Basolateral Na+-dependent HCO3- transporter NBCn1-mediated HCO3- influx in rat medullary thick ascending limb

State of knowledge on ammonia handling by the kidney

The disposal of ammonia, the main proton buffer in the urine, is important for acid-base homeostasis. Renal ammonia excretion is the predominant contributor to renal net acid excretion, both under basal condition and in response to acidosis. New insights into the mechanisms of renal ammonia production and transport have been gained in the past decades. Ammonia is the only urinary solute known to be produced in the kidney and selectively transported through the different parts of the nephron. Both molecular forms of total ammonia, NH3 and NH4+, are transported by specific proteins. Proximal tubular ammoniagenesis and the activity of these transport processes determine the eventual fate of total ammonia produced and excreted by the kidney. In this review, we summarized the state of the art of ammonia handling by the kidney and highlighted the newest processes described in the last decade.

Dietary sodium enhances the expression of SLC4 family transporters, IRBIT, L-IRBIT, and PP1 in rat kidney: Insights into the molecular mechanism for renal sodium handling

The kidney plays a central role in maintaining the fluid and electrolyte homeostasis in the body. Bicarbonate transporters NBCn1, NBCn2, and AE2 are expressed at the basolateral membrane of the medullary thick ascending limb (mTAL). In a previous study, NBCn1, NBCn2, and AE2 are proposed to play as a regulatory pathway to decrease NaCl reabsorption in the mTAL under high salt condition. When heterologously expressed, the activity of these transporters could be stimulated by the InsP3R binding protein released with inositol 1,4,5-trisphosphate (IRBIT), L-IRBIT (collectively the IRBITs), or protein phosphatase PP1. In the present study, we characterized by immunofluorescence the expression and localization of the IRBITs, and PP1 in rat kidney. Our data showed that the IRBITs were predominantly expressed from the mTAL through the distal renal tubules. PP1 was predominantly expressed in the TAL, but is also present in high abundance from the distal convoluted tubule through the medullary collecting duct. Western blotting analyses showed that the abundances of NBCn1, NBCn2, and AE2 as well as the IRBITs and PP1 were greatly upregulated in rat kidney by dietary sodium. Co-immunoprecipitation study provided the evidence for protein interaction between NBCn1 and L-IRBIT in rat kidney. Taken together, our data suggest that the IRBITs and PP1 play an important role in sodium handling in the kidney. We propose that the IRBITs and PP1 stimulates NBCn1, NBCn2, and AE2 in the basolateral mTAL to inhibit sodium reabsorption under high sodium condition. Our study provides important insights into understanding the molecular mechanism for the regulation of sodium homeostasis in the body.

Potential Novel Role of Membrane-Associated Carbonic Anhydrases in the Kidney

Carbonic anhydrases (CAs), because they catalyze the interconversion of carbon dioxide (CO₂) and water into bicarbonate (HCO₃^-) and protons (H⁺), thereby influencing pH, are near the core of virtually all physiological processes in the body. In the kidneys, soluble and membrane-associated CAs and their synergy with acid-base transporters play important roles in urinary acid secretion, the largest component of which is the reabsorption of HCO₃^- in specific nephron segments. Among these transporters are the Na⁺-coupled HCO₃^- transporters (NCBTs) and the Cl^--HCO₃^- exchangers (AEs)-members of the "solute-linked carrier" 4 (SLC4) family. All of these transporters have traditionally been regarded as "HCO₃^-" transporters. However, recently our group has demonstrated that two of the NCBTs carry CO₃^2- rather than HCO₃^- and has hypothesized that all NCBTs follow suit. In this review, we examine current knowledge on the role of CAs and "HCO₃^-" transporters of the SLC4 family in renal acid-base physiology and discuss how our recent findings impact renal acid secretion, including HCO₃^- reabsorption. Traditionally, investigators have associated CAs with producing or consuming solutes (CO₂, HCO₃^-, and H⁺) and thus ensuring their efficient transport across cell membranes. In the case of CO₃^2- transport by NCBTs, however, we hypothesize that the role of membrane-associated CAs is not the appreciable production or consumption of substrates but the minimization of pH changes in nanodomains near the membrane.

Streptolysin S targets the sodium-bicarbonate cotransporter NBCn1 to induce inflammation and cytotoxicity in human keratinocytes during Group A Streptococcal infection

Group A <i>Streptococcus</i> (GAS, <i>Streptococcus pyogenes</i>) is a Gram-positive human pathogen that employs several secreted and surface-bound virulence factors to manipulate its environment, allowing it to cause a variety of disease outcomes. One such virulence factor is Streptolysin S (SLS), a ribosomally-produced peptide toxin that undergoes extensive post-translational modifications. The activity of SLS has been studied for over 100 years owing to its rapid and potent ability to lyse red blood cells, and the toxin has been shown to play a major role in GAS virulence <i>in vivo</i>. We have previously demonstrated that SLS induces hemolysis by targeting the chloride-bicarbonate exchanger Band 3 in erythrocytes, indicating that SLS is capable of targeting host proteins to promote cell lysis. However, the possibility that SLS has additional protein targets in other cell types, such as keratinocytes, has not been explored. Here, we use bioinformatics analysis and chemical inhibition studies to demonstrate that SLS targets the electroneutral sodium-bicarbonate cotransporter NBCn1 in keratinocytes during GAS infection. SLS induces NF-κB activation and host cytotoxicity in human keratinocytes, and these processes can be mitigated by treating keratinocytes with the sodium-bicarbonate cotransport inhibitor S0859. Furthermore, treating keratinocytes with SLS disrupts the ability of host cells to regulate their intracellular pH, and this can be monitored in real time using the pH-sensitive dye pHrodo Red AM in live imaging studies. These results demonstrate that SLS is a multifunctional bacterial toxin that GAS uses in numerous context-dependent ways to promote host cell cytotoxicity and increase disease severity. Studies to elucidate additional host targets of SLS have the potential to impact the development of therapeutics for severe GAS infections.

Understanding the Physiological Role of Electroneutral Na+-Coupled HCO3− Cotransporter and Its Therapeutic Implications

Acid–base homeostasis is critical for proper physiological function and pathology. The SLC4 family of HCO₃⁻ transmembrane cotransporters is one of the HCO₃⁻ transmembrane transport carriers responsible for cellular pH regulation and the uptake or secretion of HCO₃⁻ in epithelial cells. NBCn1 (SLC4A7), an electroneutral Na⁺/HCO₃⁻ cotransporter, is extensively expressed in several tissues and functions as a cotransporter for net acid extrusion after cellular acidification. However, the expression and activity level of NBCn1 remain elusive. In addition, NBCn1 has been involved in numerous other cellular processes such as cell volume, cell death/survival balance, transepithelial transport, as well as regulation of cell viability. This review aims to give an inclusive overview of the most recent advances in the research of NBCn1, emphasizing the basic features, regulation, and tissue-specific physiology as well as the development and application of potent inhibitors of NBCn1 transporter in cancer therapy. Research and development of targeted therapies should be carried out for NBCn1 and its associated pathways.

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

NBCn1 Increases NH4 + Reabsorption Across Thick Ascending Limbs, the Capacity for Urinary NH4 + Excretion, and Early Recovery from Metabolic Acidosis

BACKGROUND

The electroneutral Na⁺/HCO₃ ^- cotransporter NBCn1 (Slc4a7) is expressed in basolateral membranes of renal medullary thick ascending limbs (mTALs). However, direct evidence that NBCn1 contributes to acid-base handling in mTALs, urinary net acid excretion, and systemic acid-base homeostasis has been lacking.

METHODS

Metabolic acidosis was induced in wild-type and NBCn1 knockout mice. Fluorescence-based intracellular pH recordings were performed and NH₄ ⁺ transport measured in isolated perfused mTALs. Quantitative RT-PCR and immunoblotting were used to evaluate NBCn1 expression. Tissue [NH₄ ⁺] was measured in renal biopsies, NH₄ ⁺ excretion and titratable acid quantified in spot urine, and arterial blood gasses evaluated in normoventilated mice.

RESULTS

Basolateral Na⁺/HCO₃ ^- cotransport activity was similar in isolated perfused mTALs from wild-type and NBCn1 knockout mice under control conditions. During metabolic acidosis, basolateral Na⁺/HCO₃ ^- cotransport activity increased four-fold in mTALs from wild-type mice, but remained unchanged in mTALs from NBCn1 knockout mice. Correspondingly, NBCn1 protein expression in wild-type mice increased ten-fold in the inner stripe of renal outer medulla during metabolic acidosis. During systemic acid loading, knockout of NBCn1 inhibited the net NH₄ ⁺ reabsorption across mTALs by approximately 60%, abolished the renal corticomedullary NH₄ ⁺ gradient, reduced the capacity for urinary NH₄ ⁺ excretion by approximately 50%, and delayed recovery of arterial blood pH and standard [HCO₃ ^-] from their initial decline.

CONCLUSIONS

During metabolic acidosis, NBCn1 is required for the upregulated basolateral HCO₃ ^- uptake and transepithelial NH₄ ⁺ reabsorption in mTALs, renal medullary NH₄ ⁺ accumulation, urinary NH₄ ⁺ excretion, and early recovery of arterial blood pH and standard [HCO₃ ^-]. These findings support that NBCn1 facilitates urinary net acid excretion by neutralizing intracellular H⁺ released during NH₄ ⁺ reabsorption across mTALs.

Multiple acid-base and electrolyte disturbances upregulate NBCn1, NBCn2, IRBIT and L-IRBIT in the mTAL

KEY POINTS

The roles of the Na⁺ /HCO₃ ^- cotransporters NBCn1 and NBCn2 as well as their activators IRBIT and L-IRBIT in the regulation of the mTAL transport of NH₄ ⁺ , HCO₃ ^- , and NaCl are investigated. Dietary challenges of NH₄ Cl, NaHCO₃ or NaCl all increase the abundance of NBCn1 and NBCn2 in the outer medulla. The three challenges generally produce parallel increases in the abundance of IRBIT and L-IRBIT in the outer medulla. Both IRBIT and L-IRBIT powerfully stimulate the activities of the mTAL isoforms of NBCn1 and NBCn2 as expressed in Xenopus oocytes. Our findings support the hypothesis that NBCn1, NBCn2, IRBIT and L-IRBIT appropriately promote NH₄ ⁺ shunting but oppose HCO₃ ^- and NaCl reabsorption in the mTAL, and thus are at the nexus of the regulation pathways for multiple renal transport processes.

ABSTRACT

The medullary thick ascending limb (mTAL) plays a key role in urinary acid and NaCl excretion. NBCn1 and NBCn2 are present in the basolateral mTAL, where NBCn1 promotes NH₄ ⁺ shunting. IRBIT and L-IRBIT (the IRBITs) are two powerful activators of certain acid-base transporters. Here we use western blotting and immunofluorescence to examine the effects of multiple acid-base and electrolyte disturbances on expression of NBCn1, NBCn2 and the IRBITs in rat kidney. We also use electrophysiology to examine the functional effects of IRBITs on NBCn1 and NBCn2 in Xenopus oocytes. NH₄ Cl-induced metabolic acidosis (MAc) substantially increases protein expression of NBCn1 and NBCn2 in the outer medulla (OM) of rat kidney. Surprisingly, NaHCO₃ -induced metabolic alkalosis (MAlk) and high-salt diet (HSD) also increase expression of NBCn1 and NBCn2 (effect of NaHCO₃  > HSD). Moreover, all three challenges generally increase OM expression of the IRBITs. In Xenopus oocytes, the IRBITs substantially increase the activities of NBCn1 and NBCn2. We propose that upregulation of basolateral NBCn1 and NBCn2 plus the IRBITs in the mTAL: (1) promotes NH₄ ⁺ shunting by increasing basolateral HCO₃ ^- uptake to neutralize apical NH₄ ⁺ uptake during MAc; (2) inhibits HCO₃ ^- reabsorption during MAlk by opposing HCO₃ ^- efflux via the basolateral anion exchanger AE2; and (3) inhibits NaCl reabsorption by mediating (with AE2) net NaCl backflux into the mTAL cell during HSD. Thus, NBCn1, NBCn2 and the IRBITs are at the nexus of the regulatory pathways for multiple renal transport processes.

Molecular Pathophysiology of Acid-Base Disorders

Acid-base balance is critical for normal life. Acute and chronic disturbances impact cellular energy metabolism, endocrine signaling, ion channel activity, neuronal activity, and cardiovascular functions such as cardiac contractility and vascular blood flow. Maintenance and adaptation of acid-base homeostasis are mostly controlled by respiration and kidney. The kidney contributes to acid-base balance by reabsorbing filtered bicarbonate, regenerating bicarbonate through ammoniagenesis and generation of protons, and by excreting acid. This review focuses on acid-base disorders caused by renal processes, both inherited and acquired. Distinct rare inherited monogenic diseases affecting acid-base handling in the proximal tubule and collecting duct have been identified. In the proximal tubule, mutations of solute carrier 4A4 (SLC4A4) (electrogenic Na⁺/HCO₃^--cotransporter Na⁺/bicarbonate cotransporter e1 [NBCe1]) and other genes such as CLCN5 (Cl^-/H⁺-antiporter), SLC2A2 (GLUT2 glucose transporter), or EHHADH (enoyl-CoA, hydratase/3-hydroxyacyl CoA dehydrogenase) causing more generalized proximal tubule dysfunction can cause proximal renal tubular acidosis resulting from bicarbonate wasting and reduced ammoniagenesis. Mutations in adenosine triphosphate ATP6V1 (B1 H⁺-ATPase subunit), ATPV0A4 (a4 H⁺-ATPase subunit), SLC4A1 (anion exchanger 1), and FOXI1 (forkhead transcription factor) cause distal renal tubular acidosis type I. Carbonic anhydrase II mutations affect several nephron segments and give rise to a mixed proximal and distal phenotype. Finally, mutations in genes affecting aldosterone synthesis, signaling, or downstream targets can lead to hyperkalemic variants of renal tubular acidosis (type IV). More common forms of renal acidosis are found in patients with advanced stages of chronic kidney disease and are owing, at least in part, to a reduced capacity for ammoniagenesis.

Deletion of the Na/HCO3 Transporter NBCn1 Protects Hippocampal Neurons from NMDA-induced Seizures and Neurotoxicity in Mice

The Na/HCO₃ cotransporter NBCn1/SLC4A7 can affect glutamate neurotoxicity in primary cultures of rat hippocampal neurons. Here, we examined NMDA-induced neurotoxicity in NBCn1 knockout mice to determine whether a similar effect also occurs in the mouse brain. In primary cultures of hippocampal neurons from knockouts, NMDA had no neurotoxic effects, determined by lactate dehydrogenase release and nitric oxide synthase (NOS)-dependent cGMP production. Male knockouts and wildtypes (6–8 weeks old) were then injected with NMDA (75 mg/kg; ip) and hippocampal neuronal damages were assessed. Wildtypes developed severe tonic-clonic seizures, whereas knockouts had mild seizure activity (motionless). In knockouts, the NOS activity, caspase-3 expression/activity and the number of TUNEL-positive cells were significantly low. Immunochemical analysis revealed decreased expression levels of the NMDA receptor subunit GluN1 and the postsynaptic density protein PSD-95 in knockouts. Extracellular recording from hippocampal slices showed no Mg²⁺/NMDA-mediated epileptiform events in knockouts. In conclusion, these results show a decrease in NMDA neurotoxicity by NBCn1 deletion. Given that acid extrusion has been known to prevent pH decrease and protect neurons from acid-induced damage, our study presents novel evidence that acid extrusion by NBCn1 stimulates neurotoxicity.

Molecular mechanisms of bi-directional ion transport in the Malpighian tubules of a lepidopteran crop pest, Trichoplusia ni

Classical studies have described in detail the complex and regionalized morphology of the Malpighian tubule (MT) in larval Lepidoptera. Recent studies revealed unusual aspects of ion transport in the Malpighian tubules of the larva of the cabbage looper, Trichoplusia ni. These included: cation reabsorption via secondary cells (SC); coupling of SCs to neighbouring PCs via gap junctions to enable reabsorption; and a reversal from cation secretion to reabsorption by the principal cells in the distal ileac plexus region of the in situ tubule in response to dietary ion loading. The current paper aimed to identify molecular components of ion transport in the MTs of T. ni and to describe their role in the recently reported reversal of ion transport in ion-loaded animals. Using a combination of molecular, immunohistochemical and electrophysiological techniques, we assigned roles to Na⁺/K⁺-ATPase (NKA), V-type H⁺-ATPase (VA), Na⁺/K⁺/Cl^- co-transporter (NKCC), K⁺/Cl^- co-transporter (KCC), inward-rectifying K⁺ channel (Kir), and Na⁺/H⁺ exchangers (NHE)-7 and -8 in the transport of Na⁺ and K⁺ by the distal ileac plexus of T. ni. The yellow region of the tubule lacked all of the above except VA, and the white region lacked all of the above transporters but expressed an amiloride-sensitive Na⁺ channel (NaC). Overall, the ion transport machinery in the distal ileac plexus of the T. ni tubule shows remarkable similarity to that in tubules of other groups of insects, yet this region transports ions very differently. Shutdown of secretory ATPases and utilisation of the same molecular machinery in the face of changing ion gradients may enable ion transport reversal in lepidopteran MTs. We propose that gap junction-based coupling of the two cell types likely aids in toggling between ion secretion and ion reabsorption in this segment.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.

Lithium increases ammonium excretion leading to altered urinary acid-base buffer composition

Previous reports identify a voltage dependent distal renal tubular acidosis (dRTA) secondary to lithium (Li⁺) salt administration. This was based on the inability of Li⁺-treated patients to increase the urine-blood (U-B) pCO₂ when challenged with NaHCO₃ and, the ability of sodium neutral phosphate or Na₂SO₄ administration to restore U-B pCO₂ in experimental animal models. The underlying mechanisms for the Li⁺-induced dRTA are still unknown. To address this point, a 7 days time course of the urinary acid-base parameters was investigated in rats challenged with LiCl, LiCitrate, NaCl, or NaCitrate. LiCl induced the largest polyuria and a mild metabolic acidosis. Li⁺-treatment induced a biphasic response. In the first 2 days, proper urine volume and acidification occurred, while from the 3rd day of treatment, polyuria developed progressively. In this latter phase, the LiCl-treated group progressively excreted more NH₄⁺ and less pCO₂, suggesting that NH₃/NH₄⁺ became the main urinary buffer. This physiological parameter was corroborated by the upregulation of NBCn1 (a marker of increased ammonium recycling) in the inner stripe of outer medulla of LiCl treated rats. Finally, by investigating NH₄⁺ excretion in ENaC-cKO mice, a model resistant to Li⁺-induced polyuria, a primary role of the CD was confirmed. By definition, dRTA is characterized by deficient urinary ammonium excretion. Our data question the presence of a voltage-dependent Li⁺-induced dRTA in rats treated with LiCl for 7 days and the data suggest that the alkaline urine pH induced by NH₃/NH₄⁺ as the main buffer has lead to the interpretation dRTA in previous studies.

Extracellular acidosis and very low [Na+ ] inhibit NBCn1- and NHE1-mediated net acid extrusion from mouse vascular smooth muscle cells

AIM

The electroneutral Na⁺ , HCO3- cotransporter NBCn1 and Na⁺ /H⁺ exchanger NHE1 regulate acid-base balance in vascular smooth muscle cells (VSMCs) and modify artery function and structure. Pathological conditions - notably ischaemia - can dramatically perturb intracellular (i) and extracellular (o) pH and [Na⁺ ]. We examined effects of low [Na⁺ ]_o and pH_o on NBCn1 and NHE1 activity in VSMCs of small arteries.

METHODS

We measured pH_i by 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein-based fluorescence microscopy of mouse mesenteric arteries and induced intracellular acidification by NH4+ prepulse technique.

RESULTS

NBCn1 activity - defined as Na⁺ -dependent, amiloride-insensitive net base uptake with CO₂ /HCO3- present - was inhibited equally when pH_o decreased from 7.4 (22 mm HCO3-/5% CO₂ ) by metabolic (pH_o 7.1/11 mm HCO3-: 22 ± 8%; pH_o 6.8/5.5 mm HCO3-: 61 ± 7%) or respiratory (pH_o 7.1/10% CO₂ : 35 ± 11%; pH_o 6.8/20% CO₂ : 56 ± 7%) acidosis. Extracellular acidosis more prominently inhibited NHE1 activity - defined as Na⁺ -dependent net acid extrusion without CO₂ /HCO3- present - at both pH_o 7.1 (45 ± 9%) and 6.8 (85 ± 5%). Independently of pH_o , lowering [Na⁺ ]_o from 140 to 70 mm reduced NBCn1 and NHE1 activity <20% whereas transport activities declined markedly (25-50%) when [Na⁺ ]_o was reduced to 35 mm. Steady-state pH_i decreased more during respiratory (ΔpH_i /ΔpH_o  = 71 ± 4%) than metabolic (ΔpH_i /ΔpH_o  = 30 ± 7%) acidosis.

CONCLUSION

Extracellular acidification inhibits NBCn1 and NHE1 activity in VSMCs. NBCn1 is equivalently inhibited when pCO₂ is raised or [HCO3-]_o decreased. Lowering [Na⁺ ]_o inhibits NBCn1 and NHE1 markedly only below the typical physiological and pathophysiological range. We propose that inhibition of Na⁺ -dependent net acid extrusion at low pH_o protects against cellular Na⁺ overload at the cost of intracellular acidification.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

cAMP-dependent secretagogues stimulate the NaHCO3 cotransporter in the villous epithelium of the brushtail possum, Trichosurus vulpecula

In the ileum of the brushtail possum, Trichosurus vulpecula, fluid secretion appears to be driven by electrogenic HCO₃^- secretion. Consistent with this, the cystic fibrosis transmembrane conductance regulator is expressed in the apical membrane of the ileal epithelial cells and the pancreatic or secretory variant of the NaHCO₃ cotransporter in the basolateral membrane. This suggests that in the possum ileum, electrogenic HCO₃^- secretion is driven by basolateral NaHCO₃ cotransporter (NBC) activity. To determine if the NBC contributes to HCO₃^- secretion in the possum ileum, intracellular pH (pHi) measurements in isolated villi were used to demonstrate NBC activity in the ileal epithelial cells and investigate the effect of cAMP-dependent secretagogues. In CO₂/HCO₃^--free solutions, recovery of the epithelial cells from an acid load was Na⁺-dependent and ≈80% inhibited by ethyl-isopropyl-amiloride (EIPA, 10 µmol L^-1), indicative of the presence of an Na⁺/H⁺ exchanger, most likely NHE1. However, in the presence of CO₂/HCO₃^-, EIPA only inhibited ≈ 50% of the recovery, the remainder was inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS, 500 µmol L^-1), indicative of NBC activity. Under steady-state conditions, NHE1 inhibition by EIPA had little effect on pHi in the presence or absence of secretagogues, but NBC inhibition with DIDS resulted in a rapid acidification of the cells, which was increased fivefold by secretagogues. These data demonstrate the functional activity of an NaHCO₃ cotransporter in the ileal epithelial cells. Furthermore, the stimulation of NBC activity by secretagogues is consistent with the involvement of an NaHCO₃ cotransporter in electrogenic HCO₃^- secretion.

Cation-coupled bicarbonate transporters

Cation-coupled HCO3(-) transport was initially identified in the mid-1970s when pioneering studies showed that acid extrusion from cells is stimulated by CO2/HCO3(-) and associated with Na(+) and Cl(-) movement. The first Na(+)-coupled bicarbonate transporter (NCBT) was expression-cloned in the late 1990s. There are currently five mammalian NCBTs in the SLC4-family: the electrogenic Na,HCO3-cotransporters NBCe1 and NBCe2 (SLC4A4 and SLC4A5 gene products); the electroneutral Na,HCO3-cotransporter NBCn1 (SLC4A7 gene product); the Na(+)-driven Cl,HCO3-exchanger NDCBE (SLC4A8 gene product); and NBCn2/NCBE (SLC4A10 gene product), which has been characterized as an electroneutral Na,HCO3-cotransporter or a Na(+)-driven Cl,HCO3-exchanger. Despite the similarity in amino acid sequence and predicted structure among the NCBTs of the SLC4-family, they exhibit distinct differences in ion dependency, transport function, pharmacological properties, and interactions with other proteins. In epithelia, NCBTs are involved in transcellular movement of acid-base equivalents and intracellular pH control. In nonepithelial tissues, NCBTs contribute to intracellular pH regulation; and hence, they are crucial for diverse tissue functions including neuronal discharge, sensory neuron development, performance of the heart, and vascular tone regulation. The function and expression levels of the NCBTs are generally sensitive to intracellular and systemic pH. Animal models have revealed pathophysiological roles of the transporters in disease states including metabolic acidosis, hypertension, visual defects, and epileptic seizures. Studies are being conducted to understand the physiological consequences of genetic polymorphisms in the SLC4-members, which are associated with cancer, hypertension, and drug addiction. Here, we describe the current knowledge regarding the function, structure, and regulation of the mammalian cation-coupled HCO3(-) transporters of the SLC4-family.

Physiological role of NBCe2 in the regulation of electrolyte transport in the distal nephron

The electrogenic Na(+)-HCO3 (-) cotransporter 2 (NBCe2) is a newly discovered protein in the distal nephron. Our understanding is minimal regarding its physiological role in renal electrolyte transport. In this mini-review, we summarize the potential function of NBCe2 in the regulation of blood pressure, acid-base, and K(+) and Ca(2+) transport in the distal nephron.

Deficient acid handling with distal RTA in the NBCe2 knockout mouse

In many circumstances, the pathogenesis of distal renal tubular acidosis (dRTA) is not understood. In the present study, we report that a mouse model lacking the electrogenic Na(+)-HCO3 (-) cotransporter [NBCe2/Slc4a5; NBCe2 knockout (KO) mice] developed dRTA after an oral acid challenge. NBCe2 expression was identified in the connecting tubule (CNT) of wild-type mice, and its expression was significantly increased after acid loading. NBCe2 KO mice did not have dRTA when on a standard mouse diet. However, after acid loading, NBCe2 KO mice exhibited complete features of dRTA, characterized by insufficient urinary acidification, hyperchloremic hypokalemic metabolic acidosis, and hypercalciuria. Additional experiments showed that NBCe2 KO mice had decreased luminal transepithelial potential in the CNT, as revealed by micropuncture. Further immunofluorescence and Western blot experiments found that NBCe2 KO mice had increased expression of H(+)-ATPase B1 in the plasma membrane. These results showed that NBCe2 KO mice with acid loading developed increased urinary K(+) and Ca(2+) wasting due to decreased luminal transepithelial potential in the CNT. NBCe2 KO mice compensated to maintain systemic pH by increasing H(+)-ATPase in the plasma membrane. Therefore, defects in NBCe2 can cause dRTA, and NBCe2 has an important role to regulate urinary acidification and the transport of K(+) and Ca(2+) in the distal nephron.

Increased Epithelial Sodium Channel Activity Contributes to Hypertension Caused by Na+-HCO3- Cotransporter Electrogenic 2 Deficiency

The gene SLC4A5 encodes the Na(+)-HCO3 (-) cotransporter electrogenic 2, which is located in the distal nephron. Genetically deleting Na(+)-HCO3 (-) cotransporter electrogenic 2 (knockout) causes Na(+)-retention and hypertension, a phenotype that is diminished with alkali loading. We performed experiments with acid-loaded mice and determined whether overactive epithelial Na(+) channels (ENaC) or the Na(+)-Cl(-) cotransporter causes the Na(+) retention and hypertension in knockout. In untreated mice, the mean arterial pressure was higher in knockout, compared with wild-type (WT); however, treatment with amiloride, a blocker of ENaC, abolished this difference. In contrast, hydrochlorothiazide, an inhibitor of Na(+)-Cl(-) cotransporter, decreased mean arterial pressure in WT, but not knockout. Western blots showed that quantity of plasmalemmal full-length ENaC-α was significantly higher in knockout than in WT. Amiloride treatment caused a 2-fold greater increase in Na(+) excretion in knockout, compared with WT. In knockout, but not WT, amiloride treatment decreased plasma [Na(+)] and urinary K(+) excretion, but increased hematocrit and plasma [K(+)] significantly. Micropuncture with microelectrodes showed that the [K(+)] was significantly higher and the transepithelial potential (Vte) was significantly lower in the late distal tubule of the knockout compared with WT. The reduced Vte in knockout was amiloride sensitive and therefore revealed an upregulation of electrogenic ENaC-mediated Na(+) reabsorption in this segment. These results show that, in the absence of Na(+)-HCO3 (-) cotransporter electrogenic 2 in the late distal tubule, acid-loaded mice exhibit disinhibition of ENaC-mediated Na(+) reabsorption, which results in Na(+) retention, K(+) wasting, and hypertension.

P2X receptors trigger intracellular alkalization in isolated perfused mouse medullary thick ascending limb

Aims

Extracellular ATP is an important regulator of renal tubular transport. Recently, we found that basolateral ATP markedly inhibits Na⁺ and Cl⁻ absorption in mouse medullary thick ascending limb (mTAL) via a P2X receptor. The underlying mechanism that mediates this ATP-dependent transport inhibition in mTAL is, however, unclear. The renal outer medullary K⁺ channel (ROMK) is sensitive to intracellular pH where a reduction leads to closing of ROMK. We speculated that P2X receptor stimulation in the TAL could lead to changes in pH_i, leading to a reduction in NaCl transport.

Methods

To test this hypothesis, we measured pH_i in single perfused mouse mTALs using the fluorescent ratiometric dye 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethylester.

Results

Interestingly, basolateral ATP (100 μm) caused a prominent, reversible intracellular alkalization of mTAL, with an average pH_i increase of 0.14 ± 0.02 (n = 14). This was completely abolished by the P2X receptor antagonist periodate-oxidized ATP (50 μm). The P2X receptor-mediated intracellular alkalization required the activity of the apical Na⁺/H⁺ exchanger (NHE3). Typically, G_q-coupled receptors cause a significant acidification of tubular epithelial cells, which was confirmed in this study, by P2Y₂ and Ca²⁺ sensing receptor stimulation.

Conclusion

This study reports that stimulation of basolateral P2X receptors causes a substantial intracellular alkalization in the isolated perfused mouse mTAL. This intracellular alkalization is mediated through an increased apical NHE3 activity, similar to what we previously observed when tubular transport is inhibited with furosemide. This increased NHE3 activity causes H⁺ secretion in the mTAL and provides further support that the TAL is a site of urinary acidification.

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.

Thick ascending limb of the loop of Henle

The thick ascending limb occupies a central anatomic and functional position in human renal physiology, with critical roles in the defense of the extracellular fluid volume, the urinary concentrating mechanism, calcium and magnesium homeostasis, bicarbonate and ammonium homeostasis, and urinary protein composition. The last decade has witnessed tremendous progress in the understanding of the molecular physiology and pathophysiology of this nephron segment. These advances are the subject of this review, with emphasis on particularly recent developments.

Direct evidence for calcineurin binding to the exon-7 loop of the sodium-bicarbonate cotransporter NBCn1

The NaHCO₃ cotransporter NBCn1 plays a role in neutralizing intracellular acid loads at the basolateral membrane in cells of the medullary thick ascending limb (mTAL). Calcineurin inhibitors (Cn-Is) are known to both downregulate NBCn1 expression in the distal nephron and cause renal tubular acidosis (RTA), a risk factor for nephrocalcinosis and nephrolithiasis. In this report, we provide a new perspective on concurrent studies of NBCn1 in various tissues by using cell-free binding assays to investigate the interaction of NBCn1 with the calcineurin (Cn) isoform PPP3CA. Surface plasmon resonance (SPR) analyses show that the protein domain Exon 7 (translated from cassette II of NBCn1) binds Cn with an equilibrium dissociation constant (K_D) of 30 +/- 15 nm. Linked-reaction tests suggest that the binding involves a conformational change. Nested PCR reactions were used to show that NBCn1-Exon 7 splice variants with alternative N-termini regions are expressed in the kidney, as well as other tissues. Additionally, we discuss NBCn1-Exon 7 implication in acid-base balance and calcium crystallization in the kidney.

Everything you need to know about distal renal tubular acidosis in autoimmune disease

Renal acid–base homeostasis is a complex process, effectuated by bicarbonate reabsorption and acid secretion. Impairment of urinary acidification is called renal tubular acidosis (RTA). Distal renal tubular acidosis (dRTA) is the most common form of the RTA syndromes. Multiple pathophysiologic mechanisms, each associated with various etiologies, can lead to dRTA. The most important consequence of dRTA is (recurrent) nephrolithiasis. The diagnosis is based on a urinary acidification test. Potassium citrate is the treatment of choice.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Na(+) dependent acid-base transporters in the choroid plexus; insights from slc4 and slc9 gene deletion studies

The choroid plexus epithelium (CPE) is located in the ventricular system of the brain, where it secretes the majority of the cerebrospinal fluid (CSF) that fills the ventricular system and surrounds the central nervous system. The CPE is a highly vascularized single layer of cuboidal cells with an unsurpassed transepithelial water and solute transport rate. Several members of the slc4a family of bicarbonate transporters are expressed in the CPE. In the basolateral membrane the electroneutral Na⁺ dependent Cl⁻/HCO₃⁻ exchanger, NCBE (slc4a10) is expressed. In the luminal membrane, the electrogenic Na⁺:HCO₃⁻ cotransporter, NBCe2 (slc4a5) is expressed. The electroneutral Na⁺:HCO₃⁻ cotransporter, NBCn1 (slc4a7), has been located in both membranes. In addition to the bicarbonate transporters, the Na⁺/H⁺ exchanger, NHE1 (slc9a1), is located in the luminal membrane of the CPE. Genetically modified mice targeting slc4a2, slc4a5, slc4a7, slc4a10, and slc9a1 have been generated. Deletion of slc4a5, 7 or 10, or slc9a1 has numerous impacts on CP function and structure in these mice. Removal of the transporters affects brain ventricle size (slc4a5 and slc4a10) and intracellular pH regulation (slc4a7 and slc4a10). In some instances, removal of the proteins from the CPE (slc4a5, 7, and 10) causes changes in abundance and localization of non-target transporters known to be involved in pH regulation and CSF secretion. The focus of this review is to combine the insights gathered from these knockout mice to highlight the impact of slc4 gene deletion on the CSF production and intracellular pH regulation resulting from the deletion of slc4a5, 7 and 10, and slc9a1. Furthermore, the review contains a comparison of the described human mutations of these genes to the findings in the knockout studies. Finally, the future perspective of utilizing these proteins as potential targets for the treatment of CSF disorders will be discussed.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Increased biological response to 1,25(OH)(2)D(3) in genetic hypercalciuric stone-forming rats

Genetic hypercalciuric stone-forming (GHS) rats, bred to maximize urine (U) calcium (Ca) excretion, have increased intestinal Ca absorption and bone Ca resorption and reduced renal Ca reabsorption, leading to increased UCa compared with the Sprague-Dawley (SD) rats. GHS rats have increased vitamin D receptors (VDR) at each of these sites, with normal levels of 1,25(OH)(2)D(3) (1,25D), indicating that their VDR is undersaturated with 1,25D. We tested the hypothesis that 1,25D would induce a greater increase in UCa in GHS rats by feeding both strains ample Ca and injecting 1,25D (25 ng · 100 g body wt(-1) · day(-1)) or vehicle for 16 days. With 1,25D, UCa in SD increased from 1.7 ± 0.3 mg/day to 24.4 ± 1.2 (Δ = 22.4 ± 1.5) and increased more in GHS from 10.5 ± 0.7 to 41.9 ± 0.7 (Δ = 29.8 ± 1.8; P = 0.003). To determine the mechanism of the greater increase in UCa in GHS rats, we measured kidney RNA expression of components of renal Ca transport. Expression of transient receptor potential vanilloid (TRPV)5 and calbindin D(28K) were increased similarly in SD + 1,25D and GHS + 1,25D. The Na(+)/Ca(2+) exchanger (NCX1) was increased in GHS + 1,25D. Klotho was decreased in SD + 1,25D and GHS + 1,25D. TRPV6 was increased in SD + 1,25D and increased further in GHS + 1,25D. Claudin 14, 16, and 19, Na/K/2Cl transporter (NKCC2), and secretory K channel (ROMK) did not differ between SD + 1,25D and GHS + 1,25D. Increased UCa with 1,25D in GHS exceeded that of SD, indicating that the increased VDR in GHS induces a greater biological response. This increase in UCa, which must come from the intestine and/or bone, must exceed any effect of 1,25D on TRPV6 or NCX1-mediated renal Ca reabsorption.

Renal ammonia metabolism and transport

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4(+) and 2 HCO3(-) for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3(-)-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4(+) trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4(+)-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K(+), and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.

Contribution of Na+,HCO3(-)-cotransport to cellular pH control in human breast cancer: a role for the breast cancer susceptibility locus NBCn1 (SLC4A7)

Genome-wide association studies recently linked the locus for Na(+),HCO(3)(-)-cotransporter NBCn1 (SLC4A7) to breast cancer susceptibility, yet functional insights have been lacking. To determine whether NBCn1, by transporting HCO(3)(-) into cells, may dispose of acid produced during high metabolic activity, we studied the expression of NBCn1 and the functional impact of Na(+),HCO(3)(-)-cotransport in human breast cancer. We found that the plasmalemmal density of NBCn1 was 20-30% higher in primary breast carcinomas and metastases compared to matched normal breast tissue. The increase in NBCn1 density was similar in magnitude to that observed for Na(+)/H(+)-exchanger NHE1 (SLC9A1), a transporter previously implicated in cell migration, proliferation and malignancy. In primary breast carcinomas, the apparent molecular weight for NBCn1 was increased compared to normal tissue. Using pH-sensitive fluorophores, we showed that Na(+),HCO(3)(-)-cotransport is the predominant mechanism of acid extrusion and is inhibited 34 ± 9% by 200 μM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in human primary breast carcinomas. At intracellular pH (pH(i) ) levels >6.6, CO(2)/HCO(3)(-)-dependent mechanisms accounted for >90% of total net acid extrusion. Na(+)/H(+)-exchange activity was prominent only at lower pH(i) -values. Furthermore, steady-state pH(i) was 0.35 ± 0.06 units lower in the absence than in the presence of CO(2)/HCO(3)(-). In conclusion, expression of NBCn1 is upregulated in human primary breast carcinomas and metastases compared to normal breast tissue. Na(+),HCO(3)(-)-cotransport is a major determinant of pH(i) in breast cancer and the modest DIDS-sensitivity is consistent with NBCn1 being predominantly responsible. Hence, our results suggest a major pathophysiological role for NBCn1 that may be clinically relevant.

Intracellular pH in the resistance vasculature: regulation and functional implications

Net acid extrusion from vascular smooth muscle (VSMCs) and endothelial cells (ECs) in the wall of resistance arteries is mediated by the Na(+),HCO(3)(-) cotransporter NBCn1 (SLC4A7) and the Na(+)/H(+) exchanger NHE1 (SLC9A1) and is essential for intracellular pH (pH(i)) control. Experimental evidence suggests that the pH(i) of VSMCs and ECs modulates both vasocontractile and vasodilatory functions in resistance arteries with implications for blood pressure regulation. The connection between disturbed pH(i) and altered cardiovascular function has been substantiated by a genome-wide association study showing a link between NBCn1 and human hypertension. On this basis, we here review the current evidence regarding (a) molecular mechanisms involved in pH(i) control in VSMCs and ECs of resistance arteries at rest and during contractions, (b) implications of disturbed pH(i) for resistance artery function, and (c) involvement of disturbed pH(i) in the pathogenesis of vascular disease. The current evidence clearly implies that pH(i) of VSMCs and ECs modulates vascular function and suggests that disturbed pH(i) either consequent to disturbed regulation or due to metabolic challenges needs to be taken into consideration as a mechanistic component of artery dysfunction and disturbed blood pressure regulation.

More actors in ammonia absorption by the thick ascending limb

This review will briefly summarize current knowledge on the basolateral ammonia transport mechanisms in the thick ascending limb (TAL) of the loop of Henle. This segment transports ammonia against a concentration gradient and is responsible for the accumulation of ammonia in the medullary interstitium, which, in turn, favors ammonia secretion across the collecting duct. Experimental data indicate that the sodium/hydrogen ion exchanger isoform 4 (NHE4; Scl9a4) is a sodium/ammonia exchanger and plays a major role in this process. Disruption of murine NHE4 leads to metabolic acidosis with inappropriate urinary ammonia excretion and decreases the ability of the TAL to absorb ammonia and to build the corticopapillary ammonia gradient. However, NHE4 does not account for the entirety of ammonia absorption by the TAL, indicating that, at least, one more transporter is involved.

Sodium-bicarbonate cotransporter NBCn1/Slc4a7 inhibits NH4Cl-mediated inward current in Xenopus oocytes

The electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1 (SLC4A7) contributes to intracellular pH maintenance and transepithelial HCO(3)(-) movement. In this study, we expressed NBCn1 in Xenopus oocytes and examined the effect of NBCn1 on oocyte NH(4)(+) transport by analysing changes in membrane potential, current and intracellular pH mediated by NH(4)Cl. In the presence of HCO(3)(-)/CO(2), applying NH(4)Cl (20 mm) produced intracellular acidification of oocytes. The acidification was faster in oocytes expressing NBCn1 than in control oocytes injected with water; however, NH(4)Cl-mediated membrane depolarization was smaller in oocytes expressing NBCn1. In HCO(3)(-)/CO(2)-free solution, NH(4)Cl produced a smaller inward current in NBCn1-expressing oocytes (56% inhibition by 20 mm NH(4)Cl, measured at --60 mV), while minimally affecting intracellular acidification. The inhibition of the current by NBCn1 was unaffected when BaCl(2) replaced KCl. Current-voltage relationships showed a positive and nearly linear relationship between NH(4)Cl-mediated current and voltage, which was markedly reduced by NBCn1. Large basal currents (before NH(4)Cl exposure) were produced in NBCn1-expressing oocytes owing to the previously characterized channel-like activity of NBCn1. Inhibiting this channel-like activity by Na(+) removal abolished the inhibitory effect of NBCn1 on NH(4)Cl-mediated currents. The currents were progressively reduced over 72-120 h after NBCn1 cRNA injection, during which the channel-like activity was high. These results indicate that NBCn1 stimulates NH(4)(+) transport by its Na(+)-HCO(3)(-) cotransport activity, while reducing NH(4)(+) conductance by its channel-like activity.

Role of NH3 and NH4+ transporters in renal acid-base transport

Renal ammonia excretion is the predominant component of renal net acid excretion. The majority of ammonia excretion is produced in the kidney and then undergoes regulated transport in a number of renal epithelial segments. Recent findings have substantially altered our understanding of renal ammonia transport. In particular, the classic model of passive, diffusive NH3 movement coupled with NH4+ "trapping" is being replaced by a model in which specific proteins mediate regulated transport of NH3 and NH4+ across plasma membranes. In the proximal tubule, the apical Na+/H+ exchanger, NHE-3, is a major mechanism of preferential NH4+ secretion. In the thick ascending limb of Henle's loop, the apical Na+-K+-2Cl- cotransporter, NKCC2, is a major contributor to ammonia reabsorption and the basolateral Na+/H+ exchanger, NHE-4, appears to be important for basolateral NH4+ exit. The collecting duct is a major site for renal ammonia secretion, involving parallel H+ secretion and NH3 secretion. The Rhesus glycoproteins, Rh B Glycoprotein (Rhbg) and Rh C Glycoprotein (Rhcg), are recently recognized ammonia transporters in the distal tubule and collecting duct. Rhcg is present in both the apical and basolateral plasma membrane, is expressed in parallel with renal ammonia excretion, and mediates a critical role in renal ammonia excretion and collecting duct ammonia transport. Rhbg is expressed specifically in the basolateral plasma membrane, and its role in renal acid-base homeostasis is controversial. In the inner medullary collecting duct (IMCD), basolateral Na+-K+-ATPase enables active basolateral NH4+ uptake. In addition to these proteins, several other proteins also contribute to renal NH3/NH4+ transport. The role and mechanisms of these proteins are discussed in depth in this review.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

Na-coupled bicarbonate transporters of the solute carrier 4 family in the nervous system: function, localization, and relevance to neurologic function

Many cellular processes including neuronal activity are sensitive to changes in intracellular and/or extracellular pH-both of which are regulated by acid-base transporter activity. HCO(3)(-)-dependent transporters are particularly potent regulators of intracellular pH in neurons and astrocytes, and also contribute to the composition of the cerebrospinal fluid (CSF). The molecular physiology of HCO(3)(-) transporters has advanced considerably over the past ∼14 years as investigators have cloned and characterized the function and localization of many Na-Coupled Bicarbonate Transporters of the solute carrier 4 (Slc4) family (NCBTs). In this review, we provide an updated overview of the function and localization of NCBTs in the nervous system. Multiple NCBTs are expressed in neurons and astrocytes in various brain regions, as well as in epithelial cells of the choroid plexus. Characteristics of human patients with SLC4 gene mutations/deletions and results from recent studies on mice with Slc4 gene disruptions highlight the functional importance of NCBTs in neuronal activity, somatosensory function, and CSF production. Furthermore, energy-deficient states (e.g., hypoxia and ischemia) lead to altered expression and activity of NCBTs. Thus, recent studies expand our understanding of the role of NCBTs in regulating the pH and ionic composition of the nervous system that can modulate neuronal activity.

Sodium-bicarbonate cotransporter NBCn1 in the kidney medullary thick ascending limb cell line is upregulated under acidic conditions and enhances ammonium transport

In this study, we examined the effect of bicarbonate transporters on ammonium/ammonia uptake in the medullary thick ascending limb cell line ST-1. Cells were treated with 1 mm ouabain and 0.2 mM bumetanide to minimize carrier-mediated NH(4)(+) transport, and the intracellular accumulation of (14)C-methylammonium/methylammonia ((14)C-MA) was determined. In CO(2)/HCO(3)(-)-free solution, cells at normal pH briefly accumulated (14)C-MA over 7 min and reached a plateau. In CO(2)/HCO(3)(-) solution, however, cells markedly accumulated (14)C-MA over the experimental period of 30 min. This CO(2)/HCO(3)(-)-dependent accumulation was reduced by the bicarbonate transporter blocker, 4,4-diisothiocyanatostilbene-2,2-disulfonate (DIDS; 0.5 mM). Replacing Cl(-) with gluconate reduced the accumulation, but the reduction was more substantial in the presence of DIDS. Incubation of cells at pH 6.8 (adjusted with NaHCO(3) in 5% CO(2)) for 24 h lowered the mean steady-state intracellular pH to 6.96, significantly lower than 7.28 for control cells. The presence of DIDS reduced (14)C-MA accumulation in control conditions but had no effect after acidic incubation. Immunoblotting showed that NBCn1 was upregulated after acidic incubation and in NH(4)Cl-containing media. The Cl(-)-HCO(3)(-) exchanger AE2 was present, but its expression remained unaffected by acidic incubation. Expressed in Xenopus oocytes, NBCn1 increased carrier-mediated (14)C-MA transport, which was abolished by replacing Na(+). Two-electrode voltage clamp of oocytes exhibited negligible current after NH(4)Cl application. These results suggest that DIDS-sensitive HCO(3)(-) extrusion normally governs NH(4)(+)/NH(3) uptake in the medullary thick ascending limb cells. We propose that, in acidic conditions, DIDS-sensitive HCO(3)(-) extrusion is inactivated, while NBCn1 is upregulated to stimulate NH(4)(+) transport.

NHE4 is critical for the renal handling of ammonia in rodents

Ammonia absorption by the medullary thick ascending limb of Henle's loop (MTALH) is thought to be a critical step in renal ammonia handling and excretion in urine, in which it is the main acid component. Basolateral Na+/H+ exchangers have been proposed to play a role in ammonia efflux out of MTALH cells, which express 2 exchanger isoforms: Na+/H+ exchanger 1 (NHE1) and NHE4. Here, we investigated the role of NHE4 in urinary acid excretion and found that NHE4-/- mice exhibited compensated hyperchloremic metabolic acidosis, together with inappropriate urinary net acid excretion. When challenged with a 7-day HCl load, NHE4-/- mice were unable to increase their urinary ammonium and net acid excretion and displayed reduced ammonium medulla content compared with wild-type littermates. Both pharmacologic inhibition and genetic disruption of NHE4 caused a marked decrease in ammonia absorption by the MTALH. Finally, dietary induction of metabolic acidosis increased NHE4 mRNA expression in mouse MTALH cells and enhanced renal NHE4 activity in rats, as measured by in vitro microperfusion of MTALH. We therefore conclude that ammonia absorption by the MTALH requires the presence of NHE4 and that lack of NHE4 reduces the ability of MTALH epithelial cells to create the cortico-papillary gradient of NH3/NH4+ needed to excrete an acid load, contributing to systemic metabolic acidosis.

Neuronal expression of sodium/bicarbonate cotransporter NBCn1 (SLC4A7) and its response to chronic metabolic acidosis

The sodium-bicarbonate cotransporter NBCn1 (SLC4A7) is an acid-base transporter that normally moves Na(+) and HCO(3)(-) into the cell. This membrane protein is sensitive to cellular and systemic pH changes. We examined NBCn1 expression and localization in the brain and its response to chronic metabolic acidosis. Two new NBCn1 antibodies were generated by immunizing a rabbit and a guinea pig. The antibodies stained neurons in a variety of rat brain regions, including hippocampal pyramidal neurons, dentate gyrus granular neurons, posterior cortical neurons, and cerebellar Purkinje neurons. Choroid plexus epithelia were also stained. Double immunofluorescence labeling showed that NBCn1 and the postsynaptic density protein PSD-95 were found in the same hippocampal CA3 neurons and partially colocalized in dendrites. PSD-95 was pulled down from rat brain lysates with the GST/NBCn1 fusion protein and was also coimmunoprecipitated with NBCn1. Chronic metabolic acidosis was induced by feeding rats with normal chow or 0.4 M HCl-containing chow for 7 days. Real-time PCR and immunoblot showed upregulation of NBCn1 mRNA and protein in the hippocampus of acidotic rats. NBCn1 immunostaining was enhanced in CA3 neurons, posterior cortical neurons, and cerebellar granular cells. Intraperitoneal administration of N-methyl-d-aspartate caused neuronal death determined by caspase-3 activity, and this effect was more severe in acidotic rats. Administering N-methyl-d-aspartate also inhibited NBCn1 upregulation in acidotic rats. We conclude that NBCn1 in neurons is upregulated by chronic acid loads, and this upregulation is associated with glutamate excitotoxicity.

Degradation of hydroxyapatite in vivo and in vitro requires osteoclastic sodium-bicarbonate co-transporter NBCn1

Dissolution of the inorganic bone matrix releases not only calcium and phosphate ions, but also bicarbonate. Electroneutral sodium-bicarbonate co-transporter (NBCn1) is expressed in inactive osteoclasts, but its physiological role in bone resorption has remained unknown. We show here that NBCn1, encoded by the SLC4A7 gene, is directly involved in bone resorption. NBCn1 protein was specifically found at the bone-facing ruffled border areas, and metabolic acidosis increased NBCn1 expression in rats in vivo. In human hematopoietic stem cell cultures, NBCn1 mRNA expression was observed only after formation of resorbing osteoclasts. To further confirm the critical role of NBCn1 during bone resorption, human hematopoietic stem cells were transduced with SLC4A7 shRNA lentiviral particles. Downregulation of NBCn1 both on mRNA and protein level by lentiviral shRNAs significantly inhibited bone resorption and increased intracellular acidification in osteoclasts. The lentiviral particles did not impair osteoclast survival, or differentiation of the hematopoietic or mesenchymal precursor cells into osteoclasts or osteoblasts in vitro. Inhibition of NBCn1 activity may thus provide a new way to regulate osteoclast activity during pathological bone resorption.

PKC{alpha}{beta}{gamma}- and PKC{delta}-dependent endocytosis of NBCe1-A and NBCe1-B in salivary parotid acinar cells

We examined membrane trafficking of NBCe1-A and NBCe1-B variants of the electrogenic Na(+)-HCO(3)(-) cotransporter (NBCe1) encoded by the SLC4A4 gene, using confocal fluorescent microscopy in rat parotid acinar cells (ParC5 and ParC10). We showed that yellow fluorescent protein (YFP)-tagged NBCe1-A and green fluorescent protein (GFP)-tagged NBCe1-B are colocalized with E-cadherin in the basolateral membrane (BLM) but not with the apical membrane marker zona occludens 1 (ZO-1). We inhibited constitutive recycling with monensin and W13 and detected that NBCe1-A and NBCe1-B accumulated in vesicles marked with the early endosomal marker early endosome antigen-1 (EEA1), with a parallel loss from the BLM. We observed that NBCe1-A and NBCe1-B undergo massive carbachol (CCh)-stimulated redistribution from the BLM into early endosomes. We showed that internalization of NBCe1-A and NBCe1-B was prevented by the general PKC inhibitor GF-109203X, the PKCalphabetagamma-specific inhibitor Gö-6976, and the PKCdelta-specific inhibitor rottlerin. We verified the involvement of PKCdelta by blocking CCh-induced internalization of NBCe1-A-cyan fluorescent protein (CFP) in cells transfected with dominant-negative kinase-dead (Lys376Arg) PKCdelta-GFP. Our data suggest that NBCe1-A and NBCe1-B undergo constitutive and CCh-stimulated endocytosis regulated by conventional PKCs (PKCalphabetagamma) and by novel PKCdelta in rat epithelial cells. To help develop a more complete model of the role of NBCe1 in parotid acinar cells we also investigated the initial phase of the secretory response to cholinergic agonist. In an Ussing chamber study we showed that inhibition of basolateral NBCe1 with 5-chloro-2,3-dihydro-3-(hydroxy-2-thienylmethylene)-2-oxo-1H-indole-1-carboxamide (tenidap) significantly decreases an initial phase of luminal anion secretion measured as a transient short-circuit current (I(sc)) across ParC10 cell monolayers. Using trafficking and functional data we propose a model that describes a physiological role of NBC in salivary acinar cell secretion.

Modular structure of sodium-coupled bicarbonate transporters

Mammalian genomes contain 10 SLC4 genes that, between them, encode three Cl-HCO(3) exchangers, five Na(+)-coupled HCO(3) transporters (NCBTs), one reported borate transporter, and what is reported to be a fourth Cl-HCO(3) exchanger. The NCBTs are expressed throughout the body and play important roles in maintaining intracellular and whole-body pH, as well as contributing to transepithelial transport processes. The importance of NCBTs is underscored by the genetic association of dysfunctional NCBT genes with blindness, deafness, epilepsy, hypertension and metal retardation. Key to understanding the action and regulation of NCBTs is an appreciation of the diversity of NCBT gene products. The transmembrane domains of human NCBT paralogs are 50-84% identical to each other at the amino acid level, and are capable of a diverse range of actions, including electrogenic Na/HCO(3) cotransport (i.e. NBCe1 and NBCe2) and electroneutral Na/HCO(3) cotransport (i.e. NBCn1 and NBCn2), as well as Na(+)-dependent Cl-HCO(3) exchange (i.e. NDCBE). Furthermore, by the use of alternative promoters and alternative-splicing events, individual SLC4 genes have the potential to generate multiple splice variants (as many as 16 in the case of NBCn1), each of which could have unique temporal and spatial patterns of distribution, unitary transporter activity (i.e. flux mediated by one molecule), array of protein-binding partners, and complement of regulatory stimuli. In the first section of this review, we summarize our present knowledge of the function and distribution of mammalian NCBTs and their multiple variants. In the second section of this review we consider the molecular consequences of NCBT variation.

Age-dependent renal expression of acid-base transporters in neonatal ureter obstruction

Congenital obstructive nephropathy accounts for a major proportion of renal insufficiency in infancy and childhood. In an earlier investigation we demonstrated that bilateral complete ureteral obstruction (BUO) in rats is associated with inadequate urinary acidification [Am J Physiol Renal Physiol. 295(2):F497-506, 2008]. The aim of the study reported here was to determine whether this defect is also associated with unilateral ureteral obstruction (UUO), which is clinically more common than BUO. The time-course of the changes in protein expression levels of major renal acid-base transporters was examined at 7 and 14 weeks in rats with neonatally induced partial unilateral ureteral obstruction (PUUO), which was performed within the first 48 h of life. We observed that protein expression of the renal acid-base transporters NHE3, NBC1, NBCn1, pendrin and Na(+)-K(+)-ATPase was increased in both obstructed and non-obstructed kidneys 7 weeks after the induction of neonatal PUUO. This was confirmed by immunocytochemistry. In contrast, 14 weeks after the induction of PUUO, there was a significant downregulation of the renal acid-base transporters NBC1, NBCn1 and Na(+)-K(+)-ATPase in the obstructed kidneys. These time/age-dependent changes in protein expression were associated with parallel changes in renal function resulting in urine acidification in response to exogenous acid loading. In conclusion, these results show that downregulation of protein expression is a time/age-dependent response to PUUO, which could contribute to the decreased net acid excretion and development of metabolic acidosis in neonatal rats with PUUO.

Calcium-sensing receptor stimulates luminal K+-dependent H+ excretion in medullary thick ascending limbs of Henle's loop of mouse kidney

The calcium-sensing receptor (CaSR) is known well as a sensor of extracellular calcium for regulating parathyroid hormone secretion. CaSR is located along all nephron segments in the kidney. While hypercalcemia strongly enhances urinary acidification, the relationship between CaSR and acid-base metabolism in the kidney is still uncertain. In the present study, we examined whether CaSR activation caused acid secretion in the medullary thick ascending limb (mTAL), which is one of the major nephron segments involved in both mineral and acid-base regulation. The effects of a potent calcimimetic neomycin (Neo) on intracellular pH (pHi) were analyzed in the in vitro miroperfused mouse mTALs. The mTALs were incubated with 2,7-bis-(2-carboxyethyl)-5(6)-carboxyfluoresceine-acetoxymethylester (BCECF-AM) for microfluorescent pHi measurements. In HCO(3)(-)/CO(2)-buffered solution, the steady-state pHi was 7.17 +/- 0.01 (n = 19). Basolateral Neo at 0.4 mM in basolateral side significantly alkalinized the mTAL cells to 7.28 +/- 0.02 (n = 19), while Neo in the lumen had no effect on pHi. Neo in the basolateral side alkalinized the mTALs in the absence of ambient Na(+) and the presence of H(+)-ATPase inhibitor bafilomycin in the lumen, indicating that the effect of Neo is unrelated to Na(+)-dependent acid-base transporters such as Na(+)-H(+) exchangers and Na(+)-HCO(3)(-) cotransporter, or to luminal H(+)-ATPase. In contrast, the effect of Neo on pHi was inhibited by K(+) removal or treatment with specific H(+)-K(+)-ATPase (HKa) inhibitors, ouabain and Sch-28080, in the lumen. Our results suggest that hypercalcemia induces urinary acidification partly by stimulating luminal K(+)-dependent H(+)-excretion via CaSR in mouse mTALs.

Ureter obstruction alters expression of renal acid-base transport proteins in rat kidney

Urinary tract obstruction impairs renal function and is often associated with a urinary acidification defect caused by diminished net H+ secretion and/or HCO3- reabsorption. To identify the molecular mechanisms of these defects, protein expression of key acid-base transporters were examined along the renal nephron and collecting duct of kidneys from rats subjected to 24-h bilateral ureteral obstruction (BUO), 4 days after release of BUO (BUO-R), or BUO-R rats with experimentally induced metabolic acidosis (BUO-A). Semiquantitative immunoblotting revealed that BUO caused a significant reduction in the expression of the type 3 Na+/H+ exchanger (NHE3) in the cortex (21 +/- 4%), electrogenic Na+/HCO3- cotransporter (NBC1; 71 +/- 5%), type 1 bumetanide-sensitive Na+-K+-2Cl- cotransporter (NKCC2; 3 +/- 1%), electroneutral Na+/HCO3- cotransporter (NBCn1; 46 +/- 7%), and anion exchanger (pendrin; 87 +/- 2%). The expression of H+-ATPase increased in the inner medullary collecting duct (152 +/- 13%). These changes were confirmed by immunocytochemistry. In BUO-R rats, there was a persistent downregulation of all the acid-base transporters including H+-ATPase. Two days of NH4Cl loading reduced plasma pH and HCO3- levels in BUO-A rats. The results demonstrate that the expression of multiple renal acid-base transporters are markedly altered in response to BUO, which may be responsible for development of metabolic acidosis and contribute to the urinary acidification defect after release of the obstruction.

Characterization of human SLC4A10 as an electroneutral Na/HCO3 cotransporter (NBCn2) with Cl- self-exchange activity

The SLC4A10 gene product, commonly known as NCBE, is highly expressed in rodent brain and has been characterized by others as a Na(+)-driven Cl-HCO(3) exchanger. However, some of the earlier data are not consistent with Na(+)-driven Cl-HCO(3) exchange activity. In the present study, northern blot analysis showed that, also in humans, NCBE transcripts are predominantly expressed in brain. In some human NCBE transcripts, splice cassettes A and/or B, originally reported in rats and mice, are spliced out. In brain cDNA, we found evidence of a unique partial splice of cassette B that is predicted to produce an NCBE protein with a novel C terminus containing a protein kinase C phosphorylation site. We used pH-sensitive microelectrodes to study the molecular physiology of human NCBE expressed in Xenopus oocytes. In agreement with others we found that NCBE mediates the 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid-sensitive, Na(+)-dependent transport of HCO(3)(-). For the first time, we demonstrated that this transport process is electroneutral. Using Cl(-)-sensitive microelectrodes positioned at the oocyte surface, we found that, unlike both human and squid Na(+)-driven Cl-HCO(3) exchangers, human NCBE does not normally couple the net influx of HCO(3)(-) to a net efflux of Cl(-). Moreover we found that that the (36)Cl efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na(+) and HCO(3)(-), actually represents a CO(2)/HCO(3)(-)-stimulated Cl(-) self-exchange not coupled to either Na(+) or net HCO(3)(-) transport. We propose to rename NCBE as the second electroneutral Na/HCO(3) cotransporter, NBCn2.