Regulation of intracellular pH in the rabbit cortical collecting tubule

The study of intercalated cells using ex vivo techniques: primary cell culture, cell lines, kidney slices, and organoids

This review summarizes methods to study kidney intercalated cell (IC) function ex vivo. While important for acid-base homeostasis, IC dysfunction is often not recognized clinically until it becomes severe. The advantage of using ex vivo techniques is that they allow for the differential evaluation of IC function in controlled environments. Although in vitro kidney tubular perfusion is a classical ex vivo technique to study IC, here we concentrate on primary cell cultures, immortalized cell lines, and ex vivo kidney slices. Ex vivo techniques are useful in evaluating IC signaling pathways that allow rapid responses to extracellular changes in pH, CO2, and bicarbonate (HCO3-). However, these methods for IC work can also be challenging, as cell lines that recapitulate IC do not proliferate easily in culture. Moreover, a "pure" IC population in culture does not necessarily replicate its collecting duct (CD) environment, where ICs are surrounded by the more abundant principal cells (PCs). It is reassuring that many findings obtained in ex vivo IC systems signaling have been largely confirmed in vivo. Some of these newly identified signaling pathways reveal that ICs are important for regulating NaCl reabsorption, thus suggesting new frontiers to target antihypertensive treatments. Moreover, recent single-cell characterization studies of kidney epithelial cells revealed a dual developmental origin of IC, as well as the presence of novel CD cell types with certain IC characteristics. These exciting findings present new opportunities for the study of IC ex vivo and will likely rediscover the importance of available tools in this field.NEW & NOTEWORTHY The study of kidney intercalated cells has been limited by current cell culture and kidney tissue isolation techniques. This review is to be used as a reference to select ex vivo techniques to study intercalated cells. We focused on the use of cell lines and kidney slices as potential useful models to study membrane transport proteins. We also review how novel collecting duct organoids may help better elucidate the role of these intriguing cells.

The K-Cl cotransporter-3 in the mammalian kidney

PURPOSE OF REVIEW

We recently localized a new K-Cl cotransporters-3 (KCC3) transporter to the apical membrane of type-B intercalated cells. This gives us an opportunity to revisit the roles of the KCC3 in kidney and integrate the new findings to our current knowledge of the biology of the bicarbonate secreting cells.

RECENT FINDINGS

Here, we review the basic properties of the K-Cl cotransporter with a particular attention to the responsiveness of the transporter to cell swelling. We summarize what is already known about KCC3b and discuss new information gained from our localizing of KCC3a in type-B intercalated cells. We integrate the physiology of KCC3a with the main function of the type-B cell, that is, bicarbonate secretion through the well characterized apical Cl-/HCO3- exchanger and the basolateral Na-HCO3 cotransporter.

SUMMARY

Both KCC3b and KCC3a seem to be needed for maintaining cell volume during enhanced inward cotransport of Na-glucose in proximal tubule and Na-HCO3 in intercalated cells. In addition, apical KCC3a might couple to pendrin function to recycle Cl-, particularly in conditions of low salt diet and therefore low Cl- delivery to the distal tubule. This function is critical in alkalemia, and KCC3a function in the pendrin-expressing cells may contribute to the K+ loss which is observed in alkalemia.

Low-Cost, Real-Time Polymerase Chain Reaction System for Point-of-Care Medical Diagnosis

Global health crises due to the prevailing Coronavirus Disease 2019 (COVID-19) pandemic have placed significant strain on health care facilities such as hospitals and clinics around the world. Further, foodborne and waterborne diseases are not only spreading faster, but also appear to be emerging more rapidly than ever before and are able to circumvent conventional control measures. The Polymerase Chain Reaction (PCR) system is a well-known diagnostic tool for many applications in medical diagnostics, environmental monitoring, and food and water quality assessment. Here, we describe the design, development, and testing of a portable, low-cost, and real-time PCR system that can be used in emergency health crises and resource-poor situations. The described PCR system incorporates real-time reaction monitoring using fluorescence as an alternative to gel electrophoresis for reaction analysis, further decreasing the need of multiple reagents, reducing sample testing cost, and reducing sample analysis time. The bill of materials cost of the described system is approximately $340. The described PCR system utilizes a novel progressive selective proportional–integral–derivative controller that helps in reducing sample analysis time. In addition, the system employs a novel primer-based approach to quantify the initial target amplicon concentration, making it well-suited for food and water quality assessment. The developed PCR system performed DNA amplification at a level and speed comparable to larger and more expensive commercial table-top systems. The fluorescence detection sensitivity was also tested to be at the same level as commercially available multi-mode optical readers, thus making the PCR system an attractive solution for medical point-of-care and food and water quality assessment.

Regulation of Blood Pressure and Salt Balance By Pendrin-Positive Intercalated Cells

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

Impaired renal HCO3 - secretion in CFTR deficient mice causes metabolic alkalosis during chronic base-loading

AIM

Cystic fibrosis patients have an increased risk of developing metabolic alkalosis presumably as a result of altered renal HCO₃ ^- handling. In this study, we directly assess the kidneys' ability to compensate for a chronic base-load in the absence of functional CFTR.

METHODS

Comprehensive urine and blood acid-base analyses were done in anaesthetized WT mice or mice lacking either CFTR or pendrin, with or without 7 days of oral NaHCO₃ loading. The in vivo experiments were complemented by a combination of immunoblotting and experiments with perfused isolated mouse cortical collecting ducts (CCD).

RESULTS

Base-loaded WT mice maintained acid-base homeostasis by elevating urinary pH and HCO₃ ^- excretion and decreasing urinary net acid excretion. In contrast, pendrin KO mice and CFTR KO mice were unable to increase urinary pH and HCO₃ ^- excretion and unable to decrease urinary net acid excretion sufficiently and thus developed metabolic alkalosis in response to the same base-load. The expression of pendrin was increased in response to the base-load in WT mice with a paralleled increased pendrin function in the perfused CCD. In CFTR KO mice, 7 days of base-loading did not upregulate pendrin expression and apical Cl^- /HCO₃ ^- exchange function was strongly blunted in the CCD.

CONCLUSION

CFTR KO mice develop metabolic alkalosis during a chronic base-load because they are unable to sufficiently elevate renal HCO₃ ^- excretion. This can be explained by markedly reduced pendrin function in the absence of CFTR.

The Renal Physiology of Pendrin-Positive Intercalated Cells

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

Section-specific H+ flux in renal tubules of fasted and fed goldfish

A recent study demonstrated that in response to a feeding-induced metabolic acidosis, goldfish (Carassius auratus) adjust epithelial protein and/or mRNA expression in their kidney tubules for multiple transporters known to be relevant for acid-base regulation. These include Na⁺/H⁺ exchanger (NHE), V-type H⁺-ATPase (V-ATPase), cytoplasmic carbonic anhydrase, HCO₃ ^- transporters and Rhesus proteins. Consequently, renal acid output in the form of protons and NH₄ ⁺ increases. However, little is known about the mechanistic details of renal acid-base regulation in C. auratus and teleost fishes in general. The present study applied the scanning ion-selective electrode technique (SIET) to measure proton flux in proximal, distal and connecting tubules of goldfish. We detected increased H⁺ efflux into the extracellular fluid from the tubule in fed animals, resulting from paracellular back-flux of H⁺ through the tight junction. By applying inhibitors for selected acid-base regulatory epithelial transporters, we found that cytosolic carbonic anhydrase and HCO₃ ^- transporters were important in mediating H⁺ flux in all three tubule segments of fed goldfish. Contrastingly, V-ATPase seemed to play a role in H⁺ flux only in proximal and distal tubules, and NHE in proximal and connecting tubules. We developed working models for transport of acid-base relevant equivalents (H⁺, HCO₃ ^-, NH₃/NH₄ ⁺) for each tubule segment in C. auratus kidney. While the proximal tubule appears to play a major role in both H⁺ secretion and HCO₃ ^- reabsorption, the distal and connecting tubules seem to mainly serve for HCO₃ ^- reabsorption and NH₃/NH₄ ⁺ secretion.

Acid-Base Homeostasis

Acid-base homeostasis and pH regulation are critical for both normal physiology and cell metabolism and function. The importance of this regulation is evidenced by a variety of physiologic derangements that occur when plasma pH is either high or low. The kidneys have the predominant role in regulating the systemic bicarbonate concentration and hence, the metabolic component of acid-base balance. This function of the kidneys has two components: reabsorption of virtually all of the filtered HCO3(-) and production of new bicarbonate to replace that consumed by normal or pathologic acids. This production or generation of new HCO3(-) is done by net acid excretion. Under normal conditions, approximately one-third to one-half of net acid excretion by the kidneys is in the form of titratable acid. The other one-half to two-thirds is the excretion of ammonium. The capacity to excrete ammonium under conditions of acid loads is quantitatively much greater than the capacity to increase titratable acid. Multiple, often redundant pathways and processes exist to regulate these renal functions. Derangements in acid-base homeostasis, however, are common in clinical medicine and can often be related to the systems involved in acid-base transport in the kidneys.

SDF1 induction by acidosis from principal cells regulates intercalated cell subtype distribution

The nephron cortical collecting duct (CCD) is composed of principal cells, which mediate Na, K, and water transport, and intercalated cells (ICs), which are specialized for acid-base transport. There are two canonical IC forms: acid-secreting α-ICs and HCO3-secreting β-ICs. Chronic acidosis increases α-ICs at the expense of β-ICs, thereby increasing net acid secretion by the CCD. We found by growth factor quantitative PCR array that acidosis increases expression of mRNA encoding SDF1 (or CXCL12) in kidney cortex and isolated CCDs from mouse and rabbit kidney cortex. Exogenous SDF1 or pH 6.8 media increased H+ secretion and decreased HCO3 secretion in isolated perfused rabbit CCDs. Acid-dependent changes in H+ and HCO3 secretion were largely blunted by AMD3100, which selectively blocks the SDF1 receptor CXCR4. In mice, diet-induced chronic acidosis increased α-ICs and decreased β-ICs. Additionally, IC-specific Cxcr4 deletion prevented IC subtype alterations and magnified metabolic acidosis. SDF1 was transcriptionally regulated and a target of the hypoxia-sensing transcription factor HIF1α. IC-specific deletion of Hif1a produced no effect on mice fed an acid diet, as α-ICs increased and β-ICs decreased similarly to that observed in WT littermates. However, Hif1a deletion in all CCD cells prevented acidosis-induced IC subtype distribution, resulting in more severe acidosis. Cultured principal cells exhibited an HIF1α-dependent increase of Sdf1 transcription in response to media acidification. Thus, our results indicate that principal cells respond to acid by producing SDF1, which then acts on adjacent ICs.

Molecular mechanisms and regulation of urinary acidification

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

ENaC inhibition stimulates HCl secretion in the mouse cortical collecting duct. II. Bafilomycin-sensitive H+ secretion

Epithelial Na(+) channel (ENaC) blockade stimulates stilbene-sensitive conductive Cl(-) secretion in the mouse cortical collecting duct (CCD). This study's purpose was to determine the co-ion that accompanies benzamil- and DIDS-sensitive Cl(-) flux. Thus transepithelial voltage, VT, as well as total CO2 (tCO2) and Cl(-) flux were measured in CCDs from aldosterone-treated mice consuming a NaCl-replete diet. We reasoned that if stilbene inhibitors (DIDS) reduce conductive anion secretion they should reduce the lumen-negative VT. However, during ENaC blockade (benzamil, 3 μM), DIDS (100 μM) application to the perfusate reduced net H(+) secretion, which increased the lumen-negative VT. Conversely, ENaC blockade alone stimulated H(+) secretion, which reduced the lumen-negative VT. Application of an ENaC inhibitor to the perfusate reduced the lumen-negative VT, increased intercalated cell intracellular pH, and reduced net tCO2 secretion. However, benzamil did not change tCO2 flux during apical H(+)-ATPase blockade (bafilomycin, 5 nM). The increment in H(+) secretion observed with benzamil application contributes to the fall in VT observed with application of this diuretic. As such, ENaC blockade reduces the lumen-negative VT by inhibiting conductive Na(+) absorption and by stimulating H(+) secretion by type A intercalated cells. In conclusion, 1) in CCDs from aldosterone-treated mice, benzamil application stimulates HCl secretion mediated by the apical H(+)-ATPase and a yet to be identified conductive Cl(-) transport pathway; 2) benzamil-induced HCl secretion is reversed with the application of stilbene inhibitors or H(+)-ATPase inhibitors to the perfusate; and 3) benzamil reduces VT not only by inhibiting conductive Na(+) absorption, but also by stimulating H(+) secretion.

Increased Na+/H+ exchanger activity on the apical surface of a cilium-deficient cortical collecting duct principal cell model of polycystic kidney disease

Pathophysiological anomalies in autosomal dominant and recessive forms of polycystic kidney disease (PKD) may derive from impaired function/formation of the apical central monocilium of ductal epithelia such as that seen in the Oak Ridge polycystic kidney or orpk (Ift88(Tg737Rpw)) mouse and its immortalized cell models for the renal collecting duct. According to a previous study, Na/H exchanger (NHE) activity may contribute to hyperabsorptive Na(+) movement in cilium-deficient ("mutant") cortical collecting duct principal cell monolayers derived from the orpk mice compared with cilium-competent ("rescued") monolayers. To examine NHE activity, we measured intracellular pH (pH(i)) by fluorescence imaging with the pH-sensitive dye BCECF, and used a custom-designed perfusion chamber to control the apical and basolateral solutions independently. Both mutant and rescued monolayers exhibited basolateral Na(+)-dependent acid-base transporter activity in the nominal absence of CO(2)/HCO(3)(-). However, only the mutant cells displayed appreciable apical Na(+)-induced pH(i) recoveries from NH(4)(+) prepulse-induced acid loads. Similar results were obtained with isolated, perfused collecting ducts from orpk vs. wild-type mice. The pH(i) dependence of basolateral cariporide/HOE-694-sensitive NHE activity under our experimental conditions was similar in both mutant and rescued cells, and 3.5- to 4.5-fold greater than apical HOE-sensitive NHE activity in the mutant cells (pH(i) 6.23-6.68). Increased apical NHE activity correlated with increased apical NHE1 expression in the mutant cells, and increased apical localization in collecting ducts of kidney sections from orpk vs. control mice. A kidney-specific conditional cilium-knockout mouse produced a more acidic urine compared with wild-type littermates and became alkalotic by 28 days of age. This study provides the first description of altered NHE activity, and an associated acid-base anomaly in any form of PKD.

Luminal flow modulates H+-ATPase activity in the cortical collecting duct (CCD)

Epithelial Na(+) channel (ENaC)-mediated Na(+) absorption and BK channel-mediated K(+) secretion in the cortical collecting duct (CCD) are modulated by flow, the latter requiring an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), microtubule integrity, and exocytic insertion of preformed channels into the apical membrane. As axial flow modulates HCO(3)(-) reabsorption in the proximal tubule due to changes in both luminal Na(+)/H(+) exchanger 3 and H(+)-ATPase activity (Du Z, Yan Q, Duan Y, Weinbaum S, Weinstein AM, Wang T. Am J Physiol Renal Physiol 290: F289-F296, 2006), we sought to test the hypothesis that flow also regulates H(+)-ATPase activity in the CCD. H(+)-ATPase activity was assayed in individually identified cells in microperfused CCDs isolated from New Zealand White rabbits, loaded with the pH-sensitive dye BCECF, and then subjected to an acute intracellular acid load (NH(4)Cl prepulse technique). H(+)-ATPase activity was defined as the initial rate of bafilomycin-inhibitable cell pH (pH(i)) recovery in the absence of luminal K(+), bilateral Na(+), and CO(2)/HCO(3)(-), from a nadir pH of ∼6.2. We found that 1) an increase in luminal flow rate from ∼1 to 5 nl·min(-1)·mm(-1) stimulated H(+)-ATPase activity, 2) flow-stimulated H(+) pumping was Ca(2+) dependent and required microtubule integrity, and 3) basal and flow-stimulated pH(i) recovery was detected in cells that labeled with the apical principal cell marker rhodamine Dolichos biflorus agglutinin as well as cells that did not. We conclude that luminal flow modulates H(+)-ATPase activity in the rabbit CCD and that H(+)-ATPases therein are present in both principal and intercalated cells.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effect of intercalated cell-specific Rh C glycoprotein deletion on basal and metabolic acidosis-stimulated renal ammonia excretion

Rh C glycoprotein (Rhcg) is an NH(3)-specific transporter expressed in both intercalated cells (IC) and principal cells (PC) in the renal collecting duct. Recent studies show that deletion of Rhcg from both intercalated and principal cells inhibits both basal and acidosis-stimulated renal ammonia excretion. The purpose of the current studies was to better understand the specific role of Rhcg expression in intercalated cells in basal and metabolic acidosis-stimulated renal ammonia excretion. We generated mice with intercalated cell-specific Rhcg deletion (IC-Rhcg-KO) using Cre-loxP techniques; control (C) mice were floxed Rhcg but Cre negative. Under basal conditions, IC-Rhcg-KO and C mice excreted urine with similar ammonia content and pH. Mice were then acid loaded by adding HCl to their diet. Ammonia excretion after acid loading increased similarly in IC-Rhcg-KO and C mice during the first 2 days of acid loading but on day 3 was significantly less in IC-Rhcg-KO than in C mice. During the first 2 days of acid loading, urine was significantly more acidic in IC-Rhcg-KO mice than in C mice; there was no difference on day 3. In IC-Rhcg-KO mice, acid loading increased principal cell Rhcg expression in both the cortex and outer medulla as well as expression of another ammonia transporter, Rh glycoprotein B (Rhbg), in principal cells in the outer medulla. We conclude that 1) Rhcg expression in intercalated cells is necessary for the normal renal response to metabolic acidosis; 2) principal cell Rhcg contributes to both basal and acidosis-stimulated ammonia excretion; and 3) adaptations in Rhbg expression occur in response to acid-loading.

Remodeling of the fetal collecting duct epithelium

Congenital urinary tract obstruction induces changes to the renal collecting duct epithelium, including alteration and depletion of intercalated cells. To study the effects of obstruction on the ontogeny of intercalated cell development, we examined normal and obstructed human fetal and postnatal kidneys. In the normal human fetal kidney, intercalated cells originated in the medullary collecting duct at 8 weeks gestation and remained most abundant in the inner medulla throughout gestation. In the cortex, intercalated cells were rare at 18 and 26 weeks gestation and observed at low abundance at 36 weeks gestation. Although early intercalated cells exhibit an immature phenotype, Type A intercalated cells predominated in the inner and outer medullae at 26 and 36 weeks gestation with other intercalated cell subtypes observed rarely. Postnatally, the collecting duct epithelium underwent a remodeling whereby intercalated cells become abundant in the cortex yet absent from the inner medulla. In 18-week obstructed kidneys with mild to moderate injury, the intercalated cells became more abundant and differentiated than the equivalent age-matched normal kidney. In contrast, more severely injured ducts of the late obstructed kidney exhibited a significant reduction in intercalated cells. These studies characterize the normal ontogeny of human intercalated cell development and suggest that obstruction induces premature remodeling and differentiation of the fetal collecting duct epithelium.

Heterogeneity of H-K-ATPase-mediated acid secretion along the mouse collecting duct

In the collecting duct (CD), H-K-ATPases function in cation reabsorption and H secretion. This study evaluated H-K-ATPase-mediated H secretion along the mouse CD, measured as EIPA- and luminal bafilomycin A(1)-insensitive intracellular pH (pH(i)) recovery from acute H loading (NH(4)) using BCECF. pH(i) recovery was measured in 1) microperfused cortical, outer medullary, and inner medullary CDs (CCD, OMCD, and IMCD) from C57BL/6J mice fed a normal diet and 2) common murine CD cell lines. H-K-ATPase activity along the native, microperfused CD was greatest in the CCD, less in the OMCD, and least in the IMCD (0.10 +/- 0.02, 0.04 +/- 0.01, and 0.01 +/- 0.002 U/min, respectively). H-K-ATPase activity was 0.30 +/- 0.03 and 0.26 +/- 0.03 in A- and B-type ICs, respectively, and was sensitive to Sch-28080 or ouabain. pH(i) recovery was greatest in the OMCD(1) cell line (0.25 +/- 0.01) and less in mpkCCD(c14) (0.17 +/- 0.01), mIMCD-K2 (0.12 +/- 0.01), and mIMCD-3 (0.05 +/- 0.01) cells. EIPA inhibited the majority of pH(i) recovery in these cells (100%, 64%, 75%, and 80% in mpkCCD(c14), OMCD(1), mIMCD-K2, and mIMCD-3, respectively). In OMCD(1) cells, where EIPA-insensitive pH(i) recovery was greatest, H-K-ATPase activity was 0.10 +/- 0.01 and was significantly inhibited (80%) by Sch-28080. We conclude that 1) H-K-ATPase-mediated H secretion in the native mouse CD is greatest in the ICs of the CCD, 2) A- and B-type ICs possess HKalpha(1) and HKalpha(2) H-K-ATPase activity, and 3) the OMCD(1) cell line best exhibits H-K-ATPase.

Impaired acid secretion in cortical collecting duct intercalated cells from H-K-ATPase-deficient mice: role of HKalpha isoforms

Two classes of H pumps, H-K-ATPase and H-ATPase, contribute to luminal acidification and HCO(3) transport in the collecting duct (CD). At least two H-K-ATPase alpha-subunits are expressed in the CD: HKalpha(1) and HKalpha(2). Both exhibit K dependence but have different inhibitor sensitivities. The HKalpha(1) H-K-ATPase is Sch-28080 sensitive, whereas the pharmacological profile of the HKalpha(2) H-K-ATPase is not completely understood. The present study used a nonpharmacological, genetic approach to determine the contribution of HKalpha(1) and HKalpha(2) to cortical CD (CCD) intercalated cell (IC) proton transport in mice fed a normal diet. Intracellular pH (pH(i)) recovery was determined in ICs using in vitro microperfusion of CCD after an acute intracellular acid load in wild-type mice and mice of the same strain lacking expression of HKalpha(1), HKalpha(2), or both H-K-ATPases (HKalpha(1,2)). A-type and B-type ICs were differentiated by luminal loading with BCECF-AM and peritubular chloride removal from CO(2)/HCO(3)-buffered solutions to identify the membrane locations of Cl/HCO(3) exchange activity. H-ATPase- and Na/H exchange-mediated H transport were inhibited with bafilomycin A(1) (100 nM) and EIPA (10 microM), respectively. Here, we report 1) initial pH(i) and buffering capacity were not significantly altered in the ICs of HKalpha-deficient mice, 2) either HKalpha(1) or HKalpha(2) deficiency resulted in slower acid extrusion, and 3) A-type ICs from HKalpha(1,2)-deficient mice had significantly slower acid extrusion compared with A-type ICs from HKalpha(1)-deficient mice alone. These studies are the first nonpharmacological demonstration that both HKalpha(1) and HKalpha(2) contribute to H secretion in both A-type and B-type ICs in animals fed a normal diet.

Immunolocalization of anion exchanger 1 (Band 3) in the renal collecting duct of the common marmoset

The purpose of this study was to determine the expression and distribution of band 3 in the collecting duct and connecting tubules of the kidney of the marmoset monkey (Callithrix jacchus), and to establish whether band 3 is expressed in type A intercalated cells. The intracellular localization of band 3 in the different populations of intercalated cells was determined by double-labeling immunohistochemistry. Immunohistochemical microscopy demonstrated that band 3 is located in the basolateral plasma membranes of all type A intercalated cells in the connecting tubule (CNT), cortical collecting duct (CCD), and outer medullary collecting duct (OMCD) of the marmoset. However, type B intercalated cells and non-A/non-B intercalated cells did not show band 3 labeling. Electron microscopy of the CNT, CCD and OMCD confirmed the light microscopic observation of the basolateral plasma membrane staining for band 3 in a subpopulation of interacted cells. Basolateral staining was seen on the plasma membrane and small coated vesicles in the perinuclear structure, some of which were located in the Golgi region. In addition, there was no labeling of band 3 in the mitochondria of the CNT, CCD and in OMCD cells. The intensity of the immunostaining of the basolateral membrane was less in the CNT than in the CCD and OMCD. In contrast, band 3 immunoreactivity was greater in the intracellular vesicles of the CNT. From these results, we suggest that the basolateral Cl^-/HCO₃^- exchanger in the monkey kidney is in a more active state in the collecting duct than in the CNT.

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Effect of reduced renal mass on renal ammonia transporter family, Rh C glycoprotein and Rh B glycoprotein, expression

Kidneys can maintain acid-base homeostasis, despite reduced renal mass, through adaptive changes in net acid excretion, of which ammonia excretion is the predominant component. The present study examines whether these adaptations are associated with changes in the ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg). We used normal Sprague-Dawley rats and a 5/6 ablation-infarction model of reduced renal mass; control rats underwent sham operation. After 1 wk, glomerular filtration rate, assessed as creatinine clearance, was decreased, serum bicarbonate was slightly increased, and Na+and K+were unchanged. Total urinary ammonia excretion was unchanged, but urinary ammonia adjusted for creatinine clearance, an index of per nephron ammonia metabolism, increased significantly. Although reduced renal mass did not alter total Rhcg protein expression, both light microscopy and immunohistochemistry with quantitative morphometric analysis demonstrated hypertrophy of both intercalated cells and principal cells in the cortical and outer medullary collecting duct that was associated with increased apical and basolateral Rhcg polarization. Rhbg expression, analyzed using immunoblot analysis, immunohistochemistry, and measurement of cell-specific expression, was unchanged. We conclude that altered subcellular localization of Rhcg contributes to adaptive changes in single-nephron ammonia metabolism and maintenance of acid-base homeostasis in response to reduced renal mass.

Expression of protein kinase C isoenzymes alpha, betaI, and delta in subtypes of intercalated cells of mouse kidney

Recent studies of the distribution of PKC isoenzymes in the mouse kidney demonstrated that PKC-alpha, -beta(I), and -delta are expressed in intercalated cells. The purpose of this study was to identify the intercalated cell subtypes that express the different PKC isoenzymes and determine the location of the PKC isoenzymes within these cells. Adult C57BL/6 mice kidney tissues were processed for multiple-labeling immunohistochemistry. Antibodies against the vacuolar H(+)-ATPase and pendrin were used to identify intercalated cell subtypes, whereas antibodies against calbindin D(28K) and aquaporin-2 (AQP2) were used to identify connecting tubule cells and principal cells of the collecting duct, respectively. Within type A intercalated cells, PKC-delta was highly expressed in the apical part of the cells, whereas immunoreactivity for both PKC-alpha and PKC-beta(I) was weak. Type B intercalated cells exhibited strong expression of PKC-alpha, -beta(I), and -delta. PKC-alpha and -beta(I) were localized throughout the cytoplasm, whereas PKC-delta was restricted to the basal domain. Within non-A-non-B cells, immunoreactivity for both PKC-alpha and PKC-beta(I) was high in intensity and localized diffusely in the cytoplasm, whereas PKC-delta was localized in the apical part of the cells. None of the PKC isoenzymes (PKC-alpha, -beta(I), or -delta) were expressed in the calbindin D(28K)-positive connecting tubule cells. Within AQP2-positive principal cells of the collecting duct, PKC-alpha was expressed on the basolateral plasma membrane, but no significant staining was detected for PKC-beta(I) and -delta. In summary, this study demonstrates distinct and differential expression patterns of PKC-alpha, -beta(I), and -delta in the three subtypes of intercalated cells in the mouse kidney.

Recent advances in our understanding of intercalated cells

PURPOSE OF REVIEW

This review will summarize newly described novel functions of renal intercalated cells.

RECENT FINDINGS

Over the past 20 years, the importance of intercalated cells in the process of renal net acid excretion has been recognized. More recently, many of the molecular mechanisms responsible for this cellular function have been described. Functionally, type A and type B intercalated cells are largely mirror images in that type A intercalated cells are H secreting cells, whereas type B intercalated cells are OH secreting cells. Whether non-A, non-B intercalated cells represent H or OH secreting cells or whether they interconvert between these functions is unclear. Transporters such as pendrin (Slc26a4, Pds), AE1 (Slc4a1), the H-ATPase, and NBC3 (Slc4a7) contribute to the ability of intercalated cells to secrete H or OH equivalents. In addition to mediating secretion of H or OH equivalents, however, intercalated cells also regulate vascular volume, and hence blood pressure, likely by regulating renal Cl excretion.

SUMMARY

The molecular mechanisms of net H/OH secretion by intercalated cell subsets have been largely defined over the past decade. Moreover, targeted genetic disruption of these transporters has revealed novel roles, such as blood pressure regulation. Thus, some of these transporters might be the target of future antihypertensive drugs.

Role of hensin in mediating the adaptation of the cortical collecting duct to metabolic acidosis

PURPOSE OF REVIEW

The cortical collecting duct is able to secrete HCO3-, a state that can be converted to acid secretion during metabolic acidosis. Bicarbonate secretion in this segment is mediated by beta-intercalated cells whereas alpha-intercalated cells perform acid secretion. During metabolic acidosis, the number of beta-intercalated cells is reduced while that of alpha-intercalated cells increases without a change in the total number of intercalated cells, suggesting conversion of one cell type to another. Using an immortalized intercalated cell line we found that this adaptation is mediated by an extracellular protein named hensin. Hensin is secreted as a monomer which is then polymerized in the extracellular environment by a complex process requiring at least three other proteins.

RECENT FINDINGS

We describe that a cyclophilin, via its cis/trans prolyl isomerase activity, is required for this polymerization. This may explain the distal renal tubular acidosis observed with cyclosporin A therapy. In addition, galectin-3 is needed to aggregate the protein. Finally, we recently found that activation of integrins is also necessary for the development of the hensin fiber. Hensin is expressed in all epithelia and deletion of its gene is embryonic lethal at an early stage when the first columnar epithelia develop.

SUMMARY

These studies suggest that the response of intercalated cells to metabolic acidosis uses a pathway that is involved in terminal differentiation of columnar epithelia.

Basolateral ammonium transport by the mouse inner medullary collecting duct cell (mIMCD-3)

The renal collecting duct is the primary site for the ammonia secretion necessary for acid-base homeostasis. Recent studies have identified the presence of putative ammonia transporters in the collecting duct, but whether the collecting duct has transporter-mediated ammonia transport is unknown. The purpose of this study was to examine basolateral ammonia transport in the mouse collecting duct cell (mIMCD-3). To examine mIMCD-3 basolateral ammonia transport, we used cells grown to confluence on permeable support membranes and quantified basolateral uptake of the radiolabeled ammonia analog [14C]methylammonia ([14C]MA). mIMCD-3 cell basolateral MA transport exhibited both diffusive and transporter-mediated components. Transporter-mediated uptake exhibited a Kmfor MA of 4.6 ± 0.2 mM, exceeded diffusive uptake at MA concentrations below 7.0 ± 1.8 mM, and was competitively inhibited by ammonia with a Kiof 2.1 ± 0.6 mM. Transporter-mediated uptake was not altered by inhibitors of Na+-K+-ATPase, Na+-K+-2Cl−cotransporter, K+channels or KCC proteins, by excess potassium, by extracellular sodium or potassium removal or by varying membrane potential, suggesting the presence of a novel, electroneutral ammonia-MA transport mechanism. Increasing the outwardly directed transmembrane H+gradient increased transport activity by increasing Vmax. Finally, mIMCD-3 cells express mRNA and protein for the putative ammonia transporter Rh B-glycoprotein (RhBG), and they exhibit basolateral RhBG immunoreactivity. We conclude that mIMCD-3 cells express a basolateral electroneutral NH4+/H+exchange activity that may be mediated by RhBG.

Diversity of the mammalian sodium/proton exchanger SLC9 gene family

Sodium/proton antiporters or exchangers (NHE) are integral membrane proteins present in most, if not all, living organisms. In mammals, these transporters chiefly catalyze the electroneutral exchange of Na(+) and H(+) down their respective concentration gradients and are crucial for numerous physiological processes, ranging from the fine control of intracellular pH and cell volume to systemic electrolyte, acid-base and fluid volume homeostasis. NHE activity also facilitates the progression of other cellular events such as adhesion, migration, and proliferation. Thus far, eight distinct NHE genes (NHE1/SLC9A1-NHE8/SLC9A8) and several pseudogenes have been identified in the human genome. The functional genes encode proteins of varying primary sequence identity (25-70%), but share a common predicted secondary structure comprising 12 conserved membrane-spanning segments at the amino-terminus and a more divergent, cytoplasmically-oriented, carboxy-terminus. They show considerable heterogeneity in their patterns of tissue/cell expression and membrane localization. Functional studies have revealed further differences in their kinetic properties, sensitivity to pharmacological antagonists, and regulation by diverse hormonal and mechanical stimuli. Altered NHE activity has been linked to the pathogenesis of several diseases, including essential hypertension, congenital secretory diarrhea, diabetes, and tissue damage caused by ischemia/reperfusion. Further characterization of their functional properties should lead to a better understanding of their unique contributions to human health and disease.

Increased expression of H+-ATPase in inner medullary collecting duct of aquaporin-1-deficient mice

Phenotype analysis has demonstrated that aquaporin-1 (AQP1) null mice are polyuric and manifest a urinary concentrating defect because of an inability to create a hypertonic medullary interstitium. We report here that deletion of AQP1 is also associated with a decrease in urinary pH from 6.15 +/- (SE) 0.1 to 5.63 +/- 0.07. To explore the mechanism of the decrease in urinary pH, we examined the expression of H+-ATPase in kidneys of AQP1 null mice. There was strong labeling for H+-ATPase in intercalated cells and proximal tubule cells in both AQP1 null and wild-type mice. Strong H+-ATPase immunostaining was also present in the apical plasma membrane of inner medullary collecting duct (IMCD) cells in AQP1 null mice, whereas no H+-ATPase labeling was observed in IMCD cells in wild-type mice. In addition, there was an increase in the prevalence of type A intercalated cells in the IMCD of AQP1 null mice, suggesting that the deletion of intercalated cells from the IMCD, which normally occurs during postnatal kidney development, was impaired. Western blot analysis of H+-ATPase expression in the different regions of the kidney demonstrated a significant increase in H+-ATPase protein in the inner medulla of AQP1 null mice compared with wild-type mice. There were no changes in H+-ATPase expression in the cortex or outer medulla. These results represent the first demonstration of apical H+-ATPase immunoreactivity in IMCD cells in vivo and suggest that the decrease in urinary pH observed in AQP1 null mice is due to upregulation of H+-ATPase in the IMCD. The induction of H+-ATPase expression in IMCD cells of AQP1 null mice may be related to the chronically low interstitial osmolality in these animals. The challenge will be to identify the molecular signal(s) responsible for the de novo H+-ATPase expression.

Immunocytochemical localization of pendrin in intercalated cell subtypes in rat and mouse kidney

Recent studies have demonstrated that a novel anion exchanger, pendrin, is expressed in the apical domain of type B intercalated cells in the mammalian collecting duct. The purpose of this study was 1) to determine the expression and distribution of pendrin along the collecting duct and connecting tubule of mouse and rat kidney and establish whether pendrin is expressed in the non-A-non-B intercalated cells and 2) to determine the intracellular localization of pendrin in the different populations of intercalated cells by immunoelectron microscopy. A peptide-derived affinity-purified antibody was generated that specifically recognized pendrin in immunoblots of rat and mouse kidney. Immunohistochemistry and confocal laser scanning microscopy demonstrated the presence of pendrin in apical domains of all type B intercalated cells in mouse and rat connecting tubule and collecting duct. In addition, strong pendrin immunostaining was observed in non-A-non-B intercalated cells. There was no labeling of type A intercalated cells. Immunoelectron microscopy demonstrated that pendrin was located in the apical plasma membrane and intracellular vesicles of both type B intercalated cells and non-A-non-B cells; the latter was identified by the presence of H(+)-ATPase in the apical plasma membrane. The results of this study demonstrate that both pendrin and H(+)-ATPase are expressed in the apical plasma membrane of non-A-non-B intercalated cells, suggesting that these cells are capable of both HCO and proton secretion. Furthermore, the presence of pendrin in both the apical plasma membrane and the apical intracellular vesicles of type B and non-A-non-B intercalated cells suggests that HCO secretion may be regulated by trafficking of pendrin between the two membrane compartments.

Mechanisms through which ammonia regulates cortical collecting duct net proton secretion

Ammonia stimulates cortical collecting duct (CCD) net bicarbonate reabsorption by activating an apical H(+)-K(+)-ATPase through mechanisms that are independent of ammonia's known effects on intracellular pH and active sodium transport. The present studies examined whether this stimulation occurs through soluble N-ethylmaleimide-sensitive fusion attachment receptor (SNARE) protein-mediated vesicle fusion. Rabbit CCD segments were studied using in vitro microperfusion, and transepithelial bicarbonate transport was measured using microcalorimetry. Ammonia's stimulation of bicarbonate reabsorption was blocked by either chelating intracellular calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester or by inhibiting microtubule polymerization with colchicine compared with parallel studies performed in the absence of these inhibitors. An inactive structural analog of colchicine, lumicolchicine, did not alter ammonia's stimulation of bicarbonate reabsorption. Tetanus toxin, a zinc endopeptidase specific for vesicle-associated SNARE (v-SNARE) proteins, prevented ammonia from stimulating net bicarbonate reabsorption. Consistent with the functional evidence for v-SNARE involvement, antibodies directed against a conserved region of isoforms 1-3 of the tetanus toxin-sensitive, vesicle-associated membrane protein (VAMP) members of v-SNARE proteins labeled the apical and subapical region of collecting duct intercalated cells. Similarly, antibodies to NSF protein, a protein involved in activation of SNARE proteins for subsequent vesicle fusion, localized to the apical and subapical region of collecting duct intercalated cells. These results indicate that ammonia stimulates CCD bicarbonate reabsorption through an intracellular calcium-dependent, microtubule-dependent, and v-SNARE-dependent mechanism that appears to involve insertion of cytoplasmic vesicles into the apical plasma membrane of CCD intercalated cells.

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

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

Immunochemical analysis of the vacuolar proton-ATPase B-subunit in the gills of a euryhaline stingray (Dasyatis sabina): effects of salinity and relation to Na+/K+-ATPase

In the gills of freshwater teleost fishes, vacuolar proton-ATPase (V-H+-ATPase) is found on the apical membrane of pavement and chloride (Na+/K+-ATPase-rich) cells, and is an important transporter for energizing Na+ uptake and H+ excretion. In the gills of elasmobranch fishes, the V-H+-ATPase has not been extensively studied and its expression in freshwater individuals has not been examined. The goals of this study were to examine the effects of environmental salinity on the expression of V-H+-ATPase in the gills of an elasmobranch (the Atlantic stingray, Dasyatis sabina) and determine if V-H+-ATPase and Na+/K+-ATPase are expressed in the same cells. We found that gills from freshwater stingrays had the highest relative abundance of V-H+-ATPase and greatest number of V-H+-ATPase-rich cells, using immunoblotting and immunohistochemistry, respectively. When freshwater animals were acclimated to sea water for 1 week, V-H+-ATPase abundance and the number of V-H+-ATPase-rich cells decreased significantly. Atlantic stingrays from seawater environments were characterized by the lowest expression of V-H+-ATPase and least number of V-H+-ATPase-rich cells. In contrast to teleost fishes, localization of V-H+-ATPase in freshwater stingray gills was not found in pavement cells and occurred on the basolateral membrane in cells that are presumably rich in mitochondria. In freshwater stingrays acclimated to sea water and seawater stingrays, V-H+-ATPase localization appeared qualitatively to be stronger in the cytoplasm, which may suggest the transporter was stored in vesicles. Using a double-immunolabeling technique, we found that V-H+-ATPase and Na+/K+-ATPase occurred in distinct cells, which suggests there may be two types of mitochondrion-rich cells in the elasmobranch gill epithelium. Based on these findings, we propose a unique model of NaCl and acid–base regulation where the V-H+-ATPase-rich cells and Na+/K+-ATPase-rich cells are the sites of Cl– uptake/HCO3– excretion and Na+ uptake/H+ excretion, respectively.

Effects of ammonia on acid-base transport by the B-type intercalated cell

Ammonia, in addition to its role as a constituent of urinary net acid excretion, stimulates cortical collecting duct (CCD) net bicarbonate reabsorption. The current study sought to begin determining the cellular transport processes through which ammonia regulates bicarbonate reabsorption by testing whether ammonia stimulates B-type intercalated cell bicarbonate secretion, bicarbonate reabsorption, or both. The effects of ammonia on single CCD intercalated cells was studied by use of measurements of intracellular pH taken from in vitro microperfused CCD segments after luminal loading of the pH-sensitive fluorescent dye BCECF. These results showed, first, that ammonia inhibited B-cell unidirectional bicarbonate secretion and that this occurred despite no effect of ammonia on apical Cl(-)/HCO(3)(-) exchange activity. Second, ammonia increased the contribution of a SCH28080-sensitive apical H(+)-K(+)-ATPase to basal intracellular pH regulation and it stimulated basolateral Cl(-)/HCO(3)(-) exchange activity. Thus, ammonia activated both apical proton secretion and basolateral base exit, consistent with stimulation of unidirectional bicarbonate reabsorption. It was concluded that ammonia regulates CCD net bicarbonate reabsorption, at least in part, through the coordinated regulation of the separate processes of B-cell bicarbonate reabsorption and bicarbonate secretion. These effects do not reflect a general activation of ion transport but, instead, reflect coordinated and specific regulation of ion transport.

A mathematical model of rat cortical collecting duct: determinants of the transtubular potassium gradient

In assessing disorders of potassium excretion, urine composition is used to calculate the transtubular gradient (TTKG), as an estimate of tubule fluid concentration, at a point when the fluid was last isotonic to plasma, namely, within the cortical collecting duct (CCD). A mathematical model of the CCD has been developed, consisting of principal cells and α- and β-intercalated cells, and which includes Na+, K+, Cl−, HCO[Formula: see text], CO2, H2CO3, phosphate, ammonia, and urea. Parameters have been selected to achieve fluxes and permeabilities compatible with data obtained from perfusion studies of rat CCD under the influence of both antidiuretic hormone and mineralocorticoid. Both epithelial (flat sheet) and tubule models have been configured, and model calculations have focused on the determinants of the TTKG. Using the epithelial model, luminal K+ concentrations can be computed at which K+secretion ceases (0-flux equilibrium), and this luminal concentration derives from the magnitude of principal cell peritubular uptake of K+ via the Na-K-ATPase, relative to principal cell peritubular membrane K+ permeability. When the model is configured as a tubule and examined in the context of conditions in vivo, osmotic equilibration of luminal fluid produces a doubling of the initial K+ concentration, which, depending on delivered load, may be substantially greater than the zero-flux equilibrium value. Under such circumstances, the CCD will be a site for K+ reabsorption, although the relatively low permeability ensures that this reabsorptive flux is likely to be small. Osmotic equilibration may also raise luminal NH3 concentrations well above those in cortical blood. In this situation, diffusive reabsorption of NH3 provides a mechanism for base reclamation without the metabolic cost of active proton secretion.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Effects of ammonia on bicarbonate transport in the cortical collecting duct

Both acidosis and hypokalemia stimulate renal ammoniagenesis, and both regulate urinary proton and potassium excretion. We hypothesized that ammonia might play an important role in this processing by stimulating H(+)-K(+)-ATPase-mediated ion transport. Rabbit cortical collecting ducts (CCD) were studied using in vitro microperfusion, bicarbonate reabsorption was measured using microcalorimetry, and intracellular pH (pH(i)) was measured using the fluorescent, pH-sensitive dye, 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Ammonia caused a concentration-dependent increase in net bicarbonate reabsorption that was inhibited by luminal addition of either of the H(+)-K(+)-ATPase inhibitors, Sch-28080 or ouabain. The stimulation of net bicarbonate reabsorption was not mediated through apical H(+)-ATPase, basolateral Na(+)-K(+)-ATPase, or luminal electronegativity. Although ammonia caused intracellular acidification, similar changes in pH(i) induced by inhibiting basolateral Na(+)/H(+) exchange did not alter net bicarbonate reabsorption. We conclude that ammonia regulates CCD proton and potassium transport, at least in part, by stimulating apical H(+)-K(+)-ATPase.

Apical proton secretion by the inner stripe of the outer medullary collecting duct

The inner stripe of outer medullary collecting duct (OMCDis) is unique among collecting duct segments because both intercalated cells and principal cells secrete protons and reabsorb luminal bicarbonate. The current study characterized the mechanisms of OMCDis proton secretion. We used in vitro microperfusion, and we separately studied the principal cell and intercalated cell using differential uptake of the fluorescent, pH-sensitive dye, 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Both the principal cell and intercalated cell secreted protons, as identified as Na+/H+ exchange-independent intracellular pH (pHi) recovery from an intracellular acid load. Two proton transport activities were identified in the principal cell; one was luminal potassium dependent and Sch-28080 sensitive and the other was luminal potassium independent and luminal bafilomycin A1 sensitive. Thus the OMCDis principal cell expresses both apical H+-K+-ATPase and H+-ATPase activity. Intercalated cell Na+/H+ exchange-independent pHi recovery was approximately twice that of the principal cell and was mediated by pharmacologically similar mechanisms. We conclude 1) the OMCDis principal cell may contribute to both luminal potassium reabsorption and urinary acidification, roles fundamentally different from those of the principal cell in the cortical collecting duct; and 2) the OMCDis intercalated cell proton transporters are functionally similar to those in the principal cell, raising the possibility that an H+-K+-ATPase similar to the one present in the principal cell may contribute to intercalated cell proton secretion.

Intercalated cell subtypes in connecting tubule and cortical collecting duct of rat and mouse

At least two populations of intercalated cells, type A and type B, exist in the connecting tubule (CNT), initial collecting tubule (ICT), and cortical collecting duct (CCD). Type A intercalated cells secrete protons via an apical H+-ATPase and reabsorb bicarbonate by a band 3-like Cl-/HCO3-exchanger, AE1, located in the basolateral plasma membrane. Type B intercalated cells secrete bicarbonate by an apical Cl-/HCO3- exchanger that is distinct from AE1 and remains to be identified. They express H+-ATPase in the basolateral plasma membrane and in vesicles throughout the cytoplasm. A third type of intercalated cell with apical H+-ATPase, but no AE1, has been described in the CNT and CCD of both rat and mouse. The prevalence of the third cell type is not known. The aim of this study was to characterize and quantify intercalated cell subtypes, including the newly described third non A-non B cell, in the CNT, ICT, and CCD of the rat and mouse. A triple immunolabeling procedure was developed in which antibodies to H+-ATPase and band 3 protein were used to identify subpopulations of intercalated cells, and segment-specific antibodies were used to identify distal tubule and collecting duct segments. In both rat and mouse, intercalated cells constituted approximately 40% of the cells in the CNT, ICT, and CCD. Type A, type B, and non A-non B intercalated cells were observed in all of the three segments, with type A cells being the most prevalent in both species. In the mouse, however, non A-non B cells constituted more than half of the intercalated cells in the CNT, 39% in the ICT, and 22% in the CCD, compared with 14, 7, and 5%, respectively, in the rat. In contrast, type B intercalated cells accounted for only 8 to 16% of the intercalated cells in the three segments in the mouse compared with 26 to 39% in the rat. It is concluded that striking differences exist in the prevalence and distribution of the different types of intercalated cells in the CNT, ICT, and CCD of rat and mouse. In the rat, the non A-non B cells are fairly rare, whereas in the mouse, they constitute a major fraction of the intercalated cells, primarily at the expense of the type B intercalated cells.

Immunochemical characterization of Na+/H+ exchanger isoform NHE4

Mammalian Na+/H+ exchangers (NHEs) are a family of transport proteins (NHE1-NHE5). To date, the cellular and subcellular localization of NHE4 has not been characterized using immunochemical techniques. We purified a fusion protein containing a portion of rat NHE4 (amino acids 565-675) to use as immunogen. A monoclonal antibody (11H11) was selected by ELISA. It reacted specifically with both the fusion protein and to a 60- to 65-kDa polypeptide expressed in NHE4-transfected LAP1 cells. By Western blot analysis, NHE4 was identified as a 65- to 70-kDa protein that was expressed most abundantly in stomach and in multiple additional epithelial and nonepithelial rat tissues including skeletal muscle, heart, kidney, uterus, and liver. Subcellular localization of NHE4 in the kidney was evaluated by Western blot analysis of membrane fractions isolated by Percoll gradient centrifugation. NHE4 was found to cofractionate with the basolateral markers NHE1 and Na+-K+-ATPase rather than the luminal marker gamma-glutamyl transferase. In stomach, NHE4 was detected by immunoperoxidase labeling on the basolateral membrane of cells at the base of the gastric gland. We conclude that NHE4 is a 65- to 70-kDa protein with a broad tissue distribution. In two types of epithelial cells, kidney and stomach, NHE4 is localized to the basolateral membrane.

Low-NaCl diet increases H-K-ATPase in intercalated cells from rat cortical collecting duct

Extracellular K+-dependent H+ extrusion after an acute acid load, an index of H/K exchange, was monitored in intercalated cells (ICs) from rat cortical collecting tubule (CCT) using ratiometric fluorescence imaging of the intracellular pH (pHi) indicator, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The hypothesis tested was that 12- to 14-day NaCl deprivation increases H-K-ATPase in rat ICs. The rate of H/K exchange in the low-NaCl ICs was double that of controls. In control ICs, H/K exchange was inhibited by Sch-28080 (10 microM). In the low-NaCl ICs, it was partially blocked by Sch-28080 or ouabain (1 mM). Simultaneous addition of both inhibitors abolished K-dependent pHi recovery. The induced H/K exchange observed with NaCl restriction was not due to elevated plasma aldosterone as evidenced by experiments on ICs from rats implanted with osmotic minipumps administering aldosterone such that plasma levels were comparable to those of NaCl-deficient rats. The results suggest that NaCl deficiency induces two isoforms of H-K-ATPase in ICs and that this effect is not mediated by elevated plasma aldosterone.

Regulation of B-type intercalated cell apical anion exchange activity by CO2/HCO3-

The cortical collecting duct (CCD) B cell possesses an apical anion exchanger dissimilar to AE1, AE2, and AE3. The purpose of these studies was to characterize this transporter more fully by examining its regulation by CO2 and HCO3. We measured intracellular pH (pHi) in single intercalated cells of in vitro microperfused CCD using the fluorescent, pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). In the absence of extracellular CO2/HCO3, luminal Cl removal caused reversible intracellular alkalinization, identifying this transporter as a Cl/base exchanger able to transport bases other than HCO3. Adding extracellular CO2/HCO3 decreased B cell pHi while simultaneously increasing Cl/base exchange activity. Since intracellular acidification inhibits AE1, AE2, and AE3, we examined mechanisms other than pHi by which the stimulation occurred. These studies showed that B cell apical anion exchange activity was CO2 stimulated and carbonic anhydrase dependent. Moreover, the stimulation was independent of luminal bicarbonate, luminal pH or pHi, and changes in buffer capacity. We conclude that the B cell possesses an apical Cl/base exchanger whose activity is regulated by CO2-stimulated, carbonic anhydrase-dependent cytoplasmic HCO3 formation.

Transitional stages in the development of the rabbit renal collecting duct

The collecting duct (CD) epithelium of the mammalian kidney is an extraordinary structure with respect to its functional changes during development and its heterogeneous composition when matured. All of the different nephron epithelia of the mammalian kidney consist of one single cell type. In contrast, the differentiated CD is composed of at least three distinct cell types [principal, alpha intercalated-, and beta intercalated cells] that are responsible for the multiple physiological functions of this kidney compartment. During development the function of the CD changes: initially, the CD ampulla serves as an embryonic inducer, while the matured epithelium plays a key role in maintaining the homeostasis of body fluids. At present the process of CD maturation is not well understood. Neither the time course of development nor the morphogenic factors leading to the heterogeneously composed epithelium are known. In the present study the differentiation of the CD epithelium was investigated using newly developed monoclonal antibodies and well-characterized antisera. The morphological changes induced during differentiation were monitored by immunohistochemistry and scanning electron microscopy. The experiments were performed on neonatal and adult rabbit kidneys. Results obtained by light microscopical techniques and scanning electron microscopy revealed that the ampullary tip can be distinguished from the ampullary neck, as well as from the maturing CD. A number of proteins that were not detectable in the ampulla were detected in the neonatal CD and were found at even higher concentrations in the adult CD (PCD8, chloride/bicarbonate exchanger). Other proteins (PCD9) were downregulated during differentiation. For the first time the transient character of the differentiation stage of the neonatal CD could be demonstrated unequivocally. Furthermore, considerable heterogeneity in protein expression patterns (PCD6 and PCD9) was demonstrated within the beta IC cell population of the mature CD.

Polarized distribution of Na+/H+ exchanger isoforms in rabbit collecting duct cells

The present study describes two Na+/H+ exchanger (NHE) isoforms in an immortalized rabbit renal cortical collecting tubule cell line (RC.SV3). Na+/H+ exchange activity was assayed using fluorescence measurements of intracellular pH (pHi) in monolayers mounted in a cuvette containing two fluid compartments, making it possible to independently measure Na+/H+ exchange activity on either the apical or basolateral surface. RC.SV3 monolayers express Na+/H+ exchange activities in both the apical and basolateral membrane domains. The two exchangers have half-saturation constants (Km) for external sodium and sensitivities to dimethylamiloride, to HOE-694 and to cimetidine and clonidine consistant with the NHE-1 isoform on the basolateral cell surface and the NHE-2 isoform on the apical surface. Protein kinase A inhibition of basolateral exchanger activity was significantly higher than that of the apical exchanger. Protein kinase C significantly stimulated both exchangers equally. RT-PCR analysis found RNA for only NHE-1 and NHE-2, and immunofluorescence with an antibody against NHE-1 demonstrated a basolateral location for this isoform. The results suggest that RC.SV3 cells have two Na+/H+ exchange activities separated spatially to the two cellular membranes, with the NHE-1 and the NHE-2 isoforms located on the basolateral and the apical membranes, respectively.

Adaptation of rabbit cortical collecting duct HCO3- transport to metabolic acidosis in vitro

Net HCO3- transport in the rabbit kidney cortical collecting duct (CCD) is mediated by simultaneous H+ secretion and HCO3- secretion, most likely occurring in a alpha- and beta-intercalated cells (ICs), respectively. The polarity of net HCO3- transport is shifted from secretion to absorption after metabolic acidosis or acid incubation of the CCD. We investigated this adaptation by measuring net HCO3- flux before and after incubating CCDs 1 h at pH 6.8 followed by 2 h at pH 7.4. Acid incubation always reversed HCO3- flux from net secretion to absorption, whereas incubation for 3 h at pH 7.4 did not. Inhibition of alpha-IC function (bath CL- removal or DIDS, luminal bafilomycin) stimulated net HCO3- secretion by approximately 2 pmol/min per mm before acid incubation, whereas after incubation these agents inhibited net HCO3- absorption by approximately 5 pmol/min per mm. Inhibition of beta-IC function (luminal Cl- removal) inhibited HCO3- secretion by approximately 9 pmol/min per mm before incubation, whereas after incubation HCO3- absorption by only 3 pmol/min per mm. After acid incubation, luminal SCH28080 inhibited HCO3- absorption by only 5-15% vs the circa 90% inhibitory effect of bafilomycin. In outer CCDs, which contain fewer alpha-ICs than midcortical segments, the reversal in polarity of HCO3- flux was blunted after acid incubation. We conclude that the CCD adapts to low pH in vitro by downregulation HCO3- secretion in beta-ICs via decreased apical CL-/base exchang activity and upregulating HCO3- absorption in alpha-ICs via increased apical H+ -ATPase and basolateral CL-/base exchange activities. Whether or not there is a reversal of IC polarity or recruitment of gamma-ICs in this adaptation remains to be established.

Localization of the Na+/H+ exchanger isoform NHE-3 in rabbit and canine kidney

The distribution and subcellular localization of Na+/H+ exchanger isoform NHE-3 was studied in rabbit and canine kidney using polyclonal antibodies to an NHE-3 fusion protein. Western blot analyses were performed against microsomal, brush-border, and basolateral membranes isolated from rabbit kidney cortex, outer medulla, and inner medulla. Immunoblots indicated that NHE-3 antibody recognized a strong band with 95-100 kDa molecular mass in cortical microsomes. Subcellular localization studies showed that NHE-3 was expressed in brush-border membranes of kidney cortex. Expression of NHE-3 in the medullary regions was studied by immunoblot analysis of NHE-3 antibody against the microsomal membranes from the outer and inner medulla. NHE-3 antibody specifically labelled a 95-100 kDa protein in outer but not inner medulla. Subcellular localization studies demonstrated that NHE-3 is localized to the brush-border membranes of the outer medulla. Immunoblot analysis against brush-border membranes from canine kidney cortex and outer medulla demonstrated the presence of an 83-90 kDa protein. The above experiments suggest that NHE-3 in rabbit kidney is a 95-100 kDa protein and is expressed in brush-border membranes of the cortex and outer medulla. The canine kidney NHE-3 is a 83-90 kDa protein and is expressed in brush-border membranes of the cortex and outer medulla. Based on its subcellular localization, we conclude that NHE-3 may be involved in vectorial Na+ and HCO3- transport and pHo regulation.

pH dependence of K+ conductances of rat cortical collecting duct principal cells

The K+ channels of the principal cells of rat cortical collecting duct (CCD) are pH sensitive in excised membranes. K+ secretion is decreased with increased H+ secretion during acidosis. We examined whether the pH sensitivity of these K+ channels is present also in the intact cell and thus could explain the coupling between K+ and H+ secretion. Membrane voltages (Vm), whole-cell conductances (gc), and single-channel currents of K+ channels were recorded from freshly isolated CCD cells or isolated CCD segments with the patch-clamp method. Intracellular pH (pHi) was measured using the pH-sensitive fluorescent dye 2'-7'-bis(carboxyethyl)-5-6-carboxyfluorescein (BCECF). Acetate (20 mmol/l) had no effect on Vm, gc, or the activity of the K+ channels in these cells. Acetate, however, acidified pHi slightly by 0.17 +/- 0.04 pH units (n = 19). Vm depolarized by 12 +/- 3 mV (n = 26) and by 23 +/- 2 mV (n = 66) and gc decreased by 26 +/- 5% (n = 13) and by 55 +/- 5% (n = 12) with 3-5 or 8-10% CO2, respectively. The same CO2 concentrations decreased pHi by 0.49 +/- 0.07 (n = 15) and 0.73 +/- 0.11 pH units (n = 12), respectively. Open probability (Po) of all four K+ channels in the intact rat CCD cells was reversibly inhibited by 8-10% CO2. pHi increased with the addition of 20 mmol/l NH4+/NH3 by a maximum of 0.64 +/- 0.08 pH units (n = 33) and acidified transiently by 0.37 +/- 0.05 pH units (n = 33) upon NH4+/NH3 removal. In the presence of NH4+/NH3 Vm depolarized by 16 +/- 2 mV (n = 66) and gc decreased by 26 +/- 7% (n = 16). The activity of all four K+ channels was also strongly inhibited in the presence of NH4+/NH3. The effect of NH4+/NH3 on Vm and gc was markedly increased when the pH of the NH4+/NH3-containing solution was set to 8.5 or 9.2. From these data we conclude that cellular acidification in rat CCD principal cells down-regulates K+ conductances, thus reduces K+ secretion by direct inhibition of K+ channel activity. This pH dependence is present in all four K+ channels of the rat CCD. The inhibition of K+ channels by NH4+/NH3 is independent of changes in pHi and rather involves an effect of NH3.