A model of the kinetics of lanthanum in human bone, using data collected during the clinical development of the phosphate binder lanthanum carbonate.

OBJECTIVE: Lanthanum carbonate (Fosrenol) is a non-calcium phosphate binder that controls hyperphosphataemia without increasing calcium intake above guideline targets. The biological fate and bone load of lanthanum were modelled with the aid of a four-compartment kinetic model, analogous to that of calcium. METHODS: The model used data from healthy subjects who received intravenous lanthanum chloride or oral lanthanum carbonate, and bone lanthanum concentration data collected from dialysis patients during three long-term trials (up to 5 years). RESULTS: Infusion of lanthanum chloride or ingestion of lanthanum carbonate led to a rapid rise in plasma lanthanum concentrations, followed by an exponential decrease. Comparison of oral and intravenous exposure confirmed that lanthanum is very poorly absorbed. On a typical intake of lanthanum (3000 mg/day as lanthanum carbonate), the rate of absorption was calculated as 2.2 microg/h, with a urinary excretion rate constant of 0.004-0.01 h(-1). The faecal content of endogenous lanthanum was estimated to be 8- to 20-fold greater than that of urine, compared with a ratio of only about 1 for calcium. The model predicts that upon multiple dosing, plasma lanthanum concentrations rise rapidly to a near plateau and then increase by about 3% per year. However, this small change is obscured by the variability of the study data, which show that a plateau is rapidly attained by 2 weeks and is thereafter maintained for at least 2 years. The initial deposition rate of lanthanum in bone was 1 microg/g/year and, after 10 years of lanthanum carbonate treatment, the model predicts a 7-fold increase in total bone lanthanum (from 10 mg to 69 mg [from 1 microg/g wet weight to 6.6 microg/g wet weight]), with lanthanum cleared after cessation of treatment at 13% per year. The model indicates that lanthanum flow from bone surface to bone interior is much lower than that of calcium. CONCLUSION: Bone is the major reservoir for metals, but bone lanthanum concentrations are predicted to remain low after long-term treatment because of very poor intestinal absorption.

Calcium nutrition and metabolism.

An adequate calcium intake throughout life is essential for maintenance of the skeleton, by far the largest body reservoir of calcium. Appropriately high calcium intake is particularly important in the first two decades, when the body calcium mass increases to near maximum. In subsequent decades, because calcium absorption is relatively modest, typically 25% or less, calcium intake must be kept near 1000 mg per day in order to minimize the possibility that the skeleton will be mined for its mineral content. The amount of calcium needed for signaling and to maintain the extracellular calcium constant is relatively small; however, skeletal turnover is enhanced in calcium deficiency, the increased turnover representing the body's attempt to preserve skeletal calcium.

Founding editorial--bone biology.

The skeleton is a complicated vertebrate structure, comprised of bone cells that form, modulate, and resorb the extracellular structure of bone. It is the extracellular structure, made up of the bone mineral (largely calcium phosphate) and the bone matrix, which constitutes the visible skeleton and the mechanical support for the vertebrate body. The matrix is the protein structure on which the bone mineral is laid down, many components of which have been identified in recent years.

Modulation of vitamin d status and dietary calcium affects bone mineral density and mineral metabolism in göttingen minipigs.

Calcium and vitamin D deficiency impairs bone health and may cause rickets in children and osteomalacia in adults. Large animal models are useful to study experimental osteopathies and associated metabolic changes. We intended to modulate vitamin D status and induce nutritional osteomalacia in minipigs. The control group (n = 9) was fed a semisynthetic reference diet with 6 g calcium and 6,500 IU vitamin D3/kg and the experimental group (n = 10) the same diet but with only 2 g calcium/kg and without vitamin D. After 15 months, the deficient animals were in negative calcium balance, having lost bone mineral density significantly (means ± SEM) with -51.2 ± 14.7 mg/cm(3) in contrast to controls (-2.3 ± 11.8 mg/cm(3)), whose calcium balance remained positive. Their osteoid surface was significantly higher, typical of osteomalacia. Their plasma 25(OH)D dropped significantly from 60.1 ± 11.4 nmol/L to 15.3 ± 3.4 nmol/L within 10 months, whereas that of the control group on the reference diet rose. Urinary phosphorus excretion and plasma 1,25-dihydroxyvitamin D concentrations were significantly higher and final plasma calcium significantly lower than in controls. We conclude that the minipig is a promising large animal model to induce nutritional osteomalacia and to study the time course of hypovitaminosis D and associated functional effects.

Recent developments in intestinal calcium absorption.

Calcium absorption proceeds by transcellular and paracellular flux, with the latter accounting for most absorbed calcium when calcium intake is adequate. Vitamin D helps regulate transcellular calcium transport by increasing calcium uptake via a luminal calcium channel and by inducing the cytosolic calcium transporting protein, calbindinD(9k). Recent studies utilizing knockout mice have challenged the functional importance of the channel and calbindin. To integrate the new findings with many previous studies, the function of the two molecules must be evaluated in the calcium transport and economy of mice. When calcium intake is high, transcellular calcium transport contributes little to total calcium absorption. Therefore, increasing calcium intake seems the most effective nutritional approach to ensure adequate absorption and prevent bone loss.

Association between the abnormal expression of matrix-degrading enzymes by human osteoarthritic chondrocytes and demethylation of specific CpG sites in the promoter regions.

OBJECTIVE: To investigate whether the abnormal expression of matrix metalloproteinases (MMPs) 3, 9, and 13 and ADAMTS-4 by human osteoarthritic (OA) chondrocytes is associated with epigenetic "unsilencing." METHODS: Cartilage was obtained from the femoral heads of 16 patients with OA and 10 control patients with femoral neck fracture. Chondrocytes with abnormal enzyme expression were immunolocalized. DNA was extracted, and the methylation status of the promoter regions of MMPs 3, 9, and 13 and ADAMTS-4 was analyzed with methylation-sensitive restriction enzymes, followed by polymerase chain reaction amplification. RESULTS: Very few chondrocytes from control cartilage expressed the degrading enzymes, whereas all clonal chondrocytes from late-stage OA cartilage were immunopositive. The overall percentage of non-methylated sites was increased in OA patients (48.6%) compared with controls (20.1%): 20% versus 4% for MMP-13, 81% versus 47% for MMP-9, 57% versus 30% for MMP-3, and 48% versus 0% for ADAMTS-4. Not all CpG sites were equally susceptible to loss of methylation. Some sites were uniformly methylated, whereas in others, methylation was generally absent. For each enzyme, there was 1 specific CpG site where the demethylation in OA patients was significantly higher than that in controls: at -110 for MMP-13, -36 for MMP-9, -635 for MMP-3, and -753 for ADAMTS-4. CONCLUSION: This study provides the first evidence that altered synthesis of cartilage-degrading enzymes by late-stage OA chondrocytes may have resulted from epigenetic changes in the methylation status of CpG sites in the promoter regions of these enzymes. These changes, which are clonally transmitted to daughter cells, may contribute to the development of OA.

Mechanisms and functional aspects of intestinal calcium absorption.

Calcium absorption, in terms of mechanisms and function, is well adapted to meet the calcium needs of mammals. When calcium levels in the food are low, the active, mediated transcellular calcium transport assumes primary importance. This process is vitamin D-dependent, largely localized in the duodenum, and involves three steps: entry across the brush border, mediated by a molecular structure, CaT1, with two components; a facilitated transport that saturates at low luminal calcium concentration; and a channel component through which most calcium enters the cell at the higher luminal concentrations. Intracellular diffusion is assured by a small, cytosolic calcium binding molecule, calbindinD(9k), which carries more than 90% of the calcium that traverses the duodenal cell, thus also serving as a buffer. Extrusion is by the CaATPase and is not a limiting step. Calcium entry is reduced by more than 90% in the absence of vitamin D, with biosynthesis of calbindinD(9k) totally vitamin D-dependent. Active transport is upregulated on low calcium intake and downregulated at high calcium intake, when paracellular calcium transport through the tight junctions of the intestine becomes the dominant process. The amount of calcium absorbed paracellularly is a function of the calcium gradient between lumen and plasma and of the time the chyme spends at a given intestinal site. The coexistence of mediated and nonmediated transport processes assures the organism of an adequate calcium supply, yet prevents excessive calcium absorption.

Mechanisms of intestinal calcium absorption.

Calcium is absorbed in the mammalian small intestine by two general mechanisms: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. The transcellular process involves three major steps: entry across the brush border, mediated by a molecular structure termed CaT1, intracellular diffusion, mediated largely by the cytosolic calcium-binding protein (calbindinD(9k) or CaBP); and extrusion, mediated largely by the CaATPase. Chyme travels down the intestinal lumen in approximately 3 h, spending only minutes in the duodenum, but over 2 h in the distal half of the small intestine. When calcium intake is low, transcellular calcium transport accounts for a substantial fraction of the absorbed calcium. When calcium intake is high, transcellular transport accounts for only a minor portion of the absorbed calcium, because of the short sojourn time and because CaT1 and CaBP, both rate-limiting, are downregulated when calcium intake is high. Biosynthesis of CaBP is fully and CaT1 function is approximately 90% vitamin D-dependent. At high calcium intakes CaT1 and CaBP are downregulated because 1,25(OH)(2)D(3), the active vitamin D metabolite, is downregulated.