Net Phosphate Absorption Research Articles

Biochemical assessment of phosphate homeostasis

Phosphate homeostasis is a requirement for normal life. Phosphate is involved in the synthesis of membrane lipids, DNA, RNA, and energy-rich molecules (ATP and GTP), and the regulation of protein activity by phosphorylation/dephosphorylation. Moreover, phosphate is a component of apatite crystals, which provide stability to the bone, and is essential for normal growth. Phosphate balance in the body is the difference between net phosphate absorption through the intestine and phosphate excretion through the kidney. Numerous disorders, both genetic and acquired, may alter phosphate homeostasis. In affected individuals, it is crucial to identify the underlying mechanism(s) to provide adequate treatment; however, phosphate homeostasis assessment remains challenging. Besides the measurement of key hormones involved in the control of phosphate homeostasis (parathyroid hormone, vitamin D and metabolites, fibroblast growth factor 23), assessing the magnitude of phosphate reabsorption by the kidney is a crucial step. It makes it possible to distinguish between a primary disorder of renal phosphate reabsorption, associated with an intrinsic defect or endocrine disturbance, and a nutritional cause of phosphate deficiency. This strategy is described, and the potential consequences for therapeutic decisions are discussed.

Métabolisme du phosphore

Phosphate homeostasis is a requirement for normal life. Phosphate is involved in synthesis of membrane lipids, DNA, and RNA molecules, of energy-rich molecules (ATP or GTP), and is implicated in the regulation of protein activity by phosphorylation/dephosphorylation. Moreover phosphate as integrated in apatite crystals provides stability to the bone and is essential for normal growth. The adequate phosphate balance in the organism results essentially from the net summation of net phosphate absorption from intestine and output fluxes through the kidney. Disorders of phosphate metabolism are multiple from genetic and acquired causes. Thus assessing the status of phosphate metabolism in individuals remains a challenge in order to provide adequate care. Besides the measurement of hormones critically involved in the control of phosphate metabolism (parathyroid hormone, vitamin D, fibroblast growth factor 23), assessing the magnitude of phosphate reabsorption by the kidney is a crucial step. It allows to discriminate between a primary disorder of renal phosphate reabsorption, of intrinsic or endocrine origin, and a nutritional cause of phosphate deficiency. This strategy is described and the potential consequences for therapeutic decisions are discussed.

Novel Approaches for Dietary Phosphorus Management in Chronic Kidney Disease

<p>Conventional approaches to alleviating hyperphosphatemia in patients with Chronic Kidney Disease (CKD) include dietary phosphorus restriction and use of phosphate binders. These approaches are, however not enough to control hyperphosphatemia and hence the need for novel nutritional approaches arises. Dietary phosphorus, protein and phytate values of some common food ingredients were obtained. Phosphates to protein ratio and net phosphorus absorption from the gut were reported on the basis of the phytate content of these ingredients. Among some common Indian food ingredients, phosphate to protein ratio was high in cereals, millets and dairy products. However due to presence of high amount of phytate, the net absorption of phosphate was lower from plant based sources of phosphorus as compared to animal based sources. It can be concluded that phosphorus to protein ratio is a metric which helps to ensure dietary phosphorus restriction with sufficient intake of proteins. Tables providing phosphorus to protein ratio along with phytate and per cent absorption may prove to be beneficial in achieving desired target levels of phosphorus for renal dieticians in case of CKD patients with hyperphosphatemia.</p>

Impact of westernization on fibroblast growth factor 23 levels among individuals of African ancestry.

The Western diet is associated with high consumption of processed foods preserved with phosphate. Higher dietary phosphate consumption stimulates production of fibroblast growth factor 23 (FGF23), which heightens risk for cardiovascular disease and mortality. We hypothesized that adults living in a more westernized society have higher levels of FGF23 due to increased phosphate consumption as measured by urinary phosphate excretion. We measured plasma C-terminal FGF23 levels and urinary phosphate and creatinine levels in timed urine collections among 100 African adults living in the rural area of Igbo-Ora, Nigeria (52 women, 48 men), and 100 African Americans (32 women, 68 men) living in Maywood, IL, an urban suburb of Chicago, IL, USA. Among these 200 participants, urine collections were adequate in 76 and 68 of the Maywood and Igbo-Ora participants, respectively. In the total group, the mean age and body mass index, respectively, were 34.6 ± 8.2 years and 22.1 ± 3.9 kg/m(2) in Igbo-Ora, and 42.8 ± 7.2 years and 25.8 ± 6.5 kg/m(2) in Maywood. Demographic characteristics for each site were very similar after excluding participants without adequate urine collections. Among all 200 participants, the median (interquartile range) FGF23 levels were significantly higher in Maywood versus Igbo-Ora [63.8 (45.0-89.9) versus 12.5 RU/mL (8.5-18.5); P < 0.0001] and these differences did not change substantially after excluding nine women from Maywood with FGF23 levels >400 RU/mL or after excluding participants with inadequate urine collections. Among participants with adequate urine collections, the mean 24-h urinary phosphate excretion was significantly higher in Maywood versus Igbo-Ora (810.6 ± 309.0 versus 347.5 ± 153.1 mg; P < 0.001) and FGF23 levels correlated significantly with total urinary phosphate excretion (r = 0.62; P < 0.001) and urinary phosphate-to-creatinine ratios (r = 0.50; P < 0.001). Living in a more westernized society may be associated with greater net phosphate absorption, as reflected by higher urinary phosphate excretion, and higher FGF23 levels.

Different phosphate transport in the duodenum and jejunum of chicken response to dietary phosphate adaptation.

Intestinal phosphate (Pi) absorption across the apical membrane of small intestinal epithelial cells is mainly mediated by the type IIb Na-coupled phosphate co-transporter (NaPi-IIb), but its expression and regulation in the chicken remain unclear. In the present study, we investigated the mRNA and protein levels of NaPi-IIb in three regions of chicken small intestine, and related their expression levels to the rate of net phosphate absorption. Our results showed that maximal phosphate absorption occurs in the jejunum, however the highest expression levels of NaPi-IIb mRNA and protein occurs in the duodenum. In response to a low-Pi diet (TP 0.2%), there is an adaptive response restricted to the duodenum, with increased brush border membrane (BBM) Na-Pi transport activity and NaPi-IIb protein and mRNA abundance. However, when switched from a low- (TP 0.2%) to a normal diet (TP 0.6%) for 4 h, there is an increase in BBM NaPi-IIb protein abundance in the jejunum, but no changes in BBM NaPi-IIb mRNA. Therefore, our study indicates that Na-Pi transport activity and NaPi-IIb protein expression are differentially regulated in the duodenum vs the jejunum in the chicken.

Comparison of Dietary Phosphate Absorption After Single Doses of Lanthanum Carbonate and Sevelamer Carbonate in Healthy Volunteers: A Balance Study

Lanthanum carbonate and sevelamer carbonate are noncalcium phosphate binders used to treat hyperphosphatemia in patients with chronic kidney disease. This is the first study to compare phosphate absorption from a standardized meal ingested with a typical clinical dose of these binders. Randomized open-label crossover study. Healthy volunteers were confined to a clinical research center during 4 study periods. Of 31 volunteers randomly assigned, 19 completed all treatments and 18 were analyzed in the pharmacodynamic set (1 was excluded because of vomiting). Participants were assigned in random order to meal alone, meal plus lanthanum carbonate (1 tablet containing 1,000 mg of elemental lanthanum), and meal plus sevelamer carbonate (three 800-mg tablets). The gastrointestinal tract was cleared, the meal was ingested (± treatment), and rectal effluent was collected. In a fourth period, volunteers repeated the study procedures while fasting. The primary end point, net phosphate absorption, was analyzed using a mixed-effect linear model. Phosphorus content of effluent and duplicate meal samples were measured using inductively coupled plasma-optical emission spectroscopy. The standard meal contained ∼375 mg of phosphate, 75% of which was absorbed (net absorption, 281.7 ± 14.1 mg [adjusted mean ± standard error]). Lanthanum carbonate decreased net phosphate absorption by 45% (net absorption, 156.0 ± 14.2 mg) compared with 21% (net absorption, 221.8 ± 14.1 mg) for sevelamer carbonate (P < 0.001). Lanthanum carbonate bound 135.1 ± 12.3 mg of phosphate, whereas sevelamer carbonate bound 63.2 ± 12.3 mg, a 71.9-mg difference (95% CI, 40.0-103.8; P < 0.001). Per tablet, this equates to 135 mg of phosphate bound with lanthanum carbonate versus 21 mg with sevelamer carbonate. A single-dose study. In healthy volunteers, 1,000 mg of lanthanum carbonate decreased phosphate absorption by 45% compared with a 21% decrease with 2,400 mg of sevelamer carbonate.

Mechanism of 24,25‐dihydroxyvitamin D3‐mediated inhibition of rapid, 1,25‐dihydroxyvitamin D3‐induced responses: Role of reactive oxygen species

In intestine, 24,25(OH)(2)D(3), which is made under conditions of calcium-, phosphate-, and 1,25(OH)(2)D(3) sufficiency, inhibits the stimulatory actions of 1,25(OH)(2)D(3) on phosphate and calcium absorption. In the current work, we provide evidence that 24,25(OH)(2)D(3)-mediated signal transduction occurs mechanistically through increased H(2)O(2) production which involves binding of 24,25(OH)(2)D(3) to catalase and resultant decreases in enzyme activity. Physiological levels of H(2)O(2) mimicked the action of 24,25(OH)(2)D(3) on inhibiting 1,25(OH)(2)D(3)-stimulated phosphate uptake in isolated enterocytes. Moreover, the molecular basis of such inhibition was suggested by the presence of two thioredoxin domains in the 1,25D(3)-MARRS protein/ERp57: Exposure of cells to either 24,25(OH)(2)D(3) or H(2)O(2) gradually reduced 1,25(OH)(2)D(3) binding to 1,25D(3)-MARRS protein, between 10 and 20 min of incubation, but not to VDR. Feeding studies with diets enriched in the antioxidants vitamins C and E showed that net phosphate absorption in vivo nearly doubled relative to chicks on control diet. Antioxidant diets also resulted in increased [(3)H]1,25(OH)(2)D(3) binding to both 1,25D(3)-MARRS and VDR, suggesting benefits to both transcription- and membrane-initiated signaling pathways. Intriguingly, phosphorous content of bones from birds on antioxidant diets was reduced, suggesting increased osteoclast activity. Because mature osteoclasts lack VDR, we analyzed a clonal osteoclast cell line by RT-PCR and found it contained the 1,25D(3)-MARRS mRNA. The combined data provide mechanistic details for the 1,25(OH)(2)D(3)/24,25(OH)(2)D(3) endocrine system, and point to a role for the 1,25D(3)-MARRS protein as a redox-sensitive mediator of osteoclast activity and potential therapeutic target.

Intestinal phosphate absorption and the effect of vitamin D: a comparison of rats with mice

Previously, it was thought that intestinal phosphate transport occurred exclusively in the proximal small intestine of rodents and humans. However, a recent study has demonstrated that the ileum of mice contributes significantly to the absorption of dietary phosphate, but it is not known whether this region is also an important site of phosphate absorption in the rat. In the present study, we have investigated the mRNA and protein levels of the sodium-phosphate cotransporter, NaPi-IIb, in three regions of rat and mouse small intestine, and related its expression levels to the rate of net phosphate absorption, as measured using the in situ intestinal loop technique. 1,25-Dihydroxyvitamin D3 is an important physiological regulator of intestinal phosphate absorption that increases phosphate transport in both the duodenum and jejunum of the rat. Based on the recently proposed regional profile of phosphate absorption along the mouse small intestine, we have re-evaluated the effects of 1,25-dihydroxyvitamin D3 using three distinct regions of the mouse and rat small intestine. Our studies have revealed important differences in the intestinal handling of phosphate between mice and rats. In mice, maximal phosphate absorption occurs in the ileum, which is paralleled by the highest expression levels of NaPi-IIb mRNA and protein. In contrast, in rats maximal absorption occurs in the duodenum with very little absorption occurring in the ileum, which is similar to the pattern reported in humans. However, in both rodent species only the jejunum shows an increase in phosphate absorption in response to treatment with 1,25-dihydroxyvitamin D3.

Phosphate transport in isolated rat inner medullary collecting duct.

Phosphate transport by the inner medullary collecting duct of normal rats was studied using an in vitro microperfusion technique. Net (Jnet), lumen-to-bath (Jlb) and bath-to-lumen (Jbl) phosphate fluxes were measured using 32PO4 as tracer, in the absence of net water absorption. A net absorption of phosphate (22.3 +/- 3.3 pmol cm-2 s-1) was observed by direct determination, and was similar to the difference between the Jlb and Jbl (57.7 +/- 8.2 and 32.2 +/- 1.5 pmol cm-2 s-1 respectively). The addition of amiloride (10 microM) to the perfusate did not change the Jlb of phosphate but blocked the efflux of sodium. Also, the withdrawal of sodium from the bath and perfusion solution did not change the Jlb of phosphate. In parallel, the addition of ouabain (10 mM) to the bath fluid decreased the Jlb of sodium more (37%) than the Jlb of phosphate (12%) and did not change the Jbl of phosphate. The addition of arsenate (10 microM) to the perfusate both in the presence and in the absence of sodium caused a decrease in Jlb, but Jbl remained unchanged, and parathyroid hormone (10 U) added to the bath did not change the Jlb. The increase in pH of the bath and perfusion fluid was associated with an increase in the Jlb of phosphate, and the decrease in pH was similarly followed by a decrease in phosphate efflux. The Jbl did not change with the pH alterations. These data demonstrate that a net phosphate absorption takes place in rat inner medullary collecting duct perfused in vitro and that this transport appears to be independent of sodium absorption and the action of parathyroid hormone. Moreover, a decrease in luminal and bath pH induces a decrease in phosphate efflux.

Effects of intraluminal pH and dietary phosphate on phosphate transport in the proximal convoluted tubule.

The proximal tubule cell adjusts its phosphate absorption appropriately to meet the needs of the organism. Studies were performed to characterize some of the cellular changes involved with dietary phosphate adaptation. First, early proximal convoluted tubules were perfused with equilibrium Ringer solutions buffered to pH 7.65 or 6.5. Saturation kinetics for phosphate transport were determined at each pH value. Rats maintained on a diet of normal phosphate composition demonstrated the apparent Jmax and Km parameters about twofold greater with intraluminal pH 7.65 vs. pH 6.5. The Jmax values increased to 53.47 +/- 3.71 and 42.73 +/- 5.48 pmol X min-1 X mm-1, respectively, when the rats were placed on a phosphate-restricted diet for 5 days. By contrast, adaptation to a high dietary phosphate content resulted in diminished phosphate absorption, 8.53 +/- 1.80 and 12.87 +/- 1.61 pmol X min-1 X mm-1, for the respective pH 7.65 and 6.5 values. Second, the effect of peritubule phosphate concentration was evaluated at constant intraluminal phosphate concentrations. Unidirectional lumen-to-blood phosphate efflux was inhibited at all plasma phosphate concentrations in animals maintained on normal dietary phosphate. By contrast, rats adapted to a low phosphate diet exhibited an increase in phosphate absorption: pH 7.65, 86.21 +/- 2.63, and pH 6.5, 140.84 +/- 86.76 pmol X min-1 X mm-1 when plasma concentrations were elevated two- or three-fold from 2.4 to 6.4 mM. This was attributed to enhanced phosphate exchange on the basolateral membrane. Further hyperphosphatemic levels, above 6.4 mM, inhibited phosphate absorption. These data suggest that net phosphate absorption is determined, in part, by factors other than sodium-dependent uptake of phosphate by the brush border membrane including intracellular pH and peritubular phosphate that act in concert to control renal phosphate absorption.

Phosphate transport in the light segment of the rabbit cortical collecting tubule.

The mode and mechanism of phosphate transport in the light portion of the rabbit cortical collecting duct were studied with isolated tubular perfusion. Control studies (n = 48) revealed a mean transepithelial potential difference (PD) of -16.5 +/- 1.3 mV and net phosphate absorption (JPO4) of 0.58 +/- 0.07 pmol.mm-1.min-1. To characterize transport, the effects of flow rate, ouabain, amiloride, and phosphate concentration, and parathyroid hormone (PTH) were analyzed. There was a significant linear relationship between JPO4 and flow rate (n = 48) (r = 0.71, P less than 0.025). Increasing the difference between lumen and bath phosphate concentration from -8.0 to +8.0 mg/dl was associated with a stepwise increase in JPO4 from -0.18 +/- 0.02 to 0.93 +/- 0.05 pmol.mm-1.min-1. PTH had no effect on JPO4. To assess the effects of PD, the tubules were treated with either amiloride (n = 6) or ouabain (n = 7). JPO4 fell significant in both circumstances with reduction of PD, although there remained significant residual transport following amiloride addition. The data demonstrate significant phosphate transport in this segment that is independent of PTH and altered by transepithelial electrical and concentration gradients as well as flow rate.

Regulation of Na-dependent phosphate influx across the mucosal border of duodenum by 1,25-dihydroxycholecalciferol.

Animals teated with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP), at doses which decrease the renal production and/or the plasma levels of 1,25-dihydroxycholecalciferol [1,25(OH)2D3], display a reduced net absorption of phosphate. In this study we investigated whether EHDP-treatment and administration of 1,25(OH)2D3 to EHDP-treated animals affected the phosphate influx across the mucosal border of rabbit duodenum. The initial rate of phosphate influx into mucosal cells was measured in isolated intestine. In control, untreated rabbits, the phosphate influx shows a saturable, Na-dependent component and a diffusional, Na-independent uptake. In tissue from rabbits treated for 3 days with EHDP, the phosphate influx was found to be strongly reduced. EHDP-treatment decreased the Na-dependent, carrier mediated phosphate influx in duodenum. Administration of 1,25(OH)2D3 to EHDP-treated animals reversed the reduced phosphate influx. These effects were mainly apparent through changes in the J(mc)(max) of the phosphate influx, which was decreased from 211 +/- 38.7 nmol/cm2h in controls to 42.1 +/- 18.1 nmole/cm2 h in the EHDP-treated group and increased to 413 +/- 43.6 nmole/cm2 h by 1,25(OH)2D3. The treatment did not appear to affect the diffusional, Na-independent phosphate influx. EHDP-treatment did not affect the influx of alanine in this segment suggesting that EHDP-treatment affects only 1,25(OH)2D3-dependent transport mechanisms. The results suggest that 1,25(OH)2D3 modulates the number of carrier sites available at the mucosal membrane for Na-dependent phosphate entry.

The use of a Thiry-Vella loop of jejunum to study the intestinal absorption of calcium and inorganic phosphate in the conscious pig

1. Pigs, each surgically prepared with a Thiry-Vella jejunal loop were used to study the absorption of calcium and inorganic phosphate from the intestine.2. The loops were perfused daily for 6–8 h with a nutrient solution and absorption from the perfusate was measured.3. The technique employed minimized atrophy of the jejunal mucosa and enabled the measurement of hourly or daily changes in absorption rate of components of the luminal fluid.4. No differences were observed when polyethylene glycol (PEG; molecular weight 4000), [14C]PEG or 51Cr-EDTA were used as markers of net water movement.5. Increasing the concentration of Ca in the perfusate resulted in the demonstration of a two-component relationship between net absorption rate of Ca and intraluminal Ca concentration. An initial rapid absorption rate from 0 to 4 mm was found, then a slower rate from approximately 5 mm upwards which did not saturate at the highest concentration tested (25 mm).6. Increasing the concentration of phosphate in the perfusion solution increased the net absorption of phosphate from that solution. Although a two-component system, similar to that for Ca, was not evident, net absorption of phosphate was not saturated at the highest concentration tested (50 mm).7. The absorption of Ca was unaffected by the presence of phosphate in the solution but Ca (2.5 mm) enhanced the absorption of phosphate.

Effect of parathyroid function on absorption and excretion of calcium, magnesium and phosphate by rats.

The effects of parathyroidectomy, of parathyroid extract administration and of vitamin D on calcium, phosphate and magnesium metabolism by adult male rats have been studied. Parathyroidectomy had no effect on net calcium absorption or calcium balance but the administration of parathyroid gland extract to parathyroidectomized rats decreased them. Parathyroidectomy also had no significant effect on net phosphate absorption or balance. The administration of parathyroid gland extract to parathyroidectomized rats did not change net phosphate absorption but lowered phosphate balance as a consequence of hyperphosphaturia. The secretion of calcium or phosphate into the intestinal tract was unaffected by the parathyroid status or the serum levels of calcium or phosphate. Supplemental vitamin D increased calcium absorption and balance by parathyroidectomized rats much more than by their controls; it had no significant effect on phosphate absorption by intact rats but increased absorption by parathyroidectomized rat...

THE ABSORPTION OF INORGANIC PHOSPHATE FROM THE SMALL INTESTINE OF THE RAT

1. The net absorption of inorganic phosphate has been measured in the jejunum and ileum of male albino rats.2. The net absorption rate of inorganic phosphate increased with decreasing hydrogen‐ion concentration. No optimum value for absorption was found in the pH range 4·4 to 7·9.3. Increasing the concentration of phosphate in the intestinal lumen increased the net absorption rate linearly. No evidence for a rate‐limiting process was found.4. The absorption rate of phosphate was relatively uninfluenced by the tonicity of the intestinal contents, but was greatly diminished in the presence of low concentrations of sodium.5. The presence of 37 mM/l. glucose in the intestinal contents had no effect on the net absorption of phosphate.6. Inorganic phosphate was absorbed more rapidly from the jejunum than from the ileum: the difference in rate may be accounted for by the difference in mucosal area between the two regions.