Effects of mucosal metallothionein in small intestine on tissue distribution of cadmium after oral administration of cadmium compounds.

The effect of mucosal metallothionein (MT) preinduced by zinc (Zn) on tissue distribution of cadmium (Cd) after administration of Cd with several chelating agents was studied in rats. After Cd-cysteine (Cd-Cys) was incubated with intestinal Zn-MT in vitro, all the Cd dissociated from Cys and exchanged the Zn bound to MT. However, dissociation of Cd bound to EDTA (Cd-EDTA) was not observed in the incubation mixture containing intestinal Zn-MT. The concentration of Cd in intestinal mucosa reached a maximum 16 hr after oral administration of Cd-Cys. The Cd level in the intestine was higher than that in the liver and kidney and was similar to that occurring after oral administration of CdCl2. The amount of Cd distributed to the liver and kidney after Cd-EDTA administration was about 30% of the level after CdCl2 administration. Even at 15 mg Cd/kg Cd-EDTA, the Cd level in the intestinal mucosa reached a plateau after 2-4 hr, as it did in the liver and kidney. When Cd-Cys was administered po to control or to Zn-pretreated rats, it was found that Zn pretreatment increased the concentration of Cd in the kidney, as was the case after oral administration of CdCl2. This effect of Zn pretreatment was not observed after oral administration of Cd-EDTA. When Cd-MT was injected into the duodenum, the intestinal absorption of Cd was 60% of that after CdCl2 administration. After the duodenal administration of Cd-MT, at all doses, the concentration of Cd in the kidney was higher than that in the liver. These results suggest that mucosal MT in the small intestine might trap Cd absorbed from the intestinal lumen and transport it to the kidney.     

Uptake of cadmium and metallothionein by rat everted intestinal sacs

The gastrointestinal uptake and transport of cadmium (Cd) and the role of metallothionein (MT) were studied in everted sacs of rat intestine (ESRI). When ESRI were incubated for 30 min in a medium containing various Cd concentrations (1-5 x 10(-4) M) as CdCl2, Cd-cysteine (Cd-Cyst) or rat liver Cd-MT-II (Cd-MT), a dose-dependent tissue uptake of Cd was observed. In ESRI incubated with Cd-MT, total Cd uptake was lower than that of CdCl2 (25% of CdCl2). Fractionation of the tissue showed that about 80% of Cd in the tissue was recovered in the particulate fraction after CdCl2 and Cd-Cyst incubation, while that after Cd-MT incubation was present mainly in the cytosol fraction (about 80%). Most of the Cd in the cytosol fraction of Cd-MT-incubated ESRI was associated with a 10,000 molecular weight protein on Sephadex G-75 column fractionation. Similar results were obtained after incubation of ESRI from Zn-pretreated rats with 109CdCl2 solution. In addition to the Cd-MT peak, there was a small peak of Cd associated with a high molecular weight fraction. Only a small percentage of Cd was leaked to serosal fluid in the everted sacs incubated at a low concentration of CdCl2 (0.8%) but this leakage of cadmium was increased at higher concentration and was higher after incubation with Cd-MT. The results suggest that the uptake of Cd from CdCl2 and Cd-MT is different. Although Cd-MT was taken up intact by everted sacs, the uptake was slow as compared to Cd salts. The intracellular presence of MT had little effect on the uptake of CdCl2 but the Cd was sequestered by MT in the intestine.     

Protective role of renal metallothionein against Cd nephropathy in rats

Rats were treated with four types of Cd compound: CdCl2, Cd bound (Cd-peptide), and Cd bound to metallothionein (Cd-MT). This treatment caused no nephropathy. Subsequently, toxic doses of Cd compounds were administered to these pretreated rats and their effects on renal function were examined. When 1.4 mg Cd/kg as Cd-Cys was administered, marked increases in urinary protein, glucose, and amino acid were observed. However, when the animals were pretreated with 1 mg Cd/kg/day as CdCl2 for 3 days, and 1.4 mg Cd/kg as Cd-Cys was administered 24 hr later, no renal damage was observed. Such a protective effect against the nephrotoxic action of Cd-Cys was also shown by pretreatment with Cd-Cys, Cd-peptide, or Cd-MT. Furthermore such a phenomenon was also observed when the nephropathy was caused by Cd-peptide or Cd-MT. The efficacy of pretreatment depended on the time before subsequent administration of Cd and the dose used for pretreatment. Incorporation of Cd into the liver and the kidney was not altered by the pretreatment. No matter in which form the nephrotoxic dose of Cd was administered, the incorporated Cd was distributed between particulates and cytosol; 3 hr after administration, cytosolic Cd was present in almost equal amounts in the high-molecular-weight and the MT fractions in the nonpretreated rats. However, after pretreatment, more of the Cd subsequently administered was found in the MT fraction. These results suggest that MT participates in the detoxication mechanism against Cd in the kidney, as it does in the liver.     

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.     

Effect of cadmium chloride and Cd-metallothionein on the nervous tissue culture

Influence of metallothionein (MT) isolated from rat liver on rat cerebellum in culture was investigated by comparison with that of CdCl2. Cd-MT at 0.2 X 10(-5) M as Cd significantly depressed the outgrowth of nerve fibers, fibroblasts and glial cells as compared to the control culture. In the range from 0.2 X 10(-5) M to 2.7 X 10(-5) M Cd, the toxicity of Cd-MT was the same as that of CdCl2. Above 5 X 10(-5) M Cd, however, the toxicity of Cd-MT was less than that of CdCl2. Cadmium added as CdCl2 was perfectly recovered at a region of higher Mr than MT on the Sephadex G-75 column. Cadmium added as Cd-MT was detected in part at the higher Mr region and in part at the MT region, depending on incubation time and Cd concentration in the medium. The toxic action of Cd-MT was proportional to the recovery level of Cd at the higher Mr region.     

The dose-dependent deposition of cadmium into organs of Japanese quail following oral administration

The accumulation and disposition of Cd2+ as CdCl2 administered orally to Japanese quail (Coturnix coturnix) was investigated. Birds received 0.01, 0.10, 1.0, 50, 500, 5000, or 50,000 micrograms Cd/kg/day for 4 consecutive days by gastric tube, and were killed 4 days after the final dose. The percentage of the total administered dose recovered in liver + kidneys + duodenum was 0.7% or less in all but the highest dose, for which recovery was approximately 2%. Only at the highest dose did the hepatic Cd concentration exceed that of the kidney, and only at this dose was there any appreciable increase in metallothionein (MT) concentrations in the liver and kidney. Duodenal cytosol was found to contain high levels (300-1300 micrograms/g) of endogenous MT-like proteins, probably due to the relatively high Zn concentration (approximately 185 ppm) of the commercial diet eaten by the quail. In the small intestine, Cd2+ taken up after trace doses of oral 109Cd2+ was found to be exclusively bound to these 10,000-MW, or lower MW, ligands. In the liver, MT synthesis was accompanied by increased concentrations of Cd and Zn (but not Cu) associated with the MT fractions, whereas in the kidney, all three metals were elevated in response to Cd-induced MT synthesis. A major conclusion of the present study is that, in response to environmentally relevant (less than 10 micrograms/kg/day po) doses of Cd2+, absorbed Cd is transported in blood primarily in a form which enhances deposition in the kidney. This behavior is consistent with the pharmacokinetics of Cd-MT.     

Role of intestinal metallothionein in absorption and distribution of orally administered cadmium

The effects of mucosal metallothionein (MT) preinduced by Zn on the intestinal absorption and tissue distribution of Cd were studied. 109CdCl2 was administered to control and Zn-pretreated rats. The total amount of Cd distributed to the liver and the kidney in the group pretreated with 100 mg/kg of Zn was about 70% that of the control group. In the control group, the Cd concentration in the intestinal mucosa reached a maximum 16-24 hr after its administration and then gradually decreased with time, unlike that in the liver and the kidney. The concentration of intestinal Cd in the pretreated group reached a maximum earlier than it did in the control group and most of the Cd was in the MT fraction. Pretreatment with Zn (100 mg/kg or higher, po) caused a reduction in the Cd concentration in the liver and an increase in the kidney. Pretreatment with Zn (5 X 10 mg/kg, sc) or Cd (5 mg/kg, po) also increased renal Cd concentration. This was effective at 24 hr but not at 0.5 hr after pretreatment. These effects of pretreatment with Zn (100 mg/kg, po) on tissue distribution of Cd were also observed after an intraintestinal injection of Cd but not after an iv injection. The results indicate that MT in intestinal mucosa plays a significant role not only in the absorption of Cd but also in its transport to the kidney.     

Induction of metallothionein synthesis by zinc in cadmium pretreated rats

The ability of zinc (Zn) salts to induce the synthesis of metallothionein (MT) in liver, kidney and pancreas of rats pretreated with cadmium (Cd) salts was investigated. Twenty-four hours after either CdCl2 (2.0 mg Cd/kg, s.c.) or saline pretreatment, rats were injected with saline, CdCl2 (2.0 mg Cd/kg, s.c.) or ZnSO4 (20 mg Zn/kg, s.c.) and the concentrations of MT and MT-1 mRNA in tissues subsequently measured. After a single injection of Cd salts, concentrations of MT and MT-1 mRNA were significantly increased in liver as compared to control. With two injections of Cd, the accumulation of MT in liver was approximately twice the levels of MT following a single injection of Cd. In kidney, MT and MT-1 mRNA expression were significantly increased only after two injections of Cd and in the pancreas, Cd injections did not alter either MT content or MT-1 mRNA expression. Treatment with Zn salts increased MT concentrations in both liver and pancreas. However, the pancreas was the most responsive to injections of Zn salts as compared to the liver in terms of increases in both protein concentration and MT-1 mRNA expression. When Zn injection was preceded by a Cd injection, induction as measured by MT-1 mRNA and MT concentrations were approximately additive in liver. In kidney, although Cd or Zn treatment separately had no effect on MT or MT-1 mRNA content, injection of Cd followed by Zn resulted in significantly increased levels of renal MT and MT-1 mRNA. Fractionation of liver cytosols on a Sephadex G-75 column revealed that in animals receiving two injections of Cd, virtually all the Cd was associated with MT whereas Zn was distributed between both high molecular weight (HMW) proteins and MT. In animals receiving both Cd and Zn injections, cytosolic Cd was still bound predominantly to the MT fraction, while the proportion of cytosolic Zn associated with MT increased. The results of this study suggest that, treatment with Cd salts followed by Zn salt injection can induce further synthesis of MT in liver, kidney and pancreas with subsequent binding of both Zn and Cd to the intracellular MT.     

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.     

Dosage-dependent absorption of cadmium in the rat intestine measured in situ

Previous studies have shown that the disposition of cadmium (Cd) following oral administration is dosage dependent and may possibly be due to dosage-dependent intestinal absorption of Cd. Though extensively studied, the precise nature of Cd absorption by the intestine remains unclear. Similarly, the role of metallothionein (MT) in the intestinal absorption of Cd remains equivocal. The present study was designed (1) to characterize the intestinal absorption of Cd in the rat, and (2) to determine the role of MT in intestinal Cd absorption. The study has been conducted with an isolated intestinal loop preparation in situ, which allows direct measurement of intestinal absorption under nearly physiological conditions. Under urethane-induced anesthesia. Cd (0.1, 10, 100, 1000, or 10,000 micrograms/kg) was injected intraluminally into the isolated intestinal loop in situ and all mesenteric venous (portal) blood exiting from the loop was collected for 90 min. Absorption of Cd into the portal circulation was low at all dosages studied. The percentage of the dosage absorbed ranged from 0.09% at the 0.1 microgram/kg dosage to 3.4% at the 10,000 micrograms/kg dosage. At low dosages (0.1 and 10 micrograms/kg), little difference was noted in the fractional absorption of Cd (0.09 and 0.14% of the dosage, respectively). However, the fractional absorption of Cd was 10-fold greater in rats administered 100 micrograms Cd/kg (1.1% of the dosage). Administration of higher dosages of Cd (1000 and 10,000 micrograms/kg) further increased the percentage of the dosage absorbed (1.8 and 3.4%, respectively). To evaluate the role of MT in the intestinal absorption of Cd. rats were subcutaneously injected with zinc (Zn) for 4 days (30 mg/kg/day) and the absorption of an intermediate dosage of Cd (100 micrograms/kg) was subsequently assessed in situ. Zn pretreatment increased the endogenous concentration of MT in the intestine 25-fold. Following intraluminal administration. 93% of Cd in intestinal cytosol of Zn-treated rats was bound to MT whereas 40% of the cytosolic Cd was bound to MT in saline-treated (control) rats. Moreover, the amount of Cd in intestinal cytosol was 2-fold greater in Zn-treated rats than in control rats. However, the intestinal absorption of Cd in rats pretreated with Zn demonstrated no difference from that in saline-treated rats. These results indicate that the intestinal absorption of Cd is dosage independent at low dosages of Cd (less than 10 micrograms/kg) and dosage dependent at high dosages (greater than 10 micrograms/kg). Furthermore, saturation of intestinal MT is not a major determinant of the observed dosage-dependent absorption of Cd.     

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.     

Tissue distribution of cadmium and nephropathy after administration of cadmium in several chemical forms

Cadmium (Cd) was administered as CdCl2, Cd-Cys, Cd-partial structural peptide of metallothionein (MT) II, Cd-MT I, and Cd-MT II to rats, and the distribution of and nephropathy caused by the corresponding Cd compounds were examined. Each Cd complex showed dissociation of Cd in vivo and in vitro in the plasma. With Cd-Cys approximately 80% dissociation was observed while Cd-MT showed only 15% dissociation. When the dissociation of the Cd complex in the plasma was less, the distribution of Cd to the liver was decreased but distribution was increased to the kidney and urine. Each Cd complex showed the presence of Cd in the kidney shortly after the administration in the high molecular weight fraction (HM-fr) and also in MT-fr. This was then followed by a decrease in the Cd level in the HM-fr but by an increase in the MT-fr. All Cd compounds except CdCl2 caused some transient renal damage. Renal damage shown by significant increases of urinary protein, glucose, and amino acids were observed at the doses of 1.3-1.7 mg Cd/kg in the Cd-Cys group, at 0.51-0.64 mg Cd/kg in the Cd-peptide group, and at 0.16-0.23 mg/kg in the Cd-MT I and II groups. The Cd level in the kidney of rats with renal damage from these complexes was approximately the same in all the groups, that is, 10 micrograms/g kidney. It is concluded that Cd causes renal damage when its concentration in the kidney is 10 micrograms/g or higher regardless of the type of Cd complex that is administered.     

Metal exposure assessment in native fish, Mullus barbatus L., from the Eastern Adriatic Sea

Metallothionein (MT) and metal (Zn, Cu, Fe, Mn, Cd) association in the cytosol of liver, kidney and intestine was studied on native Mullus barbatus specimens. Investigated parameters of all specimens (1 to 8-year-old) were related to the fish length, which reflects fish growth and aging. Cytosolic metal concentrations in liver, kidney and intestine are tissue specific but in general decrease as follows: Fe approximately Zn>Cu>Mn>Cd. Metallothionein levels are also tissue specific with the highest level in intestine, followed by kidney and liver. Percentage partition of MT and Zn in liver, kidney and intestine cytosol of red mullet is comparable. Associations of MT and metals that induce MT synthesis exist in liver for MT/Zn (r=0.28) and MT/Cu (r=0.26) and in intestine for MT/Cu (r=0.38). Unlike essential metals, there is no significant correlation between MT and the toxic metal Cd, what is ascribed to its very low cytosolic concentration. The most pronounced size-related accumulation is found for the toxic metal Cd in liver cytosol (r=0.69), indicating chronic Cd accumulation. Negative correlation between metals (Cd, Zn, Fe, Mn) and the condition factor also indicates chronic metal effects and their metabolic impact.     

Comparisons of the toxicity of CdCl2 and Cd-metallothionein in isolated rat hepatocytes

In the intact animal, inorganic Cd distributes mainly to the liver and produces hepatotoxicity, while Cd-metallothionein (CdMT) distributes primarily to the kidney and produces nephrotoxicity. CdMT has also been demonstrated to be more toxic than Cd in cultured kidney cells, but it is not known if CdMT is more toxic to all cultured cells or if there is a good correlation between in vitro and in vivo toxicity. Therefore, hepatocytes, which were isolated and grown in monolayer culture for 24 h, were incubated with CdCl2 (1-100 microM) or CdMT (3-100 microM Cd). The intracellular K+ content was quantitated 24 h later as an index of toxicity. The K+ concentration of the hepatocytes was decreased 50% by 4 microM CdCl2, whereas 25 microM CdMT was required to produce similar injury. In the intact animal, zinc induces the synthesis of MT and decreases the hepatotoxicity of Cd. ZnCl2 added to the media (100 microM) for 24 h before exposure to Cd or CdMT increased the intracellular MT concentration 700%. This elevation in MT reduced the toxicity of CdCl2 approximately 80% but did not alter the toxicity of CdMT. In summary, CdCl2 is more toxic to cultured hepatocytes than Cd-MT, and MT induction decreases the toxicity of CdCl2 in hepatocytes, as has been observed in the intact animal. This indicates that cultured hepatocytes appear to be an excellent model for examining the hepatotoxicity of Cd.     

Absorption and distribution of cadmium (Cd), copper and zinc following oral subchronic low level administration to rats of different binding forms of cadmium (Cd-acetate, Cd-metallothionein, Cd-glutathione)

Male Wistar rats received by gavage saline or about 25 micrograms cadmium (Cd)/kg/day as Cd-acetate (Cd-Ac), Cd-metallothionein (Cd-MT) or Cd-glutathione (Cd-GSH) 5 times per week for 28 times. At all treatments 0.2-0.3% of the totally administered Cd dose was found in liver, kidneys, small intestine and pancreas, whereas none of the Cd forms applied resulted in a Cd accumulation in testes. Cd in small intestine was not increased by Cd-MT. However, it was raised by Cd-Ac and even more by Cd-GSH. A smaller increase in hepatic and renal Cd resulted from Cd-GSH than from Cd-Ac or Cd-MT. Cd in pancreas increased after Cd-GSH but not after Cd-Ac or Cd-MT. Copper (Cu) rose in small intestine and testes but decreased in kidneys independent of either Cd treatment. Concomitantly, zinc (Zn) was decreased in small intestine and testes. The tissue concentration of metallothionein (MT) was only marginally increased by all treatments. The highest value (80%) above controls) was found in small intestine after Cd-GSH. Intestinal Cd as well as testicular Cu were related to the tissue MT. Therefore, the distribution of Cd between various organs depends on the Cd form applied. There is some relationship to the distribution of Cu and Zn.     

Turnover of parenterally administered zinc and cadmium and the redistribution of metallothionein bound zinc in newborn rats

The whole body retention, tissue distribution and protein binding patterns of 65Zn were compared with 109Cd in newborn rats during postnatal development. One-day-old pups received a single injection of either 65Zn (2.5 microCi) or 109Cd (2.5 microCi plus 1 mg Cd/kg as CdCl2). During the 22 days of age, the whole body retention of 109Cd was higher than that for 65Zn. The biological half times were 466 and 46.3 days for 109Cd and 65Zn, respectively. There were marked differences in tissue deposition of these metals. Both liver and kidney accumulated more 109Cd than other tissues while the 65Zn showed a uniform distribution, with a gradual decrease in radioactivity with age. At the time of weaning, 109Cd had accumulated mainly in liver and kidney whereas, 65Zn was found predominantly in bone and skin. The specific binding of 109Cd to hepatic MT in newborn rats did not change with growth. Although a significant amount of 65Zn initially accumulated in the MT fractions in the liver, it was transferred gradually to high molecular weight protein fractions during development. The administration of these 2 metals had no effect on the body weight, liver weight and total hepatic zinc concentration. However, a significantly high content of MT and zinc in MT fractions was detected in the livers of Cd-treated rats at 22 days of age. The results show the transfer of the essential metal, zinc from hepatic MT to other proteins and the specific binding of cadmium, the non-essential metal to MT during postnatal development in rats.     

Involvement of intestinal calcium transporter 1 and metallothionein in cadmium accumulation in the liver and kidney of mice fed a low-calcium diet

Essential metals can affect the metabolism of nonessential metals. Calcium (Ca) is an essential mineral that is commonly lacking in the diet. When we fed 5-week-old male mice for 4 weeks on a purified diet containing 0.005% Ca (CaDF mice), the Ca concentration in the plasma, liver and kidneys did not decreased. Cd accumulation increased in the liver and kidneys of CaDF mice given 1mg/kg Cd orally each day for 5 days, but not in those given intraperitoneal injections of Cd or Cd-metallothionein (Cd-MT). The zinc (Zn) concentration increased significantly in the intestinal cytosol and plasma during the time the mice were fed the low-Ca diet, and expression of both MT-1 and ZnT-1 sharply increased with a similar time course. Intestinal mRNA expression of CaT1, a Ca transporter, was more than 10 times higher in CaDF mice than in controls, although expression of other transporters, including DMT1, decreased in CaDF mice. These results suggest that CaT1 may stimulate the intestinal absorption of Cd and Zn, and some Cd may be distributed to the kidneys along with MT induced by Zn.     

Gastrointestinal absorption of Cd-metallothionein and cadmium chloride in mice

CdCl₂ or Cd-metallothionein (MT) (6 g Cd with 2.25 Ci (83.25 KBq)¹⁰⁹Cd) was given orally to mice, which were sacrificed at 30 min and 2 h after intubation. Although¹⁰⁹Cd in Cd-MT was excreted rapidly into the urine, its absorption was found to be significantly less than that of CdCl₂. The poor absorption was due to a decrease of Cd-MT uptake into the intestine. Cadmium chloride taken up into the mucosa could stimulate MT synthesis even 30 min after its intubation. However, the percentage of MT-bound Cd in the Cd of intestinal supernatants was lower with CdCl₂ (62% at 30 min and 2 h) than with Cd-MT (78% and 84% at 30 min and 2 h, respectively). These results suggest that the transport mode of lumenal Cd-MT to mucosal cells is different from that of lumenal CdCl₂. Lumenal Cd-MT is probably internalized into intestinal cells in an intact form. Furthermore, the Cd-MT may pass through the basolateral membrane in this form. This hypothesis was supported by the different distributions of Cd in the liver and kidney after Cd-MT and CdCl₂ intubations.     

Fate of erythrocyte Cd-metallothionein in mice

Degradation of metallothionein (MT), which appears in erythrocytes following cadmium (Cd) administration, was investigated in mice. Cd-MT underwent only slight decomposition by hemolysate in an in vitro experiment unlike an 800g supernatant fraction of the liver homogenate. In an in vivo study, [3H]diisopropylfluorophosphate was given to mice which had received 109CdCl2 to investigate the relationship between the decay of 109Cd-MT in the erythrocyte and the life span of the erythrocyte. A similar reduction pattern of radioactivity of 109Cd and 3H was observed. Erythrocytes containing 109Cd-MT obtained from mice preadministered with 109CdCl2 was transfused to normal mice. The 109Cd radioactivity of erythrocytes decreased in a manner similar to Cd in erythrocytes of 109CdCl2-administered mice. Contrary to this decrease of erythrocyte Cd in the transfused mice, Cd concentration of the spleen increased markedly. Cd increased also in the liver. These results indicate that erythrocyte MT degrades along with the erythrocyte. The Cd from this MT is deposited in the spleen and liver where blood cells are catabolized.     

Dietary iron lowers the intestinal uptake of cadmium-metallothionein in rats.

It has been shown that addition of extra calcium/phosphorus (Ca/P), zinc (Zn) and iron (Fe2+) to the diet results in a significant protection against cadmium (Cd) accumulation and toxicity in rats fed inorganic Cd salt. However, it is not clear whether the presence of these mineral supplements in the diet also protects against the Cd uptake from cadmium-metallothionein. The present study examines the influence of Ca/P, Zn and Fe2+ on the Cd disposition in rats fed diets containing either 1.5 and 8 mg Cd/kg diet as cadmium-metallothionein (CdMt) or as cadmium chloride (CdCl2) for 4 weeks. The feeding of Cd resulted in a dose-dependent increase of Cd in intestine, liver and kidneys. The total Cd uptake in liver and kidneys after exposure to CdMt was lower than after exposure to CdCl2. At the low dietary Cd level and after addition of the mineral supplement, the kidney/liver concentration ratio increased. However, this ratio was always higher with CdMt than with CdCl2, suggesting a selective renal disposition of dietary CdMt. The uptake of Cd from CdCl2 as well as from CdMt was significantly decreased by the presence of a combined mineral supplement of Ca/P, Zn and Fe2+. The protection which could be achieved was 72 and 75% for CdMt and 85 and 92% for CdCl2 after doses of 1.5 mg/kg and 8 mg/kg respectively. In a following experiment it was shown that the protective effect of the mineral mixture against CdMt was mainly due to the presence of Fe2+. It seems clear that Cd speciation and the mineral status of the diet have a considerable impact on the extent of Cd uptake in rats.