Chronic metabolic acidosis enhances NHE-3 protein abundance and transport activity in the rat thick ascending limb by increasing NHE-3 mRNA

miRNA-23a modulates sodium-hydrogen exchanger 1 expression: studies in medullary thick ascending limb of salt-induced hypertensive rats

BACKGROUND

The kidney is the main organ in the pathophysiology of essential hypertension. Although most bicarbonate reabsorption occurs in the proximal tubule, the medullary thick ascending limb (mTAL) of the nephron also maintains acid-base balance by contributing to 25% of bicarbonate reabsorption. A crucial element in this regulation is the sodium-hydrogen exchanger 1 (NHE1), a ubiquitous membrane protein controlling intracellular pH, where proton extrusion is driven by the inward sodium flux. MicroRNA (miRNA) expression of hypertensive patients significantly differs from that of normotensive subjects. The aim of this study was to determine the functional role of miRNA alterations at the mTAL level.

METHODS

By miRNA microarray analysis, we identified miRNA expression profiles in isolated mTALs from high sodium intake-induced hypertensive rats (HSD) versus their normotensive counterparts (NSD). In vitro validation was carried out in rat mTAL cells.

RESULTS

Five miRNAs involved in the onset of salt-sensitive hypertension were identified, including miR-23a, which was bioinformatically predicted to target NHE1 mRNA. Data demonstrated that miRNA-23a is downregulated in the mTAL of HSD rats while NHE1 is upregulated. Consistently, transfection of an miRNA-23a mimic in an mTAL cell line, using a viral vector, resulted in NHE1 downregulation.

CONCLUSION

NHE1, a protein involved in sodium reabsorption at the mTAL level and blood pressure regulation, is upregulated in our model. This was due to a downregulation of miRNA-23a. Expression levels of this miRNA are influenced by high sodium intake in the mTALs of rats. The downregulation of miRNA-23a in humans affected by essential hypertension corroborate our data and point to the potential role of miRNA-23a in the regulation of mTAL function following high salt intake.

Novel Treatments from Inhibition of the Intestinal Sodium-Hydrogen Exchanger 3

Plasma membrane sodium–hydrogen exchangers (NHE) transport Na⁺ into cells in exchange for H⁺. While there are nine isoforms of NHE in humans, this review focuses on the NHE3 isoform, which is abundantly expressed in the gastrointestinal tract, where it plays a key role in acid–base balance and water homeostasis. NHE3 inhibition in the small intestine results in luminal sodium and water retention, leading to a general decrease in paracellular water flux and diffusional driving force, reduced intestinal sodium absorption, and increased stool sodium excretion. The resulting softer and more frequent stools are the rationale for the development of tenapanor as a novel, first-in-class NHE3 inhibitor to treat irritable bowel syndrome with constipation. NHE3 also has additional therapeutic implications in nephrology. Inhibition of intestinal NHE3 also lowers blood pressure by reducing intestinal sodium absorption. Perhaps, the most novel effect is its ability to decrease intestinal phosphate absorption by inhibiting the paracellular phosphate absorption pathway. Therefore, selective pharmacological inhibition of NHE3 could be a potential therapeutic strategy to treat not only heart failure and hypertension but also hyperphosphatemia. This review presents an overview of the molecular and physiological functions of NHE3 and discusses how these functions translate to potential clinical applications in nephrology.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Defective bicarbonate reabsorption in Kir4.2 potassium channel deficient mice impairs acid-base balance and ammonia excretion

The kidneys excrete the daily acid load mainly by generating and excreting ammonia but the underlying molecular mechanisms are not fully understood. Here we evaluated the role of the inwardly rectifying potassium channel subunit Kir4.2 (Kcnj15 gene product) in this process. In mice, Kir4.2 was present exclusively at the basolateral membrane of proximal tubular cells and disruption of Kcnj15 caused a hyperchloremic metabolic acidosis associated with a reduced threshold for bicarbonate in the absence of a generalized proximal tubule dysfunction. Urinary ammonium excretion rates in Kcnj15- deleted mice were inappropriate to acidosis under basal and acid-loading conditions, and not related to a failure to acidify urine or a reduced expression of ammonia transporters in the collecting duct. In contrast, the expression of key proteins involved in ammonia metabolism and secretion by proximal cells, namely the glutamine transporter SNAT3, the phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase enzymes, and the sodium-proton exchanger NHE-3 was inappropriate in Kcnj15-deleted mice. Additionally, Kcnj15 deletion depolarized the proximal cell membrane by decreasing the barium-sensitive component of the potassium conductance and caused an intracellular alkalinization. Thus, the Kir4.2 potassium channel subunit is a newly recognized regulator of proximal ammonia metabolism. The kidney consequences of its loss of function in mice support the proposal for KCNJ15 as a molecular basis for human isolated proximal renal tubular acidosis.

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.

Acidic pH and short-chain fatty acids activate Na+ transport but differentially modulate expression of Na+/H+ exchanger isoforms 1, 2, and 3 in omasal epithelium

Low sodium content in feed and large amounts of salivary sodium secretion are essential requirements to efficient sodium reabsorption in the dairy cow. It is already known that Na(+)/H(+) exchange (NHE) of the ruminal epithelium plays a key role in Na(+) absorption, and its function is influenced by the presence of short-chain fatty acids (SCFA) and mucosal pH. By contrast, the functional role and regulation of NHE in omasal epithelium have not been completely understood. In the present study, we used model studies in small ruminants (sheep and goats) to investigate NHE-mediated Na(+) transport and the effects of pH and SCFA on NHE activity in omasal epithelium and on the expression of NHE isoform in omasal epithelial cells. Conventional Ussing chamber technique, primary cell culture, quantitative PCR, and Western blot were used. In native omasal epithelium of sheep, the Na(+) transport was electroneutral, and it was inhibited by the specific NHE3 inhibitor 3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride, which decreased mucosal-to-serosal, serosal-to-mucosal, and net flux rates of Na(+) by 80% each. The application of low mucosal pH (6.4 or 5.8) in the presence of SCFA activated the Na(+) transport across omasal epithelium of sheep compared with that at pH 7.4. In cultured omasal epithelial cells of goats, mRNA and protein of NHE1, NHE2, and NHE3 were detected. The application of SCFA increased NHE1 mRNA and protein expression, which was most prominent when the culture medium pH decreased from 7.4 to 6.8. At variance, the mRNA and protein expression of NHE2 and NHE3 were decreased with low pH and SCFA, which was contrary to the published data from ruminal epithelial studies. In conclusion, this paper shows that (1) NHE1, NHE2, and NHE3 are expressed in omasal epithelium; (2) NHE3 mediates the major portion of transepithelial Na(+) transport in omasal epithelium; and (3) SCFA and acidic pH acutely activate Na(+) transport but suppress the expression of NHE2 and NHE3 in the longer term. By contrast, the expression of NHE1 is increased by SCFA and acidic pH, indicating a prominent role for NHE1 in the regulation of intracellular pH of omasal epithelium. Our results suggest a regulatable Na(+) absorption in ruminal and omasal epithelium. It is of benefit for intracellular pH homeostasis and highly relevant to dairy cows fed on high-concentrate diets.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Renal acid-base regulation: new insights from animal models

Because majority of biological processes are dependent on pH, maintaining systemic acid-base balance is critical. The kidney contributes to systemic acid-base regulation, by reabsorbing HCO3 (-) (both filtered by glomeruli and generated within a nephron) and acidifying urine. Abnormalities in those processes will eventually lead to a disruption in systemic acid-base balance and provoke metabolic acid-base disorders. Research over the past 30 years advanced our understanding on cellular and molecular mechanisms responsible for those processes. In particular, a variety of transgenic animal models, where target genes are deleted either globally or conditionally, provided significant insights into how specific transporters are contributing to the renal acid-base regulation. Here, we broadly overview the mechanisms of renal ion transport participating to acid-base regulation, with emphasis on data obtained from transgenic mice models.

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.

A two-hit mechanism for sepsis-induced impairment of renal tubule function

Renal insufficiency is a common and severe complication of sepsis, and the development of kidney dysfunction increases morbidity and mortality in septic patients. Sepsis is associated with a variety of defects in renal tubule function, but the underlying mechanisms are incompletely understood. We used a cecal ligation and puncture (CLP) model to examine mechanisms by which sepsis influences the transport function of the medullary thick ascending limb (MTAL). MTALs from sham and CLP mice were studied in vitro 18 h after surgery. The results show that sepsis impairs the ability of the MTAL to absorb HCO(3)(-) through two distinct mechanisms. First, sepsis induces an adaptive decrease in the intrinsic capacity of the tubules to absorb HCO(3)(-). This effect is associated with an increase in ERK phosphorylation in MTAL cells and is prevented by pretreatment of CLP mice with a MEK/ERK inhibitor. The CLP-induced reduction in intrinsic HCO(3)(-) absorption rate appears to involve loss of function of basolateral Na(+)/H(+) exchange. Second, sepsis enhances the ability of LPS to inhibit HCO(3)(-) absorption, mediated through upregulation of Toll-like receptor 4 (TLR4)-ERK signaling in the basolateral membrane. The two inhibitory mechanisms are additive and thus can function in a two-hit capacity to impair renal tubule function in sepsis. Both effects depend on ERK and are eliminated by interventions that prevent ERK activation. Thus the TLR4 and ERK signaling pathways represent potential therapeutic targets to treat or prevent sepsis-induced renal tubule dysfunction.

Toll-like receptor 2 is required for LPS-induced Toll-like receptor 4 signaling and inhibition of ion transport in renal thick ascending limb

Previously we demonstrated that basolateral LPS inhibits HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through TLR4-dependent ERK activation. Here we report that the response of the MTAL to basolateral LPS requires TLR2 in addition to TLR4. The basolateral addition of LPS (ultrapure Escherichia coli K12) decreased HCO(3)(-) absorption in isolated, perfused MTALs from wild-type mice but had no effect in MTALs from TLR2(-/-) mice. In contrast, inhibition of HCO(3)(-) absorption by lumen LPS was preserved in TLR2(-/-) MTALs, indicating that TLR2 is involved specifically in mediating the basolateral LPS response. LPS also did not increase ERK phosphorylation in MTALs from TLR2(-/-) mice. TLR2 deficiency had no effect on expression of TLR4, MD-2, or MyD88. However, LPS-induced recruitment of MyD88 to the basolateral membrane was impaired in TLR2(-/-) MTALs. Inhibition of HCO(3)(-) absorption by LPS did not require CD14. Co-immunoprecipitation studies demonstrated an association between TLR4 and TLR2. Inhibition of HCO(3)(-) absorption by TLR2-specific ligands was preserved in MTALs from TLR4(-/-) mice. These results indicate that the effect of basolateral LPS to inhibit HCO(3)(-) absorption in the MTAL through MyD88-dependent ERK activation depends on a novel interaction between TLR4 and TLR2. TLR2 plays a dual role in the induction of intracellular signals that impair MTAL function, both through cooperation with TLR4 to mediate ERK signaling by LPS and through a TLR4-independent signaling pathway activated by Gram-positive bacterial ligands. Regulation of TLR2 expression and its interaction with TLR4 may provide new mechanisms for controlling and therapeutic targeting of TLR4-mediated LPS responses.

Basolateral LPS inhibits NHE3 and HCOFormula absorption through TLR4/MyD88-dependent ERK activation in medullary thick ascending limb

Sepsis is associated with defects in renal tubule function, but the underlying mechanisms are incompletely understood. Recently, we demonstrated that Gram-negative bacterial lipopolysaccharide (LPS) inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through activation of Toll-like receptor 4 (TLR4). Here, we examined the mechanisms responsible for inhibition of HCO(3)(-) absorption by basolateral LPS. Adding LPS to the bath decreased HCO(3)(-) absorption by 30% in rat and mouse MTALs perfused in vitro. The inhibition of HCO(3)(-) absorption was eliminated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)/ERK inhibitors U0126 and PD98059. LPS induced a rapid (<15 min) and sustained (up to 60 min) increase in ERK phosphorylation in microdissected MTALs that was blocked by PD98059. The effects of basolateral LPS to activate ERK and inhibit HCO(3)(-) absorption were eliminated in MTALs from TLR4(-/-) and myeloid differentiation factor 88 (MyD88)(-/-) mice but were preserved in MTALs from TIR (Toll/interleukin-1 receptor) domain-containing adapter-inducing interferon-β (Trif)(-/-) mice. Basolateral LPS decreased apical Na(+)/H(+) exchanger 3 NHE3 activity through a decrease in maximal velocity (V(max)). The inhibition of NHE3 by LPS was eliminated by MEK/ERK inhibitors. LPS inhibited HCO(3)(-) absorption despite the presence of physiological stimuli that activate ERK in the MTAL. We conclude that basolateral LPS inhibits HCO(3)(-) absorption in the MTAL through activation of a TLR4/MyD88/MEK/ERK pathway coupled to inhibition of NHE3. These studies identify NHE3 as a target of TLR4 signaling in the MTAL and show that bacterial molecules can impair the absorptive functions of renal tubules through inhibition of this exchanger. The ERK pathway links TLR4 to downstream modulation of ion transport proteins and represents a potential target for treatment of sepsis-induced renal tubule dysfunction.

High sodium intake increases HCO(3)- absorption in medullary thick ascending limb through adaptations in basolateral and apical Na+/H+ exchangers

A high sodium intake increases the capacity of the medullary thick ascending limb (MTAL) to absorb HCO(3)(-). Here, we examined the role of the apical NHE3 and basolateral NHE1 Na(+)/H(+) exchangers in this adaptation. MTALs from rats drinking H(2)O or 0.28 M NaCl for 5-7 days were perfused in vitro. High sodium intake increased HCO(3)(-) absorption rate by 60%. The increased HCO(3)(-) absorptive capacity was mediated by an increase in apical NHE3 activity. Inhibiting basolateral NHE1 with bath amiloride eliminated 60% of the adaptive increase in HCO(3)(-) absorption. Thus the majority of the increase in NHE3 activity was dependent on NHE1. A high sodium intake increased basolateral Na(+)/H(+) exchange activity by 89% in association with an increase in NHE1 expression. High sodium intake increased apical Na(+)/H(+) exchange activity by 30% under conditions in which basolateral Na(+)/H(+) exchange was inhibited but did not change NHE3 abundance. These results suggest that high sodium intake increases HCO(3)(-) absorptive capacity in the MTAL through 1) an adaptive increase in basolateral NHE1 activity that results secondarily in an increase in apical NHE3 activity; and 2) an adaptive increase in NHE3 activity, independent of NHE1 activity. These studies support a role for NHE1 in the long-term regulation of renal tubule function and suggest that the regulatory interaction whereby NHE1 enhances the activity of NHE3 in the MTAL plays a role in the chronic regulation of HCO(3)(-) absorption. The adaptive increases in Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL may play a role in enabling the kidneys to regulate acid-base balance during changes in sodium and volume balance.

Roles of cortisol and carbonic anhydrase in acid-base compensation in rainbow trout, Oncorhynchus mykiss

Fish compensate for acid-base disturbances primarily by modulating the branchial excretion of acid-base equivalents, with a supporting role played by adjustment of urinary acid excretion. The present study used metabolic acid-base disturbances in rainbow trout, Oncorhynchus mykiss, to evaluate the role played by cortisol in stimulating compensatory responses. Trout infused with acid (an iso-osmotic solution of 70 mmol L(-1) HCl), base (140 mmol L(-1) NaHCO(3)) or saline (140 mmol L(-1) NaCl) for 24 h exhibited significant elevation of circulating cortisol concentrations. Acid infusion significantly increased both branchial (by 328 μmol kg(-1) h(-1)) and urinary (by 5.9 μmol kg(-1) h(-1)) net acid excretion, compensatory responses that were eliminated by pre-treatment of trout with the cortisol synthesis inhibitor metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone). The significant decrease in net acid excretion (equivalent to enhanced base excretion) of 203 μmol kg(-1) h(-1) detected in base-infused trout was unaffected by metyrapone treatment. Acid- and base-infusions also were associated with significant changes in the relative mRNA expression of branchial and renal cytosolic carbonic anhydrase (tCAc) and renal membrane-linked CA IV (tCA IV). Cortisol treatment caused changes in CA gene expression that tended to parallel those observed with acid but not base infusion. For example, significant increases in renal relative tCA IV mRNA expression were detected in both acid-infused (~2x) and cortisol-treated (~10x) trout, whereas tCA IV mRNA expression was significantly reduced (~5x) in base-infused fish. Despite changes in CA gene expression in acid- or base-infused fish, neither acid nor base infusion affected CAc protein levels in the gill, but both caused significant increases in branchial CA activity. Cortisol treatment similarly increased branchial CA activity in the absence of an effect on branchial CAc protein expression. Taken together, these findings provide support for the hypothesis that in rainbow trout, cortisol is involved in mediating acid-base compensatory responses to a metabolic acidosis, and that cortisol exerts its effects at least in part through modulation of CA.

Mechanisms of the regulation of the intestinal Na+/H+ exchanger NHE3

A major of Na⁺ absorptive process in the proximal part of intestine and kidney is electroneutral exchange of Na⁺ and H⁺ by Na⁺/H⁺ exchanger type 3 (NHE3). During the past decade, significant advance has been achieved in the mechanisms of NHE3 regulation. A bulk of the current knowledge on Na⁺/H⁺ exchanger regulation is based on heterologous expression of mammalian Na⁺/H⁺ exchangers in Na⁺/H⁺ exchanger deficient fibroblasts, renal epithelial, and intestinal epithelial cells. Based on the reductionist's approach, an understanding of NHE3 regulation has been greatly advanced. More recently, confirmations of in vitro studies have been made using animals deficient in one or more proteins but in some cases unexpected findings have emerged. The purpose of this paper is to provide a brief overview of recent progress in the regulation and functions of NHE3 present in the luminal membrane of the intestinal tract.

Downregulation of claudin-2 expression in renal epithelial cells by metabolic acidosis

Chronic metabolic acidosis (CMA) is associated with an inhibition of fluid reabsorption in the renal proximal tubule. The effects of CMA on paracellular transport across the renal epithelial tight junction (TJ) is unknown. Claudin-2 is a transmembrane TJ-associated protein which confers TJ paracellular permeability to Na(+). We examined the effects of CMA on the expression of TJ transport proteins using both in vivo and in vitro models of CMA. The results showed downregulation of claudin-2 mRNA and protein expression in the cortex of rats subjected to the NH(4)Cl loading model of CMA. Madin-Darby canine kidney (MDCK) and HK-2 cells are models of renal epithelial cells and express claudin-2 protein in their TJ. We examined the effects of acidic pH exposure on the expression of claudin-2 in MDCK and HK-2 renal epithelial cells. Exposure of MDCK cells to pH 6.96 medium caused a significant and reversible decrease in claudin-2 protein abundance. A dose-response analysis of acidic medium exposure of MDCK and HK-2 cells demonstrated a downregulation of claudin-2 protein. The downregulation effect of acidic pH is specific to claudin-2 expression as the expression of other TJ-associated proteins (i.e., claudin-1, -3, -4, and -7, occludin, and zonula occludens-1) remained unchanged compared with control pH (7.40). Collectively, these data demonstrate that CMA downregulates the expression of claudin-2 likely through a direct effect of acidic pH. Potential physiological significance of these changes is discussed.

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.

Renal response to metabolic acidosis: role of mRNA stabilization

The renal response to metabolic acidosis is mediated, in part, by increased expression of the genes encoding key enzymes of glutamine catabolism and various ion transporters that contribute to the increased synthesis and excretion of ammonium ions and the net production and release of bicarbonate ions. The resulting adaptations facilitate the excretion of acid and partially restore systemic acid-base balance. Much of this response may be mediated by selective stabilization of the mRNAs that encode the responsive proteins. For example, the glutaminase mRNA contains a direct repeat of 8-nt AU sequences that function as a pH-response element (pHRE). This element is both necessary and sufficient to impart a pH-responsive stabilization to chimeric mRNAs. The pHRE also binds multiple RNA-binding proteins, including zeta-crystallin (zeta-cryst), AU-factor 1 (AUF1), and HuR. The onset of acidosis initiates an endoplasmic reticulum (ER)-stress response that leads to the formation of cytoplasmic stress granules. zeta-cryst is transiently recruited to the stress granules, and concurrently, HuR is translocated from the nucleus to the cytoplasm. On the basis of the cumulative data, a mechanism for the stabilization of selective mRNAs is proposed. This hypothesis suggests multiple experiments that should define better how cells in the kidney sense very slight changes in intracellular pH and mediate this essential adaptive response.

Effect of chronic elevated carbon dioxide on the expression of acid-base transporters in the neonatal and adult mouse

Several pulmonary and neurological conditions, both in the newborn and adult, result in hypercapnia. This leads to disturbances in normal pH homeostasis. Most mammalian cells maintain tight control of intracellular pH (pHi) using a group of transmembrane proteins that specialize in acid-base transport. These acid-base transporters are important in adjusting pHiduring acidosis arising from hypoventilation. We hypothesized that exposure to chronic hypercapnia induces changes in the expression of acid-base transporters. Neonatal and adult CD-1 mice were exposed to either 8% or 12% CO2for 2 wk. We used Western blot analysis of membrane protein fractions from heart, kidney, and various brain regions to study the response of specific acid-base transporters to CO2. Chronic CO2increased the expression of the sodium hydrogen exchanger 1 (NHE1) and electroneutral sodium bicarbonate cotransporter (NBCn1) in the cerebral cortex, heart, and kidney of neonatal but not adult mice. CO2increased the expression of electrogenic NBC (NBCe1) in the neonatal but not the adult mouse heart and kidney. Hypercapnia decreased the expression of anion exchanger 3 (AE3) in both the neonatal and adult brain but increased AE3 expression in the neonatal heart. We conclude that: 1) chronic hypercapnia increases the expression of the acid extruders NHE1, NBCe1 and NBCn1 and decreases the expression of the acid loader AE3, possibly improving the capacity of the cell to maintain pHiin the face of acidosis; and 2) the heterogeneous response of tissues to hypercapnia depends on the level of CO2and development.

Na+/H+ antiporter, V-H+-ATPase and Na+/K+-ATPase immunolocalization in a marine teleost(Myoxocephalus octodecemspinosus)

Long-term pH compensation in a marine teleost requires the transepithelial excretion of H+ across the gill epithelium. H+ efflux in the longhorn sculpin (Myoxocephalus octodecemspinosus) is dependent on external sodium ion concentration and is inhibited by known inhibitors of Na+/H+ exchangers. Our model for proton transport suggests acid-excreting cells in the gill with an apical Na+/H+ antiporter and basolateral Na+/K+-ATPase. This model is similar to mammalian kidney and elasmobranch gill epithelium in which a basolateral electrogenic-vacuolar proton pump (V-H+-ATPase) localizes to base-excreting cells. The objective of this study was to detect the presence and location of membrane transporters in marine fish gills using immunohistochemical staining. Our data indicate the presence of an apical and subapical Na+/H+-exchanger 2 (NHE2) in the sculpin gill. NHE2 is present in large, ovoid chloride cells and often colocalizes in the same cells as Na+/K+-ATPase. We also detected V-H+-ATPase immunoreactivity, predominantly in cells at the base of the lamellae, with staining patterns indicative of a basolateral location. The 85 kDa protein detected on immunoblots with anti-NHE2 antibodies was found in both control and acid-infused animals and did not change following a large acute acidosis over 8 h.

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.

Metabolic acidosis has dual effects on sodium handling by rat kidney

Chronic metabolic acidosis (CMA) is associated with decreased NaCl reabsorption in the proximal tubule (PT). However, the effect of CMA on Na(+) transport in the distal tubule (DT) and collecting duct (CD) is poorly understood. Rats were placed in metabolic cages and had access to water (control), 0.28 M NH(4)Cl, or 0.28 M KCl solutions in a pair-feeding protocol for 5 days (5d). Metabolic acidosis developed within 24 h in NH(4)Cl-, but not in KCl-loaded rats. Interestingly, NH(4)Cl- but not KCl-loaded rats exhibited a significant natriuresis after 24 h of treatment. Urinary Na(+) excretion increased from 1.94 to 2.97 meq/24 h (P < 0.001) and returned to below baseline level (1.67 meq/l) after 5d of CMA. The protein abundance of the cortical Na-Cl cotransporter (NCC) remained unchanged at 24 h, but increased significantly (P < 0.01) after 5d of CMA. The protein abundance of alpha-, beta-, and gamma-subunits of the epithelial Na(+) channel (ENaC) in the cortex decreased sharply during the first 24 h and then returned to baseline levels after 5d of CMA. Interestingly, Sgk1 expression decreased after 24 h (-31%, P < 0.05) and then returned to baseline after 5d of CMA. Nedd4-2 expression was not altered during CMA. CMA enhanced serum aldosterone levels by 54% and increased the expression of aldosterone synthase in the adrenal gland by 134% after 5d of CMA. In conclusion, metabolic acidosis has dual effects on urinary Na(+) excretion. The early natriuresis results from decreased Na(+) reabsorption in the PT and Sgk1-related decreased ENaC activity in the DT and CD. Aldosterone-induced upregulation of NCC, Sgk1, and ENaC likely contributes to the antinatriuretic phase of metabolic acidosis. This adaptation prevents Na(+) wasting and volume depletion during chronic acid insult.

Acute and Chronic Regulation of the Renal Na+/H+ Exchanger NHE3 in Rats with STZ-Induced Diabetes mellitus

BACKGROUND

Early stages of diabetic nephropathy are characterized by alterations of glomerular filtration, increased tubular sodium and water reabsorption, and systemic volume expansion, which may be a major cause for the development of hypertension. As a significant fraction of renal salt and water transport is mediated by the proximal tubular Na+/H+ exchanger NHE3, we investigated its regulation in rats with STZ-induced diabetes mellitus.

METHODS

Male Sprague-Dawley rats were injected +/- streptozotocin (STZ, 60 mg/kg), and sacrificed after 2, 7 or 14 days. Renal cortical BBM vesicles were prepared to measure Na+/H+ exchange (NHE) activity and NHE3 protein abundance. Cortical NHE3 mRNA was extracted to perform Northern blot analysis. Pharmacological inhibitors were used in vivo and in vitro in order to identify isoform specificity conferring changes in NHE activity mediated by the diabetic milieu.

RESULTS

Compared to control rats, STZ rats were clearly hyperglycemic at all time points studied. NHE activity was significantly increased by 40 and 37% in diabetic rats after 7 and 14 days, respectively, but not after 2 days. The increase in Na+/H+ exchange activity was not inhibited by HOE-642 (3 microM). Administration of exogenous insulin to diabetic rats resulted in lower blood sugars, but not NHE activity. Moreover, serum glucose concentration did not correlate with NHE activity in any subgroup nor in all animals analyzed together. However, in STZ rats supplemented with exogenous insulin NHE activity was positively correlated with serum insulin concentrations (r = 0.86, p < 0.01). In vivo, the increase in NHE activity induced by STZ could be completely inhibited when rats were fed 6 ppm of HOE-642 with the diet over 14 days. The changes in Na+/H+ exchange activity were not paralleled by changes in NHE3 protein or mRNA abundance in diabetic rats at any of the time points investigated.

CONCLUSIONS

These results suggest that proximal tubular Na/H exchange activity is modified in the early stage of diabetes mellitus.

Nongenomic regulation by aldosterone of the epithelial NHE3 Na(+)/H(+) exchanger

The relevance of nongenomic pathways to regulation of epithelial function by aldosterone is poorly understood. Recently, we demonstrated that aldosterone inhibits transepithelial HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through a nongenomic pathway. Here, we examined the transport mechanism(s) responsible for this regulation, focusing on Na(+)/H(+) exchangers (NHE). In the MTAL, apical NHE3 mediates H(+) secretion necessary for HCO(3)(-) absorption; basolateral NHE1 influences HCO(3)(-) absorption by regulating apical NHE3 activity. In microperfused rat MTALs, the addition of 1 nM aldosterone rapidly decreased HCO(3)(-) absorption by 30%. This inhibition was unaffected by three maneuvers that inhibit basolateral Na(+)/H(+) exchange and was preserved in MTALs from NHE1 knockout mice, ruling out the involvement of NHE1. In contrast, exposure to aldosterone for 15 min caused a 30% decrease in apical Na(+)/H(+) exchange activity over the intracellular pH range from 6.5 to 7.7, due to a decrease in V(max). Inhibition of HCO(3)(-) absorption by aldosterone was not affected by 0.1 mM lumen Zn(2+) or 1 mM lumen DIDS, arguing against the involvement of an apical H(+) conductance or apical K(+)-HCO(3)(-) cotransport. These results demonstrate that aldosterone inhibits HCO(3)(-) absorption in the MTAL through inhibition of apical NHE3, and identify NHE3 as a target for nongenomic regulation by aldosterone. Aldosterone may influence a broad range of epithelial transport functions important for extracellular fluid volume and acid-base homeostasis through direct regulation of this exchanger.

The effect of environmental hypercapnia and salinity on the expression of NHE-like isoforms in the gills of a euryhaline fish (Fundulus heteroclitus)

The current models for branchial acid excretion in fishes include Na(+)/H(+) exchange and the electrogenic excretion of H+ via H+-ATPase. The predominant route of acid excretion in some freshwater fishes is thought to be via the H+-ATPase/Na+ channel system. The euryhaline Fundulus heteroclitus may not fit this profile even when adapted to freshwater (FW). We hypothesize that the Na+/H+ exchanger (NHE) in this species may play a predominant role in acid-base regulation for both marine and FW adapted animals. Acidosis induced by ambient hypercapnia (1% CO2 in air), resulted in an increase in net H+ excretion to the water in F. heteroclitus pre-adapted to FW, brackish (isoosmotic; BW) and seawater (SW). Both FW and SW adapted mummichogs were tested for NHE protein expression using mammalian NHE antibodies, and we identified NHE-like immunoreactive proteins in gill membrane preparations from both groups. Hypercapnia induced a approximately three-fold elevation in gill NHE2-like protein in FW animals but SW adapted fish showed inconsistent NHE3-like protein expression. There was no change in NHE-1 levels in FW fish. In contrast, SW animals demonstrated a significant increase in both NHE1 and NHE3-like proteins following hypercapnia but limited expression of the NHE2 protein. We hypothesize that different isoforms of NHE may be preferentially expressed depending on the salinity to which the animals are adapted. Net H+ transfers during acidosis may be driven, at least in part by the action of these transporters.

Regulatory and functional interaction of vasoactive factors in the kidney and extracellular pH

A growing body of evidence suggests that vasoactive factors produced in the kidney such as nitric oxide, endothelins, angiotensin, and prostaglandins participate actively in the regulation of acid-base homeostasis under physiologic conditions. In addition, recent reports indicate that alterations in the systemic acid-base status may also influence the generation of vasoactive cytokines in the kidney, which in turn may mediate the renal effector processes that tend to restore normality under such conditions. Metabolic acidosis, which so frequently accompanies many forms of chronic renal failure (CRF), may contribute to down-regulation of intrarenal nitric oxide production that characterizes CRF. Reduced extracellular pH inhibits inducible nitric oxide production in mesangial cells by altering the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidation, an important posttranslational mechanism in the inducible nitric oxide synthase (iNOS) activation. The underlying defects resulting in the uncoupling of NADPH oxidation in acidemic microenvironment are discussed. Acidosis stimulates renal production of endothelins, which mediate proximal tubular acidification by enhancing sodium-hydrogen exchanger-3 (NHE-3) activity. Renal endothelins mediate enhanced urinary acid excretion following dietary acid ingestion, an effect that is effectively blocked by endothelin receptor blockers. Reduced extracellular pH stimulates endothelin secretion from renal microvascular endothelial cells, which may promote enhanced acid excretion from the distal tubule under conditions of acidosis. These phenomena as well as the role of angiotensin and renal prostaglandins in mediating renal acidification in normal and acidotic conditions are discussed in this review, which describe the regulatory interaction between extracellular pH and renal vasoactive factors.

An apical K+-dependent HCO3− transport pathway opposes transepithelial HCO3− absorption in rat medullary thick ascending limb

Absorption of HCO3− in the medullary thick ascending limb (MTAL) is mediated by apical membrane Na+/H+ exchange. The identity and function of other apical acid-base transporters in this segment have not been defined. The present study was designed to examine apical membrane HCO3−/OH−/H+ transport pathways in the rat MTAL and to determine their role in transepithelial HCO3− absorption. MTALs were perfused in vitro in Na+- and Cl−-free solutions containing 25 mM HCO3−, 5% CO2. Lumen addition of either 120 mM Cl− or 50 mM Na+ (50 μM EIPA present) had no effect on intracellular pH (pHi). Lumen Cl− addition also had no effect on pHi in the presence of 145 mM Na+ or in the nominal absence of HCO3−/CO2. Thus there was no evidence for apical Cl−/HCO3− (OH−) exchange, Na+-dependent Cl−/HCO3− exchange, or Na+-HCO3− cotransport. In contrast, in tubules studied in Na+- and Cl−-free solutions containing 25 mM HCO3−, 5% CO2 and 120 mM K+, removal of luminal K+ induced a rapid and pronounced decrease in pHi (ΔpHi = 0.56 ± 0.06 pH U). pHi recovered following lumen K+ readdition. The initial rate of net base efflux induced by lumen K+ removal was decreased 85% at the same pHi in the nominal absence of HCO3−/CO2, indicating a dependence on HCO3−/CO2 and arguing against apical K+/H+ exchange. A combination of the apical K+ channel blockers quinidine (0.1 mM) and glybenclamide (0.25 mM) had no effect on the lumen K+-induced pHi changes, arguing against electrically coupled K+ and HCO3− conductances. The effect of lumen K+ on pHi was inhibited by 1 mM H2DIDS. In addition, lumen addition of DIDS increased transepithelial HCO3− absorption from 10.7 ± 0.7 to 14.9 ± 0.7 pmol·min−1·mm−1 ( P < 0.001) and increased pHi slightly in MTAL studied in physiological solutions (25 mM HCO3− and 4 mM K+). Lumen DIDS stimulated HCO3− absorption in the absence and presence of furosemide. These results are consistent with an apical membrane K+-dependent HCO3− transport pathway that mediates coupled transfer of K+ and HCO3− from cell to lumen in the MTAL. This mechanism, possibly an apical K+-HCO3− cotransporter, functions in parallel with apical Na+/H+ exchange and opposes transepithelial HCO3− absorption.

An autocrine role for endothelin-1 in the regulation of proximal tubule NHE3

BACKGROUND

Chronic metabolic acidosis leads to an increase in NHE3 activity that is mediated by endothelin-1 (ET-1) expression and activation of the proximal tubule endothelin B receptor. Chronic metabolic acidosis increases preproET-1 mRNA abundance in kidney cortex, but the cell responsible has not been identified.

METHODS

PreproET-1 mRNA abundance was quantified by competitive reverse transcription-polymerase chain reaction (RT-PCR) on tissue harvested from control rats or rats in which chronic metabolic acidosis was induced by addition of NH(4)Cl to the drinking water.

RESULTS

Chronic metabolic acidosis leads to an increase in preproET-1 mRNA expression in kidney cortex, proximal tubules, and glomeruli. The increase in preproET-1 expression correlates with the decrease in blood [HCO3(-)]. ET-1 expression is also increased by acidosis in abdominal aorta, but not in cardiac muscle.

CONCLUSION

In the renal proximal tubule, chronic metabolic acidosis induces an increase in preproET-1 expression, providing a mechanism for autocrine regulation of proximal tubule NHE3 activity. This response is not unique to the proximal tubule cell, but is also not ubiquitous.

Upregulation of collecting duct aquaporin-2 by metabolic acidosis: role of vasopressin

Metabolic acidosis is associated with alteration in fluid and electrolyte reabsorption in a number of nephron segments. However, the effects of metabolic acidosis on urine osmolality and aquaporin-2 (AQP-2) remain poorly understood. In these studies, we examined the effects of chronic metabolic acidosis on water handling by the kidney. Rats were placed in metabolic cages and subjected to water (control) or 280 mM NH(4)Cl loading for 120 h to induce metabolic acidosis. The results indicated a significant increase in urine osmolality with no change in urine volume or urinary Na(+) excretion in acid-loaded animals. This effect was independent of alteration in fluid intake or salt/Cl(-) loading. Immunoblotting and Northern hybridization studies indicated that AQP-2 protein abundance and mRNA expression levels increased significantly along the collecting duct system of NH(4)Cl-but not NaCl-loaded animals. RIA results indicated that metabolic acidosis was associated with a fourfold increase in circulating levels of vasopressin (AVP) and a significant increase in brain AVP mRNA expression levels. In conclusion, metabolic acidosis upregulates the expression levels of AQP-2 and increases urine osmolality, suggesting an adaptive increase in water reabsorption in the collecting duct. A concomitant increase in AVP synthesis and secretion likely plays an essential role in the adaptation of AQP-2 in metabolic acidosis.

Differentiated thick ascending limb (TAL) cultured cells derived from SV40 transgenic mice express functional apical NHE2 isoform: effect of nitric oxide

Studying the apical Na/H exchanger NHE2 is difficult in the intact thick ascending limb (TAL) because of its weak expression and transport activity compared with the co-expressed NHE3. From a mouse transgenic for a recombinant plasmid adeno-SV(40) (PK4), we developed an immortalized TAL cell line, referred to as MKTAL, which selectively expresses NHE2 protein and activity. The immortalized cells retain the main properties of TAL cells. They have a stable homogeneous epithelial-like phenotype, express SV(40) T antigen and exhibit polarity with an apical domain bearing few microvilli and separated from lateral domains by typical epithelial-type junctional complexes expressing ZO1 protein. Tamm-Horsfall protein is present on the apical membrane. MKTAL cells express NHE2 and NHE1 proteins but not NHE3 and NHE4, whereby NHE2 protein is expressed selectively in the apical domain of the plasma membrane. NHE2 contributed about half of the total Na/H exchange activity. mRNAs for the Na-K-2Cl cotransporter-2 (NKCC2) and the anion exchangers AE2 and AE3 were also present. While acute exposure to NO donors did not alter NHE2 activity, chronic exposure inhibited NHE2 activity selectively and down-regulated NHE2 mRNA abundance. In conclusion, MKTAL cells retain structural and functional properties of their in vivo TAL counterparts and express functional NHE2 protein in the apical membrane, which may be inhibited by NO. Thus, MKTAL cells may be an appropriate model for studying the cellular mechanisms of NHE2 regulation.

Bicarbonate reabsorption and NHE-3 expression: abundance and activity are increased in Henle's loop of remnant rats

BACKGROUND

The bulk of bicarbonate reabsorption along the loop of Henle (LOH) is localized at the level of the thick ascending limb (TAL) and is mainly dependent on the presence of luminal Na+-H+ exchanger (NHE-3). We investigated whether the reduction of renal mass is associated with alterations in LOH bicarbonate transport coupled to changes in NHE-3 gene expression and in vivo activity.

METHODS

Sham-operated and remnant rats (4/6 nephrectomy) were studied 15 days after the surgery. To measure net bicarbonate reabsorption (JHCO3-) superficial loops were perfused by in vivo micropuncture. Perfusate was an end-like proximal solution containing 3H-methoxy-inulin. NHE-3 gene expression was quantified by competitive PCR using an internal standard of cDNA that differed from the wild-type NHE-3 by a deletion of 76 bp. Western blot experiments were performed on TAL suspension using anti-NHE-3 antibodies.

RESULTS

At various LOH bicarbonate loads, JHCO3- was constantly larger in remnant rats as compared to sham-operated animals. NHE-3 mRNA abundance was estimated to be 0.339 +/- 0.031 attomoles (amol)/ng-1 total RNA in sham-operated (N = 5) and it increased to 0.465 +/- 0.023 in remnant rats (N = 5, P < 0.01). Western blot experiments showed a significant increase of NHE-3 protein abundance in TAL of remnant rats as compared to sham-operated animals. Finally, by means of a specific NHE-3 inhibitor, S-3226, in vivo microperfusion experiments demonstrated that NHE-3 in vivo activity along the LOH was substantially increased in remnant rats in addition to the non-NHE-3 bicarbonate transport.

CONCLUSIONS

These data indicate that the reduction of renal mass increases mRNA, protein abundance and in vivo activity of NHE-3 along the TAL. This may explain, at least in part, the augmented transepithelial bicarbonate transport along the LOH. Such an effect will counterbalance the increased glomerular bicarbonate load, thus preventing urinary bicarbonate loss and mitigating the ensuing metabolic acidosis.

Acid-base regulation in fishes: cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na(+)/H(+) exchange (NHE) and vacuolar-type H(+)-ATPase transport H(+) from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H(+)-ATPase drives H(+) excretion and is indirectly coupled to Na(+) uptake (via Na(+) channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H(+)-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl(-)/HCO(3)(-) anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance.

Na+/H+ exchange activity and NHE-3 expression in renal tubules from the spontaneously hypertensive rat

BACKGROUND

The NHE-3 isoform of the Na+/H+ antiporter, in the apical membrane of renal proximal tubule, is responsible for the bulk transport of Na+ and fluid reabsorption. Studies have reported that apical NHE-3 translocates to internal pools, thereby facilitating natriuresis when blood pressure increases abruptly.

METHODS

The present study examined Na+/H+ exchange activity and NHE-3 expression in renal cortical tubules from the spontaneously hypertensive rat (SHR) and WKY rats before and after the development of hypertension. SHR 4 to 6 weeks of age were pre-hypertensive, 6 to 7 weeks old had mild hypertension, and 8 to 13 weeks old had severe hypertension. Renal proximal tubules (PTs) were isolated and purified by Percoll gradient centrifugation. NHE-3 protein and mRNA levels were determined by Western and Northern blots, respectively. Apical brush border membrane vesicles (BBMV) were prepared using the MgSO4 aggregation method and Na+/H+ exchange activity assessed using the acridine orange method.

RESULTS

Na+/H+ exchange activity, determined as the rate of Na+-dependent intracellular pH (pHi) recovery assessed using BCECF after an acute acid load, was significantly greater in PTs from SHR than in WKY rats at all age groups (4 to 6 weeks, 0.30 +/- 0.04 vs. 0.24 +/- 0.02 pH U/30 sec, P < 0.05; 6 to 7 weeks, 0.42 +/- 0.07 vs. 0.29 +/- 0.05 pH U/30 sec, P < 0.05; and 8 to 13 weeks, 0.48 +/- 0.07 vs. 0.40 +/- 0.07 pH U/30 sec, P < 0.05). The Na+-dependent recovery in BBMV was also greater in SHR than WKY rats (1464 +/- 62 vs. 1042 +/- 79 fluorescence. U/5 sec, P < 0.001) and was unaffected by cariporide, a specific NHE-1 inhibitor. NHE-3 protein levels also were significantly higher in SHR than age-matched WKY rats at all stages during the development of hypertension (pre-hypertensive 1.8-fold; early onset hypertension twofold; established hypertension 1.5-fold; each P < 0.05). By contrast, NHE-3 mRNA levels were not different between SHR and WKY rats at each age group.

CONCLUSIONS

Na+/H+ exchange activity and NHE-3 protein abundance in renal proximal tubules from the SHR are increased while NHE-3 mRNA is not. A post-transcriptional event(s) best explains the increase in NHE-3 protein expression since mRNA levels were not increased. The alterations in the SHR antedate the development of hypertension and fail to decrease as blood pressure increases with age in the SHR, which likely results in inappropriate renal sodium retention in the face of a chronic rise in blood pressure.

ERK mediates inhibition of Na(+)/H(+) exchange and HCO(3)(-) absorption by nerve growth factor in MTAL

Mitogen-activated protein (MAP) kinases mediate a variety of critical cellular events, but their role in the regulation of epithelial transport is largely undefined. Recently, we demonstrated that nerve growth factor (NGF) inhibits HCO(3)(-) absorption in the rat medullary thick ascending limb (MTAL) through an unusual mechanism: 1) NGF inhibits basolateral membrane Na(+)/H(+) exchange activity, an effect opposite to the stimulation of Na(+)/H(+) exchange by growth factors in other cells; and 2) inhibition of basolateral Na(+)/H(+) exchange results secondarily in inhibition of apical Na(+)/H(+) exchange, thereby inhibiting HCO(3)(-) absorption. In this study, we examined the role of MAP kinases in mediating inhibition by NGF. In tissue strips from the inner stripe of the outer medulla and in microdissected MTALs, NGF increased extracellular signal-regulated kinase (ERK) activity twofold but had no effect on c-Jun NH(2)-terminal kinase (JNK) or p38 MAP kinase activity. The selective MAP kinase kinase (MEK1/2) inhibitors U0126 and PD-98059 abolished the NGF-induced ERK activation and largely eliminated (> or = 60%) the effects of NGF to inhibit basolateral Na(+)/H(+) exchange activity and transepithelial HCO absorption in perfused MTALs. The MEK1/2 inhibitors did not affect inhibition of HCO(3)(-) absorption by bath ethylisopropyl amiloride, indicating that ERK activation is not involved in mediating interaction between the basolateral and apical Na(+)/H(+) exchangers. These results demonstrate that NGF inhibits basolateral Na(+)/H(+) exchange activity and HCO(3)(-) absorption in the MTAL through activation of the ERK signaling pathway. These findings identify a novel action of ERK to inhibit Na(+)/H(+) exchange activity and establish a role for MAP kinase pathways in the acute regulation of Na(+)/H(+) exchange activity and transepithelial acid secretion in renal tubules.

Expression of isoforms of the Na(+)/H(+) exchanger in M-1 mouse cortical collecting duct cells

Na(+)/H(+) exchanger (NHE) proteins perform a variety of functions in the kidney and are differentially distributed among nephron segments. The purpose of this study was to identify NHE isoforms in murine M-1 cells as a model of cortical collecting duct principal cells. It was found that mRNAs corresponding to NHE1, NHE2, and NHE4 are expressed in M-1 cells. NHE-dependent regulation of intracellular pH (pH(i)) was investigated in the absence of extracellular HCO. Application of a 20 mM NH(4)Cl pulse resulted in a reversible intracellular acidification from which recovery was partially inhibited by application of 1 mM amiloride to either the apical or the basolateral membranes and was abolished when amiloride was applied to both sides of the monolayers, which suggests that NHEs are expressed in both the apical and the basolateral cell membranes of M-1 cells. The purinergic agonists ATP and benzoylbenzoyl-ATP caused a reduction of pH(i) when applied to the apical membrane, which suggests pH(i) may be influenced by extracellular nucleotides in the luminal fluid of the cortical collecting duct.

Basolateral membrane Cl(-)-, Na(+)-, and K(+)-coupled base transport mechanisms in rat MTALH

Mechanisms involved in basolateral HCO transport were examined in the in vitro microperfused rat medullary thick ascending limb of Henle (MTALH) by microfluorometric monitoring of cell pH. Removing peritubular Cl(-) induced a cellular alkalinization that was inhibited in the presence of peritubular 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and blunted in the absence of external CO(2)/HCO. The alkalinization elicited by removing peritubular Cl(-) persisted in the bilateral absence of Na(+), together with a voltage clamp. When studied in Cl(-)-free solutions, lowering peritubular pH induced a base efflux that was inhibited by peritubular DIDS or by the absence of external CO(2)/HCO. Removing peritubular Na(+) elicited a cellular acidification that was accounted for by stimulation of a DIDS- and ethylisopropylamiloride (EIPA)-insensitive Na(+)-HCO cotransport and inhibition of a basolateral Na(+)/H(+) exchange. Increasing bath K(+) induced an intracellular alkalinization that was inhibited in the absence of external CO(2)/HCO. At 2 mM, peritubular Ba(2+), which inhibits the K(+)-Cl(-) cotransport, did not induce any change in transepithelial voltage but elicited a cellular alkalinization and inhibited K(+)-induced cellular alkalinization, consistent with the presence of a basolateral, electroneutral Ba(2+)-sensitive K(+)-Cl(-) cotransport that may operate as a K(+)-HCO cotransport. This cotransport was inhibited in the peritubular presence of furosemide, [(dihydroindenyl)oxy]alkanoic acid, 5-nitro-2-(3-phenylpropylamino)benzoate, or DIDS. At least three distinct basolateral HCO transport mechanisms are functional under physiological conditions: electroneutral Cl(-)/HCO exchange, DIDS- and EIPA-insensitive Na(+)-HCO cotransport, and Ba(2+)-sensitive electroneutral K(+)-Cl(-)(HCO) cotransport.

Immunological detection of Na(+)/H(+) exchangers in the gills of a hagfish, Myxine glutinosa, an elasmobranch, Raja erinacea, and a teleost, Fundulus heteroclitus

Na(+)/H(+) exchangers (NHE) are a family of ion exchangers with diverse functions that are well defined in mammals. NHE-1 is expressed in the plasma membrane of most mammalian cells where it regulates intracellular pH, and usually in the basolateral membrane of epithelial cells. It has also been detected in teleost gills where it may participate in systemic pH regulation. NHE-3 is usually expressed in the apical membrane of mammalian epithelial cells where it helps reabsorb Na(+) and HCO(3)(-); it has also been detected in teleost gills. We used Western blotting and heterologous antibodies to screen for expression of NHE-1 and NHE-3 in gills of an agnathan (Myxine glutinosa) and an elasmobranch (Raja erinacea), and NHE-3 in gills of a teleost (Fundulus heteroclitus). Positive NHE-1 bands were detected in gills from the agnathan and elasmobranch. Using the NHE-3 antibody, bands were detected in the gills of the elasmobranch and teleost. These data are some of the first direct evidence of NHEs in the gills of an agnathan and elasmobranch, and confirm the presence of NHEs in the gills of teleosts.

Carbon dioxide affects rat colonic Na+ absorption by modulating vesicular traffic

BACKGROUND & AIMS

We examined whether CO2 affects colonic Na+ absorption by endosome recycling of the Na+/H+ exchanger NHE3.

METHODS

Rat distal colon segments exposed to various acid-base conditions were examined by transmission electron microscopy at 27,500x magnification and subapical vesicles quantified. Immunocytochemistry was used to identify vesicular NHE3. Endocytosis was tested for by observing internalization of apical membrane labeled with fluorescein isothiocyanate-phytohemagglutinin and Cy-3-NHE3 antibody using confocal microscopy. The effects of mucosal 5-(N,N-dimethyl)-amiloride (DMA), which inhibits NHE2 and/or NHE3, and wortmannin, which inhibits phosphatidylinositol 3-kinase, on CO2-stimulated Na+ absorption were measured in the Ussing chamber.

RESULTS

The number of (coated and uncoated) subapical vesicles in epithelial cells was specifically and inversely related to net colonic Na+ absorption and PCO2. Immunoperoxidase labeling localized NHE3 on microvilli and vesicle membranes. Under the confocal microscope, a fluorescent band along apical membranes at PCO2 70 mm Hg became a subapical haze at PCO2 21 mm Hg. This pattern was not affected by carbonic anhydrase inhibition or when pH or [HCO3-] was changed, but PCO2 was held constant. DMA inhibition indicated that NHE3 mediates CO2-stimulated Na+ absorption. Wortmannin inhibited CO2-stimulated vesicle movement (exocytosis) and Na+ absorption.

CONCLUSIONS

CO2 affects Na+ absorption in rat distal colon epithelium in part by modulating the movement of NHE3-containing vesicles to and from the apical membrane.

Role of cartilage-derived anti-angiogenic factor, chondromodulin-I, during endochondral bone formation

OBJECTIVE

Cartilage is a typical avasclar tissue that exhibits powerful resistance to angiogenesis or vascular invasion. We previously identified a cartilage-specific 25 kDa glycosylated protein, chondromodulin-I (ChM-I), as anti-angiogenic factor. Taking advantage of ectopic bone formation and xenograft tumour model by human chondrosarcoma cell line OUMS-27, we examined how ChM-I is involved in switching of angiogenesis in cartilage.

DESIGN

Gene expression pattern of ChM-I was examined in 4-week-old mice and mouse embryos by northern blot analysis and in situ hybridization. To evaluate the effect of ChM-I on ectopic bone formation, guanidine extracts of demineralized bone matrix were mixed with the ChM-I-bound heparin-Sepharose beads and were implanted onto the fasciae of back muscle of 6-week old nude mice. To analyse the effect of ChM-I on tumour angiogenesis, the level of ChM-I mRNA in cartilaginous tumours was assessed by competitive PCR, and compared with that of articular cartilage. Then, human chondrosarcoma OUMS-27 cells were inoculated into the back of nude mice to form a tumour about 45 mm3 in size. Recombinant ChM-I protein was administrated into OUMS-27 xenograft tumours for the initial 5 days to study its effect against tumour-angiogenesis.

RESULTS

ChM-I gene was specifically expressed in cartilage of 4-week-old mice. Eye and thymus were also identified as minor expression sites. However, during endochondral bone development, cartilage changes its character from anti-angiogenic into angiogenic prior to the replacement of calcified cartilage by bone. In embryos, ChM-I mRNA was expressed in proliferative and upper hypertrophic cartilage zones in the developing cartilaginous bone rudiments, but completely abolished in lower hypertrophic and calcified cartilage zones. Purified ChM-I protein apparently inhibited vascular invasion into cartilage induced by the implantation of demineralized bone matrix in nude mice, leading to the inhibition of replacement of cartilage. The level of ChM-I transcripts in the lower-grade chondrosarcomas was substantially reduced to several hundreds or less in the lower-grade chondrosarcomas, compared with that of articular cartilage or other benign cartilage tumours. The local administration of recombinant human ChM-I almost completely blocked tumour angiogenesis and growth in the human chondrosarcoma xenografts in mice.

CONCLUSIONS

ChM-I is involved in the anti-angiogenic property of cartilage and its absence creates a permissive microenvironment for vascular invasion into cartilage under physiological and pathological conditions.

Localization and functional characterization of Na+/H+ exchanger isoform NHE4 in rat thick ascending limbs

The Na+/H+ exchanger NHE4 was cloned from a rat stomach cDNA library and shown to be expressed predominantly in the stomach and less dramatically in the kidney. The role and precise localization of NHE4 in the kidney are still unknown. A polyclonal antibody against a unique NHE4 decapeptide was used for immunohistochemistry in rat kidney. Simultaneous use of antibodies to Tamm-Horsfall glycoprotein and aquaporin-2 or -3 permitted identification of thick ascending limbs and collecting ducts, respectively. The results indicate that NHE4 is highly expressed in basolateral membranes of thick ascending limb and distal convoluted tubule, whereas collecting ducts from cortex to inner medulla and proximal tubules showed weaker basolateral NHE4 expression. Western blot analysis of NHE4 in membrane fractions prepared from the inner stripe of the outer medulla revealed the presence of a 95-kDa protein that was enriched in basolateral membrane vesicles isolated from medullary thick ascending limbs. The inhibition curve of H+-activated (22)Na uptake by 5-(N-ethyl-N-isopropyl)amiloride (EIPA) was consistent with the presence, beyond the EIPA high-affinity NHE1 isoform, of an EIPA low-affinity NHE with apparent half-maximal inhibition of 2.5 microM. Kinetic analyses showed that the extracellular Na+ dependence of NHE4 activity followed a simple hyperbolic relationship, with an apparent affinity constant of 12 mM. Intravesicular H+ activated NHE4 by a positive cooperative mechanism. NHE4 had an unusual low affinity for intravesicular H+ with a half-maximal activation value of pK 6.21. We conclude that NHE4, like NHE1, is expressed on the basolateral membrane of multiple nephron segments. Nevertheless, these two proteins exhibited dramatically different affinities for intracellular H+, suggesting that they may play distinct physiological roles in the kidney.

Expression of rat thick limb Na/H exchangers in potassium depletion and chronic metabolic acidosis

BACKGROUND

Regulation of renal transporter expression has been shown to support adaptation of transporter activities in several chronic situations. Basolateral and apical Na/H exchangers (NHE) in medullary thick ascending limb (MTAL) are involved in NH4+ and HCO3+ absorption, respectively. The NH4+ absorption rate in Henle's loop is increased in chronic metabolic acidosis (CMA) and potassium depletion (KD), which may be secondary to the increased NH4+ concentration in luminal fluid and/or to an increased NH4+ absorptive capacity of MTAL. HCO3- absorptive capacity in Henle's loop is increased in CMA and decreased in metabolic alkalosis, but is unchanged in KD despite the presence of metabolic alkalosis. The present study compared the effects of NH4Cl-induced CMA and KD on the expression of basolateral NHE-1 and the effect of KD on the expression of apical NHE-3 in MTAL.

METHODS

NHE-1 and NHE-3 mRNAs and proteins were assessed by a competitive reverse transcription-polymerase chain reaction (RT-PCR) method and semiquantitative immunoblots, respectively, in MTAL-purified suspensions from rats with CMA and KD.

RESULTS

NHE-1 protein abundance was similarly increased (approximately 90%) at two and five weeks of KD, while NHE-1 mRNA amount in MTAL cells was increased at two weeks of KD and returned to normal values by five weeks of KD. In contrast, NHE-1 mRNA and protein abundance did not change in CMA. NHE-3 protein abundance remained unchanged in both two and five weeks of KD, while NHE-3 mRNA was unchanged by two weeks of KD and reduced by approximately 50% at five weeks of KD.

CONCLUSIONS

The results suggest the following: (1) in KD, where the increased NH4+ concentration of luminal fluid that favors NH4+ absorption is counterbalanced by a decrease in BSC1 expression and activity, the increased NHE-1 expression may support an increased MTAL NH4+ absorptive capacity in CMA, NHE-1 expression is not specifically regulated and remains unchanged, suggesting that the increase in NH4+ concentration in luminal fluid is the main determinant of increased NH4+ absorption in MTAL. (2) In KD, NHE-3 expression did not decrease despite the presence of metabolic alkalosis, in agreement with the unchanged HCO3- absorptive capacity of Henle's loop.

Endothelin-1/endothelin-B receptor-mediated increases in NHE3 activity in chronic metabolic acidosis

Decreases in blood pH activate NHE3, the proximal tubular apical membrane Na/H antiporter. In cultured renal epithelial cells, activation of the endothelin-B (ET(B)) receptor increases NHE3 activity. To examine the role of the ET(B) receptor in the response to acidosis in vivo, the present studies examined ET(B) receptor-deficient mice, rescued from neonatal lethality by expression of a dopamine beta-hydroxylase promoter/ET(B) receptor transgene (Tg/Tg:ET(B)(-/-) mice). In proximal tubule suspensions from Tg/Tg:ET(B)(+/-) mice, 10(-8) M endothelin-1 (ET-1) increased NHE3 activity, but this treatment had no effect on tubules from Tg/Tg:ET(B)(-/-) mice. Acid ingestion for 7 days caused a greater decrease in blood HCO(3)(-) concentration in Tg/Tg:ET(B)(-/-) mice compared with Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice. Whereas acid ingestion increased apical membrane NHE3 by 42-46% in Tg/Tg:ET(B)(+/+) and Tg/Tg:ET(B)(+/-) mice, it had no effect on NHE3 in Tg/Tg:ET(B)(-/-) mice. In C57BL/6 mice, excess acid ingestion increased renal cortical preproET-1 mRNA expression 2.4-fold and decreased preproET-3 mRNA expression by 37%. On a control diet, Tg/Tg:ET(B)(-/-) mice had low rates of ammonium excretion, which could not be attributed to an inability to acidify the urine, as well as hypercitraturia, with increased titratable acid excretion. Acid ingestion increased ammonium excretion, citrate absorption, and titratable acid excretion to the same levels in Tg/Tg:ET(B)(-/-) and Tg/Tg:ET(B)(+/+) mice. In conclusion, metabolic acidosis increases ET-1 expression, which increases NHE3 activity via the ET(B) receptor.

Ammonium carriers in medullary thick ascending limb

Absorption of NH(4)(+) by the medullary thick ascending limb (MTAL) is a key event in the renal handling of NH(4)(+), leading to accumulation of NH(4)(+)/NH(3) in the renal medulla, which favors NH(4)(+) secretion in medullary collecting ducts and excretion in urine. The Na(+)-K(+)(NH(4)(+))-2Cl(-) cotransporter (BSC1/NKCC2) ensures approximately 50-65% of MTAL active luminal NH(4)(+) uptake under basal conditions. Apical barium- and verapamil-sensitive K(+)/NH(4)(+) antiport and amiloride-sensitive NH(4)(+) conductance account for the rest of active luminal NH(4)(+) transport. The presence of a K(+)/NH(4)(+) antiport besides BSC1 allows NH(4)(+) and NaCl absorption by MTAL to be independently regulated by vasopressin. At the basolateral step, the roles of NH(3) diffusion coupled to Na(+)/H(+) exchange or Na(+)/NH(4)(+) exchange, which favors NH(4)(+) absorption, and of Na(+)/K(+)(NH(4)(+))-ATPase, NH(4)(+)-Cl(-) cotransport, and NH(4)(+) conductance, which oppose NH(4)(+) absorption, have not been quantitatively defined. The increased ability of the MTAL to absorb NH(4)(+) during chronic metabolic acidosis involves an increase in BSC1 expression, but fine regulation of MTAL NH(4)(+) transport probably requires coordinated effects on various apical and basolateral MTAL carriers.

Hyposmolality stimulates Na(+)/H(+) exchange and HCO(3)(-) absorption in thick ascending limb via PI 3-kinase

The signal transduction mechanisms that mediate osmotic regulation of Na(+)/H(+) exchange are not understood. Recently we demonstrated that hyposmolality increases HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through stimulation of the apical membrane Na(+)/H(+) exchanger NHE3. To investigate the mechanism of this stimulation, MTALs from rats were isolated and perfused in vitro with 25 mM HCO(3)(-)-containing solutions. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (100 nM) and LY-294002 (20 microM) blocked completely the stimulation of HCO(3)(-) absorption by hyposmolality. In tissue strips dissected from the inner stripe of the outer medulla, the region of the kidney highly enriched in MTALs, hyposmolality increased PI 3-K activity 2. 2-fold. Wortmannin blocked the hyposmolality-induced PI 3-K activation. Further studies examined the interaction between hyposmolality and vasopressin, which inhibits HCO(3)(-) absorption in the MTAL via cAMP and often is involved in the development of plasma hyposmolality in clinical disorders. Pretreatment with arginine vasopressin, forskolin, or 8-bromo-cAMP abolished hyposmotic stimulation of HCO(3)(-) absorption, due to an effect of cAMP to inhibit hyposmolality- induced activation of PI 3-K. In contrast to their effects to block stimulation by hyposmolality, PI 3-K inhibitors and vasopressin have no effect on inhibition of apical Na(+)/H(+) exchange (NHE3) and HCO(3)(-) absorption by hyperosmolality. These results indicate that hyposmolality increases NHE3 activity and HCO(3)(-) absorption in the MTAL through activation of a PI 3-K-dependent pathway that is inhibited by vasopressin and cAMP. Hyposmotic stimulation and hyperosmotic inhibition of NHE3 are mediated through different signal transduction mechanisms.

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

BACKGROUND

The most abundant Na+/H+ exchanger in the apical membrane of proximal tubules is the type 3 isoform (NHE3), and its activity is acutely inhibited by parathyroid hormone (PTH). In the present study, we investigate whether changes in protein abundance as well as in mRNA levels play a significant role in the long-term modulation of NHE3 by PTH.

METHODS

Three groups of animals were compared: (1) HP: animals submitted to hyperparathyroidism by subcutaneous implantation of PTH pellets, providing threefold basal levels of this hormone (2.1 U. h-1); (2) control: sham-operated rats in which placebo pellets were implanted; (3) PTX: animals submitted to hypoparathyroidism by thyroparathyroidectomy followed by subcutaneous implantation of thyroxin pellets, which provided basal levels of thyroid hormone. After eight days, we measured bicarbonate reabsorption in renal proximal tubules by in vivo microperfusion. NHE3 activity was also measured in brush border membrane (BBM) vesicles by proton dependent uptake of 22Na. NHE3 expression was evaluated by Northern blot, Western blot and immunohistochemistry.

RESULTS

Bicarbonate reabsorption in renal proximal tubules was significantly decreased in HP rats. Na+/H+ exchange activity in isolated BBM vesicles was 6400 +/- 840, 9225 +/- 505, and 12205 +/- 690 cpm. mg-1. 15 s-1 in HP, sham, and PTX groups, respectively. BBM NHE3 protein abundance decreased 39.3 +/- 8.2% in HP rats and increased 54.6 +/- 7.8% in PTX rats. Immunohistochemistry showed that expression of NHE3 protein in apical BBM was decreased in HP rats and was increased in PTX rats. Northern blot analysis of total kidney RNA showed that the abundance of NHE3 mRNA was 20.3 +/- 1.3% decreased in HP rats and 27. 7 +/- 2.1% increased in PTX.

CONCLUSIONS

Our results indicate that the chronic inhibitory effect of PTH on the renal proximal tubule NHE3 is associated with changes in the expression of NHE3 mRNA levels and protein abundance.

Potassium deprivation upregulates expression of renal basolateral Na(+)-HCO(3)(-) cotransporter (NBC-1)

The purpose of the present experiments was to examine the effect of potassium deprivation on the expression of the renal basolateral Na(+)-HCO(3)(-) cotransporter (NBC-1). Rats were placed on a K(+)-free diet for various time intervals and examined. NBC-1 mRNA levels increased by about threefold in the cortex (P < 0.04) at 72 h of K(+) deprivation and remained elevated at 21 days. NBC activity increased by approximately 110% in proximal tubule suspensions, with the activity increasing from 0.091 in control to 0.205 pH/min in the K(+)-deprived group (P < 0.005). The inner stripe of outer medulla and cells of medullary thick ascending limb of Henle (mTAL) showed induction of NBC-1 mRNA and activity in K(+)-deprived rats, with the activity in mTAL increasing from 0.010 in control to 0.133 pH/min in the K(+)-deprived group (P < 0.004). K(+) deprivation also increased NBC-1 mRNA levels in the renal papilla (P < 0.02). We conclude that 1) K(+) deprivation increases NBC-1 expression and activity in proximal tubule and 2) K(+) deprivation causes induction of NBC-1 expression and activity in mTAL tubule and inner medulla. We propose that NBC-1 likely mediates enhanced HCO(3)(-) reabsorption in proximal tubule, mTAL, and inner medullary collecting duct in K(+) deprivation and contributes to the maintenance of metabolic alkalosis in this condition.

Acid incubation causes exocytic insertion of NHE3 in OKP cells

Incubation of opossum kidney clone P (OKP) cells in acid media (pH 6. 8) causes activation of Na(+)/H(+) exchanger 3 (NHE3) at 6, 12, and 24 h. OKP cell NHE3 protein abundance was increased by 45% at 24 h of acid incubation but was unaffected at 3-12 h. By contrast, apical membrane NHE3, measured by surface biotinylation, increased approximately twofold at 6, 12, and 24 h, mirroring the increase in activity. Acid incubation caused a 76% increase in exocytic insertion of NHE3 into the apical membrane but had no effect on endocytic internalization at 6 h. Latrunculin B, an inhibitor of microfilament organization, inhibited the acid-induced increases in apical membrane NHE3, exocytic insertion of NHE3, and NHE3 activity at 6 h. These studies demonstrate two mechanisms for acid-induced increases in NHE3 activity. Beginning at 6 h, there is an increase in apical membrane NHE3 that is due to stimulated exocytic insertion and is required for increased NHE3 activity. At 24 h, there is an additional increase in total cellular NHE3.

Effect of chronic metabolic acidosis on calbindin expression along the rat distal tubule

Calbindin D28k has been reported to be involved in the transcellular calcium transport along the rat distal tubule. It has also been shown that chronic metabolic acidosis (CMA) induces significant hypercalciuria. The present study investigated whether CMA affects the mRNA and the protein expression of calbindin D28k along isolated distal tubule (DT) of rats. The animals were made acidotic by adding 0.28 mol/L NH4Cl to the drinking water for 7 d. This maneuver was associated with an increase in plasma ionized calcium. Inulin clearance experiments demonstrated that metabolic acidosis did not affect GFR, but it significantly increased both total and fractional urinary calcium excretion. To define the role of calbindin D28k, total RNA was extracted from DT, identified, and microdissected from collagenase-treated kidneys. cDNA was synthesized from RNA using reverse transcriptase and oligo(dT)(12-18) primers. Calbindin D28k mRNA abundance was semiquantified by a competitive reverse transcription-PCR, using an internal standard of cDNA that differed from the wild-type calbindin D28k by a deletion of 86 bp. The reverse transcription-PCR was performed starting from the same amount of total RNA. For each set of experiments, control and acidotic rats were studied in parallel. The identity of the DT was further verified by the presence of the thiazide-sensitive NaCl cotransporter (rTSC1) mRNA. Calbindin D28k mRNA abundance was 0.89 +/- 0.21 amol/ng total RNA in DT of CMA rats (n = 5) compared with 0.30 +/- 0.12 amol/ng total RNA of control rats (n = 5) (P < 0.05). Using specific rabbit polyclonal anti-calbindin D28k antibody, Western blotting was performed starting from thin slices of outer cortex. Densitometric analysis revealed that in acidotic rats (n = 7) there was a 17 +/- 5% (P < 0.05) increase in calbindin D28k protein abundance compared with controls (n = 7). These results indicate that in the rat, ammonium chloride loading induces an increase in filtered ionized calcium load that is associated with a significant upregulation of calbindin D28k both at the mRNA and protein level. These last effects will help to reduce the concomitant hypercalciuria, thus mitigating the consequence of CMA on calcium metabolism.