed in non-pretreated rats after a single subcutaneous administration of 109Cd-MT at doses of 0.05 and 0.4 mg Cd/kg. Urinary total Cd, 109Cd and MT was also increased in a dose-related fashion. Cadmium concentrations in kidney were dose related and reached 19 micrograms/g wet weight. In contrast, in animals repeatedly pretreated with CdMT according to 1), no proteinuria was observed after administration of the same single doses of 109CdMT. Total Cd. 109Cd and particularly MT-concentrations in urine were lower in such pretreated animals than in in non-pretreated ones in spite of the accumulation of higher tissue concentrations of total Cd (up to 80 micrograms/g). The pretreatment was thus shown to prevent some of the acute nephrotoxicity of CdMT, possibly by means of induction of MT synthesis.

The effects of low doses of cadmium-metallothionein on the renal uptake of beta 2-microglobulin in rats

The urinary excretion of beta 2-microglobulin (beta 2-m) was followed up in rats given increasing intravenous doses (15, 30, or 60 micrograms Cd/kg) of rat hepatic cadmium-metallothionein (Cd-MT). At the two highest doses, Cd-MT was found to induce two peaks in the urinary excretion of rat beta 2-m: a first narrow peak occurring immediately after the injection, followed 20 hr later by a broader peak. While the latter peak is caused by the well-known tubular toxicity of Cd-MT, the former most likely results from competition between rat Cd-MT and beta 2-m for a common renal transport system. This explanation is supported by the fact that the renal accumulation of MT-bound Cd can be inhibited by human beta 2-m. The Cd concentration in renal cortex of rats challenged with the lowest tubulotoxic dose of Cd-MT was only 3.4 ppm 4 hr following the injection. Since the Cd-MT nephrotoxicity is caused by the non-MT-bound Cd, which at this time represents about 70% of the renal Cd, it can be tentatively estimated that the critical concentration of free Cd in renal cortex is only 2 ppm, i.e., about 100 times less than the currently accepted critical value for the total concentration of this metal in kidney cortex.

Mechanism of cadmium-metallothionein-induced nephrotoxicity: relationship to altered renal calcium metabolism

Prolonged cadmium exposure has been associated with proteinuria, calcuria and loss of calcium from bones in humans. Previous studies have shown that kidney uptake of cadmium in vivo results from proximal tubule absorption of the circulating cadmium metallothionein complex (CdMT), and intracellular release of the Cd2+ ion prior to induction of renal metallothionein. Parenteral administration of CdMT has been found to selectively damage the proximal tubule cell lysosome system with development of a tubular proteinuria pattern similar to that observed under chronic exposure conditions. The present studies also demonstrate a concomitant calcuria but no changes in the excretion of other electrolytes or glucose using this model. These marked changes in renal calcium metabolism occurred in the absence of mitochondrial damage, changes in total, Na/K or Mg-stimulated ATPase activities, renal ATP levels, membrane 45Ca2+ transport or overt tubule cell necrosis during an 8 hour period following CdMT injection. Proteinuria and calcuria were prevented by prior zinc induction of the renal MT pool. Data from these studies indicate that renal proximal tubule cell uptake and degradation of the circulating CdMT complex produces both a marked proteinuria and calcuria. The calcuria does not appear to stem from changes in renal energy metabolism or membrane transport of this element but is probably a secondary result of calcium binding to excreted proteins which are increased in urine to a similar extent. The studies also suggest that zinc status and maintenance of the renal ZnMT pool may play an important role in regulating cadmium-induced renal proteinuria and calcuria by preventing Cd2+ perturbation of the proximal tubule cell lysosome system.

On the role of metallothionein in cadmium induced renal toxicity

Metallothionein plays an important role in the metabolism and toxicity of cadmium. Human beings are exposed to increasing amounts of cadmium. The critical organ in long term cadmium exposure is the kidney. Some groups in the population fall within the risk zone for developing renal tubular dysfunction from cadmium exposure. After absorption, cadmium is to a large extent distributed to the liver where it is bound to and induces the synthesis of metallothionein. Subsequently metallothionein-bound cadmium is slowly released into blood and efficiently taken up by renal tubular cells after glomerular filtration. The biological half-time of cadmium in the human kidney is considered to be very long: 10-15 years and this explains the life-long accumulation of cadmium, reaching a toxic concentration of approximately 200 ug/g in excessively exposed individuals. Human adult renal metallothionein has a natural content of cadmium which also induces the resynthesis of this protein upon its catabolism. This could be one explanation of the long biological half-time which has been reported for cadmium. Injection of metallothionein-bound cadmium can induce renal tubular kidney dysfunction at lower renal cadmium concentrations (10-20 ug/g), probably because of the fast catabolism and relatively slow synthesis of metallothionein in renal cells. A model of these and other mechanisms causing tubular kidney damage in cadmium exposure is presented.

Uptake of cadmium in isolated kidney cells--influence of binding form and in vivo pretreatment

Uptake of cadmium as 109CdCl2, 109Cd-cysteine, 109Cd-albumin and 109Cd-metallothionein was studied in isolated kidney cells from rat. Cd as 109CdCl2 and 109Cd-albumin was taken up at similar rates. The uptake of cadmium as 109Cd-cysteine was greater and that of 109Cd-metallothionein lower compared with that of the other substances. These observations were made on non-pretreated cells. In cells taken from rats pretreated with CdCl2 in vivo, the uptake of cadmium as 109CdCl2, 109Cd-cysteine and 109Cd-albumin was lower than in cells from non-pretreated rats. However, the uptake of 109Cd-metallothionein was considerably enhanced in pretreated cells. In pretreated kidney cells the decreased uptake of Cd (as Cd-albumin) might be related to protection of the kidney against acute Cd toxicity and increased uptake of metallothionein-Cd might contribute to the explanation of renal damage in long-term Cd exposure.

Histochemical changes in protein disulphide bonds in rat liver and kidney after chronic cadmium administration, and the possible relation to metallothionein

After chronic exposure to low doses of CdCl2 an increase in disulphide bonds has been established in rat liver using a specific staining method for disulphide bonds and cytophotometric quantitation. This increase is dependent on doses and length of exposure time. Evidence is presented that this increase might be related to the accumulation of metallothionein or some other cadmium binding protein. Using the same staining method after long exposure to low doses of CdCl2 a large number of large dark blue stained granules were observed in the proximal tubule cells, with blue stained deposits in the lumen of the proximal and some renal medulla tubules of the kidney. Evidence is presented that this staining pattern corresponds to the destruction of the proximal tubule cell by the cadmium thionein complex.

Metallothionein in rabbit kidneys preserved for transplantation

Thirteen rabbits were given repeated cadmium injections to achieve cadmium concentrations in kidney cortex ranging from 0.05 to 1 mmole Cd/kg wet weight. Another four animals served as controls. One kidney from each animal was frozen directly to -70 degrees C whereas the other kidney was kept for 24 hr at +4 degrees C in a preservative (Sachs' solution) to simulate conditions for preservation of human donor kidneys before transplantation. Protein binding of cadmium, zinc and copper in kidney homogenates and the concentration of metallothionein (MT) were measured in the kidney that was frozen directly and in the kidney that had been preserved. No gross differences in either the protein binding of cadmium, zinc and copper or in the MT content were seen between the directly frozen and preserved kidneys from the same animal. This indicates that MT is not rapidly broken down in rabbit kidneys which have been preserved similarly to human donor kidneys for 24 hr in a standard preservative solution prior to a transplantation.

Chelation of cadmium

The toxicity of cadmium is determined by chelation reactions: in vivo, Cd2+ exists exclusively in coordination complexes with biological ligands, or with administered chelating agents. The Cd2+ ion has some soft character, but it is not a typical soft ion. It has a high degree of polarizability, and its complexes with soft ligands have predominantly covalent bond characteristics. Cd2+ forms the most stable complexes with soft donor atoms (S much greater than N greater than 0). The coordination stereochemistry of Cd2+ is unusually varied, including coordination numbers from 2 to 8. Even though the Cd2+ ion is a d10 ion, disturbed coordination geometries are often seen. Generally, the stability of complexes increases with the number of coordination groups contributed by the ligand; consequently, complexes of Cd2+ with polydentate ligands containing SH groups are very stable. Cd2+ in metallothionein (MT) is coordinated with 4 thiolate groups, and the log stability constant is estimated to 25.5. Complexes between Cd2+ and low molecular weight monodentate or bidentate ligands, e.g., free amino acids (LMW-Cd), seem to exist very briefly, and Cd2+ is rapidly bound to high molecular weight proteins, mainly serum albumin. These complexes (HMW-Cd) are rapidly scavenged from blood, mainly by the liver, and Cd2+ is redistributed to MT. After about 1 day the Cd-MT complex (MT-Cd) almost exclusively accounts for the total retained dose of Cd2+, independent of the route of exposure. MT-Cd is slowly transferred to and accumulated in kidney cortex. The acute toxicity and interorgan distribution of parenterally administered Cd2+ are strongly influenced by preceding MT induction, or decreased capacity for MT synthesis; however, the gastrointestinal (GI) uptake of Cd2+ seems unaffected by preceding MT induction resulting in considerable capacity for Cd2+ chelation in intestinal mucosa, and this finding indicates that endogenous MT is not involved in Cd2+ absorption. The toxicity of parenterally administered Cd2+ is strongly enhanced when administered as complexes with NTA or STPP , but it is much decreased when administered as a complex with EDTA. In chronic oral exposure the toxicity and GI uptake of Cd2+ is not changed when Cd2+ is administered as a complex with the detergent formula chelating agents NTA, EDTA and STPP . The uptake of Cd2+ from ligated intestine in vivo was not affected by administration of Cd2+ as complexes with CYS or GSH, but significantly reduced by complexation with EDTA or BAL. The acute toxicity of orally administered Cd2+ is reduced when Cd2+ is administered as a complex with EDTA.(ABSTRACT TRUNCATED AT 400 WORDS)

Cadmium-metallothionein-induced nephropathy: a morphological and autoradiographic study of cadmium distribution, the development of tubular damage and subsequent cell regeneration

A single intravenous dose of 0.8 mg thionein-bound cadmium (Cd) per kg body weight, as the isolated (Cd, Zn)-metallothionein (MT) from rat liver, in rats of the same strain is nephrotoxic, but not lethal. Apical vesiculation in epithelial cells of the renal proximal convoluted tubules is apparent within 1 h of dosing and, by 4 h, is extensive in some of these cells that surround the larger arteries in the cortex. The membranes of these cells appear undamaged. The lesion at first progresses with time; by 24 h, the initially affected cells show extensive necrosis and most proximal convoluted tubular epithelia in other regions of the cortex are hydropically or vacuolarly degenerated. The inner stripe of the outer zone of the medulla and other portions of the nephron (glomerulus, distal tubule and collecting duct), however, appear essentially unaffected. The necrotic changes are maximal at 48 h but, after this time, regeneration begins. By seven days, much of the cell debris has been eliminated and cells of the regenerating or regenerated epithelia are similar in morphology to those of the normal kidney. Electron microscopic autoradiography of kidney sections from rats after administration of 109Cd-metallothionein of high specific activity shows that Cd is not concentrated in endocytotic vesicles, lysosomes, or any other cellular organelle, even at early times after dosing, but is distributed evenly throughout the epithelial cell. Thus, although Cd-MT appears to be taken up endocytotically in the kidney tubules, it appears that liberation of Cd from the metalloprotein must occur very early in the reabsorptive process.