Chronic effect of parathyroid hormone on NHE3 expression in rat renal proximal tubules

A novel mouse model for familial hypocalciuric hypercalcemia (FHH1) reveals PTH-dependent and independent CaSR defects

The Calcium-sensing receptor (CaSR) senses extracellular calcium, regulates parathyroid hormone (PTH) secretion, and has additional functions in various organs related to systemic and local calcium and mineral homeostasis. Familial hypocalciuric hypercalcemia type I (FHH1) is caused by heterozygous loss-of-function mutations in the CaSR gene, and is characterized by the combination of hypercalcemia, hypocalciuria, normal to elevated PTH, and facultatively hypermagnesemia and mild bone mineralization defects. To date, only heterozygous Casr null mice have been available as model for FHH1. Here we present a novel mouse FHH1 model identified in a large ENU-screen that carries an c.2579 T > A (p.Ile859Asn) variant in the Casr gene (CasrBCH002 mice). In order to dissect direct effects of the genetic variant from PTH-dependent effects, we crossed CasrBCH002 mice with PTH deficient mice. Heterozygous CasrBCH002 mice were fertile, had normal growth and body weight, were hypercalcemic and hypermagnesemic with inappropriately normal PTH levels and urinary calcium excretion replicating some features of FHH1. Hypercalcemia and hypermagnesemia were independent from PTH and correlated with higher expression of claudin 16 and 19 in kidneys. Likewise, reduced expression of the renal TRPM6 channel in CasrBCH002 mice was not dependent on PTH. In bone, mutations in Casr rescued the bone phenotype observed in Pth null mice by increasing osteoclast numbers and improving the columnar pattern of chondrocytes in the growth zone. In summary, CasrBCH002 mice represent a new model to study FHH1 and our results indicate that only a part of the phenotype is driven by PTH.

A Case of Milk-Alkali Syndrome Caused by Diuretic-Induced Alkalosis and Polypharmacy

Milk-alkali syndrome, which is characterized by hypercalcemia, metabolic alkalosis, and renal dysfunction, typically results from the ingestion of large amounts of calcium and absorbable alkaline products. However, these symptoms can also manifest when alkalosis and calcium loading occur simultaneously, owing to other factors. We report a case of milk-alkali syndrome caused by loop-diuretic-induced alkaline load and polypharmacy in an 85-year-old Japanese woman with multiple comorbidities, including osteoporosis, hypertension, type 2 diabetes, dyslipidemia, and Parkinson's disease. The patient regularly took 14 drugs, including calcium L-aspartate, eldecalcitol, celecoxib, and a fixed-dose combination of losartan and hydrochlorothiazide. Immediately before admission, furosemide was administered for the treatment of edema. The patient presented with chest discomfort, general malaise, and clinical signs of dehydration, hypercalcemia, hypophosphatemia, hypokalemia, and hypomagnesemia, accompanied by electrocardiogram abnormalities, renal dysfunction, and chloride-resistant metabolic alkalosis. The hypercalcemia was specifically induced by calcium L-aspartate and eldecalcitol. The hypomagnesaemia and hypophosphatemia were caused by diuretics and hypercalcemia. Thus, all the oral medications were discontinued, and rehydration and electrolyte correction therapy were administered. The final diagnosis was milk-alkali syndrome caused by the concomitant use of loop diuretics and other medications, without absorbable alkaline preparation use. This case underscores the importance of considering drug-related factors, checking concomitant medications, and being aware of the benefits, harmful effects, and side effects of polypharmacy in older adults with multimorbidity.

Fructose-Rich Diet Is a Risk Factor for Metabolic Syndrome, Proximal Tubule Injury and Urolithiasis in Rats

Excessive consumption of fructose (FR) leads to obesity, metabolic syndrome (MS) and insulin resistance, which are known risk factors for kidney stones. The epidemiological study has suggested the association between fructose consumption and urolithiasis, but the precise mechanism is still not well understood. Male Wistar rats were assigned for 8 weeks to three groups with different FR content in diet: RD (n = 5)-regular diet with a FR < 3%; F10 (n = 6)-regular diet with an addition of 10% Fr in drinking water; F60 (n = 5)-60% FR as a solid food. Serum concentration of FR, creatinine (Cr), insulin (Ins), triglycerides (Tg), homocysteine (HCS), uric acid (UA), calcium (Ca), phosphate (Pi), magnesium (Mg) and sodium (Na) were measured. Based on 24 h urine collection the following tests were performed: urine pH, proteinuria (PCR), excretion of N-Acetyl-(D)-Glucosaminidase (NAG), monocyte chemoattractant protein (MCP-1), uric acid (uUAEx), phosphate (uPiEx), calcium (uCaEx), magnesium (uMgEx) and sodium (uNaEx). The creatinine clearance (CrCl) was calculated. Calcium deposits in kidney sections were examined using hematoxylin and eosin (HE) and von Kossa stains. The rats on F10 and F60, as compared to the RD diet, showed a tendency for lower CrCl, higher HCS level and some features of MS as higher Ins and TG levels. Interestingly, F10 (fluid) versus F60 (solid) diet led to higher serum Ins levels. F10 and F60 versus RD demonstrated higher urinary excretion of MCP-1 and NAG which were suggestive for inflammatory injury of the proximal tubule. F10 and F60 as compared to RD showed significantly lower uUAEx, although there were no differences in clearance and fractional excretion of UA. F60 versus RD induced severe phosphaturia (>30×) and natriuria (4×) and mild calciuria. F10 versus RD induced calciuria (3×), phosphaturia (2×) and mild natriuria. Calcium phosphate stones within the tubules and interstitium were found only in rats on FR diet, respectively, in two rats from the F10 group and another two in the F60 group. The rats which developed stones were characterized by significantly higher serum insulin concentration and urinary excretion of calcium and magnesium. A fructose-rich diet may promote development of calcium stones due to proximal tubule injury and metabolic syndrome.

High blood pressure induced by vitamin D deficiency is associated with renal overexpression and hyperphosphorylation of Na+-K+-2Cl- cotransporter type 2

OBJECTIVES

Clinical and epidemiological studies have suggested a correlation between vitamin D deficiency (VDD) and high blood pressure (BP). This study aimed to test the hypothesis that high BP induced by VDD is associated with altered expression and covalent modification of apical sodium transporters along the nephron. The contributions of the intrarenal renin-angiotensin system (RAS) and oxidative stress were also investigated.

METHODS

Male Wistar rats were fed a vitamin D-free (n = 26) or standard diet (n = 25) for 30 days. BP was recorded using noninvasive and invasive procedures. The expression levels of total and phosphorylated apical sodium transporters in rat renal cortex and medulla were evaluated by immunoblotting. Intrarenal RAS components were assessed by immunoblotting and ELISA. Renal oxidative stress was analyzed by measuring the concentrations of thiobarbituric acid reactive substances and reduced glutathione.

RESULTS

Higher BP levels in VDD rats than controls were accompanied by overexpression and hyperphosphorylation of renal cortical and medullary Na+-K+-2Cl- cotransporter type 2, enhanced levels of phosphorylated Na+/H+ exchanger type 3, and reduced expression levels of total and phosphorylated Na+/Cl- cotransporter. Changes in intrarenal RAS induced by VDD vs. controls included the marked elevation of medullary renin expression, higher expression of cortical angiotensinogen, higher urinary angiotensinogen excretion, and higher cortical and medullary angiotensin II content. VDD rats displayed higher thiobarbituric acid reactive substances/glutathione ratios in the renal cortex and medulla than controls.

CONCLUSION

These results suggest that the molecular mechanisms underlying the effects of VDD on BP may include the upregulation of Na+-K+-2Cl- cotransporter type 2 and activation of intrarenal RAS and oxidative stress.

Dent disease: A window into calcium and phosphate transport

This review examines calcium and phosphate transport in the kidney through the lens of the rare X-linked genetic disorder Dent disease. Dent disease type 1 (DD1) is caused by mutations in the CLCN5 gene encoding ClC-5, a Cl^- /H⁺ antiporter localized to early endosomes of the proximal tubule (PT). Phenotypic features commonly include low molecular weight proteinuria (LMWP), hypercalciuria, focal global sclerosis and chronic kidney disease; calcium nephrolithiasis, nephrocalcinosis and hypophosphatemic rickets are less commonly observed. Although it is not surprising that abnormal endosomal function and recycling in the PT could result in LMWP, it is less clear how ClC-5 dysfunction disturbs calcium and phosphate metabolism. It is known that the majority of calcium and phosphate transport occurs in PT cells, and PT endocytosis is essential for calcium and phosphorus reabsorption in this nephron segment. Evidence from ClC-5 KO models suggests that ClC-5 mediates parathormone endocytosis from tubular fluid. In addition, ClC-5 dysfunction alters expression of the sodium/proton exchanger NHE3 on the PT apical surface thus altering transcellular sodium movement and hence paracellular calcium reabsorption. A potential role for NHE3 dysfunction in the DD1 phenotype has never been investigated, either in DD models or in patients with DD1, even though patients with DD1 exhibit renal sodium and potassium wasting, especially when exposed to even a low dose of thiazide diuretic. Thus, insights from the rare disease DD1 may inform possible underlying mechanisms for the phenotype of hypercalciuria and idiopathic calcium stones.

The potential role of myosin motor proteins in mediating the subcellular distribution of NHE3 in the renal proximal tubule

Isoform 3 of the Na⁺/H⁺ exchanger (NHE3) is responsible for the majority of the reabsorption of NaCl, NaHCO₃, and, consequently, water in the renal proximal tubule. As such, this transporter plays an essential role in acid-base balance and extracellular fluid volume homeostasis and determining systemic arterial blood pressure levels. NHE3 activity is modulated by a number of mechanisms, including the redistribution of the transporter between the body of the microvilli (where NHE3 is active) and the base of the microvilli (where NHE3 is less active). Although the physiological, pathophysiological, and pharmacological importance of the subcellular distribution of NHE3 has been well established, the exact mechanism whereby NHE3 is translocated along microvilli microdomains of the proximal tubule apical membrane is unknown. Nonmuscle myosin IIA and unconventional myosin VI move cargoes in anterograde and retrograde directions, respectively, and are known to redistribute along with NHE3 in the proximal tubule in response to a variety of natriuretic and antinatriuretic stimuli, including stimulation or inhibition of the renin-angiotensin system, high dietary Na⁺ intake, and high blood pressure. Therefore, this review aims to discuss the current evidence that suggests a potential role of myosin IIA and myosin VI in mediating the subcellular distribution of NHE3 along the kidney proximal tubule microvilli and their possible contribution in modifying NHE3-mediated Na⁺ reabsorption under both physiological and pathophysiological conditions.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

Effects of phospho- and calciotropic hormones on electrolyte transport in the proximal tubule

Calcium and phosphate are critical for a myriad of physiological and cellular processes within the organism. Consequently, plasma levels of calcium and phosphate are tightly regulated. This occurs through the combined effects of the phospho- and calciotropic hormones, parathyroid hormone (PTH), active vitamin D ₃, and fibroblast growth factor 23 (FGF23). The organs central to this are the kidneys, intestine, and bone. In the kidney, the proximal tubule reabsorbs the majority of filtered calcium and phosphate, which amounts to more than 60% and 90%, respectively. The basic molecular mechanisms responsible for phosphate reclamation are well described, and emerging work is delineating the molecular identity of the paracellular shunt wherein calcium permeates the proximal tubular epithelium. Significant experimental work has delineated the molecular effects of PTH and FGF23 on these processes as well as their regulation of active vitamin D ₃ synthesis in this nephron segment. The integrative effects of both phospho- and calciotropic hormones on proximal tubular solute transport and subsequently whole body calcium-phosphate balance thus have been further complicated. Here, we first review the molecular mechanisms of calcium and phosphate reabsorption from the proximal tubule and how they are influenced by the phospho- and calciotropic hormones acting on this segment and then consider the implications on both renal calcium and phosphate handling as well as whole body mineral balance.

Sexual Dimorphic Pattern of Renal Transporters and Electrolyte Homeostasis

Compared with males, females have lower BP before age 60, blunted hypertensive response to angiotensin II, and a leftward shift in pressure natriuresis. This study tested the concept that this female advantage associates with a distinct sexual dimorphic pattern of transporters along the nephron. We applied quantitative immunoblotting to generate profiles of transporters, channels, claudins, and selected regulators in both sexes and assessed the physiologic consequences of the differences. In rats, females excreted a saline load more rapidly than males did. Compared with the proximal tubule of males, the proximal tubule of females had greater phosphorylation of Na⁺/H⁺ exchanger isoform 3 (NHE3), distribution of NHE3 at the base of the microvilli, and less abundant expression of Na⁺/Pi cotransporter 2, claudin-2, and aquaporin 1. These changes associated with less bicarbonate reabsorption and higher lithium clearance in females. The distal nephrons of females had a higher abundance of total and phosphorylated Na⁺/Cl^- cotransporter (NCC), claudin-7, and cleaved forms of epithelial Na⁺ channel (ENaC) α and γ subunits, which associated with a lower baseline plasma K⁺ concentration. A K⁺-rich meal increased the urinary K⁺ concentration and decreased the level of renal phosphorylated NCC in females. Notably, we observed similar abundance profiles in female versus male C57BL/6 mice. These results define sexual dimorphic phenotypes along the nephron and suggest that lower proximal reabsorption in female rats expedites excretion of a saline load and enhances NCC and ENaC abundance and activation, which may facilitate K⁺ secretion and set plasma K⁺ at a lower level.

Ways of calcium reabsorption in the kidney

The role of the kidney in calcium homeostasis has been reshaped from a classic view in which the kidney was regulated by systemic calcitropic hormones such as vitamin D3 or parathyroid hormone to an organ actively taking part in the regulation of calcium handling. With the identification of the intrinsic renal calcium-sensing receptor feedback system, the regulation of paracellular calcium transport involving claudins, and new paracrine regulators such as klotho, the kidney has emerged as a crucial modulator not only of calciuria but also of calcium homeostasis. This review summarizes recent molecular and endocrine contributors to renal calcium handling and highlights the tight link between calcium and sodium reabsorption in the kidney.

Significant association between parathyroid hormone and uric acid level in men

BACKGROUND

Previous reports of patients undergoing parathyroidectomy and of patients receiving teriparatide as antiosteoporotic treatment have suggested a plausible relationship between parathyroid hormone (PTH) and uric acid. However, similar data at population level were lacking. The current study aimed to determine the relationship between PTH and uric acid in a group of apparently healthy Malaysian men.

METHODS

A cross-sectional study was conducted among 380 Malay and Chinese men aged 20 years and above, residing in the Klang Valley, Malaysia. Their body anthropometry was measured, and their fasting blood samples were collected for biochemical analysis. The relationship between PTH and uric acid was analyzed using regression analysis.

RESULTS

Increased serum PTH level was significantly associated with increased serum uric acid level (β=0.165; P=0.001). Increased PTH level was also significantly associated with the condition of hyperuricemia in the study population (odds ratio [OR], 1.045; 95% confidence interval [CI], 1.017-1.075; P=0.002). All analyses were adjusted for age, body mass index, vitamin D, total calcium, inorganic phosphate, blood urea nitrogen and creatinine levels.

CONCLUSION

There is a significant positive relationship between PTH level and uric acid level in Malaysian men. This relationship and its clinical significance should be further investigated in a larger longitudinal study.

Proximal tubular NHEs: sodium, protons and calcium?

Na⁺/H⁺ exchange activity in the apical membrane of the proximal tubule is fundamental to the reabsorption of Na⁺ and water from the filtrate. The role of this exchange process in bicarbonate reclamation and, consequently, the maintenance of acid-base homeostasis has been appreciated for at least half a century and remains a pillar of renal tubular physiology. More recently, apical Na⁺/H⁺ exchange, mediated by Na⁺/H⁺ exchanger isoform 3 (NHE3), has been implicated in proximal tubular reabsorption of Ca²⁺ and Ca²⁺ homeostasis in general. Overexpression of NHE3 increased paracellular Ca²⁺ flux in a proximal tubular cell model. Consistent with this observation, mice with genetic deletion of Nhe3 have a noticable renal Ca²⁺ leak. These mice also display decreased intestinal Ca²⁺ uptake and osteopenia. This review highlights the traditional roles of proximal tubular Na⁺/H⁺ exchange and summarizes recent novel findings implicating the predominant isoform, NHE3, in Ca²⁺ homeostasis.

The epithelial sodium/proton exchanger, NHE3, is necessary for renal and intestinal calcium (re)absorption

Passive paracellular proximal tubular (PT) and intestinal calcium (Ca(2+)) fluxes have been linked to active sodium (re)absorption. Although the epithelial sodium/proton exchanger, NHE3, mediates apical sodium entry at both these sites, its role in Ca(2+) homeostasis remains unclear. We, therefore, set out to determine whether NHE3 is necessary for Ca(2+) (re)absorption from these epithelia by comparing Ca(2+) handling between wild-type and NHE3(-/-) mice. Serum Ca(2+) and plasma parathyroid hormone levels were not different between groups. However, NHE3(-/-) mice had increased serum 1,25-dihydroxyvitamin D(3). The fractional excretion of Ca(2+) was also elevated in NHE3(-/-) mice. Paracellular Ca(2+) flux across confluent monolayers of a PT cell culture model was increased by an osmotic gradient equivalent to that generated by NHE3 across the PT in vivo and by overexpression of NHE3.( 45)Ca(2+) uptake after oral gavage and flux studies in Ussing chambers across duodenum of wild-type and NHE3(-/-) mice confirmed decreased Ca(2+) absorption in NHE3(-/-) mice compared with wild-type mice. Consistent with this, intestinal calbindin-D(9K), claudin-2, and claudin-15 mRNA expression was decreased. Microcomputed tomography analysis revealed a perturbation in bone mineralization. NHE3(-/-) mice had both decreased cortical bone mineral density and trabecular bone mass. Our results demonstrate significant alterations of Ca(2+) homeostasis in NHE3(-/-) mice and provide a molecular link between Na(+) and Ca(2+) (re)absorption.

Essential regulatory elements for NHE3 gene transcription in renal proximal tubule cells

The objectives of the present study were to identify the cis-elements of the promoter absolutely required for the efficient rat NHE3 gene transcription and to locate positive and negative regulatory elements in the 5'-flanking sequence (5'FS), which might modulate the gene expression in proximal tubules, and to compare this result to those reported for intestinal cell lines. We analyzed the promoter activity of different 5'FS segments of the rat NHE3 gene, in the OKP renal proximal tubule cell line by measuring the activity of the reporter gene luciferase. Because the segment spanning the first 157 bp of 5'FS was the most active it was studied in more detail by sequential deletions, point mutations, and gel shift assays. The essential elements for gene transcription are in the region -85 to -33, where we can identify consensual binding sites for Sp1 and EGR-1, which are relevant to NHE3 gene basal transcription. Although a low level of transcription is still possible when the first 25 bp of the 5'FS are used as promoter, efficient transcription only occurs with 44 bp of 5'FS. There are negative regulatory elements in the segments spanning -1196 to -889 and -467 to -152, and positive enhancers between -889 and -479 bp of 5'FS. Transcription factors in the OKP cell nuclear extract efficiently bound to DNA elements of rat NHE3 promoter as demonstrated by gel shift assays, suggesting a high level of similarity between transcription factors of both species, including Sp1 and EGR-1.

Localization of members of MCT monocarboxylate transporter family Slc16 in the kidney and regulation during metabolic acidosis

The monocarboxylate transporter family (MCT) comprises 14 members with distinct transport properties and tissue distribution. The kidney expresses several members of the MCT family, but only little is known about their exact distribution and function. Here, we investigated selected members of the MCT family in the mouse kidney. MCT1, MCT2, MCT7, and MCT8 localized to basolateral membranes of the epithelial cells lining the nephron. MCT1 and MCT8 were detected in proximal tubule cells whereas MCT7 and MCT2 were located in the thick ascending limb and the distal tubule. CD147, a β-subunit of MCT1 and MCT4, showed partially overlapping expression with MCT1 and MCT2. However, CD147 was also found in intercalated cells. We also detected SMCT1 and SMCT2, two Na+-dependent monocarboxylate cotransporters, on the luminal membrane of type A intercalated cells. Moreover, mice were given an acid load for 2 and 7 days. Acidotic animals showed a marked but transient increase in urinary lactate excretion. During acidosis, a downregulation of MCT1, MCT8, and SMCT2 was observed at the mRNA level, whereas MCT7 and SMCT1 showed increased mRNA abundance. Only MCT7 showed lower protein abundance whereas all other transporters remained unchanged. In summary, we describe for the first time the localization of various MCT transporters in mammalian kidney and demonstrate that metabolic acidosis induces a transient increase in urinary lactate excretion paralleled by lower MCT7 protein expression.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

Mechanisms underlying the long-term regulation of NHE3 by parathyroid hormone

The activity of the Na+/H+ exchanger NHE3 is regulated by a number of factors including parathyroid hormone (PTH). In the current study, we used a renal epithelial cell line, the opossum kidney (OKP) cell, to elucidate the mechanisms underlying the long-term effects of PTH on NHE3 transport activity and expression. We observed that NHE3 activity was reduced 6 h after addition of PTH, and this reduction persisted almost unaltered after 24 h. The decrease in activity was associated with diminished NHE3 cell surface expression at 6, 16, and 24 h after PTH addition, total cellular NHE3 protein at 16 and 24 h, and NHE3 mRNA abundance at 24 h. The lower levels of NHE3 mRNA were associated to a small, but significant, decrease in mRNA stability. Additionally, by analyzing the rat NHE3 gene promoter activity in OKP cells, we verified that the regulatory region spanning the segment −152 to +55 was mildly reduced under the influence of PTH. This effect was completely abolished by the presence of the PKA inhibitor KT 5720. In conclusion, long-term exposure to PTH results in reduction of NHE3 mRNA levels due to a PKA-dependent inhibitory effect on the NHE3 promoter and a small reduction of mRNA half-life, and decrease in the total amount of protein which is preceded by endocytosis of the apical surface NHE3. The decreased NHE3 expression is likely to be responsible for the reduction of sodium, bicarbonate, and fluid reabsorption in the proximal tubule consistently perceived in experimental models of PTH disorders.

NHE3 phosphorylation at serines 552 and 605 does not directly affect NHE3 activity

Direct phosphorylation of sodium hydrogen exchanger type 3 (NHE3) is a well-established physiological phenomenon; however, the exact role of NHE3 phosphorylation in its regulation remains unclear. The objective of this study was to evaluate whether NHE3 phosphorylation at serines 552 and 605 is physiologically regulated in vivo and, if so, whether changes in phosphorylation at these sites are tightly coupled to changes in transport activity. To this end, we directly compared PKA-induced NHE3 inhibition with site-specific changes in NHE3 phosphorylation in vivo and in vitro. In vivo, PKA was activated using an intravenous infusion of parathyroid hormone in Sprague-Dawley rats. In vitro, PKA was activated directly in opossum kidney (OKP) cells using forskolin and IBMX. NHE3 activity was assayed in microvillar membrane vesicles in the rat model and by 22Na uptake in the OKP cell model. In both cases, NHE3 phosphorylation at serines 552 and 605 was determined using previously characterized monoclonal phosphospecific antibodies directed to these sites. In vivo, we found dramatic changes in NHE3 phosphorylation at serines 552 and 605 with PKA activation but no corresponding alteration in NHE3 activity. This dissociation between NHE3 phosphorylation and activity was further verified in OKP cells in which phosphorylation clearly preceded transport inhibition. We conclude that although phosphorylation of NHE3 at serines 552 and 605 is regulated by PKA both in vivo and in vitro, phosphorylation of these sites does not directly alter Na+/H+ exchange activity.

Thyroid hormone deficiency alters expression of acid-base transporters in rat kidney

Hypothyroidism in humans is associated with incomplete distal renal tubular acidosis, presenting as the inability to respond appropriately to an acid challenge by excreting less acid. Here, we induced hypothyroidism in rats with methimazole (HYPO) and in one group substituted with l-thyroxine (EU). After 4 wk, acid-base status was similar in both groups. However, after 24 h acid loading with NH(4)Cl HYPO rats displayed a more pronounced metabolic acidosis. The expression of the Na(+)/H(+) exchanger NHE3, the Na(+)-phosphate cotransporter NaPi-IIa, and the B2 subunit of the vacuolar H(+)-ATPase was reduced in the brush-border membrane of the proximal tubule of the HYPO group, paralleled by a lower abundance of the Na(+)/HCO(3)(-) cotransporter NBCe1 and a higher expression of the acid-secretory type A intercalated cell-specific Cl(-)/HCO(3)(-) exchanger AE1. In contrast to control conditions, the expression of NBCe1 was increased in the HYPO group during metabolic acidosis. In addition, net acid excretion was similar in both groups. The relative number of type A intercalated cells was increased in the connecting tubule and cortical collecting duct of the HYPO group during acidosis. Thus thyroid hormones modulate the renal response to an acid challenge and alter the expression of several key acid-base transporters. Mild hypothyroidism is associated only with a very mild defect in renal acid handling, which appears to be mainly located in the proximal tubule and is compensated by the distal nephron.

Vacuolar H+-ATPase expression is increased in acid-secreting intercalated cells in kidneys of rats with hypercalcaemia-induced alkalosis

AIMS

Hypercalcaemia is known to be associated with systemic metabolic alkalosis, although the underlying mechanism is uncertain. Therefore, we aimed to examine whether hypercalcaemia was associated with changes in the expression of acid-base transporters in the kidney.

METHODS

Rats were infused with human parathyroid hormone (PTH, 15 microg kg(-1) day(-1)), or vehicle for 48 h using osmotic minipumps.

RESULTS

The rats treated with PTH developed hypercalcaemia and exhibited metabolic alkalosis (arterial HCO: 31.1 +/- 0.8 vs. 28.1 +/- 0.8 mmol L(-1) in controls, P < 0.05, n = 6), whereas the urine pH of 6.85 +/- 0.1 was significantly decreased compared with the pH of 7.38 +/- 0.1 in controls (P < 0.05, n = 12). The observed alkalosis was associated with a significantly increased expression of the B1-subunit of the H(+)-ATPase in kidney inner medulla (IM, 233 +/- 45% of the control level). In contrast, electroneutral Na(+)-HCO cotransporter NBCn1 and Cl(-)/HCO anion exchanger AE2 expression was markedly reduced in the inner stripe of the outer medulla (to 26 +/- 9% and 65 +/- 6%, respectively). These findings were verified by immunohistochemistry.

CONCLUSIONS

(1) hypercalcaemia-induced metabolic alkalosis was associated with increased urinary excretion of H(+); (2) the increased H(+)-ATPase expression in IM may partly explain the enhanced urinary acidification, which is speculated to prevent stone formation because of hypercalciuria and (3) the decreased expression of outer medullary AE2 suggests a compensatory reduction of the transepithelial bicarbonate transport.

Na+/H+ exchangers in renal regulation of acid-base balance

The kidney plays key roles in extracellular fluid pH homeostasis by reclaiming bicarbonate (HCO(3)(-)) filtered at the glomerulus and generating the consumed HCO(3)(-) by secreting protons (H(+)) into the urine (renal acidification). Sodium-proton exchangers (NHEs) are ubiquitous transmembrane proteins mediating the countertransport of Na(+) and H(+) across lipid bilayers. In mammals, NHEs participate in the regulation of cell pH, volume, and intracellular sodium concentration, as well as in transepithelial ion transport. Five of the 10 isoforms (NHE1-4 and NHE8) are expressed at the plasma membrane of renal epithelial cells. The best-studied isoform for acid-base homeostasis is NHE3, which mediates both HCO(3)(-) absorption and H(+) excretion in the renal tubule. This article reviews some important aspects of NHEs in the kidney, with special emphasis on the role of renal NHE3 in the maintenance of acid-base balance.

Reduced expression of renal Na+transporters in rats with PTH-induced hypercalcemia

The purpose of this study was to evaluate whether the natriuresis and polyuria seen in parathyroid hormone (PTH)-induced hypercalcemia are associated with dysregulation of renal Na transporters. Rats were infused with three different doses of human PTH [PTH ( 1 - 34 ); 7.5, 10, and 15 μg·kg-1·day-1sc] or vehicle for 48 h using osmotic minipumps. The rats treated with PTH developed significant hypercalcemia (plasma total calcium levels: 2.71 ± 0.03, 2.77 ± 0.02, and 3.42 ± 0.06 mmol/l, respectively, P < 0.05 compared with corresponding controls). The rats with severe hypercalcemia induced by high-dose PTH developed a decreased glomerular filtration rate (GFR), increased urine output, reduced urinary osmolality, increased urinary Na excretion, and fractional excretion of Na. This was associated with downregulation (calculated as a fraction of control levels) of whole kidney expression of type 2 Na-Picotransporter (NaPi-2; 16 ± 6%), type 3 Na/H exchanger (NHE3; 42 ± 7%), Na-K-ATPase (55 ± 2%), and bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1; 25 ± 4%). In contrast, an upregulation of the Ca2+-sensing receptor (CaR) was observed. Rats treated with moderate-dose PTH exhibited unchanged GFR but decreased urinary concentration. The whole kidney expression of NHE3 (52 ± 8%) and NaPi-2 (26 ± 5%) was persistently decreased, whereas BSC-1 and Na-K-ATPase protein levels were not altered. CaR expression was also increased. Moreover, rats treated with low-dose PTH showed very mild hypercalcemia but unchanged GFR, normal urinary concentration, and unchanged expression of Na transporters and CaR. In conclusion, the reduced expression of major renal Na transporters is likely to play a role in the increased urinary Na excretion and decreased urinary concentration in rats with PTH-induced hypercalcemia. Moreover, the increase in the CaR in the thick ascending limb (TAL) may indicate a potential role of the CaR in inhibiting Na transport in the TAL.

PTH stimulates a Cl(-)-dependent and EIPA-sensitive current in chick proximal tubule cells in culture

The electrophysiological effects of parathyroid hormone (PTH) were studied in a primary cell culture model of the chick (Gallus domesticus) proximal tubule. In this model, confluent monolayers are grown on permeable filters and exhibit vectorial transport, including glucose-stimulated current. Under short-circuit conditions, PTH, at 10(-9) M, induced a positive current [short-circuit current (I(sc))] response, with an average 2-min peak response of 14.30 +/- 1.58 microA/cm(2) over the baseline I(sc,) followed by a slow decay. The PTH response was dose dependent, with a half-maximal response at 5 x 10(-9) M and maximal response at 5 x 10(-8) M. Forskolin and dibutyryl-cAMP also stimulated I(sc), as did the phosphodiesterase inhibitor IBMX. In contrast, the phorbol ester PMA inhibited baseline I(sc). The PTH response was nearly abolished by apical addition of 100 microM EIPA, an inhibitor of Na(+)/H(+) exchangers, and partially blocked by the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 100 microM) and glibenclamide (300 microM). Higher doses of EIPA or NPPB alone (500 microM) were almost fully effective, with no or slight additional effects of NPPB or EIPA, respectively. The anion exchange inhibitor DIDS (100 microM) and the Na(+) channel blocker amiloride (10 microM) had no effect. Bilateral reduction of Cl(-) in the buffer, from 137 to 2.6 mM, abolished the PTH response; increasing Cl(-) concentration restored the I(sc) response, with a half-maximal effect at 50 mM. These data suggest that, in the chick proximal tubule, PTH activates both an Na(+)/H(+) exchanger and a Cl(-) channel that may be functionally linked.

Upregulation of NHE3 is associated with compensatory cell growth response in young uninephrectomized rats

It is well established that after removal of renal mass, the remaining tissue undergoes compensatory growth. Several laboratories have reported that the activity of the apical membrane Na+ - H+ exchanger (NHE3) is increased after a reduction in renal mass. These studies were designed to determine whether NHE3 expression is altered early after loss of renal mass and to investigate the possible role of NHE3 activation in the compensatory tissue growth response. Experiments were performed in young male Wistar rats submitted to left nephrectomy or sham operation. At either 4 or 24 h after the surgery, the right kidney from each animal was removed and weighed. Significant increases in the wet weight of the remaining kidney were only observed 24 h after uninephrectomy (UNX). Western blot analysis of brush-border membranes and Northern blot analysis of cortex RNA showed that NHE3 protein abundance and NHE3 mRNA were greatly enhanced 4 and 24 h after UNX in relation to the sham kidney. To identify which growth pattern was mostly responsible for the enlargement of the remained kidney in our experimental models, we measured 5-bromo-2-deoxyuridine incorporation (BrdU) and protein-to-DNA ratio (protein/DNA ratio). The number of BrdU-positive nuclei increased and protein/DNA ratio slightly decreased, indicating that a hyperplastic response was the main component involved in the early compensatory renal growth in our animals. BrdU incorporation and protein/DNA were also assessed in rats treated with S3226, a selective blocker of NHE3. Neither the number of BrdU-positive nuclei nor the protein/DNA ratio was significantly altered 4 and 24 h after UNX in rats treated with S3226. In conclusion, UNX induced an upregulation of NHE3, which was evidenced at both functional and expression levels. The compensatory growth response in young UNX rats could be blocked by inhibiting NHE3 activity, suggesting that NHE3 activation may result in a facilitator state for the cell growth response in the renal proximal tubule.

Regulation of the sodium transporters NHE3, NKCC2 and NCC in the kidney

The regulation of sodium transport in the kidney is important for maintenance of extracellular fluid volume and arterial blood-pressure regulation. The major sodium transporters and channels in individual renal tubule segments have been identified via physiological techniques, and complementary DNAs for all of the key sodium transporters and channels expressed along the renal tubule have been cloned. Complementary DNA probes and antibodies are now being used to investigate the molecular basis of renal tubule sodium-transport regulation. This review summarizes some of the major observations made in the past year that are relevant to the regulation of the major sodium transporters in the proximal tubule (the type 3 sodium-hydrogen exchanger, NHE3), the thick ascending limb of Henle (the bumetanide-sensitive sodium-potassium-chloride cotransporter, NKCC2), and the distal convoluted tubule (the thiazide-sensitive sodium-chloride cotransporter, NCC).