Cellular iron metabolism

Effect of dexrazoxane on homocysteine-induced endothelial dysfunction in normal subjects

OBJECTIVE

Dexrazoxane is an antioxidant prodrug that on hydrolysis is converted into an intracellular iron chelator. We hypothesized that the antioxidant effects of dexrazoxane would prevent homocysteine-induced endothelial dysfunction in the brachial artery of normal human subjects.

METHODS AND RESULTS

Ten healthy volunteers completed a randomized, double-blind, crossover study. Plasma homocysteine levels and brachial artery endothelium-dependent (flow-mediated dilation [FMD]) and endothelium-independent (sublingual nitroglycerin) responses were measured before and 4 hours after ingestion of L-methionine (100 mg/kg), preceded by intravenous administration of dexrazoxane (500 mg/m2) or placebo over 30 minutes. After placebo, oral methionine increased plasma homocysteine (from 5.1+/-0.4 micromol/L at baseline to 14.2+/-1.3 micromol/L at 4 hours, P<0.001) and decreased FMD (from 3.8+/-0.7% at baseline to 1.2+/-0.5% at 4 hours, P=0.02). Dexrazoxane did not change homocysteine concentrations after methionine administration (14.9+/-1.1 micromol/L at 4 hours, P=0.29 versus placebo) but did completely abrogate the homocysteine-induced reduction in FMD (from 3.5+/-0.5% at baseline to 5.9+/-1.1% at 4 hours, P<0.01 versus placebo). Endothelium-independent responses to sublingual nitroglycerin did not differ after the administration of placebo and dexrazoxane.

CONCLUSIONS

Administration of the novel antioxidant agent dexrazoxane prevents homocysteine-induced impairment of vascular endothelial function in the brachial artery of healthy subjects.

Expression of the iron transporter DMT1 in kidney from normal and anemic mk mice

BACKGROUND

DMT1 (Nramp2/DCT1) is the major apical iron transporter in absorptive cells of the duodenum, but also transports transferrin-iron across the membrane of acidified endosomes in peripheral tissues. DMT1 mRNA and protein expression has been detected in rat and mouse kidney, but its role at that site remains to be clarified.

METHODS

Immunoblotting and immunohistochemistry with specific affinity purified anti-DMT1 polyclonal antibodies were used to study expression and localization of DMT1 in mouse kidney. Possible regulation of DMT1 protein expression by the body iron stores also was examined in normal mice deprived of dietary iron, and in the genetically anemic mk mice that bear a loss of function mutation at DMT1 (G185R).

RESULTS

In microsomal kidney fractions, DMT1 isoform I (encoded by the iron responsive element (IRE)-containing mRNA) is detected as an abundant 70 to 75 kD membrane protein. DMT1 is expressed in the cortex and not in the medulla, and is present at the brush border and apical pole of epithelial cells of proximal tubules. In contrast to the intestine, DMT1 protein expression in kidney is only slightly increased upon deprivation of dietary iron, suggesting different regulation at the two sites. In kidneys from mk/mk mice, the level of detectable DMT1(G185R) protein is drastically decreased compared to mk/+ controls.

CONCLUSION

These results suggest that DMT1 may act as a re-uptake system for divalent cations at the brush border of kidney proximal tubules. A pathological mutation at DMT1 affects targeting/expression of the protein in the kidney.

Iron prevents ferritin turnover in hepatic cells

It has long been assumed that iron regulates the turnover of ferritin, but evidence for or against this idea has been lacking. This issue was addressed using rat hepatoma cells with characteristics of hepatocytes subjected to a continuous influx of iron. Iron-pretreated cells were pulsed with [(35)S]Met for 60 min or with (59)Fe overnight and harvested up to 30 h thereafter, during which they were/were not cultured with ferric ammonium citrate (FAC; 180 microm). Radioactivity in ferritin/ferritin subunits of cell heat supernatants was determined by autoradiography of rockets obtained by immunoelectrophoresis or after precipitation with ferritin antibody and SDS-PAGE. Both methods gave similar results. During the +FAC chase, the concentration of ferritin in the cells increased linearly with time. Without FAC, the half-life of (35)S-ferritin was 19-20 h; with FAC there was no turnover. Without FAC, the iron in ferritin had an apparent half-life of 20 h; in the presence of FAC there was no loss of (59)Fe. Without FAC, concentrations of ferritin iron and protein also decreased in parallel. We conclude that a continuous influx of excess iron can completely inhibit the degradation of ferritin protein and that the iron and protein portions of ferritin molecules may be coordinately degraded.

Expression of transferrin receptor 2 in normal and neoplastic hematopoietic cells

Iron is essential for cell proliferation, heme synthesis, and a variety of cellular metabolic processes. In most cells, transferrin receptor-mediated endocytosis is a major pathway for cellular iron uptake. Recently, transferrin receptor 2 (TfR2), another receptor for transferrin, was cloned. High levels of expression of TfR2 messenger RNA (mRNA) occur in the liver, as well as in HepG2 (a hepatoma cell line) and K562 (an erythroid leukemia cell line). In this study, TfR2 mRNA expression was analyzed in hematological cell lines, normal erythroid cells at various stages of differentiation, and leukemia and preleukemia cells. High levels of TfR2 expression occurred in all of the erythroid cell lines that were examined. Erythroid-specific expression of TfR2 protein in bone marrow cells was confirmed by immunohistochemical staining. Expression of TfR2 mRNA was high in normal CD34(+) erythroid precursor cells, and levels decreased during erythroid differentiation in vitro. Levels of expression of TfR2-alpha mRNA were significantly higher in erythroleukemia (M6) marrow samples than in nonmalignant control marrow samples. In addition, relatively higher levels of TfR2-alpha mRNA expression occurred in some samples of myelodysplastic syndrome that had erythroid hyperplasia in bone marrow, acute myelogenous leukemia M1, M2, and chronic myelogenous leukemia. Expression profiles of normal members of the erythroid lineage suggest that TfR2-alpha may be a useful marker of early erythroid precursor cells. The clinical significance of TfR2-alpha expression in leukemia cells remains to be determined.

Iron deposition in renal biopsy specimens from patients with kidney diseases

This study was performed to investigate the correlation between clinical parameters and grading of iron deposition in renal biopsy specimens from 102 patients with various kidney diseases. Iron deposition in renal tissues was detected by Berlin blue staining. The extent of iron staining was semiquantitatively graded as negative (Fe(-)), grade 0, or positive (Fe(+)), including faint, grade 1; moderate, grade 2; or severe, grade 3, by light microscopy. Thirty-four of 102 patients (33%) showed positive iron staining. Fe(+) patients had various renal diseases, mainly consisting of 12 patients with immunoglobulin A nephropathy and 5 patients with benign nephrosclerosis. Mean arterial pressure (MAP), serum creatinine (sCr) levels, incidence of hematuria, and urinary N-acetylbeta-D-glucosaminidase (u-NAG) levels in Fe(+) patients were significantly greater than those in Fe(-) patients, and u-NAG levels correlated positively with the extent of iron deposition. Study patients were tentatively divided into two groups according to the extent of iron deposition: group A, patients with grades 2 and 3 staining, and group B, patients with grades 0 and 1 staining. In group A, MAP, sCr level, urinary protein excretion, and the incidence of hematuria were significantly greater than in group B. Our results suggest that the amount of iron deposition in renal tissue may contribute to the progression of chronic renal disease and may be an early and sensitive indicator of renal damage in certain renal diseases.

Modulation of RNA function by oligonucleotides recognizing RNA structure

Numerous RNA structures are responsible for regulatory processes either because they constitute a signal, like the hairpins or pseudoknots involved in ribosomal frameshifting, or because they are binding sites for proteins such as the trans-activating responsive RNA element of the human immunodeficiency virus whose binding to the viral protein Tat and cellular proteins allows full-length transcription of the retroviral genome. Selective ligands able to bind with high affinity to such RNA motifs may serve as tools for dissecting the molecular mechanisms in which they are involved. Such ligands might also constitute prototypes of therapeutic agents when RNA structures play a role in the expression of dysfunctional genes or in the multiplication of pathogens. Different classes of ligands (aminoglycosides, interacalating agents, peptides) are of interest to this aim. However, oligonucleotides deserve particular consideration. They have been extensively used in the frame of the antisense strategy. The apparent simplicity of this rational approach is, at first sight, very attractive. Indeed, numerous successful studies have been published describing the efficient inhibition of translation, splicing, or reverse transcription in cell-free systems, in cultured cells, or in vivo by oligomers complementary to an RNA region. However, RNA structures restrict the access of the target site to the antisense sequence: The competition between the intramolecular association of RNA regions weakens or even abolishes the antisense effect. Various possibilities have been developed to circumvent this limitation. This includes both rational and combinatorial strategies. High-affinity oligomers were designed to invade the RNA structure. Alternatively, triplex-forming oligonucleotides (TFO) and aptamers may recognize the folded RNA motif. Whereas the use of TFOs is rather limited owing to the strong sequence constraints for triple-helix formation, in vitro selection offers a way to explore vast oligoribo or oligodeoxyribo libraries to identify strong, selective oligonucleotide binders. The candidates (aptamers) selected against the TAR RNA element of HIV-1, which form stable loop-loop (kissing) complexes with the target, provide interesting examples of oligonucleotides recognizing a functional RNA structure through an important contribution of tertiary interactions.

Megalin-dependent cubilin-mediated endocytosis is a major pathway for the apical uptake of transferrin in polarized epithelia

Cubilin is a 460-kDa protein functioning as an endocytic receptor for intrinsic factor vitamin B(12) complex in the intestine and as a receptor for apolipoprotein A1 and albumin reabsorption in the kidney proximal tubules and the yolk sac. In the present study, we report the identification of cubilin as a novel transferrin (Tf) receptor involved in catabolism of Tf. Consistent with a cubilin-mediated endocytosis of Tf in the kidney, lysosomes of human, dog, and mouse renal proximal tubules strongly accumulate Tf, whereas no Tf is detectable in the endocytic apparatus of the renal tubule epithelium of dogs with deficient surface expression of cubilin. As a consequence, these dogs excrete increased amounts of Tf in the urine. Mice with deficient synthesis of megalin, the putative coreceptor colocalizing with cubilin, also excrete high amounts of Tf and fail to internalize Tf in their proximal tubules. However, in contrast to the dogs with the defective cubilin expression, the megalin-deficient mice accumulate Tf on the luminal cubilin-expressing surface of the proximal tubule epithelium. This observation indicates that megalin deficiency causes failure in internalization of the cubilin-ligand complex. The megalin-dependent, cubilin-mediated endocytosis of Tf and the potential of the receptors thereby to facilitate iron uptake were further confirmed by analyzing the uptake of (125)I- and (59)Fe-labeled Tf in cultured yolk sac cells.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.

Regulation of expression of murine transferrin receptor 2

Complementary and genomic DNA for the murine transferrin receptor 2 (TfR2) were cloned and mapped to chromosome 5. Northern blot analysis showed that high levels of expression of murine TfR2 occurred in the liver, whereas expression of TfR1 in the liver was relatively low. During liver development, TfR2 was up-regulated and TfR1 was down-regulated. During erythrocytic differentiation of murine erythroleukemia (MEL) cells induced by dimethylsulfoxide, expression of TfR1 increased, whereas TfR2 decreased. In MEL cells, expression of TfR1 was induced by desferrioxamine, an iron chelator, and it was reduced by ferric nitrate. In contrast, levels of TfR2 were not affected by the cellular iron status. Reporter assay showed that GATA-1, an erythroid-specific transcription factor essential for erythrocytic differentiation at relatively early stages, enhanced TfR2 promoter activity. Interestingly, FOG-1, a cofactor of GATA-1 required for erythrocyte maturation, repressed the enhancement of the activity by GATA-1. Also, CCAAT-enhancer binding protein, which is abundant in liver, enhanced the promoter activity. Thus, tissue distribution of TfR2 was consistent with the reporter assays. Expression profiles of TfR2 were different from those of TfR1, suggesting unique functions for TfR2, which may be involved in iron metabolism, hepatocyte function, and erythrocytic differentiation.

The role of 3'-untranslated region (3'-UTR) mediated mRNA stability in cardiovascular pathophysiology

Knowledge of transcription and translation has advanced our understanding of cardiac diseases. Here, we present the hypothesis that the stability of mRNA mediated by the 3'-untranslated region (3'-UTR) plays a role in changing gene expression in cardiovascular pathophysiology. Several proteins that bind to sequences in the 3'-UTR of mRNA of cardiovascular targets have been identified. The affected mRNAs include those encoding beta-adrenergic receptors, angiotensin II receptors, endothelial and inducible nitric oxide synthases, cyclooxygenase, endothelial growth factor, tissue necrosis factor (TNF-alpha), globin, elastin, proteins involved in cell cycle regulation, oncogenes, cytokines and lymphokines. We discuss: (a) the types of 3'-UTR sequences involved in mRNA stability, (b) AUF1, HuR and other proteins that bind to these sequences to either stabilize or destabilize the target mRNAs, and (c) the potential role of the 3'-UTR mediated mRNA stability in heart failure, myocardial infarction and hypertension. We hope that these concepts will aid in better understanding cardiovascular diseases and in developing new therapies.

Heavy chain ferritin enhances serine hydroxymethyltransferase expression and de novo thymidine biosynthesis

We have elucidated a biochemical mechanism whereby changes in iron metabolism cause changes in folate-dependent one-carbon metabolism. Although animal and clinical studies have demonstrated that perturbations in iron status and metabolism alter folate metabolism, the biochemical mechanisms underlying these associations have yet to be identified. The effect of altered ferritin expression on folate metabolism was determined in human MCF-7 cells and SH-SY5Y neuroblastoma. Cells expressing rat heavy chain ferritin (HCF) exhibited markedly increased expression of the folate-dependent enzyme cytoplasmic serine hydroxymethyltransferase (cSHMT). These effects were not seen when rat light chain ferritin was expressed. Additionally, cSHMT expression was not altered when HCF expression was induced in MCF-7 cells cultured with supplemental ferric citrate. This indicates that cSHMT expression is increased by elevated HCF concentrations, independent of increased iron availability, suggesting that cSHMT expression may respond to HCF-induced chelation of the regulatory iron pool. Increased HCF expression did not alter cSHMT mRNA levels, but did increase translation rates of cSHMT mRNA. The increase in translation was mediated, at least in part, through the cSHMT 5'-untranslated region of the transcript. MCF-7 cells with increased expression of cSHMT displayed increased efficiency of de novo thymidylate biosynthesis, indicating that thymidylate synthesis is normally limited by cSHMT activity in MCF-7 cells. Our data suggest that the iron regulatory pool may play an important role in regulating folate metabolism and thereby thymidine biosynthesis.

Low iron availability modulates the course of Chlamydia pneumoniae infection

Chlamydiae are obligate intracellular bacteria residing exclusively in host cell vesicles termed inclusions. We have investigated the effects of deferoxamine mesylate (DAM)-induced iron deficiency on the growth of Chlamydia pneumoniae and Chlamydia trachomatis serovar L2. In epithelial cells subjected to iron starvation and infected with either C. pneumoniae or C. trachomatis L2, small inclusions were formed, and the infectivity of chlamydial progeny was impaired. Moreover, for C. trachomatis L2, we observed a delay in homotypic fusion of inclusions. The inhibitory effects of DAM were reversed by adding exogenous iron-saturated transferrin, which restored the production of infectious chlamydiae. Electron microscopy examination of iron-deprived specimens revealed that the small inclusions contained reduced numbers of C. pneumoniae that were mostly reticulate bodies. We have previously reported specific accumulation of transferrin receptors (TfRs) around C. pneumoniae inclusions within cells grown under normal conditions. Using confocal and electron microscopy, we show here a remarkable increase in the amount of TfRs surrounding the inclusions in iron-starved cultures. It has been shown that iron is an essential factor in the growth and survival of C. trachomatis. Here, we postulate that, for C. pneumoniae also, iron is an indispensable element and that Chlamydia may use iron transport pathways of the host by attracting TfR to the phagosome.

The roles of cubilin and megalin, two multiligand receptors, in proximal tubule function: possible implication in the progression of renal disease

Proteins that have not been retained by the glomerulus are reabsorbed in the proximal tubule by endocytosis, a process that involves binding at the apical pole of the tubule cell, vesicular internalization and subsequent lysosomal degradation. Data presented in this review indicate that the initial recognition step involves two high molecular weight proteins, megalin and cubilin, which have multiligand properties and can therefore account for the wide variety of proteins reabsorbed. Given the potential importance of transepithelial protein traffic in the induction of interstitial fibrosis, the identification of these receptors may have implications in the progression of acute or chronic renal disease and may provide a target for therapeutic intervention.

Cytochrome P-450 as a source of catalytic iron in minimal change nephrotic syndrome in rats

We have recently demonstrated an important pathogenic role for glomerular catalytic iron in the puromycin aminonucleoside (PAN) induced minimal change nephrotic syndrome (MCNS). The source of this iron capable of catalyzing free radical reactions is not known. We examined the role of cytochrome P-450 (CYP) as a source of catalytic iron in a model MCNS induced by single injection of PAN to rats. Treatment of PAN resulted in a marked increase in the catalytic iron associated with significant loss of glomerular CYP content. Administration of CYP inhibitors significantly prevented the injury-induced loss of CYP content and the increase in the catalytic iron in the glomeruli accompanied by a marked decrease in proteinuria. In an in vitro study utilizing glomerular epithelial cells (GEC), CYP inhibitors also markedly prevented the PAN-induced increase in the catalytic iron and hydroxyl radical formation accompanied by significant protection against PAN-induced cytotoxicity. Taken together our data indicate that the CYP, a group of heme protein, may serve as a significant source of this catalytic iron.

The physiopathological significance of ceruloplasmin. A possible therapeutic approach

This article reviews and comments on the physiological roles of ceruloplasmin (Cp). We show that, in addition to its ascertained involvement in iron homeostasis, the protein, by virtue of its unique structure among multicopper oxidases, is likely involved in other processes of both an enzymatic and a nonenzymatic nature. In particular, based on the analysis of the kinetic parameters, on the one hand, and of the side-products of the oxidation, on the other, we propose that the long-recognized ability of Cp to interact with and oxidize non-iron substrates may be of physiological relevance. The striking example of 6-hydroxydopamine oxidation is presented, where we show that the catalytic action is carried out readily under physiological conditions, without release of potentially toxic oxygen intermediates.

Reactive oxygen species and acute renal failure

Acute renal failure is commonly due to acute tubular necrosis (ATN), the latter representing an acute, usually reversible loss of renal function incurred from ischemic or nephrotoxic insults occurring singly or in combination. Such insults instigate a number of processes-hemodynamic alterations, aberrant vascular responses, sublethal and lethal cell damage, inflammatory responses, and nephron obstruction-that initiate and maintain ATN. Eventually, reparative and regenerative processes facilitate the resolution of renal injury and the recovery of renal function. Focusing mainly on ischemic ATN, this article reviews evidence indicating that the inordinate or aberrant generation of reactive oxygen species (ROS) may contribute to the initiation and maintenance of ATN. This review also discusses the possibility that ROS may instigate adaptive as well as maladaptive responses in the kidney with ATN, and raises the possibility that ROS may participate in the recovery phase of ATN.

Nramp 2 (DCT1/DMT1) expressed at the plasma membrane transports iron and other divalent cations into a calcein-accessible cytoplasmic pool

Nramp2, also known as DMT1 and DCT1, is a 12-transmembrane (TM) domain protein responsible for dietary iron uptake in the duodenum and iron acquisition from transferrin in peripheral tissues. Nramp2/DMT1 produces by alternative splicing two isoforms differing at their C terminus (isoforms I and II). The subcellular localization, mechanism of action, and destination of divalent cations transported by the two Nramp2 isoforms are not completely understood. Stable CHO transfectants expressing Nramp2 isoform II modified by addition of a hemaglutinin epitope in the loop defined by the TM7-TM8 interval were generated. Immunofluorescence with permeabilized and intact cells established that Nramp2 isoform II is expressed at the plasma membrane and demonstrated the predicted extracytoplasmic location of the TM7-TM8 loop. Using the fluorescent, metal-sensitive dye calcein, and a combination of membrane-permeant and -impermeant iron chelators, Nramp2 transport was measured and quantitated with respect to kinetic parameters and at steady state. Iron transport at the plasma membrane was time- and pH-dependent, saturable, and proportional to the amount of Nramp2 expression. Iron uptake by Nramp2 at the plasma membrane was into the nonferritin-bound, calcein-accessible so-called "labile iron pool." Ion selectivity experiments show that Nramp2 isoform II can also transport Co(2+) and Cd(2+) but not Mg(2+) into the calcein-accessible pool. Parallel experiments with transfectants expressing the lysosomal Nramp1 homolog do not show any divalent cation transport activity, establishing major functional differences between Nramp1 and Nramp2. Monitoring the effect of Nramp2 on the calcein-sensisitve labile iron pool allows a simple, rapid, and nonisotopic approach to the functional study of this protein.

Gallium disrupts iron metabolism of mycobacteria residing within human macrophages

Mycobacterium tuberculosis and M. avium complex (MAC) enter and multiply within monocytes and macrophages in phagosomes. In vitro growth studies using standard culture media indicate that siderophore-mediated iron (Fe) acquisition plays a critical role in the growth and metabolism of both M. tuberculosis and MAC. However, the applicability of such studies to conditions within the macrophage phagosome is unclear, due in part to the absence of experimental means to inhibit such a process. Based on the ability of gallium (Ga(3+)) to concentrate within mononuclear phagocytes and on evidence that Ga disrupts cellular Fe-dependent metabolic pathways by substituting for Fe(3+) and failing to undergo redox cycling, we hypothesized that Ga could disrupt Fe acquisition and Fe-dependent metabolic pathways of mycobacteria. We find that Ga(NO(3))(3) and Ga-transferrin produce an Fe-reversible concentration-dependent growth inhibition of M. tuberculosis strains and MAC grown extracellularly and within human macrophages. Ga is bactericidal for M. tuberculosis growing extracellularly and within macrophages. Finally, we provide evidence that exogenously added Fe is acquired by intraphagosomal M. tuberculosis and that Ga inhibits this Fe acquisition. Thus, Ga(NO(3))(3) disruption of mycobacterial Fe metabolism may serve as an experimental means to study the mechanism of Fe acquisition by intracellular mycobacteria and the role of Fe in intracellular survival. Furthermore, given the inability of biological systems to discriminate between Ga and Fe, this approach could have broad applicability to the study of Fe metabolism of other intracellular pathogens.

Anemia in IBD: the overlooked villain

During the past decade relevant progress has been made in the understanding and treatment of IBD-associated anemia. Effective replacement of iron deficits has become safe by using novel intravenous iron preparations such as iron sucrose. The ability of erythropoietin to interfere with key mechanisms of myelosuppression in anemia of chronic diseases also benefits patients with IBD-associated anemia. Concerns about cost effectiveness have been raised and weighed against the potential improvement in quality of life. Gastroenterologists who are caring for IBD patients should be concerned with low hemoglobin levels, since the quality of life in these patients can be as low as in anemic patients with advanced cancer. Also provided is a structured approach to cost-effective therapy.

Anemia in IBD: the overlooked villain

During the past decade relevant progress has been made in the understanding and treatment of IBD-associated anemia. Effective replacement of iron deficits has become safe by using novel intravenous iron preparations such as iron sucrose. The ability of erythropoietin to interfere with key mechanisms of myelosuppression in anemia of chronic diseases also benefits patients with IBD-associated anemia. Concerns about cost effectiveness have been raised and weighed against the potential improvement in quality of life. Gastroenterologists who are caring for IBD patients should be concerned with low hemoglobin levels, since the quality of life in these patients can be as low as in anemic patients with advanced cancer. Also provided is a structured approach to cost-effective therapy.

In vivo characterization of renal iron transport in the anaesthetized rat

1. In vivo microinjections of 55FeCl3 were made to assess renal iron (Fe2+/3+) transport in the anaesthetized rat. 2. Following microinjection into proximal convoluted tubules (PCTs), 18.5 +/- 2.9 % (mean +/- s.e.m., n = 11) of the 55Fe was recovered in the urine. This recovery was not dependent on the injection site indicating that iron is not reabsorbed across the surface convolutions of the proximal tubule. 3. Following microinjection into distal convoluted tubules (DCTs) 46.1 +/- 6.1 % (n = 8) of the injected 55Fe was recovered. Taken together the recovery data from the PCT and DCT microinjection studies indicate that the transport of iron occurs in the loop of Henle (LH) and collecting duct system. 4. In vivo luminal microperfusion was used to examine iron transport by the LH in more detail. In tubules perfused with 7 micromol l-1 55FeCl3, 52.7 +/- 8. 3 % (n = 8) of the perfused 55Fe was recovered in the collected fluid, indicating significant iron reabsorption in the LH. Addition of copper (Cu2+ as 7 micromol l-1 CuSO4), manganese (Mn2+ as 7 micromol l-1 MnSO4) or zinc (Zn2+ as 7 micromol l-1 ZnSO4) to the perfusate did not affect reabsorption of water, Na+ or K+, but increased recovery of 55Fe to 83.5 +/- 6.8 % (n = 8, P < 0.04), 75.8 +/- 5.9 (n = 6, not significant, n.s.) and 67.9 +/- 3.8; (n = 9, n.s. ), respectively. 5. Thus, iron transport in the LH can be reduced by the addition of copper or manganese to the luminal perfusate suggesting that these ions may compete with iron for a common transport pathway. However, this pathway may not be shared by zinc.