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

Glycine max (soy) based diet improves antioxidant defenses and prevents cell death in cadmium intoxicated lungs

Cadmium (Cd) is a toxic metal and an important environmental contaminant. We analyzed its effects on oligoelements, oxidative stress, cell death, Hsp expression and the histoarchitecture of rat lung under different diets, using animal models of subchronic cadmium intoxication. We found that Cd lung content augmented in intoxicated groups: Zn, Mn and Se levels showed modifications among the different diets, while Cu showed no differences. Lipoperoxidation was higher in both intoxicated groups. Expression of Nrf-2 and SOD-2 increased only in SoCd. GPx levels showed a trend to increase in Cd groups. CAT activity was higher in intoxicated groups, and it was higher in Soy groups vs. Casein. LDH activity in BAL increased in CasCd and decreased in both soy-fed groups. BAX/Bcl-2 semiquantitative ratio showed similar results than LDH activity, confirmed by Caspase 3 immunofluorescence. The histological analysis revealed an infiltration process in CasCd lungs, with increased connective tissue, fused alveoli and capillary fragility. Histoarchitectural changes were less severe in soy groups. Hsp27 expression increased in both intoxicated groups, while Hsp70 only augmented in SoCd. This show that a soy-diet has a positive impact upon oxidative unbalance, cell death and morphological changes induced by Cd and it could be a good alternative strategy against Cd exposure.

Toxic trace elements at gastrointestinal level

Many trace elements are considered essential [iron (Fe), zinc (Zn), copper (Cu)], whereas others may be harmful [lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As)], depending on their concentration and chemical form. In most cases, the diet is the main pathway by which they enter our organism. The presence of toxic trace elements in food has been known for a long time, and many of the food matrices that carry them have been identified. This has led to the appearance of legislation and recommendations concerning consumption. Given that the main route of exposure is oral, passage through the gastrointestinal tract plays a fundamental role in their entry into the organism, where they exert their toxic effect. Although the digestive system can be considered to be of crucial importance in their toxicity, in most cases we do not know the events that occur during the passage of these elements through the gastrointestinal tract and of ascertaining whether they may have some kind of toxic effect on it. The aim of this review is to summarize available information on this subject, concentrating on the toxic trace elements that are of greatest interest for organizations concerned with food safety and health: Pb, Cd, Hg and As.

Dietary Deficiency of Calcium and/or Iron, an Age-Related Risk Factor for Renal Accumulation of Cadmium in Mice

The major route of cadmium (Cd) intake by non-smokers is through food ingestion. Cd is a non-essential metal absorbed through one or more transporters of essential metal ions. Expression of these transporters is affected by nutritional status. To investigate the risk factors for Cd toxicity, the effects of deficiency of essential metals on hepatic and renal accumulation of Cd were studied in mice of different ages. Mice were administered a control diet or one of the essential metal-deficient diets, administered Cd by gavage for 6 weeks, and killed; then, Cd accumulation was evaluated. Iron deficiency (FeDF) or calcium deficiency (CaDF) resulted in remarkable increases in hepatic and renal Cd accumulation compared with control-diet mice and other essential metal-deficient mice. Cd accumulation in hepatic and renal tissue was increased significantly at all ages tested in FeDF and CaDF mice. Renal Cd concentrations were higher in 4-week-old mice than in 8- and 25-week-old mice. Increase in intestinal mRNA expression of calcium transporter (CaT)1, divalent metal ion transporter-1, and metallothionein (MT)1 was also higher in 4-week-old mice than in other mice. Renal accumulation of Cd showed strong correlation with intestinal mRNA expression of CaT1 and MT1. These data suggest that CaDF and FeDF at younger ages can be a risk factor for Cd toxicity.

Relationships between absorption efficiency of elements in mammals and chemical properties

Oral absorption efficiency is an important factor to consider in human risk assessment and varies widely between elements. Linking absorption efficiency to chemical properties facilitates the understanding of underlying processes and enables extrapolation across elements. In our study, oral absorption efficiency in humans was predicted for a number of elements based on their ionization energy and electronegativity. Data on oral absorption efficiency in humans were retrieved via a literature survey. A model was developed based on the assumption that ionic species readily react with biotic ligands. Accordingly, ionization energy was presumed to represent the reactivity and absorption of atoms in the gastrointestinal tract. The coefficients of the model were parameterized by fitting the quantitative relationship between absorption efficiency and ionization energy to data collected from well-standardized studies. Generally, absorption efficiency was strongly related to ionization energy, explaining 94% of the variability in absorption efficiency between elements reported by the International Commission on Radiological Protection (ICRP). In addition, the absorption efficiencies predicted based on ionization energy were within a factor of two of those given by the ICRP (ME = -0.05; RMSE = 0.31). However, the model is not applicable to alkaline metals and molybdenum because of the uniquely high solubility of their compounds or the flexible electron configuration of these elements. Approximately 56% of the variability in absorption efficiency between elements could be explained by electronegativity. These strong relationships between absorption efficiency and ionization energy and, to a lesser extent, electronegativity indicate potential for extrapolation across elements using atomic properties.

Heavy metals in raw cow and ewe milk from north-east Iran

The presence of toxic metals in milk may create significant health problems for the population. In this study, 1440 raw cow and ewe milk samples from 18 townships in north-east Iran were analysed in four different seasons. Lead, cadmium and mercury levels were determined using atomic absorption spectrophotometry. Mean concentration of lead, cadmium and mercury in cow milk samples was 12.9 ± 6.0, 0.3 ± 0.3 and 3.1 ± 0.3 ng g⁻¹, respectively, and in ewe milk samples, these mean values were 14.9 ± 7.8, 1.6 ± 1.2 and 3.1 ± 0.3 ng g⁻¹, respectively. Statistical analyses showed that lead and cadmium concentrations in ewe milk were significantly higher than in cow milk. Concentrations of these metals in ewe milk varied significantly with different seasons. As the concentrations did not exceed the safety limits, they could not pose a serious danger to public health.

Effect of Chlorella intake on Cadmium metabolism in rats

This study was performed to investigate the effect of chlorella on cadmium (Cd) toxicity in Cd- administered rats. Sixty male Sprague-Dawley rats (14 week-old) were blocked into 6 groups. Cadmium chloride was given at levels of 0 or 325 mg (Cd: 0, 160 ppm), and chlorella powder at levels of 0, 3 and 5%. Cadmium was accumulated in blood and tissues (liver, kidney and small intestine) in the Cd-exposed groups, while the accumulation of Cd was decreased in the Cd-exposed chlorella groups. Fecal and urinary Cd excretions were remarkably increased in Cd-exposed chlorella groups. Thus, cadmium retention ratio and absorption rate were decreased in the Cd exposed chlorella groups. Urinary and serum creatinine, and creatinine clearance were not changed in experimental animals. In addition, metallothionein (MT) synthesis in tissues was increased by Cd administration. The Cd-exposed chlorella groups indicated lower MT concentration compared to the Cd-exposed groups. Moreover, glomerular filtration rate (GFR) was not changed by dietary chlorella and Cd administration. According to the results above, this study could suggest that Cd toxicity can be alleviated by increasing Cd excretion through feces. Therefore, when exposed to Cd, chlorella is an appropriate source which counteracts heavy metal poisoning, to decrease the damage of tissues by decreasing cadmium absorption.

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.

Heavy metal accumulation and metallothionein concentration in the frog Rana ridibunda after exposure to chromium or a mixture of chromium and cadmium

The accumulation of two heavy metals (chromium (Cr) and cadmium (Cd)) in the liver, kidney and gut of Rana ridibunda exposed to Cr or to a mixture of Cr and Cd was investigated. The concentration of metallothioneins (MTs) in the same tissues was also studied. Both metals accumulated mainly in the kidney. Cr accumulation in the liver and gut was not affected by the presence of Cd. Furthermore, Cr concentration in the kidney was doubled when Cd was present. MT concentration did not increase after Cr treatment but it increased two- to six-fold over control values in mixture-exposed frogs, the highest value being observed in the gut. MTs in the gut could act as a barrier preventing ingested heavy metals from entering the blood stream. MT concentration correlated positively with Cd concentrations in both the liver and the gut of mixture-exposed animals.

Accumulation of dietary cadmium (Cd) in rabbit tissues and excretions: a comparison of lettuce amended with soluble Cd salt and lettuce with plant-incorporated Cd

Quantifying the transfer of Cd from foods to mammalian target organs is key to estimating the health risk from this exposure; however, the bioaccumulation of Cd from foods is modified by many dietary components. Studies of dietary Cd absorption would be simpler if it were known that Cd added to foods as a soluble salt was as bioavailable as Cd incorporated during growth of the food species. Rabbits were fed, for 16 d, fresh lettuce containing cadmium incorporated into the lettuce during growth or added to the lettuce before feeding, or lettuce with no Cd but soluble Cd administered to the animals by gavage. There was a marked positive relationship between increased Cd dose and its accumulation in kidney; the slopes for the gavage and added treatments were not clearly different from the incorporated treatment; liver data were highly variable. In a 10-wk study of Cd-incorporated and -amended lettuce diets, for the incorporated and control diets there was less Cd accumulation in the kidneys, but not liver, per unit cumulative dose, than for the amended diet. Cd accumulation in the small intestine and Cd concentration in feces, both per unit daily dose, were smaller for the incorporated than for the control and amended diets; Cd concentrations in bile, urine, and serum, per unit daily dose, were higher in the control diet than values in the amended diet, which were higher than the incorporated diet. These differences could not be accounted for by variation in Fe or Zn contents of the diets. Thus, data suggest that Cd-amended diets overestimate bioaccumulation in kidney, an important target organ, by up to one-third, and that studies of short duration are not adequate to evaluate Cd bioavailability from food.

A piglet model for studies of gastrointestinal uptake of cadmium in neonates

Newborns are believed to have a higher gastrointestinal uptake of inorganic cadmium (Cd) than adults. However, relevant models for investigations of absorption of dietary Cd in neonates are lacking. In the present study, piglets were exposed to 2 or 20 microg Cd/kg body weight (b wt) per day by repeated oral administrations of CdCl2 dissolved in deionized water or infant follow-up formula from days 0 to 10. Elevated and dose-dependent Cd retention in blood and tissues resulted from this low-dose Cd exposure, as determined at day 11. Follow-up formula reduced Cd uptake in comparison to deionized water. However, Cd distribution to the kidneys was higher when Cd was given in formula than in water. Metallothionein (MT) levels in liver, kidney and duodenum were six, three and two times higher, respectively compared to an un-exposed control. Differences in bioavailability and distribution of low-dose Cd given in various diets to newborns could be detected by the proposed piglet model.

Molecular handling of cadmium in transporting epithelia

Cadmium (Cd) is an industrial and environmental pollutant that affects adversely a number of organs in humans and other mammals, including the kidneys, liver, lungs, pancreas, testis, and placenta. The liver and kidneys, which are the primary organs involved in the elimination of systemic Cd, are especially sensitive to the toxic effects of Cd. Because Cd ions possess a high affinity for sulfhydryl groups and thiolate anions, the cellular and molecular mechanisms involved in the handling and toxicity of Cd in target organs can be defined largely by the molecular interactions that occur between Cd ions and various sulfhydryl-containing molecules that are present in both the intracellular and extracellular compartments. A great deal of scientific data have been collected over the years to better define the toxic effects of Cd in the primary target organs. Notwithstanding all of the new developments made and information gathered, it is surprising that very little is known about the cellular and molecular mechanisms involved in the uptake, retention, and elimination of Cd in target epithelial cells. Therefore, the primary purpose of this review is to summarize and put into perspective some of the more salient current findings, assertions, and hypotheses pertaining to the transport and handling of Cd in the epithelial cells of target organs. Particular attention has been placed on the molecular mechanisms involved in the absorption, retention, and secretion of Cd in small intestinal enterocytes, hepatocytes, and tubular epithelial cells lining both proximal and distal portions of the nephron. The purpose of this review is not only to provide a summary of published findings but also to provide speculations and testable hypotheses based on contemporary findings made in other areas of research, with the hope that they may promote and serve as the impetus for future investigations designed to define more precisely the cellular mechanisms involved in the transport and handling of Cd within the body.

Effects of cadmium intake on bone metabolism of mothers during pregnancy and lactation

Cadmium (Cd) is a heavy metal that exists ubiquitously in the environment, and it interacts with essential elements such as zinc, copper, iron, and calcium (Ca). Particularly, Cd interferes with Ca and vitamin D metabolism in bone kidney and intestine. The interaction between Cd and Ca in bone, intestine, and kidney may result in the disorder of bone metabolism. On the other hand, pregnancy and lactation are also important physiological factors affecting bone metabolism in the mother. Ca absorption is decreased by competition with Cd in the intestine, and more Ca is released from maternal bone and transferred to neonate by lactation. In the intestine, Cd uptake competes with Ca uptake. Cd causes a marked decrease in bone density compared to the normal decrease in bone mineral density during lactation. Lactation is an important factor contributing to the decrease in bone mineral density and Cd has an additive effect of decreasing bone metabolism of mother animal, although the Cd intake level is relatively low (approximately 3-14 microgCd/kg/day). The relationship among maternal Cd intake, renal function and bone metabolism and the interaction between Cd and Ca during lactation are reviewed herein, together with additional data obtained recently in our laboratory.

Interactions between cadmium and zinc in the organism

It is well known that many toxic effects of cadmium (Cd) action result from interactions with essential elements, including zinc (Zn). These interactions can take place at different stages of absorption, distribution in the organism and excretion of both metals and at the stage of Zn biological functions. Exposure to Cd leads to disturbance in Zn in the organism on the one hand, while dietary Zn intake has an important effect on Cd absorption, accumulation and toxicity on the other. The Zn status of the body is important in relation to development of Cd toxicity. Numerous data show that increased Zn supply may reduce Cd absorption and accumulation and prevent or reduce the adverse actions of Cd, whereas Zn deficiency can intensify Cd accumulation and toxicity. In this review, the interactions between these two trace elements in humans and animals are discussed on the basis of the available literature and our own results, against the background of general population exposure to Cd and common nutritional deficiency of Zn.

Metallothionein-null and wild-type mice show similar cadmium absorption and tissue distribution following oral cadmium administration

Cadmium (Cd) is an environmental pollutant and is toxic to many tissues. Food is the primary source of Cd exposure for the general population. Metallothionein (MT), a cysteine-rich, Cd-binding protein, plays an important role in Cd detoxication. However, the role of MT in Cd absorption and distribution is still controversial. For example, some reports assert that MT in the intestine decreases Cd absorption and increases its distribution to the kidney, relative to the liver. Therefore, to further clarify the role of MT in Cd absorption and tissue distribution, MT-I/II knockout (MT-null) mice and their parental background wild-type mice were given a single dose of (109)Cd (1-300 micromol/kg po or 0.1-30 micromol/kg iv). Cd content in 15 organs was determined 4 h after Cd administration by gamma scintillation spectrometry. Approximately 60% of the Cd administered iv was retained in liver, and about 5% was retained in kidney in both MT-null and wild-type mice. The distribution of iv administered Cd was independent of dose. In contrast, when administered po, approximately 0.15% of the lowest dose (1 micromol/kg) and 0.75% of the highest dose (300 micromol/kg) was detected in the liver of both MT-null and wild-type mice. Similarly in kidney, approximately 0.05% of the dose was detected after the lowest dose and about 0.15% after the higher doses in both MT-null and wild-type mice. In summary, this study demonstrates that the absorption and initial distribution of orally administered Cd is dose dependent but is not influenced by MT.

Cadmium in caribou (Rangifer tarandus) kidneys: speciation, effects of preparation and toxicokinetics

Caribou kidney is a major source of cadmium (Cd) in the traditional diets of many aboriginal communities in the Arctic. In order to characterize the risk of Cd exposure, we studied the speciation of Cd in caribou kidneys and how, it can be affected by food preparation. Cd in caribou kidneys was bound to metallothionein (MT) (40%) high molecular weight proteins (50-500 kDa) (30%) and existed as free ions (10%). There was no change in Cd concentrations after the samples were baked at 350 degrees C for 30 min but the MT level was significantly decreased. Food preparation also denatured the high molecular weight proteins, resulting in a significant increase of free Cd. The toxicokinetics of Cd in caribou kidney was studied using a rodent model. Four groups (n = 6) of adult Sprague-Dawley rats were fed with rat chow with the addition of one of the following: 10% veal kidney with 0 microg/g Cd, 10% veal kidney spiked with CdCl2 to make up to 66 microg Cd; 10% caribou kidney containing 20 microg/g Cd and 10% caribou kidney containing 66 microg/g Cd. After 14 days, Cd accumulation increased with dose but less than 1% of the dosed Cd accumulated in the liver and kidney of rats. The percentage of Cd accumulated in the liver was higher in the group fed with caribou kidney than that dosed with spiked veal kidney. Our results suggest that caribou kidney is a major source of Cd exposure among the Arctic populations but moderate consumption may be considered unavoidable.

Hepatic and renal metallothionein induction by an oral equimolar dose of zinc, cadmium or mercury in mice

The hepatic and the renal subcellular distribution of zinc, cadmium or mercury and induction of tissue metallothionein (MT) at 24, 48 and 72 h following an oral equimolar dose (15 micro;mol metal/kg) of zinc (II) chloride, cadmium (II) chloride or mercury (II) chloride in male albino mice were investigated. There was a moderate increase in hepatic and renal zinc levels mainly in their nuclear mitochondrial fraction (NMF) 24 h post zinc chloride administration. Subsequently, the hepatic zinc increased and the renal zinc declined with time. The zinc-induced hepatic MT level was maximum at 48 h, which decreased slightly thereafter, while there was no marked increase in renal MT level at any time interval. The cadmium was equally distributed in liver and kidney more in their supernatant cytosol fraction (SCF) than in their NMF at 24 h after a dose of cadmium chloride. The cadmium levels showed a decreasing trend in hepatic fractions and an increasing trend in renal fractions with time. The cadmium-induced hepatic and renal MT were substantial at 24 h post cadmium administration, the former decreased thereafter while the latter enhanced at 48 h before declining. The accumulation of mercury in kidney was 1.5 times that in liver, which was localised more in their SCF than in their NMF at 24 h in response to a dose of mercuric chloride. The mercury levels of hepatic and renal subcellular fractions started declining after 24 h and at 72 h they were significantly lower. The induction of hepatic and renal MT was maximum at 24 h after mercuric chloride administration, which declined thereafter concomitant with the decrease in their mercury levels. However, the MT levels in both the organs remained considerably higher than in normal animals at 72 h post exposure. The results show that the accumulation of metal in liver and kidney follows the order: Hg > Cd > Zn and the induction of MT follows Hg > Cd > Zn in liver and Cd > Hg > Zn in kidney. The alterations in zinc and copper homeostasis were more marked in liver than in kidney and follows the order: Hg > Cd > Zn.

Energetics in Atlantic salmon (Salmo salar L.) parr fed elevated dietary cadmium

Atlantic salmon (Salmo salar L.) parr were reared for 4 months on experimental diets supplemented with Cd (0.5, 5, 25, 125, or 250 mg Cd x kg(-1)) to assess the long-term energetic changes based on the digestibility and biochemical deposition of the major dietary nutrients and to evaluate a maximum tolerable dietary toxicant concentration. Growth did not differ significantly (P>0.05) from the control groups. The biochemical composition of the carcass, but not the viscera, was negatively affected by dietary Cd exposure. The significant decreases in protein, lipid, and glycogen concentrations in the carcass (P<0.05, 25 mg x kg(-1) compared to control groups) caused a reduction in calculated whole-body energy content in fish fed 125 mg x kg(-1)compared to control groups. This reduction in calculated whole-body energy content was explained by a concurrent significant disturbance to the gastrointestinal function (measured as reduced digestibility). Only at the highest dietary Cd exposure (250 mg x kg(-1)), increased metabolic costs to cope with Cd toxicity was thought to contribute significantly to the reduction in carcass energy content. The most important factor effecting calculated total energetics was nutrient digestibility. Based on the logarithmic effective median concentration for reduced calculated energy digestibility (dietary Cd concentration corresponding to 50% reduction, EC50), the maximum tolerable dietary Cd concentration is 11 mg x kg(-1) diet.

Tissue distribution of cadmium in rats given minimum amounts of cadmium-polluted rice or cadmium chloride for 8 months

To investigate the relationship between cadmium (Cd) toxicity, intestinal absorption, and its distribution to various tissues in rats treated orally with minimum amounts of Cd, 14 female rats per dose group per time point were given diets consisting of 28% purified diet and 72% ordinary rice containing Cd-polluted rice (0. 02, 0.04, 0.12, or 1.01 ppm of Cd) or CdCl(2) (5.08, 19.8, or 40.0 ppm of Cd) for up to 8 months. At 1, 4, and 8 months after the commencement of Cd treatment, seven rats per group were euthanized for pathological examinations to determine the Cd concentrations in the liver and kidneys and metallothionein (MT) in the liver, kidneys, intestinal mucosa, serum, and urine. One week before each period of 1, 4, and 8 months, the remaining seven rats in each group were administered a single dosage of (109)Cd, a tracer, to match the amounts of the designated Cd doses (about 1.2 to 2400 microg/kg body wt). They were euthanized 5 days later to determine the distribution of Cd to various tissues. No Cd-related toxic changes were observed. The concentrations of Cd in the liver and kidneys at any time point and MT in the liver, kidney, serum, and urine at 4 and 8 months increased dose-dependently, whereas MT in the intestinal mucosa did not alter markedly at any time point. The distribution rates of Cd to the liver increased dose-dependently (40% at lower doses to 60% at higher doses), whereas those to the kidney decreased dose-dependently (20% at lower doses to 10% at higher doses). The Cd retention rates 5 days after (109)Cd administration (amounts of Cd in various tissues/amounts of Cd administered) ranged from 0.2 to 1. 0% at any time point. These results suggest that the distribution of Cd to the liver and kidneys after the oral administration vary depending on the dosage levels of Cd. The difference of the distribution pattern of Cd to the liver and kidney is probably due to the difference in the form of the absorbed Cd, i.e., free ion or Cd-MT complex, although not closely related to the MT in the intestinal mucosa.

Cadmium uptake and transepithelial transport in control and long-term exposed Caco-2 cells: the role of metallothionein

Exposure of humans to cadmium, a common environmental pollutant, is mainly through food intake. However, the mechanisms of intestinal absorption have not been clearly elucidated for this toxic metal ion. In order to investigate the effects of long-term exposure to this metal and the role of metallothioneins in cadmium absorption, we used human-derived Caco-2 cells cultured on porous membrane filters. We first validated this model by quantifying metal uptake and transepithelial transport on control cells and cells adapted to grow for 2 to 5 weeks in the presence of low doses of cadmium in the culture medium. The nontoxic doses of cadmium (0.1, 1.0, and 5 microM), in which Caco-2 cells could be cultured for many passages without deleterious effects, were determined by evaluating transepithelial resistance of the cells and lactate dehydrogenase leakage. After 24 h of 1 microM Cd exposure, intracellular cadmium levels were 3- and 6-fold higher for cells exposed for extended periods to 1 and 5 microM cadmium, respectively, compared to control cells. In control and long-term exposed cells, this accumulation was inhibited by zinc, copper, and pCMBS, but not by verapamil or ouabain. Intracellular metallothionein content was increased 1.5-, 5-, and 12-fold for the cells grown in the presence of 0.1, 1.0, and 5 microM cadmium, respectively, in the culture medium. The amount of metallothionein synthesized and released by the cells was highly correlated with cadmium accumulation and transport. Our results suggest that Caco-2 cell monolayers are a good predictive model for the study of cadmium intestinal absorption following exposure to repeated low doses of cadmium, and confirm the essential role of metallothionein in the regulation of cadmium intestinal absorption.

The influence of humic substances on the absorption and distribution of cadmium in mice

The complex binding of cadmium ions to humic and fulvic acids in water may influence the absorption and distribution of drinking-water Cd in humans. Thus, in the present study mice were given a single oral dose of Cd (109CdCl2, 25 microg/l) in 100 microl Millipore water containing different concentrations of humic compounds (0, 1, 10 and 100 mg dissolved organic carbon/l). The complex binding of Cd was studied by dialysis. At neutral pH, 1 mg dissolved organic carbon/l caused complex binding of more than 50% of the Cd, whereas more than 90% of Cd was bound at 10 and 100 mg dissolved organic carbon/l. At pH 3 the complex binding of Cd decreased somewhat, but over 90% of the Cd was bound at 100 mg dissolved organic carbon/l. Complex binding of Cd increased the lipid solubility of Cd, expressed as an octanol/ water partition coefficient, Nevertheless, more than 99% of the bound Cd was present as hydrophilic binding forms. Irrespective of the bound of Cd, the intestinal uptake and intracellular distribution (gel filtration on Sephadex G-75 column) were not affected by the humic substances 6 hr after dosage. Moreover, complex binding did not influence the intestinal absorption of Cd 24 hr after exposure. The median Cd retention in the kidneys of the 100 mg dissolved organic carbon/l group was 23% and 46% lower than that of the control group 6 and 24 hr after administration, respectively, indicating alterations in the distribution of Cd after absorption. Thus humic substances may affect the metabolism of toxic heavy metals, such as Cd, in vivo in mice, indicating that the presence of humic and fulvic acids in drinking water should be considered in future risk assessments of metals in drinking water.

The effect of combined administration of cadmium and furosemide on auditory function in the rat

A number of heavy metals have been associated with toxic effects to the peripheral or central auditory system. These include lead, arsenic, mercury, platinum and organic tin compounds. In addition, the ototoxic effects of some metals may be potentiated by other factors. However, the auditory effects of cadmium have not previously been reported. The purpose of the present study was to investigate the potential ototoxic effects of cadmium from an acute dosage, and its potentiation by furosemide. Auditory brainstem response (ABR) thresholds were measured in adult Sprague-Dawley rats. Rats received either cadmium chloride (5 mg/kg, i.p.) followed by saline (4 ml/kg, i.p.). cadmium chloride followed by furosemide (200 mg/kg, i.p.), or furosemide alone. Follow-up ABRs were carried out 7 days post-treatment and threshold changes were compared between each treatment group. No significant threshold change was seen for the cadmium chloride plus saline treated or the furosemide treated animals. However, significant threshold elevations were observed in animals receiving cadmium chloride plus furosemide. In addition, scanning electron microscopy revealed extensive hair cell loss in animals treated with cadmium chloride and furosemide. Although functional auditory changes were not seen after the administration of cadmium alone, the potentiation of threshold changes by furosemide suggests that cadmium may be ototoxic under certain conditions.

Tissue accumulation of cadmium following oral administration to metallothionein-null mice

To verify the roles of intestinal metallothionein (MT) as a barrier against ingested cadmium (Cd) and as a transporter of mucosal Cd to the kidneys, the distribution of orally administered Cd was compared between normal and MT-I and -II knock-out (MT-null) mice. Following single administration of a low dose of Cd (0.1 mg/kg), hepatic Cd levels and the sum of total Cd in the liver and kidney (K + L) were significantly less in the controls than in MT-null mice. The ratio of Cd in the kidney to the liver (K/L) was significantly lower in the MT-null mice. On the other hand, at a high Cd dose (2.0 mg/kg), K + L and K/L were not significantly different between the two groups. However, following oral pretreatment with zinc (Zn) to the high dose control mice, K + L significantly decreased and K/L significantly increased. No such effects of Zn pretreatment were observed in MT-null mice. Similar differences in K + L and K/L were also observed between the control and MT-null mice groups following the Zn pretreatment. Repeated administration of Cd for 4 weeks resulted in significantly larger K/L distribution in control mice over null mice. These results suggest that MT in the intestinal mucosa functions both as a protective barrier against Cd absorption and as an extracellular transporter of Cd to the kidney.

Cadmium uptake by Caco-2 cells. Effect of some milk components

The effect of some milk components on the cellular uptake of cadmium has been studied using a human intestinal cell line (Caco-2). Cadmium uptake by Caco-2 cells increased with the concentration of this metal in the culture medium, in a saturable way. These cells were exposed to different concentrations of cadmium and the synthesis of metallothionein was studied by a cadmium-saturation method. The levels of metallothionein increased with the cadmium concentration in the medium up to 20 microM of metal. Supplementation of the culture medium with 10% bovine milk caused a 25% decrease in the uptake of cadmium with respect to that internalized by the cells maintained in the culture medium alone. However, the uptake of cadmium from the medium supplemented with 10% human milk was similar to that with serum-free medium. beta-Lactoglobulin interacted with cadmium when studied by equilibrium dialysis, showing a stoichiometric binding constant of 5 x 10(4) l/mol. Interaction of lactoferrin with cadmium, however, was negligible. When Caco-2 cells were incubated in culture medium containing lactoferrin, cadmium uptake decreased with respect to that observed incubating the cells in a medium containing beta-lactoglobulin or in the free-protein medium. The inhibitory effect of lactoferrin on the uptake of cadmium might be due to a reduction of the cell surface charge, through its binding to the membrane.

Investigations of the potential bioavailability of 210Po in some foodstuffs

Extraction of 210Po from lamb's liver, pig's kidneys, mussel flesh and brown crabmeat with a series of solvent systems has been used to gain some insight into the uncertainty about the gastrointestinal absorption factor for Po incorporated into foodstuffs. By extracting the tissues with diethyldithiocarbamate dissolved in chloroform, and also in methanol, it has been shown that 210Po is more effectively extracted from lamb's liver and crabmeat than it is from each of the other tissues. A similar pattern of extractability is also evident for aqueous solutions of citric acid on crabmeat and mussel flesh. Of particular note is the low extractability by dimethylsulphoxide (3%) of 210Po in mussel flesh as opposed to 24% extractability of 210Po from crabmeat. 210Po-binding macromolecules of about 10 kDa have been released from crabmeat and mussel flesh by digestion with pepsin.

Fulvic and humic acids decrease the absorption of cadmium in the rat intestine

Dissolved organic carbon (DOC) in drinking water is mainly composed of fulvic and humic acids, which may form complexes with metal ions. The influence of DOC on the intestinal absorption of Cd in the rat was investigated using an "isolated intestinal segment" technique in anaesthetised rats. The lumens of segments were exposed for 60 min to different concentrations of CdCl2 and DOC in intact animals. The fractional absorption (FA) was not dose dependent in the dosage interval 0.01-0.03 microgram Cd/kg. However, at 15 and 150 micrograms Cd/kg both FA and intracellular Cd distribution in the segments were dose dependent, which is in line with results from other studies performed on similar experimental models. In the presence of 1 and 10 mg DOC/l, FA of Cd was just half as high as FA in animals that received Cd alone (0.01 microgram/kg). Moreover, a higher percentage of Cd was associated with the metallothionein fraction in the intestinal segment of the DOC-dosed rats. An in vitro speciation experiment showed that only 0.2-7.9% of the Cd in the incubation solution was complexed to DOC. In deionized water, however, more than 99% of the Cd was complexed to DOC. This result indicates that the incubation solution contained substances that negatively affect complexation of Cd to DOC. Mechanisms other than complexation of Cd to DOC in the intestinal lumen may therefore be involved in the inhibitory effect of DOC on the absorption of Cd.

Oral and subcutaneous administration of cadmium chloride and the distribution of metallothionein and cadmium along the villus-crypt axis in rat jejunum

The route of Cd uptake influences the distribution of Cd, other metals, and metallothionein (MT). Although intestinal MT levels related to the tissue mass did not show proximodistal gradients after sc administration of CdCl2, orally administered high doses of CdCl2 increased mucosal MT levels longitudinally from the duodenum to the ileum. The gradient abolished when the mucosal MT level was related to the intestinal length. To further elucidate this finding, three groups of rats were studied: a control group, a group receiving dietary CdCl2, and a group receiving sc injections of CdCl2. The small intestine was removed after a 14-d treatment. Midjejunal segments were mounted in a cryomicrotome and cut transversally into five layers along the villus-crypt axis. Mucosal enzymes were measured to control these sections. Cd was measured by AAS and MT by RIA. Alkaline phosphatase and lactase activities exhibited the typical villus-crypt gradient. Mucosal MT levels paralleled those of Cd. Although Cd and MT concentrations were high at the tip of the villi and low in the crypts after oral administration, sc treatment reversed that profile. A molar Cd-MT ratio of approx 10 or 1 was reached after po or sc treatment, respectively. This demonstrates that only oral Cd may lead to an accumulation of Cd in the mucosal tissue fairly exceeding the binding capacity of small intestinal MT. The results show that different routes of Cd intake lead to a different MT-induction pattern in the intestinal wall and that longitudinal Cd and MT concentration gradients in the small intestine observed after high oral doses are a result of their high levels at the villus tips.

Identification of the gastrointestinal absorption site for cadmium chloride in vivo

Most models for the study of the mechanism of intestinal absorption of Cadmium (Cd) have been using intestinal tissue in vitro or in situ. The in vivo experiments reported in this article were performed in an attempt to localize the site for gastrointestinal absorption of cadmium chloride during natural physiological conditions by oral exposure of mice to 109Cd-labelled CdCl2. Independent of exposure via drinking water or oral administration of a single dose, Cd was primarily deposited in the most proximal duodenum. Thus the present study as well as others indicate that absorption takes place in the proximal part of the intestine. Absorbed Cd is initially transported to the liver and deposited before being redistributed and accumulated in the kidneys. In this experimental model, dietary tetraethylthiuram disulfide exposure was shown to change the intestinal labelling profile and increase the whole-body retention as well as the intestinal deposition of Cd.

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.