@Copyright 1987 by The Hurnana Press Inc. All rights of any nature, whatsoever, reserved. 0163-4984/87/1400-0115502.40
The Relationship Between Iron Status and Lead Absorption in Rats J. N. MORRISON* AND J. QUARTERMAN The Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, Scotland Received August 28, 1986; Accepted September 24, 1986
ABSTRACT The absorption of lead from loops of small intestine in situ was investigated in rats in which iron absorption was increased by stimuli varying in type, intensity, or duration. Lead absorption was increased by a short period of severe iron restriction before any change in hematological indices became apparent. A period of hypoxia, which markedly increased iron absorption, did not influence absorption of lead. An extended period of moderate iron restriction resulted in a marked reduction in liver iron stores and increased iron absorption throughout the 17-wk experiment. Under these conditions lead absorption was initially also increased, but after 12 wk, when iron intake had become adequate to meet essential requirements, lead absorption was similar to that in iron-supplemented rats. These results are discussed in the light of evidence for a receptor-mediated absorption process for iron. Index Entries: Lead absorption, effect of iron status on; lead absorption, effect of hypoxia on; iron deficiency, lead absorption in; iron absorption, effect of hypoxia on.
INTRODUCTION M a n y studies have s h o w n that variation in iron status can have a m a r k e d influence on the absorption a n d retention of dietary lead a n d that lead retention is e n h a n c e d in animals in w h i c h iron absorption has b e e n stimulated by feeding diets low in iron (1-4). In these studies, diet*Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
] ]5
Vol. 14, 1987
116
Morrison and Quarterman
ary iron restriction has generally been accompanied by bleeding, and the animals have shown signs of a moderate-to-severe iron deficiency, with greatly reduced blood hemoglobin concentrations and packed cell volumes (1,4). However, no consistent effect on lead absorption was observed in rats that were given a diet low in iron for periods up to 7 wk and that showed no signs of anemia (5). In other investigations it was established that the intestinal absorption of lead and of other metals increased when iron absorption was stimulated by dietary iron deficiency, but not when the low iron status was induced by bleeding (6,7). It was suggested (6) that blood loss is a less potent activator of the iron absorption mechanism than dietary iron deficiency and that its lack of effect on the absorption of lead and other metals results from the weaker interaction of those metals with a partially activated absorptive process. Alternatively, either there may be selective enhancement of a single absorptive process involved in the absorption of several metals by these two stimuli or there may be two or more mucosal pathways for iron absorption, at least one of which is highly specific for iron (7). The evidence for an effect of iron status on lead absorption in humans is equivocal. A concurrence of anemia and increased lead burden has often been observed in young children (8-10), although causality has been difficult to establish. Yip et al. (11) have concluded that nutritional iron deficiency can play a significant role in childhood lead poisoning. In one study of 10 mature subjects with differing iron stores, a positive correlation was found between the absorption of orally administered radioisotopes of iron and lead, suggesting that more lead may be absorbed by individuals with low iron stores and in whom iron absorption is increased (12). However, Flanagan et al. (13) found no correlation between iron stores and lead retention in 85 adult subjects, although there was an inverse correlation between iron stores and iron retention. In view of these uncertainties about the precise relationship between lead absorption and iron status, we have measured lead absorption in situations in which iron absorption has been increased by stimuli varying in type, intensity, or duration.
MATERIALS AND METHODS Hooded Lister rats (Rowett strain) were used in all experiments and were given a commercial rat cube diet (Oxoid, Styles, Bewdley, UK) or a semipurified diet based on that of Williams and Mills (14), with the modifications described by Davies and Reid (15), but containing 25% casein instead of 20% albumen as protein source, with a corresponding reduction in sucrose content. This formulation provided 50 mg iron as ferrous sulfate/kg diet. Diets were offered ad libitum from weaning unless otherwise stated. The sex and weight of the rats used are detailed in the Results section. Biological Trace Element Research
Vol. 14, 1987
Iron Status and Lead Absorption in Rats
117
In experiments designed to investigate the effect of various treatments on lead absorption and retention, measurements were made of the transport of the isotopes 2~ and 59Fe from loops of proximal small intestine in situ. All measurements were made after an overnight fast. The 2~ was obtained from the MRC Cyclotron Unit, Hammersmith Hospital, London and 59FeCl2 from Amersham International p.l.c., Aylesbury, Bucks., England. Rats were anesthetized by intraperitoneal injection of pentobarbitone sodium (6 mg/kg body wt). Following laparotomy and ligation of the bile duct, solutions (0.5 mL) containing either 1.8 x 10-SM Fe as ferrous chloride and 1-3 ~Ci S9Fe or 2.4 x 10 5M Pb as lead acetate and 2-4 ~Ci 2~ in 0.9% (wt/v) NaC1 adjusted to pH 3 with dilute HC1 were injected into the isolated loop of approximately 10 cm in length, measured from the pylorus. The abdomen was closed using suture clips and the animals were maintained under anesthesia for 2 h while being warmed under a 100-W lamp. The gut loop was then removed, its contents rinsed out with 5 mL saline, and the distribution of radioactivity between the washed loop and the contents of its lumen determined using an I.C.N. Tracerlab counter, Hersham, Surrey, England. Blood was withdrawn from the heart and the animal killed by an intracardiac injection of anesthetic. The gut-free rat carcass was wrapped in a polythene bag for counting in a whole-body counter (Nuclear Enterprises Model NE 8112, Sighthill, Edinburgh). Carcass radioactivity was compared with that of "phantoms" consisting of polythene bottles of similar volume, filled with saline containing an equivalent quantity of the appropriate isotope. Activity in loop tissue and contents was compared with that of standards in 5 mL saline in counting tubes. All standards were prepared in triplicate. Results are expressed as the percentage of the intraluminally injected dose found in each of the three compartments, carcass, lumen contents, and loop tissue (designated "gut mucosa"). Packed cell volume and hemoglobin concentrations were determined by microhematocrit and "cyan-met" methods, respectively, and nonhem iron by the method of Torrance and Bothwell (16). All concentrations refer to fresh weight of tissue. Student's t-test was used to determine the significance of the differences between means. Comparisons that produced a p value of >0.05 were designated not significant (NS).
RES(.ILTS ExpL 1: Effect of a Short Period of Iron Restriction on Lead Absorption Male rats weighing approximately 290 g previously maintained on the commercial cube diet from weaning, were transferred to the semipurified diet for 3 wk and randomly assigned to two groups of 23 each. One Biological Trace Element Research
Vol. 14, 1987
hlorrison and Quarterman
118
g r o u p (controls) was offered the semisynthetic diet with 50 m g Fe/kg for 7 d a n d the other received the same diet w i t h o u t the iron sup,,plement. The latter diet contained 2.6 m g Fe/kg. Absorption of 59Fe a n d "u~Pb from g u t loops in situ is given in Table 1. The 7-d period of dietary iron restriction, w h i c h was insufficient to affect the w e i g h t gain or hematological status of the rats, significantly enh a n c e d the absorption of both iron a n d lead (Table 1). Significantly m o r e 2~ b u t less 59Fe, was retained by the intestinal m u c o s a of the ironrestricted rats 2 h after dosage. A l t h o u g h the mucosal n o n h e m iron content in iron-restricted rats t e n d e d to be lower than that in control animals, this difference was n o t statistically significant (0.05 < p
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Fig. 2. Blood hemoglobin concentration and packed ceil volume in controis and in iron-restricted rats during the experimental period. iron-restricted rats absorbed significantly more 2~ than the controls from the first sampling at 2 wk until 8 wk, but at both 12 and 17 wk these differences had disappeared. At no time during the course of the experiment were there any significant differences in the binding of 2~ to the mucosa. The fraction (%) of administered 2~ retained in the carcass (Rpb) was linearly related to the postweaning age (d, days) by the following expressions: Control group: Rpb = 18.9 - 0.94 d (SE of regression = +_0.16)(1) Iron-restricted: Rpb = 31.71 -
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+0.21)(2)
indicating that the rate of decrease in the fractional retention is greater (p < 0.01) in the iron-deficient than in normal rats. The fraction of the administered dose of 2~ retained by the intestinal mucosa was not related to the duration of treatment in either the control or iron-restricted groups. However, the fraction of the dose remaining in the intestinal lumen increased with time, significantly in the control group. A similar analysis of the data for 59Fe produced only one significant relationship, an increase with time in the proportion o f s9Fe (Rye, %) in
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VoL 14, 1987
Iron Status and Lead Absorption in Rats
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TABLE 3b Mucosal Nonhero Iron Concentrations~ Weeks, 2 Control Iron-restricted ~
6
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14.5 +_ 1.5 22.9 ___ 1.6 27.9 _+ 1.6 34.5 ___ 2.4 9.8 _ 2.6 16.0 _ 1.5 20.1 + 1.8 25.6 _+ 3.0 NS p < 0.01 p < 0.01 p < 0.02
wet wt of tissue.
the carcass of the iron-deficient group, the relevant equation being: a r e = 33.9 + 0.88 d (SE of regression =
+ 0.30)
DISCGSSION These results have confirmed that dietary iron deficiency can increase the efficiency of lead absorption (1-7). However, it is clearly not essential for the animals to show clinical signs of iron deficiency before this effect is evident. A short-term period of relatively severe iron deficiency in adult rats almost doubled lead absorption (Expt. 1), without any change in hematological indices. A transient anemia did develop in w e a n e d rats given only 10 mg Fe/kg diet, but enhanced lead absorption was still evident w h e n blood hemoglobin levels first returned to normal. That lead absorption is not an invariable consequence of any treatm e n t in which iron absorption is stimulated is shown by the experiment involving hypoxia, in which lead absorption was unchanged. This extends the observations on rats subjected to blood loss (6,7) and suggests that it is only in dietary iron deficiency that the absorption of lead is increased. It seems likely, nevertheless, that the increased lead absorption is related to the increased efficiency of the iron absorption process. Stimulation of the absorption of many other metals, such as cadmium, manganese, and zinc, also occurs in iron deficiency (17). The mechanism whereby lead absorption is increased in iron deficiency has yet to be established. The present results show that uptake and retention of 2~ by the intestinal mucosa is also increased, in accordance with findings on severely iron-deficient mice (/). Little is known, however, of the forms in which lead is absorbed or accumulated in the intestinal mucosa. There have been reports that 2~ is bound with 59re to a protein fraction with mol wt of about 3.7 x 105 in rat intestinal homogenates (2). However, Robertson and Worwood (5) found that although 59Fe was associated mainly with proteins of similar mol wt to ferritin and transferrin, 2~ was b o u n d to a protein fraction of intermediate mol wt. Moreover, the Biological Trace Element Research
Vol. 14, 1987
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VoL 14, 1987
Iron Status and Lead Absorption in Rats
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binding of 2~ was relatively weak, and much of the isotope was released on dialysis against Tris-HCl buffer. Kinetic studies on iron uptake by duodenal tissue from rabbits rendered iron deficient by dietary restriction or repeated bleeding have demonstrated an increase in apparent maximum velocity for influx of iron (Vmax) without significant changes in apparent affinity for iron (18). Similarly, duodenal mucosa from mice rendered hypoxic by a period in a hypobaric chamber showed a marked increase in Vmax for iron uptake, with no effect on apparent K,, (19). These results are consistent with a stimulation of iron absorption resulting from an increase in the number of receptors for the metal in the small intestine. Thus, it seems possible that w h e n the supply of iron absorbed via the intestine is inadequate, either through a reduction in dietary iron content or by increased hematopoietic d e m a n d related to hypoxia or blood loss, the normal response is an increase in intestinal receptor capacity. In the case of a reduced iron concentration in the intestine, an excess carrier capacity for iron would be generated, which would result in binding sites becoming available to metals of lower affinity for the receptor, resulting in an increased absorption of these metals. This situation would not normally arise w h e n dietary iron is adequate to meet the increased d e m a n d produced by blood loss or hypoxia. The results obtained in Expt. 3 suggest that the increased iron absorption resulting from a dietary deficiency will eventually meet requirements, restoring blood red cell volume and hemoglobin concentration to normal, even through liver iron stores remain low. In this situation it is possible that an equilibrium between available iron and receptor density will be attained, with a consequent decrease in the number of sites available to other metals present in the gut and a corresponding reduction in their absorption.
REFERENCES 1. D. L. Hamilton, Toxicol. Appl. PharmacoI. 46, 651 (1978). 2. J. C. Barton, M. E. Conrad, S. Nuby, and L. H. Harrison, J. Lab. Clin. Med. 92{4), 536 (1978). 3. P. R. Flanagan, D. L. Hamilton, J. Haist, and L. S. Valberg, Gastroenterology 77, 1074 (1979). 4. C. R. Angle, M. S. McIntyre, and G. Brunk, ]. Toxicol. Env. Health 3, 557 (1977). 5. I. K. Robertson and M. Worwood, Br. J. Nutr. 40, 253 (1978). 6. P. R. Flanagan, J. Haist, and L. S. Valberg, J. Nutr. 110, 1754 (1980). 7. J. C. Barton, C. E. Conrad, and R. Holland, Proc. Soc. Exptl. Biol. Med. 116, 64 (1981). 8. P. R. Betts, R. Astley, and D. N. Raine, Br. Med. ]. 1, 402 (1973). 9. R. J. Watson, E. Becker, and H. C. Lichman, Paediatrics 21, 40 (1958). 10. P. D. Zsold, New Engl. ]. Med. 290, 50 (1974). 11. R. Yip, T. N. Norris, and A. S. Anderson, J. Paediat. 98 (6), 922 (1981). 12. W. S. Watson, R. Hume, and M. R. Moore. Lancet 2, 236 (1980). Biological Trace Element Research
VoL 14, 1987
126
Morrison and Ouarterman
13. P. R. Flanagan, M. J. Chamberlain, and L. S. Valberg, Am. J. Clin. Nutr. 36, 823 (1982). 14. R. B. Williams and C. F. Mills, Br. J. Nutr. 24, 989 (1970). 15. N. T. Davies and H. Reid, Br. J. Nutr. 41, 579 (1979). 16. J. D. Torrance and T. H. Bothwell, S. Afr. J. Med. Sci. 33, 9 (1968). 17. C. F. Mills, Ann. Rev. Nutr. 5, 173 (1985). 18. T. M. Cox and M. W. O'Donnell, Br. J. Nutr. 47, 251 (1982). 19. I. Bjarnason and T. J. Peters, Clin. Sci. 63, 22P (1982).
Biological Trace Element Research
Vol. 14, 1987
The Relationship Between Iron Status and Lead Absorption in Rats J. N. MORRISON* AND J. QUARTERMAN The Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, Scotland Received August 28, 1986; Accepted September 24, 1986
ABSTRACT The absorption of lead from loops of small intestine in situ was investigated in rats in which iron absorption was increased by stimuli varying in type, intensity, or duration. Lead absorption was increased by a short period of severe iron restriction before any change in hematological indices became apparent. A period of hypoxia, which markedly increased iron absorption, did not influence absorption of lead. An extended period of moderate iron restriction resulted in a marked reduction in liver iron stores and increased iron absorption throughout the 17-wk experiment. Under these conditions lead absorption was initially also increased, but after 12 wk, when iron intake had become adequate to meet essential requirements, lead absorption was similar to that in iron-supplemented rats. These results are discussed in the light of evidence for a receptor-mediated absorption process for iron. Index Entries: Lead absorption, effect of iron status on; lead absorption, effect of hypoxia on; iron deficiency, lead absorption in; iron absorption, effect of hypoxia on.
INTRODUCTION M a n y studies have s h o w n that variation in iron status can have a m a r k e d influence on the absorption a n d retention of dietary lead a n d that lead retention is e n h a n c e d in animals in w h i c h iron absorption has b e e n stimulated by feeding diets low in iron (1-4). In these studies, diet*Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research
] ]5
Vol. 14, 1987
116
Morrison and Quarterman
ary iron restriction has generally been accompanied by bleeding, and the animals have shown signs of a moderate-to-severe iron deficiency, with greatly reduced blood hemoglobin concentrations and packed cell volumes (1,4). However, no consistent effect on lead absorption was observed in rats that were given a diet low in iron for periods up to 7 wk and that showed no signs of anemia (5). In other investigations it was established that the intestinal absorption of lead and of other metals increased when iron absorption was stimulated by dietary iron deficiency, but not when the low iron status was induced by bleeding (6,7). It was suggested (6) that blood loss is a less potent activator of the iron absorption mechanism than dietary iron deficiency and that its lack of effect on the absorption of lead and other metals results from the weaker interaction of those metals with a partially activated absorptive process. Alternatively, either there may be selective enhancement of a single absorptive process involved in the absorption of several metals by these two stimuli or there may be two or more mucosal pathways for iron absorption, at least one of which is highly specific for iron (7). The evidence for an effect of iron status on lead absorption in humans is equivocal. A concurrence of anemia and increased lead burden has often been observed in young children (8-10), although causality has been difficult to establish. Yip et al. (11) have concluded that nutritional iron deficiency can play a significant role in childhood lead poisoning. In one study of 10 mature subjects with differing iron stores, a positive correlation was found between the absorption of orally administered radioisotopes of iron and lead, suggesting that more lead may be absorbed by individuals with low iron stores and in whom iron absorption is increased (12). However, Flanagan et al. (13) found no correlation between iron stores and lead retention in 85 adult subjects, although there was an inverse correlation between iron stores and iron retention. In view of these uncertainties about the precise relationship between lead absorption and iron status, we have measured lead absorption in situations in which iron absorption has been increased by stimuli varying in type, intensity, or duration.
MATERIALS AND METHODS Hooded Lister rats (Rowett strain) were used in all experiments and were given a commercial rat cube diet (Oxoid, Styles, Bewdley, UK) or a semipurified diet based on that of Williams and Mills (14), with the modifications described by Davies and Reid (15), but containing 25% casein instead of 20% albumen as protein source, with a corresponding reduction in sucrose content. This formulation provided 50 mg iron as ferrous sulfate/kg diet. Diets were offered ad libitum from weaning unless otherwise stated. The sex and weight of the rats used are detailed in the Results section. Biological Trace Element Research
Vol. 14, 1987
Iron Status and Lead Absorption in Rats
117
In experiments designed to investigate the effect of various treatments on lead absorption and retention, measurements were made of the transport of the isotopes 2~ and 59Fe from loops of proximal small intestine in situ. All measurements were made after an overnight fast. The 2~ was obtained from the MRC Cyclotron Unit, Hammersmith Hospital, London and 59FeCl2 from Amersham International p.l.c., Aylesbury, Bucks., England. Rats were anesthetized by intraperitoneal injection of pentobarbitone sodium (6 mg/kg body wt). Following laparotomy and ligation of the bile duct, solutions (0.5 mL) containing either 1.8 x 10-SM Fe as ferrous chloride and 1-3 ~Ci S9Fe or 2.4 x 10 5M Pb as lead acetate and 2-4 ~Ci 2~ in 0.9% (wt/v) NaC1 adjusted to pH 3 with dilute HC1 were injected into the isolated loop of approximately 10 cm in length, measured from the pylorus. The abdomen was closed using suture clips and the animals were maintained under anesthesia for 2 h while being warmed under a 100-W lamp. The gut loop was then removed, its contents rinsed out with 5 mL saline, and the distribution of radioactivity between the washed loop and the contents of its lumen determined using an I.C.N. Tracerlab counter, Hersham, Surrey, England. Blood was withdrawn from the heart and the animal killed by an intracardiac injection of anesthetic. The gut-free rat carcass was wrapped in a polythene bag for counting in a whole-body counter (Nuclear Enterprises Model NE 8112, Sighthill, Edinburgh). Carcass radioactivity was compared with that of "phantoms" consisting of polythene bottles of similar volume, filled with saline containing an equivalent quantity of the appropriate isotope. Activity in loop tissue and contents was compared with that of standards in 5 mL saline in counting tubes. All standards were prepared in triplicate. Results are expressed as the percentage of the intraluminally injected dose found in each of the three compartments, carcass, lumen contents, and loop tissue (designated "gut mucosa"). Packed cell volume and hemoglobin concentrations were determined by microhematocrit and "cyan-met" methods, respectively, and nonhem iron by the method of Torrance and Bothwell (16). All concentrations refer to fresh weight of tissue. Student's t-test was used to determine the significance of the differences between means. Comparisons that produced a p value of >0.05 were designated not significant (NS).
RES(.ILTS ExpL 1: Effect of a Short Period of Iron Restriction on Lead Absorption Male rats weighing approximately 290 g previously maintained on the commercial cube diet from weaning, were transferred to the semipurified diet for 3 wk and randomly assigned to two groups of 23 each. One Biological Trace Element Research
Vol. 14, 1987
hlorrison and Quarterman
118
g r o u p (controls) was offered the semisynthetic diet with 50 m g Fe/kg for 7 d a n d the other received the same diet w i t h o u t the iron sup,,plement. The latter diet contained 2.6 m g Fe/kg. Absorption of 59Fe a n d "u~Pb from g u t loops in situ is given in Table 1. The 7-d period of dietary iron restriction, w h i c h was insufficient to affect the w e i g h t gain or hematological status of the rats, significantly enh a n c e d the absorption of both iron a n d lead (Table 1). Significantly m o r e 2~ b u t less 59Fe, was retained by the intestinal m u c o s a of the ironrestricted rats 2 h after dosage. A l t h o u g h the mucosal n o n h e m iron content in iron-restricted rats t e n d e d to be lower than that in control animals, this difference was n o t statistically significant (0.05 < p
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Fig. 2. Blood hemoglobin concentration and packed ceil volume in controis and in iron-restricted rats during the experimental period. iron-restricted rats absorbed significantly more 2~ than the controls from the first sampling at 2 wk until 8 wk, but at both 12 and 17 wk these differences had disappeared. At no time during the course of the experiment were there any significant differences in the binding of 2~ to the mucosa. The fraction (%) of administered 2~ retained in the carcass (Rpb) was linearly related to the postweaning age (d, days) by the following expressions: Control group: Rpb = 18.9 - 0.94 d (SE of regression = +_0.16)(1) Iron-restricted: Rpb = 31.71 -
1.67 d (SE of regression =
+0.21)(2)
indicating that the rate of decrease in the fractional retention is greater (p < 0.01) in the iron-deficient than in normal rats. The fraction of the administered dose of 2~ retained by the intestinal mucosa was not related to the duration of treatment in either the control or iron-restricted groups. However, the fraction of the dose remaining in the intestinal lumen increased with time, significantly in the control group. A similar analysis of the data for 59Fe produced only one significant relationship, an increase with time in the proportion o f s9Fe (Rye, %) in
Biological Trace Element Research
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Iron Status and Lead Absorption in Rats
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TABLE 3b Mucosal Nonhero Iron Concentrations~ Weeks, 2 Control Iron-restricted ~
6
12
17
14.5 +_ 1.5 22.9 ___ 1.6 27.9 _+ 1.6 34.5 ___ 2.4 9.8 _ 2.6 16.0 _ 1.5 20.1 + 1.8 25.6 _+ 3.0 NS p < 0.01 p < 0.01 p < 0.02
wet wt of tissue.
the carcass of the iron-deficient group, the relevant equation being: a r e = 33.9 + 0.88 d (SE of regression =
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DISCGSSION These results have confirmed that dietary iron deficiency can increase the efficiency of lead absorption (1-7). However, it is clearly not essential for the animals to show clinical signs of iron deficiency before this effect is evident. A short-term period of relatively severe iron deficiency in adult rats almost doubled lead absorption (Expt. 1), without any change in hematological indices. A transient anemia did develop in w e a n e d rats given only 10 mg Fe/kg diet, but enhanced lead absorption was still evident w h e n blood hemoglobin levels first returned to normal. That lead absorption is not an invariable consequence of any treatm e n t in which iron absorption is stimulated is shown by the experiment involving hypoxia, in which lead absorption was unchanged. This extends the observations on rats subjected to blood loss (6,7) and suggests that it is only in dietary iron deficiency that the absorption of lead is increased. It seems likely, nevertheless, that the increased lead absorption is related to the increased efficiency of the iron absorption process. Stimulation of the absorption of many other metals, such as cadmium, manganese, and zinc, also occurs in iron deficiency (17). The mechanism whereby lead absorption is increased in iron deficiency has yet to be established. The present results show that uptake and retention of 2~ by the intestinal mucosa is also increased, in accordance with findings on severely iron-deficient mice (/). Little is known, however, of the forms in which lead is absorbed or accumulated in the intestinal mucosa. There have been reports that 2~ is bound with 59re to a protein fraction with mol wt of about 3.7 x 105 in rat intestinal homogenates (2). However, Robertson and Worwood (5) found that although 59Fe was associated mainly with proteins of similar mol wt to ferritin and transferrin, 2~ was b o u n d to a protein fraction of intermediate mol wt. Moreover, the Biological Trace Element Research
Vol. 14, 1987
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Iron Status and Lead Absorption in Rats
125
binding of 2~ was relatively weak, and much of the isotope was released on dialysis against Tris-HCl buffer. Kinetic studies on iron uptake by duodenal tissue from rabbits rendered iron deficient by dietary restriction or repeated bleeding have demonstrated an increase in apparent maximum velocity for influx of iron (Vmax) without significant changes in apparent affinity for iron (18). Similarly, duodenal mucosa from mice rendered hypoxic by a period in a hypobaric chamber showed a marked increase in Vmax for iron uptake, with no effect on apparent K,, (19). These results are consistent with a stimulation of iron absorption resulting from an increase in the number of receptors for the metal in the small intestine. Thus, it seems possible that w h e n the supply of iron absorbed via the intestine is inadequate, either through a reduction in dietary iron content or by increased hematopoietic d e m a n d related to hypoxia or blood loss, the normal response is an increase in intestinal receptor capacity. In the case of a reduced iron concentration in the intestine, an excess carrier capacity for iron would be generated, which would result in binding sites becoming available to metals of lower affinity for the receptor, resulting in an increased absorption of these metals. This situation would not normally arise w h e n dietary iron is adequate to meet the increased d e m a n d produced by blood loss or hypoxia. The results obtained in Expt. 3 suggest that the increased iron absorption resulting from a dietary deficiency will eventually meet requirements, restoring blood red cell volume and hemoglobin concentration to normal, even through liver iron stores remain low. In this situation it is possible that an equilibrium between available iron and receptor density will be attained, with a consequent decrease in the number of sites available to other metals present in the gut and a corresponding reduction in their absorption.
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13. P. R. Flanagan, M. J. Chamberlain, and L. S. Valberg, Am. J. Clin. Nutr. 36, 823 (1982). 14. R. B. Williams and C. F. Mills, Br. J. Nutr. 24, 989 (1970). 15. N. T. Davies and H. Reid, Br. J. Nutr. 41, 579 (1979). 16. J. D. Torrance and T. H. Bothwell, S. Afr. J. Med. Sci. 33, 9 (1968). 17. C. F. Mills, Ann. Rev. Nutr. 5, 173 (1985). 18. T. M. Cox and M. W. O'Donnell, Br. J. Nutr. 47, 251 (1982). 19. I. Bjarnason and T. J. Peters, Clin. Sci. 63, 22P (1982).
Biological Trace Element Research
Vol. 14, 1987
