Long-term regulation of renal Na-dependent cotransporters and ENaC: response to altered acid-base intake

Nascent shifts in renal cellular metabolism, structure, and function due to chronic empagliflozin in prediabetic mice

Sodium-glucose cotransporter, type 2 inhibitors (SGLT2i) are emerging as the gold standard for treatment of type 2 diabetes (T2D) with renal protective benefits independent of glucose lowering. We took a high-level approach to evaluate the effects of the SGLT2i, empagliflozin (EMPA) on renal metabolism and function in a prediabetic model of metabolic syndrome. Male and female 12-wk-old TallyHo (TH) mice, and their closest genetic lean strain (Swiss-Webster, SW) were treated with a high-milk-fat diet (HMFD) plus/minus EMPA (@0.01%) for 12-wk. Kidney weights and glomerular filtration rate were slightly increased by EMPA in the TH mice. Glomerular feature analysis by unsupervised clustering revealed sexually dimorphic clustering, and one unique cluster relating to EMPA. Periodic acid Schiff (PAS) positive areas, reflecting basement membranes and mesangium were slightly reduced by EMPA. Phasor-fluorescent life-time imaging (FLIM) of free-to-protein bound NADH in cortex showed a marginally greater reliance on oxidative phosphorylation with EMPA. Overall, net urine sodium, glucose, and albumin were slightly increased by EMPA. In TH, EMPA reduced the sodium phosphate cotransporter, type 2 (NaPi-2), but increased sodium hydrogen exchanger, type 3 (NHE3). These changes were absent or blunted in SW. EMPA led to changes in urine exosomal microRNA profile including, in females, enhanced levels of miRs 27a-3p, 190a-5p, and 196b-5p. Network analysis revealed "cancer pathways" and "FOXO signaling" as the major regulated pathways. Overall, EMPA treatment to prediabetic mice with limited renal disease resulted in modifications in renal metabolism, structure, and transport, which may preclude and underlie protection against kidney disease with developing T2D.NEW & NOTEWORTHY Renal protection afforded by sodium glucose transporter, type 2 inhibitors (SGLT2i), e.g., empagliflozin (EMPA) involves complex intertwined mechanisms. Using a novel mouse model of obesity with insulin resistance, the TallyHo/Jng (TH) mouse on a high-milk-fat diet (HMFD), we found subtle changes in metabolism including altered regulation of sodium transporters that line the renal tubule. New potential epigenetic determinants of metabolic changes relating to FOXO and cancer signaling pathways were elucidated from an altered urine exosomal microRNA signature.

Dietary anions control potassium excretion: it is more than a poorly absorbable anion effect

The urinary potassium (K+) excretion machinery is upregulated with increasing dietary K+, but the role of accompanying dietary anions remains inadequately characterized. Poorly absorbable anions, including [Formula: see text], are thought to increase K+ secretion through a transepithelial voltage effect. Here, we tested if they also influence the K+ secretion machinery. Wild-type mice, aldosterone synthase (AS) knockout (KO) mice, or pendrin KO mice were randomized to control, high-KCl, or high-KHCO3 diets. The K+ secretory capacity was assessed in balance experiments. Protein abundance, modification, and localization of K+-secretory transporters were evaluated by Western blot analysis and confocal microscopy. Feeding the high-KHCO3 diet increased urinary K+ excretion and the transtubular K+ gradient significantly more than the high-KCl diet, coincident with more pronounced upregulation of epithelial Na+ channels (ENaC) and renal outer medullary K+ (ROMK) channels and apical localization in the distal nephron. Experiments in AS KO mice revealed that the enhanced effects of [Formula: see text] were aldosterone independent. The high-KHCO3 diet also uniquely increased the large-conductance Ca2+-activated K+ (BK) channel β4-subunit, stabilizing BKα on the apical membrane, the Cl-/[Formula: see text] exchanger, pendrin, and the apical KCl cotransporter (KCC3a), all of which are expressed specifically in pendrin-positive intercalated cells. Experiments in pendrin KO mice revealed that pendrin was required to increase K+ excretion with the high-KHCO3 diet. In summary, [Formula: see text] stimulates K+ excretion beyond a poorly absorbable anion effect, upregulating ENaC and ROMK in principal cells and BK, pendrin, and KCC3a in pendrin-positive intercalated cells. The adaptive mechanism prevents hyperkalemia and alkalosis with the consumption of alkaline ash-rich diets but may drive K+ wasting and hypokalemia in alkalosis.NEW & NOTEWORTHY Dietary anions profoundly impact K+ homeostasis. Here, we found that a K+-rich diet, containing [Formula: see text] as the counteranion, enhances the electrogenic K+ excretory machinery, epithelial Na+ channels, and renal outer medullary K+ channels, much more than a high-KCl diet. It also uniquely induces KCC3a and pendrin, in B-intercalated cells, providing an electroneutral KHCO3 secretion pathway. These findings reveal new K+ balance mechanisms that drive adaption to alkaline and K+-rich foods, which should guide new treatment strategies for K+ disorders.

Regulation of Blood Pressure and Salt Balance By Pendrin-Positive Intercalated Cells: Donald Seldin Lecture 2020

Intercalated cells make up about a third of all cells within the connecting tubule and the collecting duct and are subclassified as type A, type B and non-A, non-B based on the subcellular distribution of the H+-ATPase, which dictates whether it secretes H+ or HCO3-. Type B intercalated cells mediate Cl- absorption and HCO3- secretion, which occurs largely through the anion exchanger pendrin. Pendrin is stimulated by angiotensin II via the angiotensin type 1a receptor and by aldosterone through MR (mineralocorticoid receptor). Aldosterone stimulates pendrin expression and function, in part, through the alkalosis it generates. Pendrin-mediated HCO3- secretion increases in models of metabolic alkalosis, which attenuates the alkalosis. However, pendrin-positive intercalated cells also regulate blood pressure, at least partly, through pendrin-mediated Cl- absorption and through their indirect effect on the epithelial Na+ channel, ENaC. This aldosterone-induced increase in pendrin secondarily stimulates ENaC, thereby contributing to the aldosterone pressor response. This review describes the contribution of pendrin-positive intercalated cells to Na+, K+, Cl- and acid-base balance.

Selective Deletion of the Mechanistic Target of Rapamycin From the Renal Collecting Duct Principal Cell in Mice Down-Regulates the Epithelial Sodium Channel

The mechanistic target of rapamycin (mTOR), a serine-threonine-specific kinase, is a cellular energy sensor, integrating growth factor and nutrient signaling. In the collecting duct (CD) of the kidney, the epithelial sodium channel (ENaC) essential in the determination of final urine Na+ losses, has been demonstrated to be upregulated by mTOR, using cell culture and mTOR inhibition in ex vivo preparations. We tested whether CD-principal cell (PC) targeted deletion of mTOR using Cre-lox recombination would affect whole-body sodium homeostasis, blood pressure, and ENaC regulation in mice. Male and female CD-PC mTOR knockout (KO) mice and wild-type (WT) littermates (Cre-negative) were generated using aquaporin-2 (AQP2) promoter to drive Cre-recombinase. Under basal conditions, KO mice showed a reduced (∼30%) natriuretic response to benzamil (ENaC) antagonist, suggesting reduced in vivo ENaC activity. WT and KO mice were fed normal sodium (NS, 0.45% Na+) or a very low Na+ (LS, <0.02%) diet for 7-days. Switching from NS to LS resulted in significantly higher urine sodium losses (relative to WT) in the KO with adaptation occurring by day 2. Blood pressures were modestly (∼5-10 mm Hg) but significantly lower in KO mice under both diets. Western blotting showed KO mice had 20-40% reduced protein levels of all three subunits of ENaC under LS or NS diet. Immunohistochemistry (IHC) of kidney showed enhanced apical-vs.-cellular localization of all three subunits with LS, but a reduction in this ratio for γ-ENaC in the KO. Furthermore, the KO kidneys showed increased ubiquitination of α-ENaC and reduced phosphorylation of the serum and glucocorticoid regulated kinase, type 1 [serum glucocorticoid regulated kinase (SGK1)] on serine 422 (mTOR phosphorylation site). Taken together this suggests enhanced degradation as a consequence of reduced mTOR kinase activity and downstream upregulation of ubiquitination may have accounted for the reduction at least in α-ENaC. Overall, our data support a role for mTOR in ENaC activity likely via regulation of SGK1, ubiquitination, ENaC channel turnover and apical membrane residency. These data support a role for mTOR in the collecting duct in the maintenance of body sodium homeostasis.

The Renal Physiology of Pendrin-Positive Intercalated Cells

Intercalated cells (ICs) are found in the connecting tubule and the collecting duct. Of the three IC subtypes identified, type B intercalated cells are one of the best characterized and known to mediate Cl- absorption and HCO3- secretion, largely through the anion exchanger pendrin. This exchanger is thought to act in tandem with the Na+-dependent Cl-/HCO3- exchanger, NDCBE, to mediate net NaCl absorption. Pendrin is stimulated by angiotensin II and aldosterone administration via the angiotensin type 1a and the mineralocorticoid receptors, respectively. It is also stimulated in models of metabolic alkalosis, such as with NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, of probably more physiological or clinical significance is the role of these pendrin-positive ICs in blood pressure regulation, which occurs, at least in part, through pendrin-mediated renal Cl- absorption, as well as their effect on the epithelial Na+ channel, ENaC. Aldosterone stimulates ENaC directly through principal cell mineralocorticoid hormone receptor (ligand) binding and also indirectly through its effect on pendrin expression and function. In so doing, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. In addition to its role in Na+ and Cl- balance, pendrin affects the balance of other ions, such as K+ and I-. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contribution of pendrin-positive ICs in the kidney to distal nephron function and blood pressure.

Regulation of renal Na transporters in response to dietary K

Changes in the expression of Na transport proteins were measured in the kidneys of mice with increased dietary K intake for 1 wk. The epithelial Na channel (ENaC) was upregulated, with enhanced expression of full-length and cleaved forms of α-ENaC and cleaved γ-ENaC. At the same time, the amount of the NaCl cotransporter NCC and its phosphorylated form decreased by ~50% and ~80%, respectively. The expression of the phosphorylated form of the Na-K-2Cl cotransporter NKCC2 also decreased, despite an increase in overall protein content. The effect was stronger in males (80%) than in females (40%). This implies that less Na⁺ is reabsorbed in the thick ascending limb of Henle's loop and distal convoluted tubule along with Cl^-, whereas more is reabsorbed in the aldosterone-sensitive distal nephron in exchange for secreted K⁺. The abundance of the proximal tubule Na/H exchanger NHE3 decreased by ~40%, with similar effects in males and females. Time-course studies indicated that NCC and NHE3 proteins decreased progressively over 7 days on a high-K diet. Expression of mRNA encoding these proteins increased, implying that the decreased protein levels resulted from decreased rates of synthesis or increased rates of degradation. The potential importance of changes in NHE3, NKCC2, and NCC in promoting K⁺ excretion was assessed with a mathematical model. Simulations indicated that decreased NHE3 produced the largest effect. Regulation of proximal tubule Na⁺ transport may play a significant role in achieving K homeostasis.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Mass spectrometric imaging of metabolites in kidney tissues from rats treated with furosemide

In the kidney, metabolic processes are different among the cortex (COR), outer medulla (OM), and inner medulla (IM). Using matrix-assisted laser desorption/ionization (MALDI) and imaging mass spectrometry (IMS), we examined the change of metabolites in the COR, OM, and IM of the rat kidney after furosemide treatment compared with vehicle-treated controls. Osmotic minipumps were implanted in male Sprague-Dawley rats to deliver 12 mg·day(-1)·rat(-1) of furosemide. Vehicle-treated (n = 14) and furosemide-treated (furosemide rats, n = 15) rats in metabolic cages received a fixed amount of rat chow (15 g·220 g body wt(-1)·day(-1) for each rat) with free access to water intake for 6 days. At day 6, higher urine output (32 ± 4 vs. 9 ± 1 ml/day) and lower urine osmolality (546 ± 44 vs. 1,677 ± 104 mosmol/kgH2O) were observed in furosemide rats. Extracts of COR, OM, and IM were analyzed by ultraperformance liquid chromatography coupled with quadrupole time-of-flight (TOF) mass spectrometry, where multivariate analysis revealed significant differences between the two groups. Several metabolites, including acetylcarnitine, betaine, carnitine, choline, and glycerophosphorylcholine (GPC), were significantly changed. The changes of metabolites were further identified by MALDI-TOF/TOF and IMS. Their spatial distribution and relative quantitation in the kidneys were analyzed by IMS. Carnitine compounds were increased in COR and IM, whereas carnitine and acetylcarnitine were decreased in OM. Choline compounds were increased in COR and OM but decreased in IM from furosemide rats. Betaine and GPC were decreased in OM and IM. Taken together, MALDI-TOF/TOF and IMS successfully provide the spatial distribution and relative quantitation of metabolites in the kidney.

Chronic Metabolic Acidosis Activates Renal Tubular Sodium Chloride Cotransporter through Angiotension II-dependent WNK4-SPAK Phosphorylation Pathway

The mechanism by which chronic metabolic acidosis (CMA) regulates sodium (Na(+))-chloride (Cl(-)) cotransporter (NCC) in the renal distal convoluted tubules remains unexplored. We examined the role of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and with-no-lysine kinase 4 (WNK4) on expression of NCC in mouse models of CMA. CMA was induced by NH4Cl in wild type mice (WTA mice), SPAK, and WNK4 knockout mice. The quantities of Ncc mRNA, expression of total NCC, phosphorylated (p)-NCC, SPAK and WNK4 in the kidneys as well as NCC inhibition with hydrochlorothiazide and Na(+) balance were evaluated. Relative to WT mice, WTA mice had similar levels of Ncc mRNA, but increased expression of total and p-NCC, SPAK, and WNK4 and an exaggerated response to hydrochlorothiazide which could not be observed in SPAK or WNK4 knockout mice with CMA. In WTA mice, increased plasma renin activity, aldosterone and angiotensin II concentrations accompanied by a significantly negative Na(+) balance. High Na(+) diet abolished the enhanced NCC expression in WTA mice. Furthermore, an angiotensin II type 1 receptor blocker rather than a mineralocorticoid receptor antagonist exerted a marked inhibition on Na(+) reabsorption and NCC phosphorylation in WTA mice. CMA increases WNK4-SPAK-dependent NCC phosphorylation and appears to be secondary to previous natriuresis with volume-dependent angiotensin II activation.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Alkali therapy attenuates the progression of kidney injury via Na/H exchanger inhibition in 5/6 nephrectomized rats

Metabolic acidosis is a cause of renal disease progression, and alkali therapy ameliorates its progression. However, there are few reports on the role of renal acid-base transporters during alkali therapy. We evaluated the effect of sodium bicarbonate therapy and the role of acid-base transporters on renal disease progression in rats with a remnant kidney. Sprague-Dawley rats consumed dietary sodium bicarbonate (NaHCO3) or sodium chloride (NaCl) with 20% casein after a 5/6 nephrectomy. After being provided with a casein diet, the NaHCO3-treated group had higher levels of serum bicarbonate than the control group. At week 4, the glomerular filtration rate in the NaHCO3 group was higher than that in the NaCl group, and the difference became prominent at week 10. The glomerulosclerosis and tubulointerstitial damage indices in the NaHCO3 group were less severe compared with controls at week 4 and 10. The expression of the Na/H exchanger (NHE) was decreased, and apical reactivity was decreased in the NaHCO3 group, compared with the NaCl group. Endothelin-1 levels in the kidney were also decreased in the NaHCO3 group. Dietary sodium bicarbonate has the effects of ameliorating renal disease progression, which may be related to the altered expression of NHE in the remaining kidney.

Targeted proteomics using immunoblotting technique for studying dysregulation of ion transporters in renal disorders

Renal salt and water transport physiology has benefited tremendously from the rapid advance of proteomics. Proteomics developed as a fast-throughput means of screening for global changes in proteins in a selected tissue, organ or cell type, as a logical offshoot of similar comprehensive, messenger RNA array-type technology. Targeted proteomics utilizes similar techniques but examines a predetermined set of proteins. One approach that has been rigorously employed over the last 10 years to evaluate differences in renal protein abundances due to a treatment or genotype has been parallel semiquantitative immunoblotting using antibody arrays. This approach, and newer ones on the horizon, provide a rapid global overview of regulation of the individual proteins whose integrated action determines overall renal sodium or water reabsorption.

Altered regulation of renal Acid base transporters in response to ammonium chloride loading in rats

The role of the kidney in combating metabolic acidosis has been a subject of considerable interest for many years. The present study was aimed to determine whether there is an altered regulation of renal acid base transporters in acute and chronic acid loading. Male Sprague-Dawley rats were used. Metabolic acidosis was induced by administration of NH₄Cl for 2 days (acute) and for 7days (chronic). The serum and urinary pH and bicarbonate were measured. The protein expression of renal acid base transporters [type 3 Na⁺/H⁺ exchanger (NHE3), type 1 Na⁺/HCO₃^- cotransporter (NBC1), Na-K⁺ ATPase, H⁺-ATPase, anion exchanger-1 (AE-1)] was measured by semiquantitative immunoblotting. Serum bicarbonate and pH were decreased in acute acid loading rats compared with controls. Accordingly, urinary pH decreased. The protein expression of NHE3, H⁺-ATPase, AE-1 and NBC1 was not changed. In chronic acid loading rats, serum bicarbonate and pH were not changed, while urinary pH was decreased compared with controls. The protein expression of NHE3, H⁺-ATPase was increased in the renal cortex of chronic acid loading rats. These results suggest that unaltered expression of acid transporters combined with acute acid loading may contribute to the development of acidosis. The subsequent increased expression of NHE3, H⁺-ATPase in the kidney may play a role in promoting acid excretion in the later stage of acid loading, which counteract the development of metabolic acidosis.

NHLBI-AbDesigner: an online tool for design of peptide-directed antibodies

Investigation of physiological mechanisms at a cellular level often requires production of high-quality antibodies, frequently using synthetic peptides as immunogens. Here we describe a new, web-based software tool called NHLBI-AbDesigner that allows the user to visualize the information needed to choose optimal peptide sequences for peptide-directed antibody production (http://helixweb.nih.gov/AbDesigner/). The choice of an immunizing peptide is generally based on a need to optimize immunogenicity, antibody specificity, multispecies conservation, and robustness in the face of posttranslational modifications (PTMs). AbDesigner displays information relevant to these criteria as follows: 1) "Immunogenicity Score," based on hydropathy and secondary structure prediction; 2) "Uniqueness Score," a predictor of specificity of an antibody against all proteins expressed in the same species; 3) "Conservation Score," a predictor of ability of the antibody to recognize orthologs in other animal species; and 4) "Protein Features" that show structural domains, variable regions, and annotated PTMs that may affect antibody performance. AbDesigner displays the information online in an interactive graphical user interface, which allows the user to recognize the trade-offs that exist for alternative synthetic peptide choices and to choose the one that is best for a proposed application. Several examples of the use of AbDesigner for the display of such trade-offs are presented, including production of a new antibody to Slc9a3. We also used the program in large-scale mode to create a database listing the 15-amino acid peptides with the highest Immunogenicity Scores for all known proteins in five animal species, one plant species (Arabidopsis thaliana), and Saccharomyces cerevisiae.

Large-scale phosphoproteomic analysis of membrane proteins in renal proximal and distal tubule

Recent advances in mass spectrometry (MS) have provided means for large-scale phosphoproteomic profiling of specific tissues. Here, we report results from large-scale tandem MS [liquid chromatography (LC)-MS/MS]-based phosphoproteomic profiling of biochemically isolated membranes from the renal cortex, with focus on transporters and regulatory proteins. Data sets were filtered (by target-decoy analysis) to limit false-positive identifications to <2%. A total of 7,125 unique nonphosphorylated and 743 unique phosphorylated peptides were identified. Among the phosphopeptides identified were sites on transporter proteins, i.e., solute carrier (Slc, n = 63), ATP-binding cassette (Abc, n = 4), and aquaporin (Aqp, n = 3) family proteins. Database searches reveal that a majority of the phosphorylation sites identified in transporter proteins were previously unreported. Most of the Slc family proteins are apical or basolateral transporters expressed in proximal tubule cells, including proteins known to mediate transport of glucose, amino acids, organic ions, and inorganic ions. In addition, we identified potentially important phosphorylation sites for transport proteins from distal nephron segments, including the bumetanide-sensitive Na-K-2Cl cotransporter (Slc12a1 or NKCC2) at Ser(87), Thr(101), and Ser(126) and the thiazide-sensitive Na-Cl cotransporter (Slc12a3 or NCC) at Ser(71) and Ser(124). A subset of phosphorylation sites in regulatory proteins coincided with known functional motifs, suggesting specific regulatory roles. An online database from this study (http://dir.nhlbi.nih.gov/papers/lkem/rcmpd/) provides a resource for future studies of transporter regulation.

Pendrin modulates ENaC function by changing luminal HCO3-

The epithelial Na(+) channel, ENaC, and the Cl(-)/HCO(3)(-) exchanger, pendrin, mediate NaCl absorption within the cortical collecting duct and the connecting tubule. Although pendrin and ENaC localize to different cell types, ENaC subunit abundance and activity are lower in aldosterone-treated pendrin-null mice relative to wild-type mice. Because pendrin mediates HCO(3)(-) secretion, we asked if increasing distal delivery of HCO(3)(-) through a pendrin-independent mechanism "rescues" ENaC function in pendrin-null mice. We gave aldosterone and NaHCO(3) to increase pendrin-dependent HCO(3)(-) secretion within the connecting tubule and cortical collecting duct, or gave aldosterone and NaHCO(3) plus acetazolamide to increase luminal HCO(3)(-) concentration, [HCO(3)(-)], independent of pendrin. Following treatment with aldosterone and NaHCO(3), pendrin-null mice had lower urinary pH and [HCO(3)(-)] as well as lower renal ENaC abundance and function than wild-type mice. With the addition of acetazolamide, however, acid-base balance as well as ENaC subunit abundance and function was similar in pendrin-null and wild-type mice. We explored whether [HCO(3)(-)] directly alters ENaC abundance and function in cultured mouse principal cells (mpkCCD). Amiloride-sensitive current and ENaC abundance rose with increased [HCO(3)(-)] on the apical or the basolateral side, independent of the substituting anion. However, ENaC was more sensitive to changes in [HCO(3)(-)] on the basolateral side of the monolayer. Moreover, increasing [HCO(3)(-)] on the apical and basolateral side of Xenopus kidney cells increased both ENaC channel density and channel activity. We conclude that pendrin modulates ENaC abundance and function, at least in part by increasing luminal [HCO(3)(-)] and/or pH.

Upregulation of renal sodium transporters in D5 dopamine receptor-deficient mice

D(5) dopamine receptor (D(5)R)-deficient (D(5)(-/-)) mice have hypertension that is aggravated by an increase in sodium intake. The present experiments were designed to test the hypothesis that a dysregulation of renal sodium transporters is related to the salt sensitivity in D(5)(-/-) mice. D(5)R was expressed in the renal proximal tubule, thick ascending limb, distal convoluted tubule, and cortical and outer medullary collecting ducts in D(5)(+/+) mice. On a control Na(+) diet, renal protein expressions of NKCC2 (sodium-potassium-2 chloride cotransporter), sodium chloride cotransporter, and alpha and gamma subunits of the epithelial sodium channel were greater in D(5)(-/-) than in D(5)(+/+) mice. Renal renin abundance and urine aldosterone levels were similar but renal angiotensin II type 1 receptor (AT(1)R) protein expression was increased in D(5)(-/-) mice. An elevated Na(+) diet increased further the elevated blood pressure of D(5)(-/-) mice but did not affect the normal blood pressure of D(5)(+/+) mice. The increased levels of NKCC2, sodium chloride cotransporter, and alpha and gamma subunits of the epithelial sodium channel persisted with the elevated Na(+) diet and unaffected by chronic AT(1)R blockade (losartan) in D(5)(-/-) mice. The expressions of proximal sodium transporters NHE3 (sodium hydrogen exchanger type 3) and NaPi2 (sodium phosphate cotransporter type 2) were increased by the elevated Na(+) diet in D(5)(-/-) mice; the increased expression of NHE3 but not NaPi2 was abolished by AT(1)R blockade. Our findings suggest that the increased protein expression of sodium transporters/channels in distal nephron segments may be the direct consequence of the disruption of D(5)R, independent of the renin-angiotensin aldosterone system.

Increased Na reabsorption via the Na-Cl cotransporter in autosomal recessive pseudohypoaldosteronism

BACKGROUND

The autosomal recessive form of pseudohypoaldosteronism type 1 (AR-PHA1) is caused by loss-of-function mutations in the epithelial sodium channel subunit genes and is characterized by a multisystemic and lifelong severe salt-wasting tendency. However, we observed a male AR-PHA1 patient who exhibited less frequent salt wasting with advancing age, despite the cessation of daily salt supplementation.

OBJECTIVE

To elucidate the mechanism for the above phenomenon.

METHODS

We evaluated the sodium-reabsorption ability of his distal nephrons (from the distal convoluted tubules to the collecting ducts) and compared it to that of a patient with the dominant form of PHA1 (AD-PHA1) carrying a heterozygous NR3C2 (mineralocorticoid receptor) gene mutation. In addition, immunoblotting of the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) protein was conducted using urine samples from the AR- and AD-PHA1 patients.

RESULTS

The levels of sodium reabsorption that occurred via the distal nephrons were almost identical in the two PHA1 patients, despite their different molecular pathogeneses. Immunoblotting showed an increased urinary NCC protein level in the AR-PHA1 patient.

CONCLUSION

Taken together, increased sodium reabsorption via the upregulation of the expression of NCC might have been responsible, at least in part, for the clinical improvement seen in an AR-PHA1 patient.

Sex differences in adaptive downregulation of pre-macula densa sodium transporters with ANG II infusion in mice

An increase in blood pressure (BP) due to angiotensin II (ANG II) infusion or other means is associated with adaptive pressure natriuresis due to reduced sodium reabsorption primarily in proximal tubule (PT) and thick ascending limb (TAL). We tested the hypothesis that male and female mice would show differential response to ANG II infusion with regard to the regulation of the protein abundance of sodium transporters in the PT and TAL and that these responses would be modulated by aging. Young (approximately 3 mo) and old (approximately 21 mo) male and female mice were infused with ANG II at 800 ng x kg body wt(-1) x min(-1) by osmotic minipump for 7 days or received a sham operation. ANG II increased mean arterial pressure (MAP), measured by radiotelemetry, significantly more in male mice of both ages (increased approximately 30-40 mmHg), compared with females (increased approximately 15-25 mmHg). On day 1, MAP was also significantly increased in old mice, relative to young (P = 0.01). ANG II infusion was associated with a significant decline in plasma testosterone (to <30% of control male) in male mice and rise in young female mice (to 478% of control female). No sex differences were found in the upregulation of the sodium hydrogen exchanger abundance on Western blots observed with ANG II infusion or the downregulation of the sodium phosphate cotransporter; however, aging did impact on some of these changes. Male mice (especially young) also had significantly reduced levels of the TAL bumetanide-sensitive Na-K-2Cl cotransporter (to 60% of male control), while young females showed an increase (to 126% of female control) with ANG II infusion. These sex differences do not support impaired pressure natriuresis in male mice, but might reflect a greater need and attempt to mount an appropriately BP-metered natriuretic response by additional downregulation of TAL sodium reabsorption.

Sex and age result in differential regulation of the renal thiazide-sensitive NaCl cotransporter and the epithelial sodium channel in angiotensin II-infused mice

BACKGROUND/AIMS

We determined the effects of age and sex on the blood pressure (BP) response to angiotensin II (Ang II) infusion and evaluated the potential mechanistic role of the thiazide-sensitive NaCl cotransporter (NCC) and the epithelial sodium channel (ENaC).

METHODS

Male and female mice (approximately 3 or 21 months of age) were infused with Ang II or control for 7 days.

RESULTS

Males had a greater BP response to Ang II, somewhat enhanced by aging. Mean systolic BPs (at 7 days) were (mm Hg): 161, 143, 172, and 157 in young male, young female, old male, and old female mice, respectively. Immunoblotting changes in the whole kidney that supported this BP profile included a 51 and 52% increase in NCC band density in the old females and old males (as compared to sex-respective controls) with Ang II infusion, while the young males and young females showed an increase of 40 and 0%, respectively. Young males also had a greater reduction in major bands of beta- and gamma-ENaC, than did young female mice. The natriuretic response to hydrochlorothiazide supported an increase in activity of NCC with Ang II in aged mice only.

CONCLUSIONS

Increased sensitivity to Ang II in aging and male mice may involve overactivity of NCC.

Identification of phosphorylation-dependent binding partners of aquaporin-2 using protein mass spectrometry

Vasopressin-mediated control of water permeability in the renal collecting duct occurs in part through regulation of the distribution of aquaporin-2 (AQP2) between the apical plasma membrane and intracellular membrane compartments. Phosphorylation of Ser-256 at AQP2's cytoplasmic COOH-terminus is well-accepted as a critical step for translocation. The aim of this study was to identify binding partners to phosphorylated versus nonphosphorylated forms of the AQP2 COOH-terminus via a targeted comparative proteomic approach. Cytosol from inner medullary collecting ducts isolated from rat kidneys was incubated with "bait" peptides, representing the COOH-terminal AQP2 tail in its nonphosphorylated and phosphorylated forms, to capture differentially bound proteins prior to LC-MS/MS analysis. Mass spectrometric results were confirmed by immunoblotting. Immunoprecipitation was performed using an AQP2 COOH-terminal antibody combined with immunblotting against the proposed binding partners to demonstrate interactions with native AQP2. Our studies confirmed previously identified interactions between AQP2 and hsc70, hsp70-1 and -2, as well as annexin II. These proteins were found to bind less to the Ser-256-phosphorylated AQP2 than to the nonphosphorylated form. In contrast, another heat shock protein, hsp70-5 (BiP/grp78), bound to phosphorylated AQP2 more avidly than to nonphosphorylated AQP2. Immunogold EM studies demonstrated that BiP is present not only in the ER but also in the cytoplasm and apical plasma membrane of rat collecting duct cells. Furthermore, confocal immunofluorescence studies showed partial colocalization of BiP with AQP2 in non-ER compartments. These results suggest that phosphorylation of AQP2 at Ser-256 may regulate AQP2 trafficking in part by mediating differential binding of hsp70 family proteins to the COOH-terminal tail.

Expression and phosphorylation of the Na+-Cl- cotransporter NCC in vivo is regulated by dietary salt, potassium, and SGK1

The Na-Cl cotransporter NCC is expressed in the distal convoluted tubule, activated by phosphorylation, and has been implicated in renal NaCl and K(+) homeostasis. The serum and glucocorticoid inducible kinase 1 (SGK1) contributes to renal NaCl retention and K(+) excretion, at least in part, by stimulating the epithelial Na(+) channel and Na(+)-K(+)-ATPase in the downstream segments of aldosterone-sensitive Na(+)/K(+) exchange. In this study we confirmed in wild-type mice (WT) that dietary NaCl restriction increases renal NCC expression and its phosphorylation at Thr(53), Thr(58), and Ser(71), respectively. This response, however, was attenuated in mice lacking SGK1 (Sgk1(-/-)), which may contribute to impaired NaCl retention in those mice. Total renal NCC expression and phosphorylation at Thr(53), Thr(58), and Ser(71) in WT were greater under low- compared with high-K(+) diet. This finding is consistent with a regulation of NCC to modulate Na(+) delivery to downstream segments of Na(+)/K(+) exchange, thereby modulating K(+) excretion. Dietary K(+)-dependent variation in renal expression of total NCC and phosphorylated NCC were not attenuated in Sgk1(-/-) mice. In fact, high-K(+) diet-induced NCC suppression was enhanced in Sgk1(-/-) mice. The hyperkalemia induced in Sgk1(-/-) mice by a high-K(+) diet may have augmented NCC suppression, thereby increasing Na(+) delivery and facilitating K(+) excretion in downstream segments of impaired Na(+)/K(+) exchange. In summary, changes in NaCl and K(+) intake altered NCC expression and phosphorylation, an observation consistent with a role of NCC in NaCl and K(+) homeostasis. The two maneuvers dissociated plasma aldosterone levels from NCC expression and phosphorylation, implicating additional regulators. Regulation of NCC expression and phosphorylation by dietary NaCl restriction appears to involve SGK1.

Initial FGF23-mediated signaling occurs in the distal convoluted tubule

Fibroblast growth factor-23 (FGF23), a hormone central to phosphate and vitamin D metabolism, reduces renal absorption of phosphate by downregulating the sodium-phosphate cotransporter Npt2a. However, the mechanisms of FGF23 action in the kidney are unclear, as Npt2a localizes to the proximal tubule (PT) and the FGF23 coreceptor alpha-Klotho (KL) localizes to the distal convoluted tubule (DCT). Immunofluorescent analyses following FGF23 injection in mice showed robust staining for phospho-ERK1/2, a marker of FGF23 bioactivity, only within the DCT in a subset of KL-positive cells. This activity colocalized with the FGF23 receptor FGFR1 and was present in DCT cells that were adjacent to Npt2a-expressing PT segments. Although KL is expressed as both secreted and membrane-bound isoforms, only the membrane-bound isoform was capable of mediating FGF23 bioactivity. These findings provide novel insight into the mechanisms of hormone-regulated phosphate metabolism by identifying an intrarenal signaling axis for FGF23.

Enhanced expressions of sodium-glucose cotransporters in the kidneys of diabetic Zucker rats

Diabetes-mediated changes in mRNA expressions of kidney glucose transporters SGLT1 and SGLT2 were investigated in Zucker rats. SGLTs expressions in pre-diabetic obese rats were similar to leans. SGLT1 and SGLT2 levels in diabetic obese rats were 1.6 (P<0.03) and 4.8 (P<0.002) folds higher than age-matched leans, respectively.

Long-term regulation of proximal tubule acid-base transporter abundance by angiotensin II

In the proximal tubule, angiotensin II (Ang-II) regulates HCO(-)(3) reabsorption and H+ secretion by binding the type 1 Ang-II (AT1) receptor, stimulating Na(+)/HCO(-)(3) cotransport and Na(+)/H(+) exchange. Studies were carried out to determine if long-term changes in Ang-II receptor occupation alter the abundance of the basolateral Na(+)/HCO(-)(3) cotransporter (NBC1) or the apical membrane type 3 Na(+)/H(+) exchanger (NHE3). In the first set of experiments, rats eating a low-sodium diet were infused with the AT1 blocker, candesartan, or vehicle. In the second, lisinopril-infused rats were infused with either Ang II or vehicle. Transporter abundances were determined in whole kidney homogenates (WKH) and in brush border membrane (BBM) preparations by semiquantitative immunoblotting. Tissue distribution of transporters was assessed by immunocytochemistry. Blockade of the AT1 receptor by candesartan caused decreased abundance of NBC1 in WKH (59 +/- 9% of control; P<0.05) and Ang-II infusion increased abundance (130 +/- 7% of control; P<0.05). Changes in NBC1 in response to candesartan were confirmed immunohistochemically. Neither candesartan nor Ang II infusion affected the abundance of NHE3 in WKH or cortical homogenates. Candesartan decreased type 2 sodium-phosphate cotransporter abundance in both WKH (52 +/- 7% of control; P<0.05) and BBM (32 +/- 7% of control; P<0.05). Serum bicarbonate was decreased by candesartan and increased by Ang-II. Candesartan also decreased urinary ammonium excretion (P<0.05). The long-term effects of Ang-II in the proximal tubule may be mediated in part by regulation of NBC1 abundance, modifying bicarbonate reabsorption.

Regulation of the renal sodium-dependent phosphate cotransporter NaPi2 (Npt2) in acute renal failure due to ischemia and reperfusion

BACKGROUND

Acute renal failure (ARF) is associated with hyperphosphatemia and decreased urinary phosphate excretion. The present study was undertaken to characterize the effects of ARF due to ischemia and reperfusion on renal phosphate transport and on gene and protein expression of type IIa NaPi cotransporter (Npt2) the physiologically most relevant renal sodium-dependent phosphate cotransporter.

METHODS

The following groups of rats with intact parathyroid glands were studied: (1) sham operated (sham); (2) after 1 h ischemia by bilateral renal artery clamping (I), and after 1 h ischemia and reperfusion of 1 h (I + R 1 h); (3) 24 h (I + R 24 h); (4) 48 h (I + R 48 h), and (5) 72 h (I + R 72 h) duration. The effect of ARF on Npt2 mRNA and protein expression was also examined after parathyroidectomy (PTX) of 2 and 4 days' duration.

RESULTS

Ischemia and reperfusion were associated with increases in plasma creatinine, hyperphosphatemia, and with decreased tubular phosphate reabsorption. Npt2 mRNA was significantly downregulated in the cortex, maximal at 24 and 48 h of reperfusion. The degree of Npt2 mRNA downregulation was not affected by PTX of 2-4 days' duration. The abundance of Npt2 protein in proximal tubular apical brush border membrane was markedly decreased after reperfusion. Npt2 protein, however, was more abundant in PTX animals than in those with intact parathyroids and a similar degree of renal insufficiency. The immunohistochemical analysis of proximal tubular apical brush border membrane showed a progressive decrease of Npt2 protein labeling after ischemia and reperfusion, with progressive regeneration after 72 h.

CONCLUSION

These results suggest that downregulation of Npt2 protein may contribute to the decreased tubular reabsorption of phosphate in acute ischemic renal failure and hyperphosphatemia.

Sodium retention in cirrhotic rats is associated with increased renal abundance of sodium transporter proteins

BACKGROUND

Liver cirrhosis with ascites is associated with a decrease in renal sodium excretion and therefore sodium retention.

METHODS

In this paper, we utilize transporter-specific antibodies to address the hypothesis that dysregulation of one or more sodium transporters or channels is associated with sodium chloride (NaCl) retention in a rat model of cirrhosis induced by repeated exposure to carbon tetrachloride. Age-matched controls and cirrhotic rats were pair fed to ensure identical NaCl and water intake for 4 days prior to euthanasia for quantitative immunoblotting studies.

RESULTS AND CONCLUSION

The rats manifested marked extracellular fluid volume expansion with massive ascites. Plasma aldosterone levels were markedly elevated. Analysis of immunoblots revealed marked increases in the abundances of both of the major aldosterone-sensitive apical transport proteins of the renal tubule, namely the thiazide-sensitive NaCl cotransporter NCC and the epithelial sodium channel alpha subunit (alpha-ENaC). These results are consistent with an important role for hyperaldosteronism in the pathogenesis of sodium retention and ascites formation in cirrhosis. In addition, we observed a large decrease in cortical NHE3 abundance (proximal tubule) and a large increase in NKCC2 abundance (thick ascending limb), potentially shifting premacula densa sodium absorption from proximal tubule to loop of Henle (which powers urinary concentration and dilution).

Upregulation of Na+ transporter abundances in response to chronic thiazide or loop diuretic treatment in rats

Furosemide and hydrochlorothiazide (HCTZ) exert their diuretic actions by binding to apical Na(+) transporters, viz., the Na(+)-K(+)-2Cl(-) cotransporter in the thick ascending limb and the Na(+)-Cl(-) cotransporter in the distal convoluted tubule, respectively. We carried out semiquantitative immunoblotting and immunohistochemistry of rat kidneys to investigate whether chronic administration of furosemide or HCTZ is associated with compensatory changes in the abundance of Na(+) transporters downstream from the primary site of action. Osmotic minipumps were implanted into Sprague-Dawley rats to deliver furosemide (12 mg/day) or HCTZ (3.75 mg/day) for 7 days. To prevent volume depletion, all animals were offered tap water and a solution containing 0.8% NaCl and 0.1% KCl as drinking fluid. The diuretic/natriuretic response was quantified in response to both agents by using quantitative urine collections. Semiquantitative immunoblotting revealed that the abundances of thick ascending limb Na(+)-K(+)-2Cl(-) cotransporter and all three subunits of the epithelial Na(+) channel (ENaC) were increased by furosemide infusion. HCTZ infusion increased the abundances of thiazide-sensitive Na(+)-Cl(-) cotransporter and beta-ENaC in the cortex and beta- and gamma-ENaC in the outer medulla. Consistent with these results, beta-ENaC immunohistochemistry showed a remarkable increase in immunoreactivity in the principal cells of collecting ducts with either diuretic treatment. These increases in the abundance of Na(+) transporters in response to chronic diuretic treatment may account for the generation of diuretic tolerance associated with long-term diuretic use.

Time course of renal Na-K-ATPase, NHE3, NKCC2, NCC, and ENaC abundance changes with dietary NaCl restriction

We have used peptide-directed antibodies to each major renal Na transporter and channel proteins to screen renal homogenates for changes in Na transporter protein expression after initiation of dietary NaCl restriction. After equilibration on a NaCl-replete diet (2.0 meq. 200 g body wt(-1). day(-1)), rats were switched to a NaCl-deficient diet (0.02 meq. 200 g body wt(-1). day(-1)). Na excretion fell to 25% of baseline levels on day 1, followed by a further decrease <4% of baseline levels on day 3, of NaCl restriction. The decreased Na excretion at day 1 occurred despite the absence of a significant increase in plasma aldosterone level or in the abundance of any of the major renal Na transporters. However, after a 1-day lag, plasma aldosterone levels increased in association with increases in abundances of three aldosterone-regulated Na transporter proteins: the thiazide-sensitive Na-Cl cotransporter (NCC), the alpha-subunit of the amiloride-sensitive epithelial Na channel (alpha-ENaC), and the 70-kDa form of gamma-ENaC. RNase protection assays of transporter mRNA levels revealed an increase in renal alpha-ENaC mRNA coincident with the increase in alpha-ENaC protein abundance. However, there was no change in NCC mRNA abundance, suggesting that the increase in NCC protein in response to dietary NaCl restriction was not a result of altered gene transcription. These results point to early regulatory processes that decrease renal Na excretion without an increase in the abundance of any Na transporter, followed by a late aldosterone-dependent response associated with upregulation of NCC and ENaC.

Targeted disruption of the mouse NHERF-1 gene promotes internalization of proximal tubule sodium-phosphate cotransporter type IIa and renal phosphate wasting

Na+/H+ exchanger regulatory factor (NHERF)-1 and NHERF-2, two structurally related protein adapters containing tandem PSD-95/Discs large/ZO-1 (PDZ) domains, were identified as essential factors for protein kinase A-mediated inhibition of the sodium-hydrogen exchanger, NHE3. NHERF-1 and NHERF-2 also bound other cellular targets including the sodium-phosphate cotransporter type IIa encoded by the NPT2 gene. Targeted disruption of the mouse NHERF-1 gene eliminated NHERF-1 expression in kidney and other tissues of the mutant mice without altering NHERF-2 levels in these tissues. NHERF-1 (+/-) and (-/-) male mice maintained normal blood electrolytes but showed increased urinary excretion of phosphate when compared with wild-type (+/+) animals. Although the overall levels of renal NHERF-1 targets, NHE3 and Npt2, were unchanged in the mutant mice, immunocytochemistry showed that the Npt2 protein was aberrantly localized at internal sites in the renal proximal tubule cells. The mislocalization of Npt2 paralleled a reduction in the transporter protein in renal brush-border membranes isolated from the mutant mice. In contrast, NHE3 was appropriately localized at the apical surface of proximal tubules in both wild-type and mutant mice. These data suggested that NHERF-1 played a unique role in the apical targeting and/or trafficking of Npt2 in the mammalian kidney, a function not shared by NHERF-2 or other renal PDZ proteins. Phosphate wasting seen in the NHERF-1(-/-) null mice provided a new experimental system for defining the role of PDZ adapters in the hormonal control of ion transport and renal disease.

Upregulation of renal BSC1 and TSC in prenatally programmed hypertension

Prenatal factors, especially intrauterine growth retardation, have been shown to correlate with the risk of essential hypertension in adult life, but the mechanisms are unknown. An experimental model of prenatal programming of hypertension in the rat, induced by a maternal low-protein diet during pregnancy, was employed to study the role of renal Na reabsorption in the pathogenesis. The abundance of the apical Na transporter type III Na/H exchanger (NHE3), bumetanide-sensitive Na-K-2Cl cotransporter (BSC1), thiazide-sensitive Na-Cl cotransporter (TSC), and the amiloride-sensitive epithelial Na channel (ENaC) was determined by semiquantitative immunoblotting in kidneys from the offspring at 4 wk of age, before hypertension became manifest. There were no significant differences between the experimental and control rats in the abundance of NHE3 or any of the ENaC subunits. In contrast, the quantity of BSC1 in the experimental group was increased to 302% of control (P < 0.001) and that of TSC to 157% of control (P < 0.05). Determination of specific mRNA levels by ELISA-linked RT-PCR revealed a significantly increased BSC1 mRNA at 1 day (P < 0.01), 4 wk (P < 0.01), and 8 wk (P < 0.001) of age, and a significantly increased TSC mRNA at 4 wk of age (P < 0.05) in the experimental group. The results suggest that prenatal programming of hypertension involves transcriptional upregulation of Na transport in thick ascending limb and distal convoluted tubule.

Posttranscriptional compensation for heterozygous disruption of the kidney-specific NaK2Cl cotransporter gene

Mice homozygous for a loss of function mutation of the kidney-specific NaK2Cl cotransporter, BSC1/NKCC2, do not survive. Here the effects of loss of one copy of the gene are studied. NKCC2 mRNA of NKCC2 +/- kidney was 55 +/- 6% of +/+, yet no differences were found between NKCC2 +/+ and +/- mice in BP, blood gas, electrolytes, creatinine, plasma renin concentration, urine volume and osmolality, ability to concentrate and dilute urine, and response to furosemide. When mice were challenged with 180 mM NH(4)Cl, plasma ammonia and urinary ammonia excretion were increased twofold and fivefold, respectively, but there was still no difference between the two genotypes. NKCC2 +/- mice had a near-normal level of NKCC2 protein and no clear change in the distribution of NKCC2 in the thick ascending limb (TAL) cells. In vitro microperfusion of isolated TAL showed no significant difference between the two genotypes in the basal and vasopressin-stimulated capacity to reabsorb NaCl. There was no difference in the mRNA expressions of thiazide-sensitive NaCl cotransporter, epithelial Na channel (ENaC), aquaporin-2, ROMK, and NaKATPase. Halving the mRNA expression of NKCC2 does not affect BP or fluid balance because of compensatory factors that restore the protein level to near normal. One possible factor is a regulated increase in the movement of cytoplasmic protein to the luminal membrane leading to a restoration of functional transporter to an essentially wild type level.

Chronic metabolic acidosis upregulates rat kidney Na-HCO cotransporters NBCn1 and NBC3 but not NBC1

Several members of the Na-HCO cotransporter (NBC) family have recently been identified functionally and partly characterized, including rkNBC1, NBCn1, and NBC3. Regulation of these NBCs may play a role in the maintenance of intracellular pH and in the regulation of renal acid-base balance. However, it is unknown whether the expressions of these NBCs are regulated in response to changes in acid-base status. We therefore tested whether chronic metabolic acidosis (CMA) affects the abundance of these NBCs in kidneys using two conventional protocols. In protocol 1, rats were treated with NH(4)Cl in their drinking water (12 +/- 1 mmol. rat(-1). day(-1)) for 2 wk with free access to water (n = 8). Semiquantitative immunoblotting demonstrated that whole kidney abundance of NBCn1 and NBC3 in rats with CMA was dramatically increased to 995 +/- 87 and 224 +/- 35%, respectively, of control levels (P < 0.05), whereas whole kidney rkNBC1 was unchanged (88 +/- 14%). In protocol 2, rats were given NH(4)Cl in their food (10 +/- 1 mmol. rat(-1). day(-1)) for 7 days, with a fixed daily water intake (n = 6). Consistent with protocol 1, whole kidney abundances of NBCn1 (262 +/- 42%) and NBC3 (160 +/- 31%) were significantly increased compared with controls (n = 6), whereas whole kidney rkNBC1 was unchanged (84 +/- 17%). In both protocols, immunocytochemistry confirmed upregulation of NBCn1 and NBC3 with no change in the segmental distribution along the nephron. Consistent with the increase in NBCn1, measurements of pH transients in medullary thick ascending limb (mTAL) cells in kidney slices revealed two- to threefold increases in DIDS- sensitive, Na(+)-dependent HCO uptake in rats with CMA. In conclusion, CMA is associated with a marked increase in the abundance of NBCn1 in the mTAL and NBC3 in intercalated cells, whereas the abundance of NBC1 in the proximal tubule was not altered. The increased abundance of NBCn1 may play a role in the reabsorption of NH in the mTAL and increased NBC3 in reabsorbing HCO.

Increased renal Na-K-ATPase, NCC, and beta-ENaC abundance in obese Zucker rats

Renal sodium retention, as a result of increased abundance of sodium transporters, may play a role in the development and/or maintenance of the increased blood pressure in obesity. To address this hypothesis, we evaluated the relative abundances of renal sodium transporters in lean and obese Zucker rats at 2 and 4 mo of age by semiquantitative immunoblotting. Mean systolic blood pressure was higher in obese rats relative to lean at 3 mo, P < 0.02. Furthermore, circulating insulin levels were 6- or 13-fold higher in obese rats compared with lean at 2 or 4 mo of age, respectively. The abundances of the alpha(1)-subunit of Na-K-ATPase, the thiazide-sensitive Na-Cl cotransporter (NCC or TSC), and the beta-subunit of the epithelial sodium channel (ENaC) were all significantly increased in the obese rats' kidneys. There were no differences for the sodium hydrogen exchanger (NHE3), the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2 or BSC1), the type II sodium-phosphate cotransporter (NaPi-2), or the alpha-subunit of ENaC. These selective increases could possibly increase sodium retention by the kidney and therefore could play a role in obesity-related hypertension.

Ammonium interaction with the epithelial sodium channel

The purpose of this study was to investigate the direct effect of NH(3)/NH on mouse epithelial Na(+) channels (mENaC) expressed in Xenopus oocytes. Two-electrode voltage-clamp and ion-selective microelectrodes were used to measure the Na(+) current, intracellular pH (pH(i)), and ion activities in oocytes expressing mENaC. In oocytes expressing mENaC, removal of external Na(+) reversibly hyperpolarized membrane potential by 129 +/- 5.3 mV in the absence of 20 mM NH(4)Cl but only by 100 +/- 7.8 mV in its presence. Amiloride completely inhibited the changes in membrane potential. In oocytes expressing mENaC, butyrate (20 mM) caused a decrease in pH(i) (0.43 +/- 0.07) similar to the NH(4)Cl-induced pH(i) decrease (0.47 +/- 0.12). Removal of Na(+) in the presence of butyrate caused hyperpolarization that was not significantly different from that in the absence of butyrate at high pH(i) (in the absence of NH(4)Cl). Removal of external Na(+) resulted in an outward current of 3.7 +/- 0.8 microA (at -60 mV). The magnitude of this change in current was only 2.7 +/- 0.7 microA when Na(+) was removed in the presence of NH(4)Cl. In oocytes expressing mENaC, NH(4)Cl also caused a decrease in whole cell conductance at negative potential and an outward current at positive potential. In the presence of amiloride, steady-state current and the change in current caused by removal of Na(+) were not different from zero. These results indicate that NH(4)Cl inhibits Na(+) transport when mENaC is expressed in oocytes. The inhibition of voltage changes is not due to intracellular acidification caused by NH(4)Cl. Permeability and selectivity of ENaC to NH may play a role.

Targeted proteomics in the kidney using ensembles of antibodies

Building on extensive physiological characterization of sodium transport mechanisms along the renal tubule over the past 30 years, complementary DNAs for almost all of the major transporters and channels responsible for renal tubular sodium reabsorption have been cloned over the past 10 years. The consequence is the generation of a broad range of cDNA and antibody probes which can be used to investigate physiological mechanisms on a molecular level. An ensemble of such probes can be exploited for comprehensive analysis of integrative physiological processes, approaches which are referred to as 'physiological genomics' or 'physiological proteomics'. In this review, we describe a targeted proteomic approach to comprehensive analysis of sodium transporter and water channel protein abundance along the renal tubule using an ensemble of rabbit polyclonal antibodies directed to the major sodium transporters and water channels expressed in each renal tubule segment. We discuss preparation and characterization of the antibodies, strategies for quantification of transporter protein abundance, and provide examples of the application of antibody-based targeted proteomics analysis of kidney tissue, revealing the effects of elevations of circulating aldosterone levels and circulating vasopressin levels on sodium transporter, sodium channel, and water channel abundance in kidney.

The renal thiazide-sensitive Na-Cl cotransporter as mediator of the aldosterone-escape phenomenon

The kidneys "escape" from the Na-retaining effects of aldosterone when circulating levels of aldosterone are inappropriately elevated in the setting of normal or expanded extracellular fluid volume, e.g., in primary aldosteronism. Using a targeted proteomics approach, we screened renal protein extracts with rabbit polyclonal antibodies directed to each of the major Na transporters expressed along the nephron to determine whether escape from aldosterone-mediated Na retention is associated with decreased abundance of one or more of renal Na transporters. The analysis revealed that the renal abundance of the thiazide-sensitive Na-Cl cotransporter (NCC) was profoundly and selectively decreased. None of the other apical solute-coupled Na transporters displayed decreases in abundance, nor were the total abundances of the three ENaC subunits significantly altered. Immunocytochemistry showed a strong decrease in NCC labeling in distal convoluted tubules of aldosterone-escape rats with no change in the cellular distribution of NCC. Ribonuclease protection assays (RPAs) revealed that the decrease in NCC protein abundance was not associated with altered NCC mRNA abundance. Thus, the thiazide-sensitive Na-Cl cotransporter of the distal convoluted tubule appears to be the chief molecular target for regulatory processes responsible for mineralocorticoid escape, decreasing in abundance via a posttranscriptional mechanism.

Sequencing and characterization of the human thiazide-sensitive Na-Cl cotransporter (SLC12A3) gene promoter

The thiazide-sensitive Na-Cl cotransporter SLC12A3 displays expression restricted to distal convoluted tubule cells where it catalyzes the uptake of sodium and chloride through the apical membrane. We sequenced 1959 bp of the 5' flanking region of human SLC12A3, located the area of transcription initiation, and used deletion constructs of the flanking region to determine areas that affect reporter gene expression in two cell lines, MDCT and CHO. Amplification of the 5' end of SLC12A3 cDNA from an adapter-ligated human kidney cDNA library demonstrated that transcription initiation is confined to an area from -18 to -6 bp upstream of the translation start codon. Maximum promoter activity (9.815 +/- 0.864 times control) was observed in MDCT cells using a promoter containing 1019 bp of the 5' flanking region. A promoter containing only 134 bp of the 5' flanking region upstream of the translation initiation codon maintained reporter gene expression at levels equal to 75% of that maximally observed (7.375 +/- 0.533 times control). Sequence analysis of this minimal promoter responsible for most of the SLC12A3 promoter activity revealed a TATA element, two Sp binding sites, a potential E box, and a potential binding site for NF-1/CTF or NY-I/CP-I. This promoter, and all other promoter constructs from SLC12A3, displayed repressor activity in CHO cells. A construct containing sequence 94 bp upstream of the initiation codon with two potential Sp binding sites was required for this repression. Protein-DNA interactions between the 182 bp region immediately upstream of the start codon and the nuclear proteins from rat kidney cortex and HeLa cells were examined to further clarify the role of the putative binding sites for SLC12A3 expression. Physiological studies investigating the effects of osmolarity, pH, and mineralocorticoid steroid on promoter activity demonstrated that the promoter activity was repressed by acidification, whereas no effects of increased osmolarity or deoxycorticosterone acetate addition were observed.

Altered expression of renal aquaporins and Na+ transporters in rats treated with L-type calcium blocker

Nifedipine, a calcium antagonist, has diuretic and natriuretic properties. However, the molecular mechanisms by which these effects are produced are poorly understood. We examined kidney abundance of aquaporins (AQP1, AQP2, and AQP3) and major sodium transporters [type 3 Na/H exchanger (NHE-3); type 2 Na-Pi cotransporter (NaPi-2); Na-K-ATPase; type 1 bumetanide-sensitive cotransporter (BSC-1); and thiazide-sensitive Na-Cl cotransporter (TSC)] as well as inner medullary abundance of AQP2, phosphorylated-AQP2 (p-AQP2), AQP3, and calcium-sensing receptor (CaR). Rats treated with nifedipine orally (700 mg/kg) for 19 days had a significant increase in urine output, whereas urinary osmolality and solute-free water reabsorption were markedly reduced. Consistent with this, immunoblotting revealed a significant decrease in the abundance of whole kidney AQP2 (47 ± 7% of control rats, P< 0.05) and in inner medullary AQP2 (60 ± 7%) as well as in p-AQP2 abundance (17 ± 6%) in nifedipine-treated rats. In contrast, whole kidney AQP3 abundance was significantly increased (219 ± 28%). Of potential importance in modulating AQP2 levels, the abundance of CaR in the inner medulla was significantly increased (295 ± 25%) in nifedipine-treated rats. Nifedipine treatment was also associated with increased urinary sodium excretion. Consistent with this, semiquantitative immunoblotting revealed significant reductions in the abundance of proximal tubule Na+ transporters: NHE-3 (3 ± 1%), NaPi-2 (53 ± 12%), and Na-K-ATPase (74 ± 5%). In contrast, the abundance of the distal tubule Na-Cl cotransporter (TSC) was markedly increased (240 ± 29%), whereas BSC-1 in the thick ascending limb was not altered. In conclusion, 1) increased urine output and reduced urinary concentration in nifedipine-treated-rats may, in part, be due to downregulation of AQP2 and p-AQP2 levels; 2) CaR might be involved in the regulation of water reabsorption in the inner medulla collecting duct; 3) reduced expression of proximal tubule Na+ transporters (NHE-3, NaPi-2, and Na, K-ATPase) may be involved in the increased urinary sodium excretion; and 4) increase in TSC expression may occur as a compensatory mechanism.

Profiling of renal tubule Na+ transporter abundances in NHE3 and NCC null mice using targeted proteomics

The Na+-H+ exchanger NHE3 and the thiazide-sensitive Na+-Cl- cotransporter NCC are the major apical sodium transporters in the proximal convoluted tubule and the distal convoluted tubule of the kidney, respectively. We investigated the mechanism of compensation that allows maintenance of sodium balance in NHE3 knockout mice and in NCC knockout mice. We used a so-called 'targeted proteomics' approach, which profiles the entire renal tubule with regard to changes in Na+ transporter and aquaporin abundance in response to the gene deletions. Specific antibodies to the Na+ transporters and aquaporins expressed along the nephron were utilized to determine the relative abundance of each transporter. Semiquantitative immunoblotting was used which gives an estimate of the percentage change in abundance of each transporter in knockout compared with wild-type mice. In NHE3 knockout mice three changes were identified which could compensate for the loss of NHE3-mediated sodium absorption. (a) The proximal sodium-phosphate cotransporter NaPi-2 was markedly upregulated. (b) In the collecting duct, the 70 kDa form of the y-subunit of the epithelial sodium channel, ENaC, exhibited an increase in abundance. This is thought to be an aldosterone-stimulated form of y-ENaC. (c) Glomerular filtration was significantly reduced. In the NCC knockout mice, amongst all the sodium transporters expressed along the renal tubule, only the 70 kDa form of the y-subunit of the epithelial sodium channel, ENaC, exhibited an increase in abundance. In conclusion, both mouse knockout models demonstrated successful compensation for loss of the deleted transporter. More extensive adaptation occurred in the case of the NHE3 knockout, presumably because NHE3 is responsible for much more sodium absorption in normal mice than in NCC knockout mice.