Renal Tubular Reabsorption Of Phosphate Research Articles

Hepatocyte Nuclear Factor 1, a Transcription Factor at the Crossroads of Glucose Homeostasis

Abstract. Hepatocyte nuclear factor 1 (HNF1) is a transcription factor involved in the regulation of a large set of hepatic genes, including albumin, β-fibrinogen, and α1-antitrypsin. HNF1 is expressed in the liver, digestive tract, pancreas, and kidney. Mice lacking HNF1 exhibit hepatic, pancreatic, and renal dysfunctions. HNF1-deficient mice fail to express the hepatic phenylalanine hydroxylase gene, giving rise to hyperphenylalaninemia. Renal proximal tubular reabsorption of glucose, phosphate, arginine, and other metabolites is affected, producing severe renal glucosuria, phosphaturia, and amino aciduria. Homozygous mutant mice also exhibit a dramatic insulin secretion defect. This dysfunction resembles that exhibited by patients with maturity-onset diabetes mellitus of the young type 3, who carry mutations in the human HNF1 gene in the heterozygous state. These data show that HNF1 is a major regulator of glucose homeostasis, regulating the expression of genes that are expressed in the liver, kidney, and pancreas.

Close Correlation between Estrogen Treatment and Renal Phosphate Reabsorption Capacity

To determine the influence of estrogen on the activity of renal proximal tubular reabsorption of inorganic phosphate (Pi) in women, we examined the changes of the renal threshold phosphate concentration (also denoted as TmP/GFR), as well as the changes in the concentrations of mineral components in the circulation in two groups of women—one receiving hormone replacement therapy (HRT) and one receiving gonadotropin-releasing hormone agonists (GnRH-a) therapy. We also examined the changes in the concentrations of serum PTH in the GnRH-a group. The patients in the HRT group were continuously treated with 0.625 mg conjugated equine estrogens plus 2.5 mg medroxyprogesterone acetate per day. The patients in the GnRH-a group were treated with a monthly injection of 3.75 mg leuprolide acetate depot for 6 months. The values of TmP/GFR decreased in all of the patients who received HRT. The mean percentage change in TmP/GFR was− 14.5% (range, −24.3% to −9.6%). In contrast, in all of the patients treated with GnRH-a, the values of TmP/GFR increased after 6 months of treatment (the mean percentage change was 28.5%; range, 18.2–78.3%) and returned to the preadministration level at 12 weeks after stopping therapy. In these patients, both the values of TmP/GFR and the concentrations of serum Pi correlated significantly with circulating estradiol levels (r = −0.767, P< 0.01 and r = −0.797, P < 0.01, respectively), but the concentrations of serum corrected calcium did not correlate. Moreover, in the same patients, the levels of serum intact PTH decreased significantly (P < 0.05) after 6 months of treatment, but at 12 weeks after stopping therapy the trends of these levels varied among individual patients. These results suggest that estrogen could act directly to suppress sodium-dependent Pi reabsorption in the renal proximal tubules.

Calculation of renal tubular reabsorption of phosphate: the algorithm performs better than the nomogram.

The ratio of the renal tubular maximum reabsorption rate of phosphate to the glomerular ®ltration rate (TmP/GFR) can be used to indicate the need for phosphate replacement (and the extent of its intracellular repletion), to monitor recovery of renal tubular function after damage and to help with the diagnosis of some rare diseases, e.g. X-linked hypophosphataemic rickets, hereditary hypophosphataemic rickets with hypercalciuria, and oncogenic osteomalacia. Calculation of the ratio requires measurement of the concentrations of plasma and urine phosphate (Pp and Up) and creatinine (Pcr and Ucr) in specimens taken in a fasting steady state. The TmP/GFR is calculated with the nomogram derived by Walton and Bijvoet or with the algorithm derived by Kenny and Glen from the data of Bijvoet, Morgan and Fourman. Both methods require the calculation of the fractional tubular reabsorption of phosphate (TRP):

Isoproterenol Increases Renal Tubular Reabsorption of Phosphate in X-Linked Hypophosphatemic (Hyp) Mice

Hypophosphatemic (Hyp) mice have a reduced renal retention of phosphate. The β-adrenergic agonist isoproterenol reverses phosphaturia induced by parathyroid hormone in the rat. This study determined whether isoproterenol could raise the renal tubular maximum transport of phosphate (TmP) in Hyp mice. Male 10- to 12-week-old normal and Hyp mice were infused with ³H-inulin and isoproterenol (0.0, 0.25, 0.5 and 1.0 µg/kg/min) in 0.9% NaCl. Glomerular filtration rate, plasma phosphate, urinary phosphate excretion, TmP, urinary sodium excretion, and urinary cyclic adenosine monophosphate were measured. As expected, Hypcontrols (0.0 dose) had a TmP which was significantly below that of normal controls: 1.15±(SEM) 0.6 (n = 9) versus 2.13±0.10 (n = 11) µmol P/ml glomerular filtrate (p<0.001). Isoproterenol at doses of 0.5 and 1.0 µg/kg/min elevated the TmP of Hyp mice to the level of normal controls: 1.94±0.19 (n = 7) and 1.98±0.10 (n = 9) µmol P/ml glomerular filtrate, respectively. These results indicate that the low tubular reabsorption of phosphate in Hyp mice is not due to a fixed low level of phosphate transport, but to decreased stimulation of phosphate transporters, since Hyp mouse kidneys increase phosphate reabsorption with suitable stimulus.

Sodium-dependent phosphate co-transporters

The renal proximal tubular reabsorption of inorganic phosphate (Pi) mediated by sodium-dependent phosphate (Na+/Pi) co-transporters plays a critical role in the maintenance of Pi homeostasis. Two nonhomologous Na+/Pi co-transporters (type I and type II) have been identified in the renal cortex of various species. The role of the type I co-transporter in Pi regulation remains to be clarified. Type II co-transporters play a major role in the regulation of renal Pi reabsorption by dietary Pi and parathyroid hormone, which regulate the rapid endocytosis/exocytosis of the transporters. Type III Na+/Pi co-transporters, which are expressed in a wide variety of tissues and are regulated by changes in the Pi concentration, have recently been described. The presence of a novel Pi-regulating hormone called `phosphatonin' has been postulated in studies of the mechanisms of X-linked hypophosphatemic rickets and oncogenic osteomalacia. The regulation of phosphatonin and Na+/Pi co-transporters may provide novel pharmacological approaches to the treatment of these diseases.

Renal tubular reabsorption of phosphate is positively related to the extent of bone metastatic load in patients with prostate cancer.

Osteolytic metastases are often associated with decreased renal tubular reabsorption of phosphate. There is, however, no specific data on phosphate metabolism in metastases from prostatic cancer, which are generally osteoblastic. The aim of the present study was to investigate renal handling of inorganic phosphate (Pi) in prostatic cancer, in patients without or with skeletal metastases of various extents. Forty-eight patients were the subjects of this study. There were 39 with malignant disease, of whom 27 had bony metastases. Nine other patients had benign prostate hyperplasia. Biochemical indexes of prostatic tumor, renal tubular reabsorption of calcium and Pi, biochemical markers of bone remodeling, and relevant calciotropic hormones were measured and analyzed in relation to the extent of skeletal metastases, as assessed by bone scintigraphy. A higher bone metastatic load was associated with significantly greater prostate-specific antigen and prostatic acid phosphatase levels (P < 0.05), increased levels of biochemical markers of bone formation (P < 0.05) and resorption (P < 0.001), higher maximal renal tubular reabsorption of Pi (TmPi/GFR; P < 0.05), and higher urinary cAMP excretion (P < 0.05). Nine patients among those with bone metastases (n = 27) had higher TmPi/GFR than metastasis-free patients. These had a greater value of osteocalcin (P < 0.001). Also, 8 of these had relatively more extensive skeletal metastatic load. In patients with prostatic cancer, high skeletal metastatic load was accompanied by increased TmPi/GFR despite higher urinary cAMP excretion, which is supposed to reduce the TmPi/GFR. These results support the hypothesis that renal tubular reabsorption of Pi is capable of adaptation to meet demands for minerals in the face of enhanced bone formation.

Renal Tubular Reabsorption of Phosphate Is Positively Related to the Extent of Bone Metastatic Load in Patients with Prostate Cancer

Renal Tubular Reabsorption of Phosphate Is Positively Related to the Extent of Bone Metastatic Load in Patients with Prostate Cancer

Regulation of renal phosphate transport by acute and chronic metabolic acidosis in the rat

Metabolic acidosis results in impaired renal tubular phosphate reabsorption and proximal tubular apical brush border membrane (BBM) sodium gradient-dependent phosphate transport (Na/Pi cotransport) activity. In the present study we investigated the cellular mechanisms responsible for decreased Na/Pi cotransport activity following six hours to 10 days of metabolic acidosis induced by ingestion of NH4Cl. Urinary Pi excretion was significantly increased and BBM Na/Pi cotransport activity was progressively and significantly decreased by 18% at six hours, 24% at 12 hours, 32% at 24 hours, and 61% after 10 days of metabolic acidosis. The progressive and time-dependent decreases in BBM cotransport activity were associated with progressive decreases in BBM NaPi-2 protein (43% at 12 hr, 54% at 24 hr and 66% at 10 days) and cortical NaPi-2 mRNA (22% at 12 hr, 54% at 24 hr and 56% at 10 days) abundance. Interestingly, following six hours of metabolic acidosis, there was a significant 29% decrease in BBM NaPi-2 protein abundance that was not associated with decreases in either cortical homogenate NaPi-2 protein or cortical NaPi-2 mRNA abundance. In additional studies we found that the effects of chronic metabolic acidosis on Na/Pi cotransport activity were independent of endogenous parathyroid hormone activity, but were somewhat dependent on dietary Pi intake. In rats fed a high or a normal Pi diet metabolic acidosis caused significant decreases in Na/Pi cotransport activity, NaPi-2 protein and NaPi-2 mRNA abundance, however, in rats fed a low Pi diet the inhibitory effect of metabolic acidosis on Na/Pi cotransport were minimal and not significant. These results indicate that in chronic (> or = 12 hr) metabolic acidosis the progressive decrease in BBM Na/Pi cotransport activity is most likely mediated by decrease in BBM NaPi-2 protein and cortical mRNA abundance. In contrast, in acute (< or = 6 hr) metabolic acidosis the decrease in BBM Na/Pi cotransport activity is likely mediated by changes in the trafficking of the NaPi-2 protein that is, enhanced internalization from and/or impaired delivery of the NaPi-2 protein to the apical BBM.

Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation.

Renal tubular reabsorption of phosphate (TmP/GFR): indications and interpretation.

Saccharated ferric oxide (SFO)-induced osteomalacia: In vitro inhibition by SFO of bone formation and 1,25-dihydroxy-vitamin D production in renal tubules

A 60-year-old man with portal hypertensive gastrophathy due to type C liver cirrhosis developed severe bone pains, marked hypophosphatemia with inappropriately increased urinary excretion of phosphate (%TRP; 9.6%), and hyperalkaline phosphatasia, after intravenous administration of saccharated ferric oxide (SFO) at a dose of 80–240 mg/week over a period of more than 5 years. The total iron infused was estimated to be more than 25 g. On a diagnosis of SFO-induced osteomalacia, the infusion of iron was immediately discontinued, and phosphate and vitamin D 2 (1000 IU/day) were adminestered. Serum levels of 25-OHD 2 increased after 1 week, whereas levels of 1,25-(OH) 2D 2 did not increase until 3 months later, accompanied by improvement of renal tubular reabsorption of phosphate and gradual improvement of the bone pains. The patient has been doing well for the last 2 years, with normal serum levels of phosphate, calcium, and alkaline phosphatase, without any supplementation of phosphate, vitamin D, or iron-containing agents. In primary culture of neonatal mouse renal tubules, in which 1,25-(OH) 2D 3 was produced from 25-OHD 3 in response to PTH, SFO significantly inhibited PTH-induced production of 1,25-(OH) 2D 3 at 30 μmol/L, which is attainable in the urine of patients receiving a therapeutic intravenous dose of SFO. Furthermore, SFO decreased the calcium content and inhibited 45Ca incorporation in cultured fetal mouse parietal bones at 3 μmol/L. Such SFO concentration may be transiently observed in the plasma of patients receiving excessive intravenous doses of SFO for a prolonged period. These in vitro findings together with the clinical observations suggest that SFO, after filtration through the glomerulus and reabsorption in the proximal renal tubules, impaired proximal renal tubular function, such as tubular reabsorption of phosphate and 1α-hydroxylase activity, leading to hypophosphatemic osteomalacia. Furthermore, it is highly likely that SFO in the peripheral blood, when transferrin is saturated with iron, may impair bone formation and aggravate osteomalacia. Although SFO-induced osteomalacia is reversible simply by discontinuation of the agent, excessive and prolonged administration of SFO should be avoided.

Oncogenic Osteomalacia Caused by a Phosphaturic Mesenchymal Tumor of the Oral Cavity

We report a case of oncogenic osteomalacia associated with a phosphaturic mesenchymal tumor in a 31-year-old woman. She was presented with severe generalized bone and muscle pain and was restricted to bed. She lost 20cm in height over the 8 years since she had first noticed a pain in her thigh. A walnut-sized, hard, soft tissue tumor was found very easily beside her lower molar teeth. Radiologic examination revealed a remarkable decrease in bone density and multiple pathologic fractures of spine, femur and phalangeal bones. Severe hypophosphatemia, hyperphosphaturia, low plasma 1,25-dihydroxyvitamin D3 level and high plasma PTH level were disclosed at presentation. Histomorphometric examination revealed an extensive area of unmineralized osteoid and little mineralizing activity. A pharmacologic dose of 1α-hydroxyvitamin D3 or 1,25-dihydroxyvitamin D3 slightly increased the serum phosphate level and renal tubular reabsorption of phosphate, and slightly decreased plasma PTH level without any symptomatic improvement. Histologic examination of the tumor revealed a mixed connective tissue tumor that consisted of central woveh bones and surrounding primitive spindle cells with prominent vascularities. After removal of the tumor, all biochemical, hormonal and radiologic abnormalities disappeared with remarkable symptomatic improvement.

X chromosome inactivation pattern in female carriers of X linked hypophosphataemic rickets.

X linked hypophosphataemia (XLH) results from an abnormality of renal tubular phosphate reabsorption. The disorder is inherited as an X linked dominant trait and the gene has been mapped to Xp22.1-p22.2. A candidate gene (PEX) has recently been isolated. The most striking clinical features are growth retardation and skeletal abnormalities. As expected for X linked dominant disorders, females are less affected. However, such a gene dosage effect does not exist for renal phosphate reabsorption. Preferential X chromosome inactivation has been proposed as a possible explanation for this lack of gene dosage. We have examined the X inactivation pattern in peripheral blood cells from 12 females belonging to seven families with XLH using PCR analysis at the androgen receptor locus. The X inactivation pattern in these patients did not differ significantly from the pattern in 30 healthy females. The X inactivation pattern in peripheral blood cells does not necessarily reflect the X inactivation pattern in renal cells. However, the finding of a normal distribution of X inactivation in peripheral blood cells indicates that the similarity in the renal handling of phosphate in male and female patients is not related to a ubiquitous preferential X inactivation.

Positional cloning of the HYP gene: A review

X-linked hypophosphatemic rickets (HYP) is the most common form of familial hypophosphatemic rickets. Patients variably prcsent with lower extremity deformities, rickets, short stature, bone pain, dental abscesses, enthesopathy, and osteomalacia [1]. It is an X-linked dominant disorder [21 characterized by decreased renal tubular phosphate reabsorption and consequent hypophosphatemia. Observations in the Hyp mouse, one of the murine homologues of the human diseasc, indicate that the disorder is not due to an intrinsic renal defect [3—6]. However, the HYP gene may directly or indirectly regulate the expression of the sodium dependent phosphate cotransporter (NPT2) in the renal proximal tubule [7]. Despite extensive study, the pathophysiology of the disorder has not been elucidated. To gain better understanding of this disorder we have used the positional cloning approach to locate and clone the HYP gene.

Expression of parathyroid hormone receptors in MDCK and LLC-PK1 cells.

Parathyroid hormone (PTH) inhibits renal proximal tubular phosphate (Pi) and bicarbonate reabsorption by regulating the activity of apical Na/Pi cotransport and Na/H exchange. Two renal epithelial cell lines ["proximal tubular", LLC-PK1; "distal tubular", Madin-Darby canine kidney, (MDCK) cells] were stably transfected with complementary deoxyribonucleic acids (cDNAs) encoding a cloned PTH receptor in order to examine the polarity of transfected receptor function and whether or not intrinsic Pi transport is regulated by the transfected PTH receptor. The receptors are functionally coupled to the stimulation of adenosine 3':5' cyclic monophosphate (cAMP) production at both cell surfaces in LLC-PK1 cells, whereas this response is primarily limited to the basolateral surface in MDCK cells. Immunocytochemistry suggests an apical and basolateral localization of the transfected PTH receptor in LLC-PK1 cells and only a basolateral localization in MDCK cells. PTH activation of the transfected receptors is not coupled to the regulation of intrinsic Pi transport in either LLC-PK1 or MDCK cells.

Chromosomal localization of the human renal sodium phosphate transporter to chromosome 5: implications for X-linked hypophosphatemia

ABSTRACT: Hypophosphatemic vitamin D-resistant rickets, an X-linked dominant disorder, is the most common form of vitamin D–resistant rickets in humans (McKusick number 307800). Biochemically, these patients exhibit hypophosphatemia due to a defect in the renal tubular reabsorption of phosphate. The human cDNA encoding for the renal phosphate transporter has been recently cloned using the expression system in the Xenopus laevis oocytes. Because hypophosphatemic vitamin D–resistant rickets has an X-linked mode of transmission, we hypothesized that the gene encoding the renal phosphate transporter might map to the X chromosome. In this report, we determined the chromosomal localization of the human renal phosphate transporter using three independent methods. First, DNA from somatic cell hybrid panels was examined by Southern blotting for the phosphate transporter. Second, the polymerase chain reaction was used to amplify DNA from somatic cell hybrids. Third, fluorescent in situ hybridization was used to sublocalize the renal phosphate transporter. All three methods localized the renal phosphate transporter to chromosome 5q13. Our results indicate that either derangement of a gene other than the phosphate transporter gene that is encoded on chromosome 5 is responsible for X-linked hypophosphatemic rickets or, alternatively, a gene encoded on the X chromosome has an epistatic effect on the expression of the renal phosphate transporter on chromosome 5.

Effects of growth hormone on phosphocalcium homeostasis and bone metabolism.

In this review the effects of growth hormone (GH) on phosphocalcium homeostasis and bone metabolism are reported. Some in vitro effects of GH on chondrocytes and osteoblasts are discussed too. The main GH effects on phosphocalcium homeostasis are the permissive action on renal 1 alpha-hydroxylase activity by the hypophosphatemic stimulus and the antiphosphaturic effect by the stimulation of the maximum rate of renal tubular reabsorption of phosphate. On bone, GH is able to stimulate bone turnover and to increase bone mass. In addition, GH stimulates type I and type III collagen metabolism. In vitro, GH increases the proliferation of chondrocytes from the human growing cartilage together with the levels of interleukin-6 in the supernatant. The hormone increases also the proliferation of the human osteosarcoma-derived osteoblast-like cells and augments the osteocalcin levels in the supernatant. Thus, GH markedly influences phosphocalcium homeostasis and bone metabolism in childhood and adolescence. In addition, it is possible that GH continues to play a role in bone physiology during adulthood when final height is reached.

The 5-year reproducibility of calcium-related biochemical variables in postmenopausal women.

A total of 19 measured and derived bone-related biochemical variables were determined in 307 postmenopausal volunteers on two occasions, 5 years apart. The plasma variables with the highest coefficients of determination (r2) were plasma globulins, alkaline phosphatase, creatinine and calculated ionized and ultrafiltrable calcium. In the urine, the highest r2 values were in respect of fasting urine calcium excretion corrected for urine sodium, hydroxyproline excretion, and the maximal renal tubular reabsorption of calcium and phosphate (TmCa/GFR and TmP/GFR). The components of variance of TmCa/GFR and TmP/GFR show marked individuality but their methods determination meet the criterion for acceptable analytical goals. We conclude that most of the measured and derived bone-related biochemical variables in fasting plasma and urine are sufficiently reproducible in postmenopausal women to be useful for ranking individuals for a period up to 5 years.

Molecular biology of hypophosphataemic rickets and oncogenic osteomalacia

Phosphate plays a central role in many of the basic processes essential to the cell and organism. In particular, skeletal mineralisation is dependent on the appropriate regulation of phosphate in the body, and any disturbances in phosphate homeostasis can have severe repercussions on the integrity of bone. The kidney regulates the serum levels of phosphate by tubular mechanisms which are not fully understood. Furthermore, the processes involved in regulating renal tubular phosphate reabsorption are complex, and involve a large number of factors. It is not surprising therefore that defects in renal phosphate handling result in a failure of bone mineralisation. There are three well characterised conditions which are associated with renal tubulopathies resulting in a phosphate leak, with consequent bone disease. Two are familial, hypophosphataemic rickets (HYP), and hereditary hypophosphataemic rickets with hypercalciuria (HHRH). The third is acquired via a tumour, oncogenic hypophosphataemic osteomalacia (OHO), and may well have relevance to the inherited hypophosphataemias. Recent advances in molecular genetics are permitting the identification of genes involved in human diseases from their chromosomal location. These approaches are now being applied to the analysis of the hypophosphataemias. The isolation of the genes responsible for the renal tubulopathies will be an important achievement. Ultimately this will help to increase our understanding of the mechanisms involved in the control of phosphate handling in the body.

Fine structure mapping of the human X-linked hypophosphatemic rickets gene locus.

X-linked hypophosphatemic rickets (HYP) is an X-linked dominant disorder characterized by decreased renal tubular phosphate reabsorption and consequent hypophosphatemia. Renal cross-transplantation studies in Hyp mice indicate that the disorder is secondary to the elaboration of an as yet unidentified humoral factor. A full understanding of the pathophysiology of the disease and the nature of this factor will be facilitated by identification of the HYP gene. Efforts to isolate the HYP gene have been deterred by limited precision in the map of the Xp22.1 region and the consequent distance between DXS365 and DXS274, the previously discovered flanking markers for the HYP gene. To map the HYP region precisely, HYP family resources from two groups of investigators were combined, and several newly available microsatellite repeat probes were tested for linkage to HYP. Our data indicate that DXS365, DXS3424, DXS443, DXS1052, DXS274, and DXS1683 are tightly linked to the HYP gene and suggest a locus order of: Xtel-DXS315-(GLR/DXS43)-DXS257-(DXS443+ ++-DXS3424)-DXS365-HYP-DXS1683-DXS1052-DXS 274-(DXS41/DXS92)-DXS451-Xcen. The HYP gene is located in the 350- to 650-kilobase region between DXS365 and DXS1683. These results will provide a basis for the isolation of candidate genes from the region.

Nutrient balance, metabolic response, and bone growth in VLBW infants fed fortified human milk

The effects of fortification of preterm human milk were evaluated by comparing two groups of very low birth weight infants (birth weight ⪯ 1300 g, gestational age ⪯ 30 weeks): six fed preterm human milk fortified with a commercially available protein-mineral supplement (protein 0.7 g/dl, calcium 90 mg/dl, phosphorus 45 mg/dl) and seven fed unfortified preterm human milk. Nitrogen and energy balance studies were performed at an average age of 56 postnatal days. Nitrogen retention in the fortified group (348.2 ± 70.5 mg/kg/day) was significantly greater than that in the unfortified group (196.0 ± 50.0 mg/kg/day) and similar to that of fetuses of comparable gestational age. Energy stored by the two groups did not differ. At age 8 weeks, the infants in the fortified group had higher serum protein, higher serum albumin, and better mineral status (higher serum calcium and phosphorus and lower alkaline phosphatase and renal tubular reabsorption of phosphate). The bone density and width of the distal third radius, as measured by X-ray microdensitometry, were greater in the fortified group than in the unfortified group 12 weeks after birth. These results suggest that the supplement corrects any nutritional inadequacies of preterm human milk for very low birth weight infants.