Toxicity of cadmium oxide instilled into the rat lung. I. Metabolism of cadmium oxide in the lung and its effects on essential elements

The metabolism of cadmium oxide (CdO, insoluble form) and cadmium chloride (CdCl2, soluble form) instilled intratracheally into the rat lung was investigated. CdO might be solubilized rapidly in the lung and consequently pulmonary clearance rate of CdO was not so different from that of CdCl2. At a dose of 5 micrograms Cd/rat about 20% of the dose was translocated to the liver within 12 h, whereas gradual and consistent accumulation of Cd was observed in the kidney up to 7 days. Both pulmonary clearance and translocation of Cd to the liver were accelerated with the dose of instilled CdO, however, Cd accumulated in the kidney was proportional to the dose. Lung weight was increased by the instillation of CdO. Lung essential elements such as S, P, Mg, Zn and Mn were not affected in the inflammatory-reparative proliferative process, but Cu content of unit lung weight was slightly decreased.     

A comparative study of the effects of inhaled cadmium chloride and cadmium oxide: pulmonary response

The effects of aerosols of cadmium chloride (CdCl2) and cadmium oxide (CdO) on pulmonary biochemical function were compared. Rats and rabbits were exposed to 0.25, 0.45, or 4.5 mg Cd/m3 for 2 h. Pulmonary toxicity was determined histologically and biochemically. Cadmium chloride and CdO showed a deposition response that was linearly related to the chamber concentration. Both compounds caused multifocal, interstitial pneumonitis 72 h after exposure, but the CdO lesion was more severe with proliferation of fibrocytic-like cells as well as pneumocytes. Comparing the two Cd compounds at the highest concentration (4.5 mg Cd/m3), the biochemical responses in the rat were similar. The majority of the effects occurred 72 h after exposure, with significant increases in lung weight, lung-to-body weight ratio, GSH reductase, GSH transferase, and G-6-PDH. However, GSH peroxidase was inhibited immediately after the CdO exposure. Cadmium oxide-related alterations in the parameters studied could easily be distinguished from those of CdCl2 at the exposure concentration of 0.45 mg Cd/m3. The response pattern in the rabbit resembled that of the rat. In both species Cd had a consistent inhibitory effect on pulmonary GSH peroxidase, even at the lowest concentration of 0.25 mg Cd/m3. Based on these findings, inhaled CdO appeared to be more toxic to the lung than inhaled CdCl2.     

Fate and toxic effects of inhaled ultrafine cadmium oxide particles in the rat lung

Female Fischer 344 rats were exposed to ultrafine cadmium oxide particles, generated by spark discharging, for 6 h at a concentration of 70 microg Cd/m(3) (1 x 10(6)/cm(3)) (40 nm modal diameter). Lung morphology and quantification of Cd content/concentration by inductively coupled plasma (ICP)-mass spectrometry were performed on days 0, 1, 4, and 7 after exposure. Cd content in the lung on day 0 was 0.53 +/- 0.12 microg/lung, corresponding to 19% of the estimated total inhaled cumulative dose, and the amount remained constant throughout the study. In the liver no significant increase of Cd content was found up to 4 days. A slight but statistically significant increase was observed in the liver on day 7. We found neither exposure-related morphological changes of lungs nor inflammatory responses in lavaged cells. Another group of rats were exposed to a higher concentration of ultrafine CdO particles (550 microg Cd/m(3) for 6 h, 51 nm modal diameter). The rats were sacrificed immediately and 1 day after exposure. The lavage study performed on day 0 showed an increase in the percentage of neutrophils. Multifocal alveolar inflammation was seen histologically on day 0 and day 1. Although the Cd content in the lung was comparable between day 0 and day 1 (3.9 microg/lung), significant elevation of Cd levels in the liver and kidneys was observed on both days. Two of 4 rats examined on day 0 showed elevation of blood cadmium, indicating systemic translocation of a fraction of deposited Cd from the lung in this group. These results and comparison with reported data using fine CdO particles indicate that inhalation of ultrafine CdO particles results in efficient deposition in the rat lung. With regard to the deposition dose, adverse health effects of ultrafine CdO and fine CdO appear to be comparable. Apparent systemic translocation of Cd took place only in animals exposed to a high concentration that induced lung injury.     

Cadmium toxicity in kidney cells. Resistance induced by short term pretreatment in vitro and in vivo

Epithelial cells from the kidney were freshly isolated from rats pretreated by daily subcutaneous doses of CdCl2 in vivo (0.5-2 mg Cd/kg X 5). Such cells were incubated in vitro in media with different concentrations of cadmium chloride (0-200 micrograms Cd/ml). There was no inhibition of cell growth in such cells. However, in cells isolated from non-treated rats, in vitro exposure to the same concentrations of CdCl2 caused a dose dependent decrease in viability. When cells, isolated from non-treated rats were pretreated in vitro with CdCl2 (10 micrograms/ml) and subsequently exposed to cadmium chloride (0-200 micrograms/ml), a protective effect was observed, which was similar to the one observed in cells isolated from animals pretreated with CdCl2. The concentration of metallothionein in the cells treated with cadmium was increased. A lower uptake of cadmium chloride, in vitro has been observed in kidney cells pretreated in vivo or in vitro compared to nonpretreated cells. Subcellular distribution studies indicate that Cd-distribution was similar in pretreated and non-pretreated cells, but concentrations were generally lower in the pretreated cells. The decreased uptake of Cd by pretreated kidney cells is a sign of Cd-interference with cellular function. These changes are suggested as a contributing mechanism to the prevention of acute toxic effects of cadmium on the kidney.     

Biochemical changes in the rat lung and liver following intratracheal instillation of cadmium oxide

Acute biochemical changes in the rat lung and liver following intratracheal instillation of cadmium oxide (CdO) were observed at a dose of 5 micrograms Cd/rat to investigate the defense mechanism to Cd intoxication via airway. In the lung metallothionein (MT) was induced, reaching a maximum at 2 days. A slight increase in reduced glutathione (GSH) concentration was observed at 4 days. The activity of glucose-6-phosphate dehydrogenase (G6PDH) was increased and superoxide dismutase (SOD) activity was slightly decreased, but glutathione peroxidase (GPx) and glutathione reductase (GR) activities were not changed. These observations suggested that MT played a key role in detoxification of instilled CdO, but that the antioxidant enzymes had a minimal role. In the liver MT and GSH concentrations were diminished 7 h after instillation and returned to their control levels. Hepatic GPx activity was increased 1 day after instillation and the significantly elevated level lasted up to 7 days, while hepatic GR activity was decreased. These hepatic biochemical changes are suggested to be due to the secondary effects of the lung injury.     

Stimulation of rat alveolar macrophage fibronectin release in a cadmium chloride model of lung injury and fibrosis.

Rats were exposed to saline or cadmium chloride (CdCl2) at 25, 100, or 400 micrograms/kg body weight by intratracheal instillation. At 3, 7, 14, and 28 days after exposure five animals/treatment were euthanized, the lungs were lavaged, and bronchoalveolar lavage fluid (BALF) was analyzed for lactate dehydrogenase (LDH), total protein, N-acetylglucosamindase (NAG), and cell number, type, and viability. Lung hydroxyproline concentration was characterized as a marker of lung collagen. Alveolar macrophages (AM) obtained in BALF were cultured and the release of fibronectin and TNF was determined. Lung tissue was examined microscopically at 28 and 90 days after exposure. Exposure to CdCl2 resulted in lung injury and inflammation demonstrated by increases in BALF LDH, total protein, NAG, and inflammatory cells. AM TNF release was not significantly changed by CdCl2 treatment. All doses of CdCl2 stimulated AM fibronectin secretion, a response which persisted throughout the 28-day postexposure period examined. Pulmonary fibrosis was demonstrated biochemically and/or histologically (trichrome staining tissue) at all CdCl2 dose levels. The association of CdCl2-induced AM fibronectin release with lung fibrosis confirms and extends previous observations relating AM-derived fibronectin to the development of interstitial lung disease and provides further evidence that the persistent increase in AM fibronectin release represents an early indicator of fibrosis.     

Distribution, localization, and pulmonary effects of yttrium chloride following intratracheal instillation into the rat

Metabolic behavior and pulmonary toxicity of yttrium chloride (YCl3) deposited in the lung was investigated. Yttrium chloride was instilled intratracheally into rats and the time-course and dose-related changes in distribution of Y between lung tissue and bronchoalveolar lavage fluid (BALF) and pulmonary inflammatory responses were investigated. Pulmonary clearance of Y was very slow and the half-life was estimated to be 168 days. Yttrium content in the supernatant of BALF did not exceed 5 micrograms Y/lung even when a dose of 200 micrograms Y/rat was administered, suggesting that the alveolar surface fluid could retain at most 5 micrograms Y. On the other hand, Y content in the pellet of BALF changed with the number of macrophages retrieved in BALF in both time-course and dose-response experiments. Transmission electron microscopy and X-ray microanalysis suggested that Y was localized in lysosomes of alveolar and interstitial macrophages, and basement membranes. These results clearly explain the long pulmonary half-life of Y. beta-Glucuronidase activity and calcium and phosphorous contents in the supernatant of BALF increased significantly even at the lowest dose (10 micrograms Y/rat). Comparative dose-effect profiles of lactate dehydrogenase activity in BALF supernatant revealed that 1 mol of YCl3 is equivalent to about one-third mole of cadmium compounds and about 3 mol of zinc oxide in the potency for acute pulmonary toxicity.     

Effects of particulate and soluble cadmium species on biochemical and functional parameters in cultured murine macrophages

Cultured murine macrophages (RAW 264.7) were used to evaluate the temporal relationships between cytotoxicity, phagocytosis, tumor necrosis factor-alpha (TNF-alpha), and nitric oxide (NO) production, and alterations in expression of stress proteins after exposure to cadmium oxide (CdO) or cadmium chloride (CdCl(2)), particulate and soluble forms of cadmium, respectively. Macrophages were exposed in vitro to CdO (25 or 50 microg) or CdCl(2) (30 or 40 microM) for 2 to 72 h. Cytotoxicity was not evident until 18 h when exposed to 30 microM CdCl(2) or 25 microg CdO, but occurred as early as 12 h after exposure to 40 microM CdCl(2) or 50 microg CdO. Relative to untreated controls, phagocytic activity decreased progressively from 2 to 24 h after exposure to both forms of cadmium. TNF-alpha levels increased to 2- to 3-fold after 4 h and remained elevated until 24 h after exposure to 25 and 50 microg CdO and 30 microM CdCl(2), but decreased by 18-24 h at 40 microM CdCl(2). CdCl(2) or CdO alone did not induce NO; however, both cadmium species reduced lipopolysaccharide (LPS)-stimulated NO production in a dose-dependent manner. Enhanced de novo synthesis of 70- and 90-kD heat shock, or stress, proteins was observed 2 to 8 h after exposure to both CdCl(2) and CdO; however, synthesis of these proteins returned to control levels by 24 h. Stress protein synthesis was enhanced by CdCl(2) or CdO prior to cytotoxicity, but coincided with a decrease in phagocytic capacity and an increase in TNF-a levels. The data suggest that cultured macrophages respond similarly in vitro to a particulate form and a soluble form of cadmium in a cell type that plays a pivotal role in inflammatory and immune responses.     

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.     

Toxic effects of cadmium on LHRH-induced LH release and ovulation in rats.

CFY rats were given 5 or 10 mg/kg bw cadmium chloride (CdCl2) or 0.9% NaCl solution (1 mL/kg) subcutaneously on the day of diestrus II. Six days later (proestrus) at 1200 h they were anesthetized with pentobarbital, 0.5 or 2 micrograms/kg LHRH was injected intravenously at 1400 h, and blood was collected for LH determination. A second group of animals pretreated with 10 mg/kg bw CdCl2 and treated with 2 or 4 micrograms/kg LHRH was allowed to recover from the anesthesia and checked for ovulation the next day (estrus). In rats treated with 10 mg/kg of CdCl2, the LH content of pituitary gland diminished, but no significant difference was found in the LH response to LHRH. In controls (ovulation blocked by anesthesia) 2 as well as 4 micrograms/kg of LHRH completely restored ovulation, while after Cd pretreatment, ovulation recovered depending on the dose of LHRH. It is concluded that Cd-induced anovulation is related to altered function of the pituitary gland and ovary, which can be restored by excess LHRH.     

Response of rat hepatocyte cultures to cadmium chloride and cadmium-diethyldithiocarbamate

Cellular effects of cadmium (Cd) were studied in primary cultures of rat hepatocytes incubated with cadmium chloride (CdCl2) or cadmium-diethyldithiocarbamate (Cd(DTC)2), labelled with 109Cd. The lipid-soluble complex Cd(DTC)2 was rapidly taken up into the cells and a maximal concentration was reached after 4 h incubation. On the other hand, incubation with CdCl2 resulted in a slow, continuous accumulation of Cd for up to 20 h. Cd was found to be associated with proteins to a higher extent when added to the incubation medium as CdCl2 than when added as Cd(DTC)2, which in addition to differences in lipophilicity of the Cd compounds partly explains the differences in Cd uptake. Subcellular distribution studies showed that a significantly higher proportion of Cd was associated with the total particulates fraction in cells after incubation with Cd(DTC)2 compared to CdCl2 (32 and 19%, respectively). The activities of glutathione reductase and succinic dehydrogenase were inhibited to a similar extent by the 2 Cd compounds. Alcohol dehydrogenase was more strongly affected by CdCl2 than by Cd(DTC)2, although the uptake of Cd was 3-4 times higher in cells incubated with Cd(DTC)2 than in those incubated with CdCl2. The results from the present study show that DTC can increase the transport of Cd into the cell by complex formation with Cd. Compared to CdCl2 the Cd(DTC)2 complex was less toxic as indicated by the biochemical parameters used.     

Relationship between toxicity and cadmium accumulation in rats given low amounts of cadmium chloride or cadmium-polluted rice for 22 months

To clarify toxic effects of long-term oral administration of low dose cadmium (Cd) on the liver and kidney, six groups of female Sprague-Dawley rats were fed a diet containing Cd-polluted rice or CdCl2 at concentrations up to 40 ppm, and killed after 12, 18, and 22 months. With toxicological parameters, including histopathology, there was no evidence of Cd-related hepato-renal toxicity, despite a slight decrease of mean corpuscular volume and mean corpuscular hemoglobin of red blood cells with 40 ppm CdCl2. Dose-dependent accumulation of Cd was observed in the liver and kidneys with peak levels of 130 +/- 42 micrograms/g and 120 +/- 20 micrograms/g, respectively, at 18 months in animals treated with 40 ppm CdCl2. A dose-dependent increase in urinary Cd levels became evident with time. Induction of metallothionein (MT) was also observed in the liver and kidney with a high correlation to the corresponding Cd levels. In the proximal renal tubular epithelia of 40 ppm CdCl2-treated rats at 22 months, prominent accumulation of Cd was observed in secondary lysosomes associated with MT deposits in their exocytotic residual bodies. The results demonstrated that, in contrast to the case with high-dose Cd-administration, renal toxicity is not induced by long-term oral administration of low amounts of Cd, although tissue accumulation does occur. Possible protective mechanisms may be operating.     

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.     

Augmented depression and reduced excitability of the central nervous system (CNS) by cadmium in the rat

Reports indicating that low doses of cadmium caused vasodilation, but that larger quantities elicited a pressor response, apparently mediated by a CNS reflex, prompted an examination of cadmium-induced changes in CNS responsiveness and activity. Rats were injected intraperitoneally with either 2 mg/kg or 4 mg/kg of CdCl2 solution, after which the CNS was either depressed by pentobarbital or excited by strychnine at different dose levels. Cadmium treatment, administered before pentobarbital, decreased the time required for sleep induction and prolonged sleep duration at doses of either 20 mg/kg or 30 mg/kg: at 40 mg/kg only induction was affected and at 60 mg/kg neither was influenced. At a dosage of 60 micrograms/kg, strychnine caused convulsions in all control animals, but in none pretreated with CdCl2. When either 75 or 120 micrograms/kg of strychnine was used, cadmium at either dosage failed to prevent convulsions, although the onset was delayed and duration curtailed. The rapidity with which Cd modified CNS activity indicated that the effect can not depend upon cadmium-induced synthesis of metallothionine, but represents a direct effect of Cd on the CNS. Cadmium treatment did not substantially improve the survival of rats that convulsed when treated with strychnine.     

Efficacy of diphenyl diselenide against cerebral and pulmonary damage induced by cadmium in mice

This study was designed to examine if diphenyl diselenide (PhSe)(2), an organoselenium compound, attenuates pulmonar and cerebral oxidative stress caused by sub-chronic exposure to CdCl(2). Male adult Swiss albino mice received CdCl(2) (10 micromol/kg, subcutaneously), 5 times/week, for 4 weeks. (PhSe)(2) (10 micromol/kg or 20 micromol/kg, orally) was given concomitantly with CdCl(2) to mice. A number of toxicological parameters in lung and brain of mice were examined including delta-aminolevulinic acid dehydratase (delta-ALA-D), superoxide dismutase (SOD) and catalase activities, lipid peroxidation, non-protein thiols (NPSH) and ascorbic acid content. Na(+),K(+)-ATPase activity, acetylcholinesterase (AChE) activity, [(3)H]glutamate uptake and [(3)H]glutamate release were also carried out in brain. Cadmium concentration and histopathological analysis were carried out in lung tissue. (PhSe)(2) at the dose of 20 micromol/kg protected the inhibition of delta-ALA-D, SOD and CAT activities, the reduction of vitamin C content and the increase of lipid peroxidation levels caused by CdCl(2) in lungs. At 10 micromol/kg, (PhSe)(2) protected cerebral AChE and CAT activities inhibited by CdCl(2). There were no histopathological alterations in the lung of mice after CdCl(2) exposure. The pulmonary cadmium concentration was higher (2.8-fold) in the group exposed to CdCl(2) than in control mice. (PhSe)(2) at dose of 20 micromol/kg reduced cadmium concentration towards the control level. The results suggest that oral administration of (PhSe)(2) attenuated the oxidative damage induced by CdCl(2) in lung and brain of mice.     

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.     

Effect of N-benzyl-D-glucamine dithiocarbamate on distribution and excretion of cadmium in rats

The effect of sodium N-benzyl-D-glucamine dithiocarbamate (NBG-DTC) on the distribution and excretion of cadmium in rats exposed to cadmium and the chemical form and intestinal reabsorption of cadmium compound excreted in the bile were studied. Rats were injected intraperitoneally with 109CdCl2 (1 mg Cd and 10 microCi 109Cd/kg) and 24 h later, they were injected with NBG-DTC (400 or 1200 mumol/kg). The biliary excretion of cadmium was remarkably increased by intraperitoneally injection of NBG-DTC, while there was only a small increase in urinary excretion of cadmium. Such an enhancement effect of NBG-DTC on the biliary excretion of cadmium was much larger at the high dose (1200 mumol/kg) of NBG-DTC. The treatment with NBG-DTC significantly decreased the cadmium content in the liver at the dose of 1200 mumol/kg and did not result in the undesirable redistribution of cadmium to the tissues, such as brain, testes, heart and lung. In addition, it was found that the cadmium compound excreted in the bile was mainly characterized as cadmium-NBG-DTC and which was not reabsorbed from the intestinal tracts.     

Induction of metallothionein by cadmium-metallothionein in rat liver: a proposed mechanism.

Distribution of Cd to various organs following iv administration of CdCl2 (3.5 mg Cd/kg) resulted in more than 43% of total tissue Cd accumulating in the liver. In contrast, after CdMT administration (0.5 mg Cd/kg), only 1% of the Cd was found in liver. Rats administered CdCl2 (1.0 mg Cd/kg) had hepatic MT values 30-fold greater than controls and a hepatic Cd concentration of 17 micrograms/g. In comparison, rats treated with CdMT (0.4 mg Cd/kg) had hepatic MT concentrations 7-fold greater than controls and a hepatic Cd concentration of 0.80 micrograms/g. However, when hepatic MT levels were normalized to tissue Cd concentrations, induction of MT by CdMT was 5-fold greater than by CdCl2. Northern and slot-blot analyses of mRNA showed that both CdCl2 and CdMT coordinately increased MT mRNA. These data suggest that both CdMT and CdCl2 increase hepatic MT by similar mechanisms. A dose-response increase in MT produced by CdCl2 indicated a biphasic response, with low doses producing relatively more hepatic MT than higher doses. In addition, the amount of MT produced per unit Cd after CdMT treatment was similar to those observed after low doses of CdCl2 in the dose-response experiment. These data provide strong evidence to support the conclusion that the apparent potency of CdMT observed here and in previous studies is most likely due to the small amount of Cd distributed to the liver, which is relatively more effective in inducing MT than are higher concentrations.     

Cadmium accumulation and metallothionein concentrations after 4-week dietary exposure to cadmium chloride or cadmium-metallothionein in rats.

The distribution of cadmium was examined in rats fed diets containing either cadmium-metallothionein (CdMt) or cadmium chloride (CdCl2) for 4 weeks. The test diets contained 3, 10, or 30 mg Cd/kg diet (3, 10, or 30 ppm) as CdMt or 30 mg Cd/kg diet (30 ppm) as CdCl2. A second study was performed to establish the Cd content in liver and kidneys after exposure to low doses of both CdMt and CdCl2 (1.5 and 8 ppm Cd). The feeding of CdMt resulted in a dose- and time-dependent increase of the Cd concentration in liver, kidneys, and intestinal mucosa. Rats fed 30 ppm CdMt consistently showed less Cd accumulation in liver and intestinal mucosa than did rats fed 30 ppm CdCl2. However, renal accumulation in rats fed 30 ppm was similar until Day 28 regardless of Cd form. At lower dietary Cd levels (1.5 and 8 ppm), relatively more Cd is deposited in the kidneys, although even at these doses the kidney/liver ratio of Cd is still higher with CdMt than with CdCl2. Tissue metallothionein (Mt) levels in the intestinal mucosa were relatively constant but always higher after CdCl2 exposure than after CdMt exposure. Mt levels in both liver and kidney increased after CdCl2 or CdMt exposure during the course of study. Although Mt levels in liver were higher after CdCl2 intake (30 ppm) than after CdMt intake (30 ppm), renal Mt concentrations were the same for both groups. In fact on Day 7, CdMt administration resulted in slightly higher Mt levels than CdCl2 administration, suggesting a direct accumulation of exogenous CdMt in the kidneys. In conclusion, after oral exposure to CdMt in the diet there is a relatively higher Cd accumulation in the kidneys. However, the indirect renal accumulation via redistribution of Cd from the liver might be lower than after CdCl2 exposure. Which of these two phenomena is decisive in the eventual level of renal toxicity of Cd after long-term oral intake could determine the toxicological risk of the chronic intake of biologically incorporated Cd.     

Effects of detergent formula chelating agents on the metabolism and toxicity of cadmium in mice.

Chelating agents like NTA (nitrilotriacetic acid) STPP (sodiumtripolyphosphate, Na5P3O10) and EDTA (ethylenediaminetetraacetic acid) are used as components of detergents. An increased toxicity of some metal compounds when combined with NTA has led to decreased use of this chelating agent in relation to STPP. In the present studies short-term and long-term effects of these chelating agents on cadmium toxicity in mice were investigated. I: In the short-term study, mice subcutaneously exposed to CdCl2 (3.2 mg Cd/kg b. wt.) in combination with STPP (32 mg/kg b. wt.) demonstrated a markedly higher mortality compared to animals given CdCl2 alone. This increase in mortality was similar to the one encountered when CdCl2 (3.2 mg Cd/kg b. wt.) and NTA (32 mg/kg b. wt.) were combined. Animals exposed subcutaneously to CdCl2 + STPP or CdCl2 + NTA showed histological evidence of liver necrosis 24 hrs after exposure not seen in animals given the same dose of CdCl2 alone and also had markedly lower cadmium concentrations in the livers compared to only Cd-exposed animals. II: In the long-term study, mice were exposed orally to CdSO4 (50p.p.m Cd) alone or in combination with STPP (500 p.p.m.), NTA (500 p.p.m.) or EDTA (50 p.p.m.) by continuous administration via the drinking water for 18 months. A decreased total excretion of urine proteins was seen in all Cd-treated animals irrespectively of the combination with various chelating agents. The conclusion of the present work was the NTA and STPP given by subcutaneous injection to mice markedly increased the toxicity of cadmium but that neither NTA, STPP nor EDTA given orally altered the toxicity of cadmium during a period of long-term exposure of 18 months.