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NDT Advance Access published online on September 5, 2007
Nephrology Dialysis Transplantation, doi:10.1093/ndt/gfm486
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Regulation of the basolateral chloride/base exchangers AE1 and SLC26A7 in the kidney collecting duct in potassium depletion

Sharon Barone1, Hassane Amlal1, Minna Kujala2, Jie Xu1, Fiona Karet3, Ann Blanchard4, Juha Kere5 and Manoocher Soleimani1,6

1Department of Medicine, University of Cincinnati, Cincinnati, OH, USA, 2Medical Genetics, University of Helsinki, Finland, 3Department of Medical Genetics and Division of Renal Medicine, University of Cambridge, Cambridge, UK, 4Université Paris Descartes, faculté de médecine, Paris, France, 5Biosciences at Novum, Karolinska Institute, Huddinge Sweden and 6Research Services, Veterans Affairs Medical Center at Cincinnati, OH, USA

Correspondence and offprint requests to: M. Soleimani, MD, Department of Medicine, University of Cincinnati, 231 Albert Sabin Way, MSB G259, Cincinnati, OH 45267-0585, USA. Email: manoocher.soleimani{at}uc.edu



   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
In the present study, the effect of potassium depletion on the expression of acid-base transporters in the collecting duct was examined. Toward this end rats were fed a potassium-free diet for 3 weeks. Thereafter, the expression of the basolateral chloride/bicarbonate exchangers AE1 and SLC26A7 and the apical H+-ATPase was examined by northern hybridization, immunoblot analysis and immunofluorescence labelling. The mRNA expression of AE1 increased by a robust ~500% in the cortex and ~70% in the outer medulla, which translated into a huge increase in AE1 protein abundance in the cortex and a moderate increase in the outer medulla in K-depletion. The mRNA expression of SLC26A7 did not change significantly but its protein abundance showed a robust increase in the outer medulla. The expression of SLC26A7 remained undetected in the cortex in K-depleted rats. The post translational increase in SLC26A7 membrane abundance in potassium depletion was recapitulated in vitro using epitope-tagged SLC26A7. H+-ATPase displayed enhanced apical plasma membrane immunoreactivity in the OMCD in K-depletion. We suggest that the up-regulation of SLC26A7 and AE1 on the basolateral membrane of A-intercalated cells in the OMCD and CCD, respectively, along with H+-ATPase on the apical membrane, contributes to enhanced bicarbonate absorption in the collecting duct in K-depletion.

Keywords: bicarbonate absorption; kidney tubules; metabolic alkalosis



   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 
Selective potassium restriction causes hypokalaemia and increases serum bicarbonate concentration [1–3]. The increase in serum bicarbonate is associated with the excretion of excess acid into the lumen in CCD, OMCD and IMCD, and is referred to as ‘paradoxical aciduria’ [4,5]. The decrease in urine pH is not affected by amiloride, an inhibitor of the Na channel, suggesting that enhanced renal acid excretion in hypokalaemia is independent of distal nephron Na+ transport, and is therefore distinct from the mineralocorticoid-stimulated acid secretion [6].

Bicarbonate absorption in the OMCD is mediated via coordinated actions of acid extruders, H+-ATPase and H-K-ATPase, on the apical membrane, and bicarbonate transporters, AE1 and SLC26A7, on the basolateral membrane of A-intercalated cells [7–9]. In the CCD, SLC26A7 is absent but AE1 is abundant and is responsible for the bulk of bicarbonate absorption in the normal state [10,11]. H+-ATPase and colonic H+-K+-ATPase are up-regulated in potassium depletion in CCD and OMCD [12–16], however, little is known about the regulation of AE1 and SLC26A7 in this condition.

In the present studies, we examined the expression of H+-ATPase, AE1 and SLC26A7 in CCD and OMCD in potassium depletion. Further, the trafficking of SLC26A7 and AE1 was examined in potassium-depleted media in cultured kidney cells in vitro.



   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Acknowledgements
 References
 
Animal model
Potassium depletion was induced by placing male Sprague Dawley rats (150–200 gm) on potassium-free diet for 3 weeks. This is an established model of potassium depletion in rats and results in severe hypokalaemia [17–19]. The potassium-free diet (Catalog # 960189) and its corresponding control diet (Catalog # 905453) were purchased from MP Biomedicals (Solon, OH). Animals had free access to food and water. Potassium-depleted rats became polyuric as early as 24 h after placement on the experimental diet and remained polyuric for the duration of the studies, confirming previous studies from our laboratory as well as by other investigators [20]. Rats were euthanized with an overdose of sodium pentobarbital. All procedures were approved by the Institutional Animal Care and Use Committee at the University of Cincinnati.

RNA isolation and northern blot hybridization
Total cellular RNA was extracted from rat kidney zones (cortex and outer medulla) according to established methods [20], quantitated spectrophotometrically, and stored at –80°C. Total RNA samples (30 µg/lane) were fractionated on a 1.2% agarose-formaldehyde gel, transferred to Magna NT nylon membranes, cross-linked by UV light and baked. Hybridization was performed according to Church and Gilbert [21]. The membranes were washed, blotted dry and exposed to a Phosphor Imager screen (Molecular Dynamics, Sunnyvale, CA). A DNA fragment encompassing rat AE1 nucleotides 445-3038 or mouse Slc26a7 nucleotides 1478-2208 were used for northern hybridization. Each hybridization was performed on separate samples of 3 or 4 different animals.

Single and double immunofluorescence labelling
Rats were euthanized with an overdose of sodium pentobarbital and perfused through the left ventricle with 0.9% saline followed by cold 4% paraformaldehyde in 0.1 M sodium phosphate buffer (pH 7.4). Kidneys were removed, cut in tissue blocks, and fixed in formaldehyde solution overnight at 4°C. Single and double immunofluorescence labelling were performed as described [22]. Primary antibodies were diluted 1:60 (SLC26A7), 1:100 (AE1), 1:500 (H+-ATPase) and 1:2000 (AQP2) in -0.3% Triton X-100–PBS solution and applied to sections overnight at room temperature [10,22–24]. Alexa Fluor 488 (green) or Alexa Fluor 568 (red) goat anti-rabbit antibody was used as secondary antibodies. Only slides processed at the same time with the same concentrations of the primary and secondary antibodies applied under identical protocols were compared. Acquisition parameters were kept constant between the control and K+-depletion samples to allow for comparisons of the intensity of fluorescent labelling. First, images were obtained from the control samples and then, using the same acquisition parameters, K–depletion samples were processed.

Immunoblot analysis of AE1 and SLC26A7
Microsomal membrane proteins were isolated from kidney outer medulla or cortex according to established protocols [23]. Proteins were resolved by SDS–PAGE (40 µg/lane) and transferred to nitrocellulose membrane. The membrane was blocked with 5% milk proteins, and then incubated for 6 h with antibodies against AE1 or SLC26A7. The secondary antibody was a donkey anti-rabbit IgG conjugated to horseradish peroxidase (Pierce). The results were visualized using chemiluminescence method (SuperSignal Substrate, Pierce) and captured on light-sensitive imaging film (Kodak). The abundance of AE1 or SLC26A7 in potassium depletion was quantitated as fold change vs control after adjustment for protein loading, as detected by blotting with beta actin antibody on the same membrane.

Antibodies
For SLC26A7 and AE1, specific antibodies were used as before [10,23]. H-ATPase antibody is a rabbit polyclonal antibody raised against {alpha}4-subunit of H-ATPase [24]. Aquaporin 2 was a monoclonal antibody raised in the laboratory of our collaborator, Dr Blanchard.

Construction of epitope-tagged SLC26A7 and AE1
The full-length SLC26A7 and AE1 cDNAs were generated by PCR, using the human full-length SLC26A7 cDNA (5280 bp, Genebank NM_052832 [GenBank] ) and rat AE1 cDNA (4375 bp, Genebank NM_011403 [GenBank] ). The primers for SLC26A7 were 5'-AAA ATG ACA GGA GCA AAG AG-3' and 5'-CTT ATT GTA GCA GAG GTC ATC-3'. The PCR fragment encodes 2092 bp that includes the entire open reading frame. The primers for rat AE1 were 5'-ATG ATG GAC CAG AGG AAC C-3' and 5'-CTA TCA CAG GCA TGG GCA C-3'. The PCR fragment encodes 2550 bp that includes the entire open reading frame. The SLC26A7 PCR product was subcloned into the pcDNA3.1/NT-GFP-TOPO vector. The AE1 PCR product was subcloned into pcDNA3.1/CT-GFP-TOPO vector. Both vectors were from Invitrogen (Carlsbad, CA).

Transient expression of epitope-tagged SLC26A7 or AE1 in MDCK cells
MDCK cells were grown on glass coverslips, transiently transfected with the epitope-tagged SLC26A7 or AE1 and studied 48 h later according to established methods [25]. Briefly, cells were plated in 24-well plates and transfected with various cDNA fragments at 80% confluence using 0.8 g of DNA and 4 l of Lipofectamine 2000 (Invitrogen, Carlsbad, CA). All cells were co-labelled with Alexa Fluor 568 phalloidin (Molecular Probes, Inc.) as marker of apical membrane labelling. Cells were switched to either low potassium (0.5 or 2 mEq K+/l) or normal potassium media (4 mEq K+/l) 32 h after transfection, and reaction was stopped 16 h later.

In separate studies, MDCK cells were grown on permeable polycarbonate membrane Transwell filters (Catalog No. 3401, Corning Inc, Corning, NY) at a density of ~105 cells/cm2. Cells achieved confluence within 5–7 days and were then transiently transfected from the apical surface with the GFP-SLC26A7 construct or AE1 and studied 48 h later.

Confocal microscopy and immunofluorescence labelling
MDCK cells were washed with PBS, and fixed with 3% formaldehyde in PBS. Afterward, cells were permeabilized with 0.1% TX-100 in PBS, and co-stained with Alexa Fluor 568 phalloidin(Molecular Probes, Inc.). Cells were mounted on glass slides in Fluoromount-G (Southern Biotechology Associates, Inc.). Images were taken on a Zeiss LSM510 confocal microscope. Both Z-line and Z-stack images were obtained using the LSM 5 Image software.

Materials
All chemicals were purchased from Sigma Chemical Co. (St Louis, MO). RadPrime DNA labelling kit was purchased from Gibco BRL, USA. mMACHINETM kit was purchased from Ambion (Austin, TX). The CT-GFP fusion expression kit, which contains the pcDNA3.1/CT-GFP TOPO vector, was purchased from Invitrogen (Carlsbad, CA). Alexa Fluor-conjugated secondary antibodies and Hoechst 33342 were purchased from Molecular Probes Inc. (Eugene, OR). Beta actin polyclonal antibodies were from Abcam (Cambridge, MA).

Statistical analyses
The experiments were performed with at least three separate samples for each manoeuvre. Values are expressed as arithmetic mean +/– standard error. Comparisons were done by using unpaired Student t-test or ANOVA and P < 0.05 was considered statistically significant. Microsoft Excel, ProStat and PSI-Plot were commercial software packages used for statistical analysis.



   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Acknowledgements
 References
 
In the first series of experiments, we examined the expression of AE1 and SLC26A7 in the CCD in potassium depletion. Figure 1A is a northern hybridization and shows that the expression of AE1 in the cortex is increased by ~5-fold in potassium depletion (n = 4, P < 0.001). Immunoblot analysis studies indicated that the abundance of AE1 increased by ~3.1-fold in the cortex in K+-depleted rats, when the protein loading was adjusted for the control protein ß actin (P < 0.01 vs control) (Figure 1B). Immunofluorescence labelling (Figure 1C) demonstrated that the expression of AE1 is abundantly increased in the CCD in potassium depletion.


Figure 1
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  		Fig. 1. Expression of AE1 in the cortex in potassium depletion. (A) Northern hybridization. (B) Immunoblot analysis. (C) Immunofluorescence labelling in the CCD.

 
Figure 2 examines the expression of SLC26A7 in the kidney cortex. Northern hybridization shows that SLC26A7 expression in the cortex is very low (Figure 2A). Figure 2B is an immunofluorescence labelling and indicates that SLC26A7 expression is not detected in the cortex in both control and potassium-depleted rats.


Figure 2
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  		Fig. 2. Expression of SLC26A7 in the cortex in potassium depletion. (A) Northern hybridization. (B) Immunofluorescence labelling.

 
In the next series of experiments, we examined the expression of AE1 and SLC26A7 in the OMCD in potassium depletion. Recent reports on the localization of SLC26A7 in the kidney have been conflicting, with several studies localizing it to the basolateral membrane of OMCD A-intercalated cells [10,23], and one report detecting it on the basolateral membrane of the thick ascending limb and sub-apical region in the proximal tubule [26]. The experiments in Figure 3 are double immunofluorescence labelling of AQP2 with either AE1 or SLC26A7 in normal rat in order to ascertain the localization of SLC26A7 in the kidney. Figure 3 (top panel) demonstrates the localization of SLC26A7 (PAT2) to the basolateral membrane of a subpopulation of cells (right panel) that are distinct from principal cells, which express the AQP2 on their apical membrane (left panel). The merged image is depicted in the middle panel and clearly shows the localization of SLC26A7 on the basolateral membrane of OMCD A-intercalated cells. Figure 3 (bottom panel) demonstrates the localization of AE1 to the basolateral membrane of cells in the OMCD (right panel) distinct from principal cells, which are identified by AQP2 labelling on their apical membrane (left panel). The merged image in the middle panel clearly demonstrates the distinct localization of AE1 and AQP2 in OMCD. Taken together, these studies confirm the co-localization of SLC26A7 and AE1 on the basolateral membrane of A-intercalated cells in OMCD.


Figure 3
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  		Fig. 3. Double immunofluorescence labelling of SLC26A7 or AE1 with AQP2. Top panel: right: SLC26A7 (green); left: AQP2 (red); middle: merged image. Bottom panel: right: AE1 (green); left: AQP2 (red); middle: merged image.

 
The expression of AE1 in the OMCD was next examined in potassium depletion. Figure 4A is a northern hybridization and indicates that the expression of AE1 is increased by ~70% in the outer medulla in potassium depletion (P < 0.05 vs control). Western blotting indicates the up-regulation of AE1 in the outer medulla, which was estimated at ~80% when the intensity of the AE1-specific band was adjusted for the protein loading (Figure 4B, P < 0.05 vs control). Immunofluorescence labelling (Figure 4C) demonstrates significant enhancement in AE1 immunoreactivity in the OMCD in potassium depletion.


Figure 4
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  		Fig. 4. Expression of AE1 in the outer medulla in potassium depletion. (A) Northern hybridization. (B) Immunoblot analysis. (C) Immunofluorescence labelling in the OMCD.

 
Next, the expression of SLC26A7 (PAT2) in OMCD was examined in potassium depletion. Figure 5A is a northern hybridization and indicates that the expression of SLC26A7 was not significantly altered in the outer medulla in potassium depletion (P > 0.05 vs control). Intriguingly, western blotting indicates that the abundance of SLC26A7 in the outer medulla increased by ~280% in potassium depletion when the loading in each lane was normalized to ß-actin level (Figure 5B, P < 0.05 vs control). Immunofluorescence labelling (Figure 5C) confirms the results of western blotting and demonstrates enhanced immunoreactivity of SLC26A7 in the OMCD in potassium depletion.


Figure 5
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  		Fig. 5. Expression of SLC26A7 in the outer medulla in potassium depletion. (A) Northern hybridization. (B) Immunoblot analysis. (C) Immunofluorescence labelling in the OMCD.

 
The results of expression studies in Figure 5 demonstrate that the membrane abundance of SLC26A7 is increased while its mRNA levels remain unchanged in potassium depletion. To ascertain the role of potassium depletion in post-translational modification of SLC26A7, the trafficking of SLC26A7 in cultured kidney cells was examined in vitro in low potassium media and compared to AE1. Accordingly, epitope-tagged SLC26A7 and AE1 cDNA were transiently expressed in MDCK cells. The cells were exposed to either a low potassium (0.5 mEq/l) or normal potassium (4 mEq/l) media 32 h after transfection. Cells were fixed after 16 h in experimental media and examined by confocal microscopy. Expression of GFP alone (no AE1 or SLC26A7 insert) was detected in the cytoplasm (data not shown). Figure 6A shows the expression of epitope-tagged AE1 in normal potassium (left panel) or low potassium media (right panel) for 16 h. As shown, GFP-tagged AE1 is predominantly detected in the plasma membrane in normal potassium and remains in the membrane in low potassium media. Figure 6B is a representative experiment depicting the expression of the epitope-tagged SLC26A7 in normal potassium (left panel) or low potassium media (right panel) for 16 h. As shown, GFP-tagged SLC26A7 is predominantly detected in the cytoplasm in normal potassium media (left panel) but is detected predominantly in the plasma membrane in low potassium media (right panel). The analysis of 30 transfected cells from three separate experiments demonstrated that >90% of SLC26A7 labelling was detected in the plasma membrane in low potassium media vs ~80% in the cytoplasm in normal potassium (P < 0.001). The abundance of AE1 in the membrane did not change in low potassium media.


Figure 6
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  		Fig. 6. Expression of epitope-tagged AE1 or SLC26A7 in normal or low potassium media in cultured cells. (A) GFP-AE1 expression in normal potassium (4 mEq/l) (left panel) or low potassium (0.5 mEq/l) (right panel). (B) GFP-SLC26A7 expression in normal potassium (4 mEq/l) (left panel) or low potassium (0.5 mEq/l) media (right panel).

 
To ascertain the role of H+-ATPase in acid secretion and bicarbonate absorption in OMCD, immunofluorescence labelling was performed. As shown in Figure 7, H+-ATPase expression shows discrete apical membrane localization in potassium depletion (bottom panel) vs cytoplasmic vesicular distribution pattern in control animals (top panel).


Figure 7
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  		Fig. 7. Expression of H+-ATPase in the outer medulla in potassium depletion. H+-ATPase shows tight apical H+-ATPase staining (right panel) in the OMCD in K+-depletion (top panel) vs diffuse cytoplasmic staining in control rats (bottom panel).

 


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Acknowledgements
 References
 
Potassium depletion, irrespective of its aetiology, plays an important role in the pathogenesis of metabolic alkalosis. While the role of hypokalaemia in the majority of cases of chloride/volume-dependent alkalosis is mostly limited to sustaining the maintenance of alkalosis, studies demonstrating severe isolated potassium deficiency causing alkalosis in man and rat have suggested that hypokalaemia can both generate and maintain metabolic alkalosis [27,28]. The generation of metabolic alkalosis is likely due to the up-regulation of enzymes mediating ammoniagenesis, which can result in new bicarbonate generation [29,30]. The maintenance of the alkalosis is in a large part due to the up-regulation of bicarbonate absorbing transporters. Potassium depletion can affect the activity and/or expression of ion transporters in almost every nephron segment, including the proximal tubule, thick limb of Henle, distal convoluted tubule and the collecting duct [12–16,27,31]. Relevant to acid-base regulation, both proximal and distal tubules show adaptive regulation of acid and bicarbonate-dependent transporters.

Transmission and scanning electron microscopy following perfusion-fixation of kidneys revealed that increasing K+ depletion leads to both hyperplasia as well as hypertrophy in the intercalated cells in the medulla and cortex [32]. Further, there are early increases in the formation of cell membrane phospholipids which correlate with specific morphologic changes in different zones within the kidney [32]. The most dramatic changes in the kidney in potassium depletion occur in the OMCD cells where a tremendous hypertrophic response, along with alterations of ion and water transporters, occurs [3]. In rats placed on a K-deficient diet for 2 weeks, the expression of colonic H-K-ATPase (HKAc) was significantly up-regulated in OMCD [12,13,15,16]. Induction of HKAc is mostly evident in the medulla and specifically OMCD [12,13,15,16], but also shows a mild increase in the cortex [16]. This occurs as early as 72 h after the start of the K-deficient diet and precedes the onset of hypokalaemia, indicating that the signal is likely activated by intracellular K-depletion. Interestingly, the up-regulation of HKAc in K-depletion is significantly blocked in hypophysectomized rats, indicating that pituitary hormones could play an important role in HKAc adaptive regulation in K-depletion [16]. The activation of HKAc was associated with enhanced potassium-dependent ouabain-sensitive bicarbonate absorption in the OMCD in K-depleted rats [32].

In addition to HKAc, H+-ATPase shows adaptive regulation in the cortical collecting duct in K-depletion [14]. Immunocytochemical studies showed that K-depletion increased the number of intercalated cells with tight apical H+-ATPase staining and decreased the number of intercalated cells with diffuse or basolateral H+-ATPase staining [14]. The changes in the distribution pattern of H+-ATPase were associated with increased activity [14]. The current studies demonstrate that H+-ATPase show a similar pattern of up-regulation in the OMCD, with intercalated cells showing abundant apical immuno-reactivity in K-depleted rats vs diffuse staining in A-intercalated cells (Figure 4). Taken together, these results, along with published reports [12–16,33] indicate that both H+-ATPase and HKAc contribute to enhanced acid secretion in OMCD in K-depletion.

One intriguing aspect of the current studies is the adaptive regulation of the SLC26A7 Cl–/HCO3– exchanger in OMCD A-intercalated cells. SLC26A7 is predominantly located on the basolateral membrane of A-intercalated cells in OMCD, where it co-localizes with AE1 (Figure 3). Recent studies demonstrated differential regulation of AE1 and SLC26A7 in certain pathophysiological states. SLC26A7 was up-regulated in the OMCD in rats subjected to water deprivation and in BrattleBoro rats treated with dDAVP [23,34]; AE1 was down-regulated in both conditions. In the present studies, both AE1 and SLC26A7 showed enhanced expression in A-intercalated cells in the OMCD in K+ depletion, with AE1 showing mild up-regulation, whereas SLC26A7 demonstrated dramatic increase in their abundance, as examined by immunofluorescence labelling (Figures 4 and 5). Northern hybridization showed no alteration in SLC26A7 expression in the outer medulla in K-depletion (Figures 4 and 5) suggesting that the up-regulation of SLC26A7 in the OMCD in K+ depletion was predominantly at post-translational level. In support of this possibility, epitope-tagged SLC26A7 was detected predominantly in the cytoplasm in cultured kidney cells in normal potassium media but was detected predominantly in the membrane in potassium-depleted media (Figure 6). AE1, on the other hand, did not display any post-translational regulation in K-depletion (Figure 6). Taken together, we propose that SLC26A7 abundance in OMCD is increased by enhanced trafficking (and or decreased degradation) in K-depletion.

The expression of AE1 in the CCD was robustly enhanced in K-depletion (Figure 1). This is the most abundant increase in AE1 expression observed in any pathophysiological state and correlates very well with published reports on enhanced H+-ATPase apical staining and activity in A-intercalated cells in the CCD in K+-depletion. The increase in AE1-positive cells is consistent with a robust increase in AE1 expression in existing cells rather than an increase in the number of A-IC cells in K-depletion. Given the absence of SLC26A7 in the CCD, these results suggest enhanced acid secretion and bicarbonate absorption, mediated via H+-ATPase and AE1, respectively, in the CCD in K-depletion.

Hypokalaemia enhances bicarbonate reabsorption in the proximal tubule [35,36], via apical Na+/H+ exchanger NHE-3 and H+ ATPase and basolateral Na+:HCO3– co-transporter (NBC-1) acting in series [27,37–39]. Studies in luminal membrane vesicles isolated from kidney cortex indicated that apical NHE and basolateral NBC activity are increased in the proximal tubule of potassium-depleted rats [27]. These results were verified by western blot analysis which indicated enhanced cortical expression of NHE3 in kidneys of K+-depleted rats [40], and correlate with micropuncture studies demonstrating enhanced bicarbonate absorption in the proximal tubule in K+ depletion [35,36].

Enhanced reabsorption of bicarbonate in the kidney proximal tubule inversely correlates with Cl– reabsorption, as the sum of Cl– and HCO3– remains unchanged. Micropuncture studies in rat proximal tubules have demonstrated that Cl– reabsorption is decreased in potassium depletion [35], likely due to the down-regulation of apical Cl–/base exchanger. A preliminary study indicates that the apical chloride/base exchanger SLC26A6 (PAT1) is down-regulated in the kidney proximal tubule in K+ depletion (Barone S, Amlal H, Wang, Z and Soleimani M. J. Am. Soc. Neph. 17: 579A, 2006), suggesting that it could contribute to enhanced chloride excretion and the maintenance of metabolic alkalosis in hypokalaemia by generating hypochloraemia, which in turn can increase bicarbonate reabsorption in the proximal tubule.

The schematic diagram in Figure 8 summarizes adaptive regulation of acid-base transporters in OMCD A-intercalated cells in potassium depletion. According to this diagram, H+-ATPase is heavily up-regulated and gastric-like H-K-ATPase is replaced by colonic H-K-ATPase on the apical membrane, and the AE1 and SLC26A7 are up-regulated on the basolateral membrane of A-intercalated cells in the OMCD. Taken together, this unique adaptive regulation of acid-base transporters contributes to enhanced bicarbonate absorption in the collecting duct in K-depletion.


Figure 8
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  		Fig. 8. Schematic diagram illustrating adaptive regulation of acid-base transporters in the outer medullary collecting duct in potassium depletion. The basolateral AE1 and SLC26A7 are up-regulated and contribute to enhanced bicarbonate absorption in K+-depletion (right panel). On the apical membrane H+-ATPase and colonic H-K-ATPase contribute to enhanced acid secretion and intracellular bicarbonate generation in K+- depletion (right panel).

 



   Acknowledgements
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
References
 
This study was supported by National Institute of Health Grant DK 62809, a Merit Review Grant, a Cystic Fibrosis Foundation grant and grants from Dialysis Clinic Incorporated (to M.S.).

Conflict of interest statement. None declared.



   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Acknowledgements
 References
 

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Received for publication: 27. 3.07
Accepted in revised form: 26. 6.07


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