Iron retention and distribution in the cadmium-induced iron deficiency

ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 10, 128-141 (1985)

Iron Retention and Distribution in the Cadmium- Induced Iron Deficiency’

S. G. SCHAFER AND B. ELSENHANS

With the assistance of Petra Schuster and Therese Widmann

Institut fir Pharmakologie und Toxikologie, Med. Fakultiit der Ludwig-Maximilians UniversitCt, Nussbaumstrasse 26, D-8000 Munich 2, Federal Republic of Germany

Received March 4, I985

The retention and distribution of iron-59 after oral administration of a single iron dose in the presence and absence of various cadmium doses were tested in normal rats acutely or subchronically treated with various amounts of dietary cadmium (28, 56, 112 ppm). In a second series of experiments the kinetic parameters (K,,, and I’,,,,.) of iron absorption were estimated in normal and iron-deficient rats after oral administration and from tied-off jejunal and duodenal segments. In acute experiments the retention of iron decreased inversely with respect to the cadmium dose administered simultaneously. After subchronic exposure to dietary cadmium for 4 weeks the retention of iron was increased if iron was administered alone or together with cadmium in comparison to normal controls. I f iron was administered alone to subchronically treated animals, iron retention was of the same order of magnitude as in iron- deficient controls. However, if iron was administered together with cadmium (molar ratios l/0.5, l/l, l/2 rmol/kg body wt) the retention of iron was decreased in a dose-dependent manner. The iron content in the liver and spleen of acutely exposed rats decreased, whereas the tissue content of the treated rats increased according to the increased body retention of iron. The utilization of iron for hemoglobin synthesis remained unchanged in all groups investigated. 0 1985 Academic Press. Inc.

1. INTRODUCTION

Cadmium is a well-known industrial and environmental toxic agent and its toxic effects on experimental animals and humans have been extensively studied (see Webb, 1979; Friberg et al., 1980). Experimental and environmental exposure to excessive amounts of cadmium has caused serious states of disease, such as testicular atrophy (Parizek and Zahor, 1956), renal dysfunction (Axelsson and Piscator, 1966; Gompertz et al., 1983), and hypochromic, microcytic anemia associated with increased levels of plasma transferrin (Jacobs et al., 1974) and decreased iron stores in the organism (Onosaka and Cherian, 1982). The general antagonism between dietary cadmium and iron has been observed in man and animals by several authors (Murata et al., 1973; Freeland and Cousins, 1973; Hamilton and Val- berg, 1974).

Pollack et al. (1965) and Forth et al. (1966) described independently that cobalt, zinc, and nickel absorption is enhanced in iron deficiency. Valberg et al. (1976) and Flanagan et al. (1978) reported that cadmium absorption is increased in anemia.

’ This paper was presented at the international symposium “Bioavailability of Environmental Chemicals,” September 12- 14, 1984, Schmallenberg-Grafschafi, Federal Republic of Germany.

0147-6513185 $3.00 Copyright 0 1985 by Academic Press. Inc.

All rights of reproduction in any form reserved.

128

CADMIUM-INDUCED IRON DEFTCIENCY 129

Furthermore, the simultaneous administration of iron together with tin, nickel, or cadmium resulted in a decreased absorption of iron from the gastrointestinal tract (GIT) as described by Schafer and Forth ( 1983) and Hamilton and Valberg (1974).

Therefore it was suggested that iron and cadmium may compete at the binding sites of the iron-transfer system in the GIT. Since all studies reporting a decreased iron absorption in the presence of cadmium were based on acute experiments, the influence of subchronic cadmium exposure on the. iron absorption had to be investigated. If the cadmium-induced anemia is due to a competition between iron and cadmium at the iron-transfer system, the absorption of iron should be inhibited only in the presence of cadmium. However, if cadmium affects the properties of the iron-transfer system, an inhibition of iron absorption should also be observed in pretreated animals in the absence of cadmium.

The aim of the present investigation was to clarify whether the development of a hypochromic, microcytic anemia is due to a competition between the two cations or to changed properties of the iron-transport system in the GIT resulting from subchronic administration of dietary cadmium. Iron absorption of acutely and subchronically cadmium-exposed animals will be compared with the absorption of normal and iron-deficient unexposed animals.

2. MATERIALS AND METHODS

2.1. ANIMALS

Male Sprague-Dawley rats (initial body weight: 220 & 6 g) were purchased from Gassner, Sulzfeld, FRG). The rats were housed in groups of four animals each in PVC cages in an animal room (temperature 21°C humidity 55%, 12 hr light per day). After 1 week of adaptation the rats were assigned to 12 experimental groups of 8 animals each (Table 1). The animals had free access to water and diet. Groups 1 to 4 were fed a commercial diet (Altromin 132 1, Altrogge, Lage, FRG) containing 0.5 mmol/kg of iron and less than 0.01 ppm of cadmium as measured after wet digestion by atomic absorption spectrophotometry (Perkin-Elmer, HGA 500, Ueb- erlingen, FRG). Groups 5 to 11 were fed the experimental diet (Forth and Andres, 1969) shown in Table 2 containing 0.5 mmol/kg of iron and 0, 0.25, 0.5, or 1 .O mmol/kg of cadmium as CdClz, corresponding to 0, 28.1, 56.2, or 112.4 ppm CdCl*.

2.2. EXPERIMENTALIRON DEFICIENCY

Group 12 (iron-deficient rats) received the same diet as compiled in Table 2 without addition of cadmium and iron as FeS04, a so-called low-iron diet (0.2 mmol/kg of iron). The rats of this group were bled six times in 3 weeks from the sublingual vein to produce an iron deficiency. The loss of protein caused by bleeding (approx 1.0 ml blood) was compensated for by oral administration of 0.3 g casein hydrolysate dissolved in 2.0 ml of distilled water. The stage of anemia was monitored by measuring hemoglobin content of the blood individually and iron concentration, free iron-binding capacity (FIBC), and total protein in pooled samples of plasma.

2.3. IRON-RETENTION EXPERIMENTS

At the end of the pretreatment period the rats were fasted overnight with free access to water. Between 9 and 10 AM, 1 ml of the test solution (0.9% NaCl, pH

130 SCHAFER AND ELSENHANS

TABLE I

EXPERIMENTALDESIGNOF IRON-RETENTION EXPERIMENTS

Treatment

Group Time on diet Molar ratio in diet Test solution

(weeks) (Fe/W (molar ratio, Fe/Cd)”

1 2 3 4

4+3 4-l-3 4+3 4+3

5 6 7 8 9

10 11

12

4+3 4+3 4+3 4+3 4+3 4+3 4+3

4+3

Acute cadmium exposure

l/O l/O l/O l/O

Subchronic cadmium exposure

l/O 110.5 l/l 112 l/O.5 l/l 112

l/O

110 l/0.5 l/l 112

110 l/0.5 l/l 112 l/O l/O l/O

l/O

No&. Groups I to 4 (normal rats) were fed a commercial stock diet containing 0.5 mmol iron/kg. Groups 5 to 1 I (normal rats) were fed experimental diets containing 0.5 mmol iron and 0, 0.25, 0.5, or 1.0 mmol cadmium/kg. Group 12 [iron-deficient (i.d.) rats] received a “low-iron” diet containing 0.2 mmol iron/kg. Each diet was fed throughout the whole experimental time. After 4 weeks of the respective diet a single dose of iron (1 rmol Fe/kg body wt) was administered orally.

a cmol/kg body wt.

TABLE 2

COMP~~ITIONOFTHECADMIUM-SUPPLEMENTED DIET

Component

Milk powder Corn starch Casein hydrolysate Salt mix” Cadmium chloride Water ad libitum b

mm

616 316

43 10

o/o.05 JO. 10/0.20

0 Ingredients of salt mix (ppm): CaC03, 1.75; CaCl* X HZO, 0.75; KCI, 1.25; KH2P04, 3.60; MgClz, 1.25; NaCI, 1.25; FeSO,, 0. I3 (0.0 in low-iron diet); CoClz, 0.0002; CuS04 X 5HZ0, 0.0050; MnQ X 4Hz0, 0.0020; ZnClz, 0.0005; choline chloride, 0.0015; inosite, 0.0001; KJ, 0.0006.

b Water was added in sufficient amounts for mixing the dough. Subsequently it was baked at 200°C for 20 min. After cooling it was sprayed with 8 ml of an ethanolic vitamin suspension containing 396 mg vitamins (ppb): thiamine, 4.3; riboflavin, 1.8; nicotinamide, 3.7; pantothenic acid, 4.3; pyr- idoxaf phosphate, I .8; biotin, 0.12, menadion, 1.22; cr-tocoph- erol, 5.2; retinal, 2.76; cholecalciferol, 0.005 1.

CADMIUM-INDUCED IRON DEFICIENCY 131

2.0) containing 1 pmol/kg body wt of iron as 59FeS04 and various amounts of cadmium as CdCl* (Table 2) were administered orally by stomach tube. Radioactivity administered (about 150,000 cpm) was measured immediately after administration as 100% value in a whole-body counter for small animals (Packard gamma spectrometer, Type Armac, Frankfurt/M, FRG). After 2 1 days whole-body retention of 59Fe was measured. Results are expressed as percentages of the dose as well as in nanomoles per kilogram body weight.

2.4. ORGAN DISTRIBUTION OF 59Fe

The animals were sacrificed in an ether anesthesia by bleeding from the arteria abdominalis. Blood was collected from each animal (8- 10 ml) and stored for up to 1 hr at 0°C. Hemoglobin content of the blood was measured as described elsewhere (Schafer et al., 1982) using the cyanohemoglobin method. Total protein as well as iron concentration and FIBC was determined by test kits (Merckotest 3327, 3307, 3313, Merck, Darmstadt, FRG). An aliquot of blood (0.02 ml) was taken and the red blood cells were counted in a Coulter counter (Coulter Electronics LTD, Harpenden Her&, England). The liver was perfused from the portal vein with ice- cold saline (20-30 ml) to remove the blood from the organ as well as possible. Subsequently the liver and the spleen were removed and stored at 0°C until measurement of the 59Fe content. The data are expressed as percentages of the dose administered and as percentages of body retention (Tables 3a and b).

2.5. CHEMICALS

Milk powder and corn starch were purchased from BAKO (Munich, FRG). They were of usual quality for human diets. Casein hydrolysate was obtained from Serva (Heidelberg, FRG) in micobiological quality. All salts and vitamins used for the diet and test solutions as well as all other chemicals used were of analytical grade and commercially available (Merck, Darmstadt, FRG).

2.6. STATISTICAL EVALUATION

Differences between mean values were assessed by the analysis of variance or the Student t test. The apparent K, and V,,,, values of iron absorption after oral administration or in tied-off duodenal and jejunal segments of normal and iron-deficient rats were calculated according to common methods (Lineweaver- Burk plot).

3. RESULTS

3.1. BODY WEIGHT

Body weight gain increased continuously throughout the experimental period in normal and iron-deficient rats as well as after oral administration of different single doses of cadmium (0.5 to 2.0 pmol Cd/kg body wt). The growth of the rats fed Cd- containing diets (28.1 to 112.4 ppm) for 7 weeks was smaller than that of the controls (Tables 4a and b).


TABLE 3a

DISTRIBUTION OF 59Fe IN LIVER AND SPLEEN TISSUE AFTER ACUTE TREATMENT WITH VARIOUS AMOUNTS OF CADMWM~

Cd in the test solution (rmol/kg body wt):

Fe in test solution (pmol/kg body wt):

0.0 0.5 1.0 2.0

1.0 1.0 1.0 1.0

59Fe in liver 90 of dose % of retention

59Fe in spleen % of dose % of retention

1.66 -+ 0.30 0.76 f 0.06 0.53 f 0.04 0.32 f 0.03 11.16 f 2.02 8.33 f 0.69 8.73 f 0.72 7.94 k 0.83

0.14 f 0.03 0.06 + 0.01 0.06 -t 0.01 0.04 f 0.01 0.94 + 0.20 0.66 f 0.11 0.98 f 0.16 0.98 + 0.25

a No Cd in diets. Means f SE of eight experiments.

3.2. HEMATOLOGICAL DATA

Several authors have reported that subchronic exposure to dietary cadmium results in the development of a hypochromic microcytic anemia. Therefore the hemoglobin content of the blood, the free iron-binding capacity, and the iron

TABLE 3b

DISTRIBUTION OF 59Fe IN LIVER AND SPLEEN TISSUE AFTER SUBCHRONIC EXFQSURE TO DIETARY CADMIUM

Cd (ppm) in diet:

Cd in test solution (eoVk8 body wt):

Fe in test solution (pmol/kg body wt):

0.0 28.1 56.2 112.4 0.0”

0.0 0.5 1.0 2.0 0.0

1.0 1.0 1.0 1.0 1.0

59Fe in liver % of dose % of retention

59Fe in spleen % of dose % of retention

59Fe in liver ‘3 of dose 90 of retention

s9Fe in spleen % of dose % of retention

Treatment: Cadmium and iron in test solution

0.98 f 0.14 6.05 +- 0.62 5.75 + 1.25 4.98 + 0.94 13.74 k 1.96 17.69 + 1.81 22.25 + 4.84 29.61 + 5.59

0.11 f 0.02 0.14 + 0.03 0.10 + 0.04 0.12 k 0.04 1.54 + 0.28 0.41 + 0.09 0.39 + 0.15 0.71 + 0.24

Treatment: Iron alone in test solution

0.98 f 0.14 3.67 f 0.52 7.30 * 0.77 13.30 f 0.78 2.34 + 0.15 13.74 + 1.96 6.79 f 0.96 14.05 -+ 1.48 24.83 + 1.46 4.60 + 0.29

0.11 + 0.02 0.32 f 0.02 0.48 k 0.06 0.79 +- 0.28 0.87 f 0.15 1.54 f 0.28 0.59 f 0.04 0.92 +- 0.12 .1.47 f 0.52 1.71 + 0.29

u Iron-deficient rats not exposed to cadmium.


TABLE 4a

EFFECTS OF ACUTE CADMIUM EXPOSURE

Cadmium in the diet (ppm):

Cadmium administered together with iron (pmol/kg body wt):

0.01 0.01 0.01 0.01

0 0.5 1.0 2.0

Hemoglobin in blood WOO ml)

FIBC (pmol/lOO ml)

Iron concentration in plasma (pmol/lOO ml)

“Fe in total blood volume (% of body retention)

Body weight gain (g/7 weeks)

Iron retention nmol % of dose

13.6 + 0.3 13.8 + 0.3 13.3 + 0.4 14.2 + 0.2

4.52 + 0.21 5.21 f 0.20 5.05 + 0.21 5.60 + 0.30

1.93 + 0.41 1.93 f 0.11 2.22 -t 0.23 2.25 f 0.10

88.96 + 4.21 85.43 + 8.12 87.21 + 5.79 86.23 f 7.32

143.1 f 15.2 138.9 f 10.7 141.6 f 10.1 145.1 + 7.9

31.20 + 4.18 22.85 + 1.08 15.30 + 1.35 10.20 + 1.05 14.88 + 1.91 9.14 f 0.43 6.12 + 0.54 4.08 + 0.42

concentration of the plasma were measured in all animals, unexposed and exposed to dietary cadmium, to estimate the effects of the experimental conditions on these parameters. The data are compiled in Tables 4a and b. The total protein content of the plasma (6.05 + 0.15 g/100 ml) and the number of RBC (5.5 + 0.1 X lo6 ~1~‘) were not changed by oral administration of single doses of the toxic metal or by subchronic exposure to any of the cadmium fortified diets.

The content of hemoglobin in the blood, the iron concentration (PIC), and the free iron-binding capacity of the plasma (FIBC) were influenced only in animals subchronically exposed to cadmium-containing diets. Hemoglobin content decreased in these groups depending upon the amount of cadmium added to the diets to 80% (28.1 ppm), 68% (56.2 ppm), and 60% (112.4 ppm) as compared to the values of normal controls. PIC and FIBC were markedly influenced by Cd exposure. PIC decreased in a dose-dependent manner to 65, 35, and 9% of the control values, respectively. PIC was decreased to a greater extent as in unexposed iron-deficient anemia. FIBC was enhanced by a factor of about 2.5 in all rats subchronically exposed to cadmium, independent of the Cd dose administered with the diet. The utilization of iron for the synthesis of hemoglobin calculated from the total blood volume, estimated to be 6% of body weight (Schafer and Forth, 1984), and the concentration of 59Fe in the whole blood were not influenced by acute or subchronic exposure to Cd. About 90% of the amount of 59Fe remaining in the body was found in the blood. The hematological data are compiled in Tables 3a and b.

3.3. TISSUE CONTENT OF 59Fe

3.3. I. Organ Weights

The relative weight of the liver (% body wt) remained unchanged in acutely treated rats. Relative liver weights of subchronically cadmium-exposed animals,

134 SCHh-ER AND EJSENHANS


O-

a

O-

b

FelCd in the test dose @mollkg bw) ::::: ::z$ ::y ::::: ::::: a ::::: l

8

: : : : :

g ..:;. t :<

: . : . : :.;:.y< . : . : . : . ; : . : . : 3 0;

n. :s; , , . , - . ; . . .*. : . , . , . . ; .~

110 I IO.5 l/l Fe/Cd in the test dose lpmdlkg b.w.1

FIG. I. Blood hemoglobin content (a) and plasma iron concentration (b) of rats acutely (open bars) and subchronically (stippled bars) exposed to cadmium. The data are given in g hemoglobin/100 ml blood (a) and prnol Fe/100 ml plasma (b) and represent the means (and SE) of eight animals each in acute experiments. Data from subchronically treated rats are combined because there were no statistically significant differences, whether cadmium was administered with the test solution or not. From these groups the means (and SE) of 16 animals are given. Asterisks indicate a significant difference as compared to normal controls, P i 0.05. The figure is based on the data of Schafer and Forth (1984b).

however, increased in a dose dependent manner from 2.53 f 0.38% body wt in controls to 2.79 + 0.037 (28.1 ppm Cd; P < O.OOl), 2.89 f 0.041 (56.2 ppm Cd), and 2.94 f 0.039 (112.4 ppm), respectively. Spleen weight was not changed by long- term cadmium exposure.

3.3.2. Acute Experiments

Twenty-one days after administration of the iron dose to normal unexposed rats between 5 and 10% of whole-body retention was stored in the liver tissue. When iron and cadmium were administered simultaneously the relative iron storage of the liver tissue was not changed by single cadmium doses. In other words, acute exposure to rather small amounts of cadmium did not change the iron storage properties of the liver tissue. The 59Fe content of the spleen was ranged between 1 .O and 1.5% of whole-body retention independent of the dose of cadmium administered (Table 3a).

3.3.3. Subchronic Exposure to Cadmium

Whether cadmium was administered with iron or not the pattern of iron storage after long-term treatment with dietary cadmium was markedly changed as compared


to the pattern obtained in the acute experiments. As in controls, in rats treated with 28.1 ppm Cd about the same relative amount was stored in the liver when expressed as percentage of whole-body retention. After feeding diets with higher Cd additions the amount of iron stored in the liver increased to 14-18% (56.2 ppm) and 25-29% (112.4 ppm). The relative content of 59Fe in the spleen, expressed as a percentage of body retention, decreased to 0.32-0.42% (28.1 ppm), 0.38-48% (56.2 ppm), and 0.70-0.79% (112.4 ppm) of body retention (P < 0.05). These data are inversely related to those for the Cd-induced iron deficiency state.

3.4. IRON ABSORPTION KINETICS IN NORMALAND IRON-DEFICIENT RATS

The general interrelationship between iron and cadmium absorption from the intestine was described by two different methods: whole-body retention of iron (Becker et al., 1979b), and use of tied-off segments of the upper small intestine (Schafer and Forth, 1983).

To estimate whether both methods are comparable, the kinetic parameters for iron absorption were evaluated with these two methods. In the first series of experiments the absorption of iron from duodenal and jejunal tied-off segments of normal and iron-deficient rats was tested in the range between 0.1 and 200 nmol/ cm corresponding to 0.005 to 5 pmol/kg body wt. These segments were chosen, because in normal and iron-deficient rats iron is absorbed mainly from the upper small intestine. In the second series of experiments whole-body retention of iron was measured with doses between 0.04 and 10 pmol/kg body wt. The apparent K,,, and Max values were calculated from absorption experiments with tied-off duodenal or jejunal segments using the Lineweaver-Burk plot. These data were compared with those obtained from the whole-body retention experiments (Table 5). In both

TABLES

ESTIMATION OF KINETIC PARAMETERS (K,,, AND I’,& OF IRON ABSORPTION IN NORMAL AND IRON-DEFICIENT RATS NOT EXPOSED TO CADMIUM

Experimental Km procedure/segment (nmol/cm) (nmol X%i X 2-i)

Normal rats

Duodenum 14.23 3.044 Jejunum 2.60 0.08 1

Whole-body retention 0.28 0.213

Iron-deficient rats

Duodenum 20.42 14.75 Jejunum 396.90 75.59

Whole-body retention 2.32 1.11

Notes. The data were calculated from the dose-dependent iron absorption in tied-off duodenal and jejunal loops (time of incubation: 10 min) as well as from whole-body retention experiments (6 days after oral iron administration) according to common methods (Lineweaver-Burk plot). Dose range: duodenum 1.0 to 200 nmol/cm or 0.025 to 5.0 pmoll kg body wt; jejunum: 0.1 to 100 nmol/cm or 0.005 to 5.0 pmol/kg hody wt; whole-body retention: 0.04 to 10 pmol/kg body wt. The data were calculated from the means of eight animals each.


experiments the apparent I’,,,, and K, values increased markedly in iron deficiency. Therefore it appears appropriate to compare the results of authors using different methods in order to elaborate a hypothesis for the interaction between iron and cadmium at the site of intestinal absorption.

3.5. EFFJXTSOFCADMIUMONTHE RETENTIONOF IRON

3.5.1. Acute Administration of Iron and Cadmium

The retention of iron is decreased by the simultaneous administration of cadmium as shown in Fig. 2. Inhibition of iron retention is strongly correlated with the cadmium dose administered. When the test solution contained iron and cadmium in a molar ratio of l/OS (1 pmol/OS pmol/kg body wt) iron retention decreased to about 60% of the control value. Increasing cadmium doses with molar Fe/Cd ratios of l/l and l/2 resulted in further inhibition of the retention to about 40 and 27% of the controls, respectively.

3.5.2. Subchronic Exposure to Dietary Cadmium

Two series of iron retention experiments were carried out to study the influence of subchronic exposure to cadmium on the iron-transfer system. After they were fed cadmium-fortified diets for 4 weeks, the animals of one series of experiments received orally a solution containing only iron. In the second series of experiments iron and cadmium were administered simultaneously by stomach tube to animals pretreated with the same diets. Iron-retention data of these two groups were compared with the retention data of iron-deficient rats not exposed to cadmium.

3.5.2.1. Iron retention in the absence of cadmium. When no cadmium was administered with the iron-59-containing test solution, iron retention was increased by a factor greater than 7 to about 50% of the dose administered, independent of the amount of dietary cadmium to which animals were exposed during the

;, 0:5 ;0 2r cadmium in the test dose I pmol Cd I kg b.w. )

FIG. 2. Iron retention in rats acutely exposed to cadmium. The rats received 1 rmol Fe/kg body wt together with various amounts of cadmium by stomach tube. The data represent the means (and SE) of eight animals each. Asterisks indicate P < 0.05. The figure is based on the data of Schlfer and Forth (1984b).


pretreatment period (Fig. 3b). Iron retention was enhanced to the same order of magnitude as in rats with experimentally produced iron deficiency.

3.5.2.2. Iron retention in the presence of cadmium. Compared to normal controls, iron retention also increased markedly when cadmium and iron were orally administered simultaneously to pretreated rats (Fig. 3a). However, a comparison showed that groups fed the cadmium-fortified diets for 4 weeks but without cadmium in the test solution showed the same pattern of iron retention as rats acutely exposed to the toxic metal. Iron retention was inversely related to the dose of cadmium administered and decreased to about 60, 50, and 30%, respectively, compared to the corresponding pretreated groups, which received no cadmium with the test solution.

4. DISCUSSION

The transfer system for iron is, on the one hand, dependent on metabolic energy and, on the other hand, capable of adapting its capacity to the iron requirement of the organism (see Forth and Rummel, 1973; 1975; Forth, 1983; Becker et al., 1979). The changes in the properties of the iron-transfer system in anemia can be characterized by the transport parameters of the system: the apparent K, and V,,,. The two different methods used in the present study are comparable tools for

test SOIution: iron and cadmium

o-1 , 0 0.25 0.5 1.0

a dietary cadmium I mmol Cd I kg diet )

test solution: iron alone

0 0.25 0.5 1.0 b dietary cadmium lmmol Cd/kg diet1

FIG. 3. Iron retention of rats treated for 4 weeks with a cadmium-fortified diet containing 0, 28, 56, and I12 ppm cadmium and 28 ppm iron corresponding to molar ratios between iron and cadmium of l/O, l/0.5, l/l, and l/2. The test solution contained 1 pmol Fe/kg body wt. Cadmium was added (a) in

the indicated amounts to the test solution (a) or omitted (b). The data represent the means (and SE) of eight animals each. An asterisk indicates a statistically significant difference as compared to the controls. The figure is based on the data of Schafer and Forth (1984b).


estimating the adaptability of the transfer system in the case of increased iron requirement of the organism.

The transfer system is highly specific for iron; it reacts, however, with other divalent metals, especially with transition group metals. The general interrelationship between iron and some other transition metals is well documented by Pollack et al. (1965) and Forth et al. (1966). The interaction of iron and cadmium has long been recognized by several authors (Jacobs et al., 1969; Freeland and Cousins, 1973; Hamilton and Valberg, 1974; Schafer and Forth, 1983). From studies of this interaction in acute experiments it was suggested that cadmium and iron compete at the transfer system for iron (Hamilton and Valberg, 1974; Schafer and Forth, 1983). In the case of subchronic administration of toxic cadmium, an iron-deficient anemia can be observed (Jacobs et al., 1969), but the mode of the development of this anemia is unclear. Iron absorption during subchronic cadmium exposure is not yet fully understood. To obtain information about the mechanism of the interaction between iron and cadmium in acutely and subchronically exposed rats, iron and cadmium were administered with the test solution in molar ratios of l/0.5, l/l, and l/2. Although higher than the cadmium concentrations reported for the epidemic exposure of man in Japan known as “Itai-Itai disease” [ 1 ppm Cd in the rice of the polluted area (Lauwerys, 1979)], the cadmium contents of these diets were chosen so as to evaluate a possible competition between iron and cadmium at the iron-transfer system in the intestine.

The present study shows that in subchronically exposed animals a general antagonism between iron and cadmium can be observed. The dose-dependent decrease in iron absorption in acute experiments agrees well with data from previous studies (Hamilton and Valberg, 1974; Schafer and Forth, 1983). After subchronic exposure to cadmium, iron retention was changed twofold. First, subchronic feeding of cadmium fortified diets resulted in an increase of iron retention, an effect observed in a similar manner in an experimental produced iron deficiency. Second, the simultaneous administration of iron and cadmium only resulted in a decrease of the iron retention which was comparable to the decrease after acute administration of both metals. Therefore, the following effects of cadmium on the intestinal iron metabolism can be suggested from the present study.

First, iron and cadmium interfere competitively in the intestinal iron-transfer system in both cases, in acute as well as in subchronic treatment of the animals. However, when no cadmium was administered with the test solution, iron absorption increased to the same level as that of experimentally produced iron deficiency. The influence of cadmium on the absorption of water from the intestine (Schafer and Forth, 1983) was obviously without effect upon the capability of the iron-transfer system. This interpretation of the present data is supported by the observation of Hamilton and Valberg (1974) that in the presence of cadmium the I’,,, of iron absorption is increased only slightly, whereas the apparent K,,, is increased markedly. Therefore it can be concluded that iron and cadmium appear to use the same transfer system in the mucosal epithelium of the intestine. Inhibition of iron absorption by cadmium can be observed at the mucosal-uptake step as well as at the rate-limiting step of iron absorption, the release of iron from the mucosal cells to the blood (Hamilton and Valberg, 1974; Schafer and Forth, 1983). The hypothesis that cadmium and iron share, at least in part, the same transfer system is emphasized by the results of Hamilton and Valberg (1974) that the absorption of cobalt is affected by cadmium in a similar manner as the absorption of iron. Mucosal proteins, i.e., mucosal transferrin and ferritin, appear to play a key role in iron


absorption (Huebers and Rummel, 1977). Cadmium belongs to a group of heavy metals which are bound with the same stoichiometry as iron to plasma transferrin (Aisen, 1980). Because plasma transferrin levels are elevated in a cadmium-induced anemia (Jacobs et al., 1969) and iron concentration in the plasma of the subchronically treated rats is dramatically decreased, it is conceivable that cadmium competes with iron at the binding sites of the mucosal transferrin. This competition would result in a decreased transfer of iron and if the concentration of the protein is enhanced, as in anemia (Osterloh and Forth, 1981), in an increased absorption of cadmium (Flanagan et al., 1978) and iron. Similar antagonisms were described for cobalt (Becker et al., 1979a,b), copper (El-Shobaki and Rummel, 1979), manganese (Forth, 1970), and zinc (Forth and Rummel, 1973).

Second, from the increase in iron absorption in rats, which developed an iron deficiency as a result of cadmium exposure, it can be concluded that the important physiological property of the intestine, the adaptation of the iron absorption to the iron requirement of the organism, is not influenced by the subchronic feeding of cadmium-fortified diets. The transfer system appears to be induced by the anemia produced by the exposure to dietary cadmium in the same order of magnitude as in experimentally produced iron deficiency. The utilization of the absorbed iron for hemoglobin synthesis is obviously not influenced by dietary cadmium. The impor- tance of the different organ-distribution patterns of iron during the various forms of treatment has to be investigated in further experiments. An explanation for the increased content of iron-59 in the liver could be that the text dose of radiolabeled iron was administered when the iron stores contained very small amounts of iron. A consequence could be an increased uptake of 59Fe into the storage proteins, as observed by Onosaka and Cherian (1982).

The proposed hypothesis, however, must be challenged further by experiments. Competitive binding of iron and cadmium at the binding sites of the mucosal transfer system, possibly the mucosal transferrin, without influencing the capability of the intestine to adapt iron absorption to the increased requirement would explain the observation that an excess of iron prevents cadmium absorption (Hill et al., 1966; Radi and Ponds, 1979).

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Iron retention and distribution in the cadmium-induced iron deficiency