Oxalobacter formigenes treatment confers protective effects in a rat model of primary hyperoxaluria by preventing renal calcium oxalate deposition.

In primary hyperoxaluria, increased hepatic oxalate production sometimes leads to severe nephrocalcinosis and early end-stage kidney disease. Oral administration of Oxalobacter formigenes (O. formigenes), an oxalate-degrading bacterium, is thought to derive oxalate from systemic sources by inducing net enteric oxalate secretion. Here, the impact of O. formigenes on nephrocalcinosis was investigated in an ethylene glycol rat model mimicking hepatic oxalate overproduction in primary hyperoxaluria. Eighteen rats were administered ethylene glycol (0.75% in drinking water) for 6 weeks, of which 9 were treated by oral gavage with O. formigenes and 9 received vehicle. Five control rats did not receive ethylene glycol or O. formigenes. Plasma and urinary oxalate levels, calcium oxalate crystalluria, urinary volume, fluid intake, and serum creatinine were monitored during the study. On killing, nephrocalcinosis was quantified. Ethylene glycol intake induced pronounced hyperoxalemia, hyperoxaluria, calcium oxalate crystalluria and nephrocalcinosis. Concomitant O. formigenes treatment partially prevented the ethylene glycol-induced increase in plasma oxalate and completely prevented nephrocalcinosis. Urinary oxalate excretion was not reduced by O. formigenes treatment. Nevertheless, absence of crystals in renal tissue of O. formigenes-treated ethylene glycol animals indicates that the propensity for oxalate to crystallize in the kidneys was reduced compared to non-treated animals. This is supported by the lower plasma oxalate concentrations in O. formigenes-treated animals. This study shows a beneficial effect of O. formigenes treatment on ethylene glycol-induced hyperoxalemia and nephrocalcinosis, and thus supports a possible beneficial effect of O. formigenes in primary hyperoxaluria.

Update on the role of bone biopsy in the management of patients with CKD-MBD.

Patients with chronic kidney disease (CKD) are at increased risk of fractures. The fracture risk steadily increases along with the progression of renal disease to become several-fold higher in end-stage renal disease (ESRD) patients as compared to age and sex-matched controls. Renal osteodystrophy (ROD) is a heterogeneous group of metabolic bone diseases complicating progressive chronic kidney disease. Bone biomarkers and bone imaging techniques may help to assess bone health and predict fractures in CKD, but do have important inherent limitations. The gold standard for the diagnosis and specific classification of renal osteodystrophy (ROD) remains the (quantitative) histomorphometric analysis of the bone biopsy. By informing on bone turnover and mineralization, a bone biopsy may help guide prevention and treatment of ROD and its consequences. This review aims to present an update on epidemiological and procedural aspects, clinical indications, and histomorphometric analysis of bone biopsies and to define the role of bone biopsy in current CKD-MBD care.

Inflammation and the bone-vascular axis in end-stage renal disease.

UNLABELLED: Bone loss and vascular calcification coincide in patients with end-stage renal disease, similar as to what is observed in the general population. In the present bone biopsy study, we provide further evidence that (micro-)inflammation may represent a common soil for both diseases. INTRODUCTION: Vascular calcification is a common complication of end-stage renal disease (ESRD) and is predictive of subsequent cardiovascular disease and mortality. Mounting evidence linking bone disorders with vascular calcification has contributed to the development of the concept of the bone-vascular axis. Inflammation is involved in the pathogenesis of both disorders. The aim of the present study was to evaluate the relationship between aortic calcification, inflammation, and bone histomorphometry in patients with ESRD. METHODS: Parameters of inflammation and mineral metabolism were assessed in 81 ESRD patients (55 ± 13 year, 68 % male) referred for renal transplantation. Static bone histomorphometry parameters were determined on transiliac bone biopsies performed during the transplant procedure. Aortic calcification was quantified on lateral lumbar X-rays using the Kauppila method. RESULTS: Aortic calcification, low bone turnover, and low bone area were observed in 53, 37, and 21 % of patients respectively. Inflammatory markers were found to be independently associated with aortic calcification (hsIL-6) and low bone area (TNF-α). Low bone area associated with aortic calcification, independent of age, diabetes, and inflammation. CONCLUSIONS: Low bone area and inflammation associates with aortic calcification, independent of each other and traditional risk factors. Our data emphasize the role of (micro-)inflammation in the bone-vascular axis in CKD.

Co-exposure to lead increases the renal response to low levels of cadmium in metallurgy workers.

PURPOSE: Research on the effect of co-exposure to Cd and Pb on the kidney is scarce. The objective of the present study was to assess the effect of co-exposure to these metals on biomarkers of early renal effect. METHODS: Cd in blood (Cd-B), Cd in urine (Cd-U), Pb in blood (Pb-B) and urinary renal biomarkers, i.e., microalbumin (µ-Alb), beta-2-microglobulin (ß2-MG), retinol binding protein (RBP), N-acetyl-ß-d-glucosaminidase (NAG), intestinal alkaline phosphatase (IAP) were measured in 122 metallurgic refinery workers examined in a cross-sectional survey. RESULTS AND CONCLUSIONS: The median Cd-B, Cd-U, Pb-B were: 0.8 µg/l (IQR = 0.5, 1.2), 0.5 µg/g creatinine (IQR = 0.3, 0.8) and 158.5 µg/l (IQR = 111.0, 219.3), respectively. The impact of Cd-B on the urinary excretion of NAG and IAP was only evident among workers with Pb-B concentrations ≥ 75th percentile. The association between Cd-U and the renal markers NAG and RBP was also evidenced when Pb-B ≥ 75th percentile. No statistically significant interaction terms were observed for the associations between Cd-B or Cd-U and the other renal markers under study (i.e., µ-Alb and ß2-MG). Our findings indicate that Pb increases the impact of Cd exposure on early renal biomarkers.

Possibilities and limits of X-ray microtomography for in vivo and ex vivo detection of vascular calcifications.

In the present paper, vascular calcifications due to chronic renal failure in rats are studied by X-ray microtomography (micro-CT). Although micro-CT is traditionally used as an imaging technique, a quantitative analysis of data obtained by in vivo and ex vivo micro-CT is described and discussed. By comparison with traditional destructive methods, such as histomorphometry and atomic absorption, the detection limits for calcium were determined in living rats and in extracted aortas. micro-CT proved to be an effective non-invasive imaging technique allowing non-destructive quantification of ectopic calcifications.

Balanced calcitriol treatment to make children grow.

Short stature is an important clinical problem in children with chronic kidney disease. Calcitriol is used as standard therapy to control secondary hyperparathyroidism, but its effect on linear growth remains controversial. Sanchez and He report multiple effects of calcitriol on chondrocyte proliferation and maturation that might help to clarify this controversy.

Endochondral bone formation is involved in media calcification in rats and in men.

Arterial media calcification is often considered a cell-regulated process resembling intramembranous bone formation, implying a conversion of vascular tissue into a bone-like structure without a cartilage intermediate. In this study, we examined the association of chondrocyte-specific marker expression with media calcification in arterial samples derived from rats with chronic renal failure (CRF) and from human transplant donors. CRF was induced in rats with a diet supplemented with adenine. Vascular calcification was evaluated histomorphometrically on Von Kossa-stained sections and the expression of the chondrocyte markers sox9 and collagen II with the osteogenic marker core-binding factor alpha1 (cbfa1) was determined immunohistochemically. Media calcification was detected in more than half of the rats with CRF. In over half of the rats with severe media calcification, a typical cartilage matrix was found by morphology. All of the animals with severe calcification showed the presence of chondrocyte-like cells expressing the markers sox9, collagen II, and cbfa1. Human aorta specimens showing mild to moderate media calcification also showed sox9, collagen II, and cbfa1 expression. The presence of chondrocytes in association with calcification of the media in aortas of rats with CRF mimics endochondral bone formation. The relevance of this association is further demonstrated by the chondrogenic conversion of medial smooth muscle cells in the human aorta.

Uremia-related vascular calcification: more than apatite deposition.

In the present study, we characterized and compared the mineral phase deposited in the aortic wall of two different frequently used chronic renal failure rat models of vascular calcification. Vascular calcification was induced in rats by either a 4-week adenine treatment followed by a 10-week high-phosphate diet or 5/6 nephrectomy followed by 6 weeks of 0.25 microg/kg/day calcitriol treatment and a high-phosphate diet. Multi-element mapping for calcium and phosphate together with mineral identification was performed on several regions of aortic sections by means of synchrotron X-ray-mu-fluorescence and diffraction. Bulk calcium and magnesium content of the aorta was assessed using flame atomic absorption spectrometry. Based on the diffraction data the Von Kossa-positive precipitate in the aortic regions (N=38) could be classified into three groups: (1) amorphous precipitate (absence of any diffraction peak pattern, N=12); (2) apatite (N=16); (3) a combination of apatite and magnesium-containing whitlockite (N=10). The occurrence of these precipitates differed significantly between the two models. Furthermore, the combination of apatite and whitlockite was exclusively found in the calcitriol-treated animals. These data indicate that in adenine/phosphate-induced uremia-related vascular calcification, apatite is the main component of the mineral phase. The presence of magnesium-containing whitlockite found in addition to apatite in the vitamin D-treated rats, has to be seen in view of the well-known vitamin D-stimulated gastrointestinal absorption of magnesium.

Demonstration of lanthanum in liver cells by energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy and high-resolution transmission electron microscopy.

The appearance of lanthanum in liver cells as a result of the injection of lanthanum chloride into rats is investigated by advanced transmission electron microscopy techniques, including electron energy loss spectroscopy and high-resolution transmission electron microscopy. It is demonstrated that the lysosomes contain large amounts of lanthanum appearing in a granular form with particle dimensions between 5 and 25 nm, whereas no lanthanum could be detected in other surrounding cellular components.

Effects of strontium on the physicochemical characteristics of hydroxyapatite.

In a previous experimental study using a chronic renal failure rat model, a dose-related multiphasic effect of strontium (Sr) on bone formation was found that could be reproduced in an in vitro set-up using primary rat osteoblasts. The results from the latter study allowed us to distinguish between a reduced nodule formation in the presence of an intact mineralization at low Sr-doses (1 microg/ml) and an interference of the element with the hydroxyapatite (HA) formation at high doses (20-100 microg/ml). To further investigate the latter effect of Sr on physicochemical bone mineral properties, an in vitro study was set up in which the UMR-106 rat osteosarcoma cell line was exposed to Sr, added to the cell culture medium in a concentration range varying between 0-100 microg/ml. Temporal growth and functionality of the culture was investigated by measurement of the alkaline phosphatase activity and calcium (Ca) concentration in the culture medium (used as an index of Ca-incorporation, i.e., HA formation) at various time points. At the end of the culture period (14 days post-confluence), samples of the mineralized cultures were taken for further analysis using X-ray diffraction (XRD) and Fourier Transform Infra-Red Spectroscopy (FTIR). Synthetic HA doped with various Sr concentrations (based on the cell culture and previous experimental studies and yielding Sr/(Sr + Ca) ratios ranging from 0-60%), was prepared and examined for crystal growth and solubility. Crystal size was assessed using scanning electron microscopy (SEM). Ca incorporation indicated a reduced mineralization in the 20 and 100 microg/ml Sr groups vs. controls. Sr-doped synthetic HA showed a significant dose-dependent reduction in crystal growth, as assessed by SEM, and an increase in solubility, apparent from 12.7% Sr/(Sr + Ca) on. Moreover, in both mineralized cultures and synthetic HA, XRD and FTIR analysis showed a reduced crystallinity and altered crystal lattice at similar concentrations. These new data support our previous in vivo and in vitro findings and point to a potential physicochemical interference of Sr with HA formation and crystal properties in vivo.

Dialysis.

Willem Kolff designed his "kunstmatige nier" in the early 1940s using spare parts obtained from the Wehrmacht; with it, he treated 14 patients with acute renal failure. Although there has been a tremendous improvement in the design and construction of dialysis machines, the basic concepts are unchanged. In this review we show that dialysis dose and adequacy can now be predicted using simple clinical methodology. The second part of the article discusses the accumulation or excess removal of important biologically active substances which can result in hitherto unseen clinical syndromes and even pose a threat to life.

Transferrin-mediated uptake of aluminium by human parathyroid cells results in reduced parathyroid hormone secretion.

BACKGROUND: The present study investigates whether aluminium-transferrin (Al-Tf) uptake by Tf receptor-mediated endocytosis induces hypoparathyroidism and thus might contribute to the increasing prevalence of adynamic bone disease (ABD) in the current dialysis population. METHODS AND RESULTS: Human parathyroid glands as well as in vitro cultured human parathyroid cells were shown to express Tf receptors. Five-day-old cultures of parathyroid cells were incubated for 48 h in serum-free DMEM/F12 supplemented with 12 microM apo-Tf: 12 microM Tf to which 150 microg/l Al or 150 microg/l Al-citrate (Al-ci) was bound. The amount of Al taken up by the parathyroid cells either as Al-Tf or Al-ci did not differ. However, incubation of cell cultures with Al-Tf showed a significant proportional decrease (mean+/-SEM, -23.1+/-4.5%) in iPTH secretion as compared to the reference apo-Tf cultures. Al-ci did not suppress PTH secretion (+3.4+/-6.5%). The Al uptake after incubation with Al-Tf was found to be dose-dependent. With regard to iPTH secretion, a tendency toward a dose response relationship was observed. Northern blot analysis of parathyroid cells incubated in 12 microM apo-Tf or 12 microM Al-Tf demonstrated that the PTH mRNA synthesis was unaffected by the Tf-mediated uptake of Al. These observations suggest an effect of Al on PTH release rather than on PTH synthesis. Since the cytoskeleton can play an important role in the release of secretory vesicles, the influence of Al on the structure of actin, beta-tubulin and vimentin was investigated by confocal microscopy. Comparison of cultures incubated with apo-Tf and Al-Tf revealed no difference in the organization of these cytoskeletal proteins in relation to the inhibitory effect of Al-Tf on PTH secretion. CONCLUSION: In summary, data in the present paper demonstrate that the (i) human parathyroid gland/parathyroid cells exhibit Tf receptors; (ii) Al-Tf complex is taken up by the parathyroid gland in a dose-dependent manner; and (iii) uptake of Al by Tf receptor-mediated endocytosis reduces the secretion of PTH but not its synthesis. These in vitro findings allow us to suggest that Tf receptor-mediated uptake of Al might, besides other factors such as vitamin D, high calcium dialysate or CaCO(3) intake, play a role in the development of hypoparathyroidism associated with ABD. The exact mechanism by which Al-Tf suppresses iPTH secretion remains to be elucidated.

Increased bone strontium levels in hemodialysis patients with osteomalacia.

BACKGROUND: In this study, we report on the association between increased bone strontium levels and the presence of osteomalacia in end-stage renal failure patients treated by hemodialysis. METHODS: We performed a histologic examination and determined the strontium content and strontium/calcium ratios in bone biopsies of 100 hemodialysis patients recruited from various centers all over the world. Aside from the bone strontium concentration, the bone aluminum content was assessed. The bone zinc concentration, a nonrelevant element for bone toxicity, was also measured. RESULTS: Bone strontium levels and bone strontium/calcium ratios were increased in subjects with osteomalacia when compared with those with the other types of renal osteodystrophy. Bone strontium and bone calcium levels correlated with each other. The slope of the linear regression curve correlating these parameters was much steeper in the osteomalacic group (Y = 2.22X - 120) as compared with the other types of renal osteodystrophy (Y = 0.52X - 5.7). Within the group of patients with osteomalacia, bone strontium levels also significantly correlated with the bone aluminum content (r = 0.72, P = 0.018). No such correlation was found for the other types of renal osteodystrophy. The bone zinc concentration of subjects with normal renal function did not differ significantly from the values noted for the various types of renal osteodystrophy taken as separate groups, nor could increased bone zinc concentrations be associated with a particular bone lesion. CONCLUSIONS: Our data demonstrate an association between osteomalacia and increased bone strontium concentrations in dialysis patients. Further studies are warranted to establish whether strontium plays either a primary, secondary, or contributive role in the development of the latter type of renal osteodystrophy.

Increased serum strontium levels in dialysis patients: an epidemiological survey.

BACKGROUND: We previously reported on increased bone strontium levels in dialysis patients with osteomalacia versus those presenting other types of renal osteodystrophy. A causal role of strontium in the development of osteomalacia was established in a chronic renal failure rat model. METHODS: To further elucidate the latter issue and to find out whether dialysis patients from particular centers/countries are at an increased risk for strontium accumulation, a worldwide multicenter study was established. In total, 834 patients from 34 dialysis centers in 23 countries were included. In each of the patients, a serum sample was taken for strontium determination, and water and dialysate samples were taken at the various steps of the water purification process. For each patient clinical data and for each center dialysis modalities were recorded. RESULTS: Strontium levels in serum of dialysis patients showed major differences between the various centers, ranging from mean values of 25 +/- 8 microgram/liter in the center with the lowest level up to 466 +/- 90 microgram/liter in the center with the highest concentration. It is of interest that these high levels were mainly found in developing countries. Furthermore, our data point toward a role of the final dialysate in the accumulation of the element, as indicated by the strong correlation (r = 0.74, P < 0.001) between mean serum and dialysate strontium levels. As the high tap water concentration of strontium was adequately reduced during the water purification process, contamination of the final dialysis fluid occurred by the addition of concentrates contaminated with strontium. Besides the dialysate, other factors, such as duration of dialysis, vitamin D supplements, or types of phosphate binders, played a less important role in the accumulation of the element. CONCLUSIONS: Data of this multicenter study indicate patients of particular dialysis centers to be at an increased risk for strontium accumulation, the clinical consequence of which is under current investigation.

Aluminum, iron, lead, cadmium, copper, zinc, chromium, magnesium, strontium, and calcium content in bone of end-stage renal failure patients.

BACKGROUND: Little is known about trace metal alterations in the bones of dialysis patients or whether particular types of renal osteodystrophy are associated with either increased or decreased skeletal concentrations of trace elements. Because these patients are at risk for alterations of trace elements as well as for morbidity from skeletal disorders, we measured trace elements in bone of patients with end-stage renal disease. METHODS: We analyzed bone biopsies of 100 end-stage renal failure patients enrolled in a hemodialysis program. The trace metal contents of bone biopsies with histological features of either osteomalacia, adynamic bone disease, mixed lesion, normal histology, or hyperparathyroidism were compared with each other and with the trace metal contents of bone of subjects with normal renal function. Trace metals were measured by atomic absorption spectrometry. RESULTS: The concentrations of aluminum, chromium, and cadmium were increased in bone of end-stage renal failure patients. Comparing the trace metal/calcium ratio, significantly higher values were found for the bone chromium/calcium, aluminum/calcium, zinc/calcium, magnesium/calcium, and strontium/calcium ratios. Among types of renal osteodystrophy, increased bone aluminum, lead, and strontium concentrations and strontium/calcium and aluminum/calcium ratios were found in dialysis patients with osteomalacia vs the other types of renal osteodystrophy considered as one group. Moreover, the concentrations of several trace elements in bone were significantly correlated with each other. Bone aluminum was correlated with the time on dialysis, whereas bone iron, aluminum, magnesium, and strontium tended to be associated with patient age. Bone trace metal concentrations did not depend on vitamin D intake nor on the patients' gender. CONCLUSIONS: The concentration of several trace elements in bone of end-stage renal failure patients is disturbed, and some of the trace metals under study might share pathways of absorption, distribution, and accumulation. The clinical significance of the increased/decreased concentrations of several trace elements other than aluminum in bone of dialysis patients deserves further investigation.

Low bone turnover in patients with renal failure.

Renal failure inevitably leads to metabolic bone disease. Low turnover disease or adynamic bone disease (ABD) is characterized by a low number of osteoblasts with normal or reduced numbers of osteoclasts. Mineralization proceeds at a normal rate, resulting in normal or decreased osteoid thickness. Recently, it became clear that the relative contribution of the various types of renal osteodystrophy (ROD) to the spectrum of the histologic picture in renal failure patients underwent profound changes during the last 25 years. At the moment, the exact physiopathological mechanisms behind ABD are not yet elucidated, and thus the reason(s) for its increasing prevalence remains poorly understood. A number of epidemiological and experimental data suggest a multifactorial pathophysiologic process, in which hypoparathyroidism and suppression of the osteoblast are the main actors. Compared to adynamic bone disease, osteomalacia has now become a much rarer disease (around 4%), at least in Western countries. On the other hand, recent studies indicate that this particular bone disease entity might still regularly occur in less developed countries. Osteomalacia originates from a direct effect on the mineralization process. With this type of renal bone disease, the effects of secondary hyperparathyroidism on bone are overridden by a number of metabolic abnormalities that finally result in a defective bone mineralization, as occurs, for instance, when the lag time between osteoid deposition and its mineralization is increased. The relationship between exogenous and endogenous vitamin D deficiency (mainly calcitriol) and the histologic finding of osteomalacia in uremic patients is well known. Recent data showed distinctly lowered 25-(OH) vitamin D3 levels in the presence of unaffected calcitriol concentrations in patients with osteomalacic lesions, as assessed radiologically by the presence of Looser's zones. Recently, we found that bone strontium levels were increased in patients with osteomalacia as compared to all other types of ROD. Strontium accumulation appeared to originate mainly from the use of strontium-contaminated dialysate, which resulted from the addition of strontium-containing acetate-based concentrates. Evidence for a causal role of the element in the development of a mineralization defect could be tested experimentally by adding strontium to drinking water in a chronic renal failure rat model.

Low serum aluminum values in dialysis patients with increased bone aluminum levels.

BACKGROUND: Using an HPLC/ETAAS hybrid speciation technique we previously demonstrated iron to have a multifold effect on the binding of aluminum to transferrin by limiting the number of available binding sites and decreasing the affinity of transferrin for aluminum. Theoretically, at a 60% iron-transferrin saturation the aluminum-transferrin fraction in serum should not exceed 30 microg/l. In the present study previous experimental data were confronted with recent clinical observations in patients with either normal iron status or iron overload. PATIENTS AND RESULTS: Serum aluminum levels and iron overload: In 38 dialysis patients with a normal iron status and of whom 63% received Al(OH)3 for phosphate binding 26 (68%) had a serum aluminum level >30 microg/l. On the other hand out of 28 transfusional iron overloaded patients; 68% of them taking Al(OH)3, only 1 subject (4%) had a serum aluminum value in excess of 30 microg/l. Taking patients of both groups receiving Al(OH)3 together a significant (p = 0.001) negative correlation (r = -0.5017) was found between the iron-transferrin saturation and the serum aluminum levels. Iron status and parenteral aluminum loading: Also could a significant (p = 0.001) negative correlation (r = -0.6383) between these parameters be found in an independent group of 44 patients which were acutely intoxicated by the use of aluminum-contaminated dialysis fluids. Since in this population aluminum loading occurred parenterally and not via the gastrointestinal tract, a direct effect of iron on the transferrin binding of aluminum rather than on the element's gastrointestinal absorption must have been responsible for the inverse relationship. Bone aluminum and iron overload: Out of 22 patients with a normal iron status (mean + SD serum ferritin: 216 +/- 245 microg/l; iron-transferrin saturation 20.4 +/- 9.6%), all of them having aluminum overload (bone aluminum level >15 microg/g and/or positive Aluminon staining) none of them presented with a serum aluminum <30 microg/l (mean +/- SD: 82.2 +/- 51.6 microg/l). On the other hand out of 13 iron overloaded patients (serum ferritin >800 microg/l; iron-transferrin saturation 61.4 +/- 17.6%) 10 (77%) presented the proposed criteria of aluminum overload in the presence of a serum aluminum level <30 microg/l. CONCLUSIONS: Our data indicate that in dialysis patients with iron overload (iron-transferrin saturation >60%; serum ferritin >800 microg/l) serum aluminum levels are low (<30 microg/l) despite exposure to aluminum by the intake of Al(OH)3 or the use of aluminum-contaminated dialysis fluids. Low serum aluminum nevertheless may be associated with aluminum overload and even aluminum-related bone disease. An effect of iron on the serum aluminum speciation may at least in part explain our observations. Our findings allow a more accurate interpretation of baseline serum aluminum values.

Strontium causes osteomalacia in chronic renal failure rats.

BACKGROUND: We recently reported an association between increased bone strontium (Sr) levels and osteomalacia in dialysis patients. METHODS: To delineate whether or not Sr acts as a causal factor in the development of osteomalacia, we devised the following study: four groups of chronic renal failure (CRF) rats were given Sr, aluminum (Al), both of these compounds or none of the elements (controls). RESULTS: Administration of Sr and/or A1 resulted in increased bone levels of the respective elements. Histological examination revealed impairment of mineralization in the Sr group and to a lesser extent in the Al group as compared to the control group. There was also a significant increase in osteoid area in the Sr group, but not in the Al group. No differences in bone surface or erodic perimeter were noted between the various study groups. Histochemically, Sr could be localized in calcified bone, mainly in new bone close to the osteoid/calcification front, a critical site of bone mineralization. Histochemical findings were confirmed by electron probe X-ray microanalysis. CONCLUSIONS: These findings indicate that Sr accumulation in chronic renal failure rats resulted in the development of osteomalacic lesions, in contrast to the Al group where adynamic bone disease was induced in the present set-up. Further studies are required to define the mechanism by which way Sr causes osteomalacia in chronic renal failure rats.

Al and Si: their speciation, distribution, and toxicity.

OBJECTIVES: In dialysis patients both aluminum (AI) and silicon (Si) may accumulate. Whereas the toxic effects of AI within this population are clearly established, little is known on the role of Si in the development/protection of particular dialysis-related diseases. A clear insight in the protein binding and speciation of trace elements is important to better understand the mechanisms underlying their toxicity/essentiality. Research in this field however is complex and often prone to analytical difficulties and inaccuracies. DESIGN AND METHODS: In the first part of this review techniques used for speciation studies of AI and Si in biological fluids are discussed. Notwithstanding recent technical advances (a) extraneous metal contamination, (b) unrecognized aspecific binding of metals to proteins, and (c) unwanted interactions with separation equipment such as chromatography columns and ultrafiltration membranes remain important pitfalls and often lead to erroneous conclusions. The factors that determine the speciation of AI and Si and their ultimate tissue distribution and toxicity are dealt with in the second part. Here, experimental data obtained with various speciation techniques are linked to in vivo data on the tissue distribution, localization/toxicity of both elements. CONCLUSIONS: A model in which the AI tissue distribution/toxicity is mediated by either its citrate or transferrin bound form is proposed.