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

Assessment of trace metals in four bird species from Korea

In birds, heavy metal concentrations are influenced by diet intake, migratory pattern, and residence time. In the present study, heavy metal concentrations (in microgram per gram dry weight) were measured in livers of four bird species from Korea. Iron concentrations were greater in Eurasian Woodcocks (Scolopax rusticola) than in Grey Herons (Ardea cinerea), Little Egrets (Egretta garzetta), and Schrenck's Bitterns (Ixobrychus eurhythmus). Copper concentrations in Grey Herons were significantly higher than in other species. Lead concentrations were greater in Schrenck's Bitterns and Eurasian Woodcocks than in Grey Herons and Little Egrets. Eurasian Woodcocks had higher cadmium concentrations than in other species. Zinc and manganese concentrations did not differ among species. Iron, zinc, manganese, and copper concentrations from this study were within the range of other Korean bird studies, and these concentrations were far below toxic levels. Cadmium and lead accumulation trends in each species were different, and the results might be associated with their migration pattern and residence time in Korea. Grey Herons, Little Egrets, and Schrenck's Bitterns are usually summer visitors, and Eurasian Woodcocks are passage migrants. But herons and egrets were collected in spring, autumn, and winter, but not during breeding season. They might be residents, so they could more reflect Korean cadmium and lead contaminations than Schrenck's Bitterns. However, Eurasian Woodcocks could more reflect habitats outside Korea because of their short staying time in Korea.

Effects of soil properties on food web accumulation of heavy metals to the wood mouse (Apodemus sylvaticus)

Effects of soil properties on the accumulation of metals to wood mice (Apodemus sylvaticus) were evaluated at two sites with different pH and organic matter content of the soil. pH and organic matter content significantly affected accumulation of Cd, Cu, Pb and Zn in earthworms and vegetation. For Cd, Cu and Zn these effects propagated through the food web to the wood mouse. Soil-to-kidney ratios differed between sites: Cd: 0.15 versus 3.52, Cu: 0.37 versus 1.30 and Zn: 0.33-0.83. This was confirmed in model calculations for Cd and Zn. Results indicate that total soil concentrations may be unsuitable indicators for risks that metals pose to wildlife. Furthermore, environmental managers may, unintentionally, change soil properties while taking specific environmental measures. In this way they may affect risks of metals to wildlife, even without changes in total soil concentrations.

A new index for evaluation of cadmium pollution in birds and mammals

The degree of cadmium (Cd) contamination in wildlife is often used as an indicator in the environmental monitoring of Cd poisoning. However, previous studies have not clarified the correlation between levels in wildlife and levels in the environment by comparing levels among different species of animals; therefore, assessing the level of pollution in this manner is not considered a reliably accurate indicator of levels in the environment. The aim of this study was to establish a new indicator for the non-polluted warm-blooded animals, one that is not species-dependent, which will facilitate using different species for Cd monitoring. First, the previous publications regarding Cd contents in wildlife, 27 reports in which Cd contents were represented as arithmetic means and described for both kidney and liver were selected. A regression line (CSRL) between Cd contents of kidney and that of liver was obtained in a high correlation coefficient (R (2) = 0.943, P < 0.01). The mean values from land and waterfowl, terrestrial mammals, seabirds, marine mammals, and non-polluted humans were located on the line and aligned in order. CSRL might allow an accurate determination of whether an animal is polluted by Cd. CSRL was confirmed using well-established and widely recognized pollution models such as Itai-itai patients and Cd-exposed experimental animals. The Cd contents from these models were located in different positions relative to CSRL depending on the level of contamination. Thus, this new indicator determining the Cd-pollution status of animals would be useful for environmental monitoring.

Coaccumulation of cadmium and zinc in tissues of sentinel mallards (Anas platyrhynchos) using a former dredge-disposal impoundment

Six- to eight-month-old female farm-raised mallards (Anas platyrhynchos) were used to examine the accumulation of and association among cadmium (Cd), zinc (Zn), and copper (Cu) from an impoundment constructed to contain sediments dredged from a lake contaminated by a zinc smelter. Cd was not detectable in the livers t of farm-raised mallards (day 0), although sentinel mallards accumulated hepatic Cd in the first 7 days after release. By day 14, mean concentrations of Cd in kidneys (= 2.82 mg/kg wet weight) had increased 3.4-fold. The mean pancreatic Cd concentration increased 59% between days 7 and 14. Renal Zn increased nominally, whereas pancreatic Zn increased 63% in sentinel ducks after release. Hepatic Zn increased significantly in the first week of release. Renal and pancreatic Cu concentrations did not change significantly, whereas concentrations of Cu in livers of ducks increased 50% in the 7 days after release before decreasing by nearly the same degree. Concentrations of Cd and Zn were correlated in livers of sentinel mallards on days 7 and 14. Cd and Cu were not correlated in the tissues of any cohort. Cu and Zn were correlated in the livers of farm-raised mallards, in the pancreases of sentinel mallards at day 7, and in the kidneys of the ducks in all three treatments. The relationship between Cd and Zn in tissues of ducks in our study was complicated by simultaneous exposure to increased and heterogeneous concentrations of Cd and Zn, both of which can induce metallothionein and compete for this and other ligands.

Safe levels of cadmium intake to prevent renal toxicity in human subjects

The present review attempts to provide an update of the scientific knowledge on the renal toxicity which occurs in human subjects as a result of chronic ingestion of low-level dietary Cd. It highlights important features of Cd toxicology and sources of uncertainty in the assessment of health risk due to dietary Cd. It also discusses potential mechanisms for increased susceptibility to Cd toxicity in individuals with diabetes. Exposure assessment on the basis of Cd levels in foodstuffs reveals that vegetables and cereals are the main sources of dietary Cd, although Cd is also found in meat, albeit to a lesser extent. Cd accumulates particularly in the kidney and liver, and hence offal contains relatively high amounts. Fish contains only small quantities of Cd, while crustaceans and molluscs may accumulate larger amounts from the aquatic environment. Data on Cd accumulation in human kidney and liver obtained from autopsy studies are presented, along with results of epidemiological studies showing the relationship between renal tubular dysfunction and kidney Cd burden. These findings suggest that a kidney Cd level of 50 μg/g wet weight is a maximum tolerable level in order to avoid abnormal kidney function. This renal Cd burden corresponds to a urinary Cd excretion of 2 μg/d. Accordingly, safe daily levels of Cd intake should be kept below 30 μg per person. Individual variations in Cd absorption and sensitivity to toxicity predicts that a dietary Cd intake of 30 μg/d may result in a slight renal dysfunction in about 1 % of the adult population. The previous guideline for a maximum recommended Cd intake of 1 μg/kg body weight per d is thus shown to be too high to ensure that renal dysfunction does not occur as a result of dietary Cd intake.

Kidney Dysfunction and Hypertension: Role for Cadmium, P450 and Heme Oxygenases?

Cadmium (Cd) is a metal toxin of continuing worldwide concern. Daily intake of Cd, albeit in small quantities, is associated with a number of adverse health effects which are attributable to distinct pathological changes in a variety of tissues and organs. In the present review, we focus on its renal tubular effects in people who have been exposed environmentally to Cd at levels below the provisional tolerable intake level set for the toxin. We highlight the data linking such low-level Cd intake with tubular injury, altered abundance of cytochromes P450 (CYPs) in the kidney and an expression of a hypertensive phenotype. We provide updated knowledge on renal and vascular effects of the eicosanoids 20-hydroxyeicosatetraenoic acid (20-HETE) and eicosatrienoic acids (EETs), which are biologically active metabolites from arachidonate metabolism mediated by certain CYPs in the kidney. We note the ability of Cd to elicit "oxidative stress" and to alter metal homeostasis notably of zinc which may lead to augmentation of the defense mechanisms involving induction of the antioxidant enzyme heme oxygenase-1 (HO-1) and the metal binding protein metallothionein (MT) in the kidney. We hypothesize that renal Cd accumulation triggers the host responses mediated by HO-1 and MT in an attempt to protect the kidney against injurious oxidative stress and to resist a rise in blood pressure levels. This hypothesis predicts that individuals with less active HO-1 (caused by the HO-1 genetic polymorphisms) are more likely to have renal injury and express a hypertensive phenotype following chronic ingestion of low-level Cd, compared with those having more active HO-1. Future analytical and molecular epidemiologic research should pave the way to the utility of induction of heme oxygenases together with dietary antioxidants in reducing the risk of kidney injury and hypertension in susceptible people.

A global perspective on cadmium pollution and toxicity in non-occupationally exposed population

Cadmium is a non-essential element that has high rates of soil to plant transference compared with other non-essential elements, and certain plant species accumulate large amounts of cadmium from low cadmium content soils. In this paper, levels of cadmium found in major food groups are highlighted together with cadmium levels found in liver and kidney samples from non-occupationally exposed populations. Data on human kidney cadmium levels identified recently, including the study in our own laboratory, are compared with older studies. Human-tissue cadmium contents showed large variations among individuals, but sources of the variation remain unknown. Exposure levels of 30-50 microg per day have been estimated for adults and these levels have been linked to increased risk of bone fracture, cancer, kidney dysfunction and hypertension. Increased mortality was found among individuals showing signs of cadmium renal toxicity compared with those without such signs, suggesting that renal toxicity may be an early warning of complications, sub-clinical or clinical morbidity.

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.

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.

Small-intestinal absorption of cadmium and the significance of mucosal metallothionein

1 Although food intake is among the most important routes of Cd exposure, not many details are known about the intestinal absorption mechanisms of Cd. In this respect Cd is representative of most other nonessential, merely toxic metals. 2 Based on a concept of two distinguishable steps, intestinal absorption of Cd is characterized by high accumulation within the intestinal mucosa and a low rate of diffusive transfer into the organism. 3 After uptake into the mammalian organism, Cd is sequestered into hepatic metallothionein (MT). It is assumed that hepatic Cd-MT then gradually redistributes Cd to the kidney, which is the main target organ for chronic Cd toxicity. 4 When feeding low levels of dietary CdCl2, however, Cd accumulates preferentially in the kidney and to a lesser degree in the liver, a distribution pattern also found after intravenous and peroral administration of the Cd-MT complex itself. As dietary Cd induces intestinal MT, intestinal Cd-MT complexes could be at least partly responsible for the renal accumulation of dietary Cd. 5 For this mechanism, however, serosal release of mucosal Cd-MT is required. In fact, in vitro findings in rats reveal a concentration-dependent release of intestinal MT to the serosal side of the small intestine. These results indicate that endogenous intestinal MT may deliver Cd-MT to other inner organs, thus contributing to the preferential renal accumulation of ingested Cd.

Alterations in isoforms of glutathione S-transferase in liver and kidney of cadmium exposed rhesus monkeys: purification and kinetic characterization

Exposure of animals to cadmium (Cd) (25 mg kg(-1) body wt day(-1)) for 10 weeks resulted in preferential accumulation of the metal in liver and kidney. Cd accumulation concomitantly increased zinc (Zn) concentration in both the organs. However, significant decrease in copper level was observed in liver, whereas kidney showed increase in copper (Cu) level. Cd exposure resulted in decreased total GST activity in liver (63%) and kidney (41%) as compared to control group monkeys on normal diet (group I). On isoelectric focusing (IEF) control liver GST segregated into thirteen isoenzymes, while in Cd-treated experimental animals (group II) liver GST resolved into nine isoenzymes. Similarly kidney GST from control animals separated into seven isoenzymes as compared to four isoenzymes from Cd-treated animals. Kinetic analysis showed that Cd exposure did not alter the affinity constant (Km) of GST for GSH and CDNB whereas maximal velocity (Vmax) for these substrates decreased as compared to controls in both the organs, indicating inhibition in GST synthesis by Cd. Cd resulted in a noncompetitive type of inhibition with respect to GSH in vitro. On isoelectric focussing GST of liver and kidney in group II resolved into nine and four isoenzymes as compared to thirteen and seven in group I, showing loss of four basic isoenzymes in case of liver and three isoenzymes in case of kidney. Monkey liver and kidney expressed all the three classes of GST isoenzymes i.e. alpha, mu and pi, which were serologically identical to human alpha, mu and pi GSTs.

The influence of nutritional deficiencies on gastrointestinal uptake of cadmium and cadmium-metallothionein in rats

The intestinal uptake and tissue distribution of cadmium (Cd) were studied in control rats and those deficient in zinc (Zn), iron (Fe) or cysteine (SH) using an in situ model where an intestinal loop of 5 cm was incubated with CdCl2 or Cd-MT (MT, metallothionein) for 30 and 60 min. The intestinal content of Cd after incubation with CdCl2 or Cd-MT was not affected by nutritional deficiencies, but the Cd uptake from CdCl2 was always higher than that from Cd-MT. However, both Fe and Zn deficiencies had a marked effect on distribution of Cd in liver, kidney and pancreas. After 30 min incubation in situ with CdCl2, Cd was deposited only in liver in control and SH deficient rats, while Cd was detected also in kidney and pancreas of both Fe and Zn deficient rats. After 60 min incubation with CdCl2, the deposition of Cd in the liver, kidney and pancreas of Fe deficient rat was significantly higher than that in the control. The deposition of Cd after Cd-MT incubation in situ was mainly found in kidney, and nutritional deficiencies increased the tissue deposition of Cd from Cd-MT. Similarly, the renal deposition of Cd absorbed from CdCl2 was markedly increased in Fe deficient rats. These results suggest that the intestinal uptake mechanisms of Cd from CdCl2 and Cd-MT are different and nutritional deficiencies can markedly increase the deposition of Cd in the kidney, the critical organ in chronic cadmium exposure.

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.

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.

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.

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.

Pathological study on beagles after long-term oral administration of cadmium

Histopathological, histochemical, and electron microscopic examinations were performed on beagles after a long-term oral cadmium (Cd) administration of greater than 8 years. Although renal atrophy was remarkable in groups receiving doses of 50 and 100 mg/kg body weight/day, bone lesions could not be demonstrated by roentgenological and histopathologic examination. It was noticed that concomitant regeneration or recovery and cell death of the epithelium occurred in the proximal convoluted tubules. The cell death was consistent with apoptosis, a special feature of cell death, which was shown to play a major part in the tubular damage of cadmium by electron microscopic examination. Fatty degeneration of the pars recta tubules was seen to show dose-dependence. The intrarenal cadmium was localized predominantly in the cytoplasm of the proximal tubular epithelium by histochemical and ultracentrifugal cell fractionation examinations. Although no remarkable changes were found in the other organs, aggregates of siderophages in the liver and focal hemorrhage in the spleen, known as spontaneous lesions, might be related to Cd intoxication. In conclusion, the present study revealed that no bone lesions occur with Cd administration in adult beagles in spite of long-term administration. An excessive cell death to regeneration or recovery in the proximal tubules might result in the renal cortical atrophy. No remarkable changes were seen in the glomeruli and distal nephrons, which were in good agreement with Cd distribution.

Effects of acidification on the availability of toxic metals and calcium to wild birds and mammals

The effects of acidification on wildlife inhabiting aquatic or semi-aquatic environments are reviewed, with particular reference to the possibility for increased dietary exposure to Hg, Cd, Pb and/or Al, and decreased availability of essential dietary minerals such as Ca. It is concluded that: (1) piscivores risk increased exposure to dietary methyl-Hg in acidified habitats, and Hg concentrations in prey may reach levels known to cause reproductive impairment in birds and mammals; (2) piscivores do not risk increased exposure to dietary Cd, Pb or Al because these metals are either not increased in fish due to acidification, or increase are trivial from a toxicological perspective; (3) insectivores and omnivores may, under certain conditions, experience increased exposure to toxic metals in some acidified environments. Exposure levels are likely to be sufficiently low, however, that significant risks to health or reproduction are unlikely. More importantly, these wildlife species may experience a drastic decrease in the availability of dietary Ca due to the pH-related extinction of high-Ca aquatic invertebrate taxa (molluscs, crustaceans). Decreased availability of dietary Ca is known to adversely affect egg laying and eggshell integrity in birds, and the growth of hatchling birds and neonatal mammals. Acidification-related changes in the dietary availability of other essential elements, such as Mg, Se and P, have not been established and require further investigation; (4) herbivores may risk increased exposure to Al and Pb, and perhaps Cd, in acidified environments because certain macrophytes can accumulate high concentrations of these metals under acidic conditions. The relative importance of pH in determining the metal concentrations of major browse species, and the toxicological consequences for herbivores wildlife, is not well established and requires further study. A decreased availability of dietary Ca is also likely for herbivores inhabiting acidified environments.

Metallothionein production: similar responsiveness of avian liver and kidney to chronic cadmium administration

The accumulation of hepatic and renal Cd, Zn, Cu, and metallothionein (MT) was investigated in ringed turtle doves (Streptopelia risoria) chronically exposed to 3 different concentrations of dietary Cd. When only tissue-Cd was considered as an inducer of MT, kidney was found to be 35% as responsive as liver in producing MT. However, when all potentially relevant inducing metals (Cd + Zn + Cu) were taken into account, kidney was found to be 85% as responsive as liver. The greater production of MT/mol Cd in liver was accounted for mainly by a greater co-accumulation of Zn/mol Cd in liver than in kidney. We conclude that the apparent tissue specificity in expression of MT may be overestimated by failure to consider fluctuations in multiple inducers. Variability in tissue-MT concentrations after chronic dietary Cd administration is best accounted for by a consideration of tissue-Cd, -Zn, and -Cu, rather than tissue-Cd alone.

The concept of critical levels of toxic heavy metals in target tissues

The high reactivity of heavy metals with biological systems is well documented, although some disagreement remains on the precise dose-effect relationships involved. This represents a question of considerable importance, especially in attempts to assess the risks of exposure. The implicit assumption is usually made that a threshold concentration of specific metals exists in the most sensitive target organ, so that an increased frequency of functional lesions will be expected if this threshold is exceeded. The threshold for the metal defines its so-called critical level, and this review was written in order to examine the theoretical and practical difficulties in establishing such a level. Among these may be cited, for instance, the dependence of what constitutes the target tissue on the speciation of the metal, the changes in apparent critical level with rate and route of metal administration, the short half-life of some of the metals as well as their compartmentation in the tissues, and the considerable initiation delay frequently preceding the appearance of lesions. For these and other reasons a useful approximate value for a critical concentration has only been proposed so far for the total Cd concentration in the renal cortex of chronically exposed human adults.

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.