Regulation of Intestinal Epithelial Brush Border Na+/H+ Exchanger Isoforms, NHE2 and NHE3, in C2bbe Cells

Until recently, studies to characterize the intestinal epithelial Na⁺/H⁺ exchangers had to be done in nonepithelial, mutated fibroblasts. In these cells, detection of any Na⁺/H⁺ exchange activity requires prior acid loading. Furthermore, most of these experiments used intracellular pH changes to measure NHE activity. Because changes in pH _i only approximate Na⁺/H⁺ exchange activity, and may be confounded by alterations in buffering capacity and/or non-NHE contributions to pH regulation, we have used ²²[Na] unidirectional apical to cell uptake to measure activities specific to NHE2 or NHE3. Furthermore, we performed these measurements under basal, nonacid-stimulated conditions to avoid bias from this nonphysiological experimental precondition. Both brush border NHEs, when expressed in the well-differentiated, intestinal villuslike Caco-2 subclone, C2bbe (C2), localize to the C2 apical domain and are regulated by second messengers in the same way they are regulated in vivo. Increases in intracellular calcium and cAMP inhibit both isoforms, while phorbol ester affects only NHE3. NHE2 inhibition by cAMP and Ca⁺⁺ involves changes to both K _Na and V _max . In contrast, the same two second messengers inhibit NHE3 by a decrease in V _max exclusively. Phorbol ester activation of protein kinase C alters both V _max and K _Na of NHE3, suggesting a multilevel regulatory mechanism. We conclude that NHE2 and NHE3, in epithelial cells, are basally active and are differentially regulated by signal transduction pathways. Received: 28 January 1999/Revised: 18 May 1999     

The Basis of Higher Na+ Transport by Inner Medullary Collecting Duct Cells from Dahl Salt-Sensitive Rats: Implicating the Apical Membrane Na+ Channel

The present experiments were designed to examine the function of Na/K pumps from Dahl salt-sensitive (S) and salt-resistant (R) rats. Previous reports have suggested that there is a difference in primary sequence in the α₁ subunit, the major Na/K pump isoform in the kidney. This sequence difference might contribute to differences in NaCl excretion in these two strains which in turn could influence the systemic blood pressure. Using ``back-door' phosphorylation of pumps isolated from basolateral membranes of kidney cortex, we found no differences between S and R strains. We also examined the Na/K pumps from cultured inner medullary collecting duct (IMCD) cells. This approach takes advantage of the fact that monolayers cultured from S rats transport about twice as much Na⁺ as monolayers cultured from R rats. In cells whose apical membrane was made permeable with amphotericin B, comparison of the affinities for ouabain, Na⁺, and K⁺, respectively, showed only small or no differences between S and R monolayers. Ouabain binding showed no difference in the number of Na/K pumps on the basolateral membrane of cultured cells, despite a 2-fold difference in Na⁺ transport rates. The analysis of the steady-state Na⁺ transport indicates that Na/K pumps in IMCD monolayers from S rats operate at a higher fraction of their maximum capacity than do pumps in monolayers from R rats. The results, taken together, suggest that the major reason for the higher rate of Na⁺ transport in S monolayers is because of a primary increase in the conductive permeability of the apical membrane to Na⁺. They suggest that the epithelial Na⁺ channel is intrinsically different or differently regulated in S and R rats. Received: 6 May 1996/Revised: 16 October 1996     

Characterization of Na+/H+ Exchanger Isoform (NHE1, NHE2 and NHE3) Expression in Prairie Dog Gallbladder

Gallbladder Na⁺ absorption is linked to gallstone formation in prairie dogs. Na⁺/H⁺ exchange (NHE) is one of the major Na⁺ absorptive pathways in gallbladder. In this study, we measured gallbladder Na⁺/H⁺ exchange and characterized the NHE isoforms expressed in prairie dogs. Na⁺/H⁺ exchange activity was assessed by measuring amiloride-inhibitable transepithelial Na⁺ flux and apical ²²Na⁺ uptake using dimethylamiloride (DMA). HOE-694 was used to determine NHE2 and NHE3 contributions. Basal J ^Na _ms was higher than J ^Na _sm with J ^Na _net absorption. Mucosal DMA inhibited transepithelial Na⁺ flux in a dose-dependent fashion, causing J ^Na _ms equal to J ^Na _sm and blocking J ^Na _net absorption at 100 μm. Basal ²²Na⁺ uptake rate was 10.9 ± 1.0 μmol · cm⁻²· hr⁻¹ which was inhibited by ∼43% by mucosal DMA and ∼30% by mucosal HOE-694 at 100 μm. RT-PCR and Northern blot analysis demonstrated expression of mRNAs encoding NHE1, NHE2 and NHE3 in the gallbladder. Expression of NHE1, NHE2 and NHE3 polypeptides was confirmed using isoform-specific anti-NHE antibodies. These data suggest that Na⁺/H⁺ exchange accounts for a substantial fraction of gallbladder apical Na⁺ entry and most of net Na⁺ absorption in prairie dogs. The NHE2 and NHE3 isoforms, but not NHE1, are involved in gallbladder apical Na⁺ uptake and transepithelial Na⁺ absorption. Received: 9 February 2001/Revised: 11 April 2001     

Inverse Relationship Between Membrane Lipid Fluidity and Activity of Na+/H+ Exchangers, NHE1 and NHE3, in Transfected Fibroblasts

This report presents a study of the effects of the membrane fluidizer, benzyl alcohol, on NHE isoforms 1 and 3. Using transfectants of an NHE-deficient fibroblast, we analyzed each isoform separately. An increase in membrane fluidity resulted in a decrease of ≈50% in the specific activities of both NHE1 and NHE3. Only V _max was affected; K _Na was unchanged. This effect was specific, as Na⁺, K⁺, ATPase activity was slightly stimulated. Inhibition of NHE1 and NHE3 was reversible and de novo protein synthesis was not required to restore NHE activity after washout of fluidizer. Inhibition kinetics of NHE1 by amiloride, 5-(N,N-dimethyl)amiloride (DMA), 5-(N-hexamethyl)amiloride (HMA) and 5-(N-ethyl-N-isopropyl)amiloride (EIPA) were largely unchanged. Half-maximal inhibition of NHE3 was also reached at approximately the same concentrations of amiloride and analogues in control and benzyl alcohol treated, suggesting that the amiloride binding site was unaffected. Inhibition of vesicular transport by incubation at 4°C augmented the benzyl alcohol inhibition of NHE activity, suggesting that the fluidizer effect does not solely involve vesicle trafficking. In summary, our data demonstrate that the physical state of membrane lipids (fluidity) influences Na⁺/H⁺ exchange and may represent a physiological regulatory mechanism of NHE1 and NHE3 activity. Received: 23 January 1997/Revised: 1 August 1997     

Activity and Stoichiometry of Na+:HCO− 3 Cotransport in Immortalized Renal Proximal Tubule Cells

The proximal tubule Na⁺-HCO⁻ ₃ cotransporter is located in the basolateral plasma membrane and moves Na⁺, HCO⁻ ₃, and net negative charge together out of the cell. The presence of charge transport implies that at least two HCO⁻ ₃ anions are transported for each Na⁺ cation. The actual ratio is of physiological interest because it determines direction of net transport at a given membrane potential. To determine this ratio, a thermodynamic approach was employed that depends on measuring charge flux through the cotransporter under defined ion and electrical gradients across the basolateral plasma membrane. Cells from an immortalized rat proximal tubule line were grown as confluent monolayer on porous substrate and their luminal plasma membrane was permeabilized with amphotericin B. The electrical properties of these monolayers were measured in a Ussing chamber, and ion flux through the cotransporter was achieved by applying Na⁺ or HCO⁻ ₃ concentration gradients across the basolateral plasma membrane. Charge flux through the cotransporter was identified as difference current due to the reversible inhibitor dinitro-stilbene disulfonate. The cotransporter activity was Cl⁻ independent; its conductance ranged between 0.12 and 0.23 mS/cm² and was voltage independent between −60 and +40 mV. Reversal potentials obtained from current-voltage relations in the presence of Na⁺ gradients were fitted to the thermodynamic equivalent of the Nernst equation for coupled ion transport. The fit yielded a cotransport ratio of 3HCO⁻ ₃:1Na⁺. Received: 19 January 1996/Revised: 24 April 1996     

Interferon‐γ suppresses Na+–H+ exchanger in cultured human endolymphatic sac epithelial cells

Adequate regulation of endolymphatic pH is essential for maintaining inner ear function. The Na⁺–H⁺ exchanger (NHE) is a major determinant of intracellular pH (pH_i), and facilitates Na⁺ and fluid absorption in various epithelia. We determined the functional and molecular expression of NHEs in cultured human endolymphatic sac (ES) epithelial cells and examined the effect of IFN‐γ on NHE function. Serial cultures of human ES epithelial cells were generated from tissue samples. The molecular expression of NHE1, ‐2, and ‐3 isoforms was determined by real‐time RT‐PCR. The functional activity of NHE isoforms was measured microfluorometrically using a pH‐sensitive fluorescent dye, 2′,7′‐bis(carbonylethyl)‐5(6)‐carboxyfluorescein (BCECF), and a NHE‐inhibitor, 3‐methylsulfonyl‐4‐piperidinobenzoyl guanidine methanesulfonate (HOE694). NHE1, ‐2, and ‐3 mRNAs were expressed in human ES epithelial cells. Functional activity of NHE1 and ‐2 was confirmed in the luminal membrane of ES epithelial cells by sequentially suppressing Na⁺‐dependent pH_i recovery from intracellular acidification using different concentrations of HOE694. Treatment with IFN‐γ (50 nM for 24 h) suppressed mRNA expression of NHE1 and ‐2. IFN‐γ also suppressed functional activity of both NHE1 and ‐2 in the luminal membrane of ES epithelial cells. This study shows that NHEs are expressed in cultured human ES epithelial cells and that treatment with IFN‐γ suppresses the expression and functional activity of NHE1 and ‐2. J. Cell. Biochem. 107: 965–972, 2009. © 2009 Wiley‐Liss, Inc.     

Renal Cortical Basolateral Na+/HCO− 3 Cotransporter: IV Characterization and Localization with Polyclonal Antibodies

We have previously partially purified the basolateral Na⁺/HCO⁻ ₃ cotransporter from rabbit renal cortex and this resulted in a 400-fold purification, and an SDS-PAGE analysis showed an enhancement of a protein band with a MW of approximately 56 kDa. We developed polyclonal antibodies against the Na⁺/HCO⁻ ₃ cotransporter by immunizing Dutch-belted rabbits with a partially purified protein fraction enriched in cotransporter activity. Western blot analysis of renal cortical basolateral membranes and of solubilized basolateral membrane proteins showed that the antibodies recognized a protein with a MW of approximately 56 kDa. The specificity of the purified antibodies against the Na⁺/HCO⁻ ₃ cotransporter was tested by immunoprecipitation. Solubilized basolateral membrane proteins enriched in Na⁺/HCO⁻ ₃ cotransporter activity were incubated with the purified antibody or with the preimmune IgG and then reconstituted in proteoliposomes. The purified antibody fraction caused a concentration-dependent inhibition of the Na⁺/HCO⁻ ₃ cotransporter activity, while the preimmune IgG failed to elicit any change. The inhibitory effect of the antibody was of the same magnitude whether it was added prior to (inside) or after (outside) reconstitution in proteoliposomes. In the presence of the substrates (NaHCO₃ or Na₂CO₃) for the cotransporter, the inhibitory effect of the antibody on cotransporter activity was significantly blunted as compared with the inhibition observed in the absence of substrates. Western blot analysis of rabbit kidneys showed that the antibodies recognized strongly a 56 kDa protein band in microsomes of the inner stripe of outer medulla and inner medulla, but not in the outer stripe of outer medulla. A 56 kDa protein band was recognized in microsomes of the stomach, liver, esophagus, and small intestine but was not detected in red blood cell membranes. Localization of the Na⁺/HCO⁻ ₃ cotransporter protein by immunogold technique revealed specific labeling of the cotransporter on the basolateral membranes of the proximal tubules, but not in the brush border membranes. These results demonstrate that the polyclonal antibodies against the 56 kDa basolateral protein inhibit the activity of the Na⁺/HCO⁻ ₃ cotransporter suggesting that the 56 kDa protein represents the cotransporter or a component thereof. These antibodies interact at or near the substrate binding sites. The Na⁺/HCO cotransporter protein is expressed in different regions of the kidneys and in other tissues. Received: 27 January 1996/Revised: 23 July 1996     

The Na+/H+ Exchanger Present in Trout Erythrocyte Membranes is Not Responsible for the Amiloride-insensitive Na+/Li+ Exchange Activity

The protein responsible for the Na⁺/Li⁺ exchange activity across the erythrocyte membrane has not been cloned or isolated. It has been suggested that a Na⁺/H⁺ exchanger could be responsible for the Na⁺/Li⁺ exchange activity across the erythrocyte membrane. Previously, we reported that in the trout erythrocyte, the Li⁺/H⁺ exchange activity (mediated by the Na⁺/H⁺ exchanger βNHE) and the Na⁺/Li⁺ exchange activity respond differently to cAMP, DMA (dimethyl-amiloride) and O₂. We concluded that the DMA insensitive Na⁺/Li⁺ exchange activity originates from a different protein. To further examine these findings, we measured Li⁺ efflux in fibroblasts expressing the βNHE as the only Na⁺/H⁺ exchanger. Moreover, the internal pH of these cells was monitored with a fluorescent probe. Our findings indicate that acidification of fibroblasts expressing the Na⁺/H⁺ exchanger βNHE, induces a Na⁺ stimulated Li⁺ efflux activity in trout erythrocytes. This exchange activity, however, is DMA sensitive and therefore differs from the DMA insensitive Na⁺/Li⁺ exchange activity. In these fibroblasts no significant DMA insensitive Na⁺/Li⁺ exchange activity was found. These results support the hypothesis that the trout erythrocyte Na⁺/Li⁺ exchange activity is not mediated by the Na⁺/H⁺ exchanger (βNHE) present in these membranes. Received: 6 December 1996/Revised: 11 August 1997     

High capacity Na+/H+ exchange activity in mineralizing osteoblasts

Osteoblasts synthesize bone in polarized groups of cells sealed by tight junctions. Large amounts of acid are produced as bone mineral is precipitated. We addressed the mechanism by which cells manage this acid load by measuring intracellular pH (pHi) in non‐transformed osteoblasts in response to weak acid or bicarbonate loading. Basal pHi in mineralizing osteoblasts was ～7.3 and decreased by ～1.4 units upon replacing extracellular Na⁺ with N‐methyl‐D ‐glucamine. Loading with 40 mM acetic or propionic acids, in normal extracellular Na⁺, caused only mild cytosolic acidification. In contrast, in Na⁺‐free solutions, weak acids reduced pHi dramatically. After Na⁺ reintroduction, pHi recovered rapidly, in keeping with Na⁺/H⁺ exchanger (NHE) activity. Sodium‐dependent pHi recovery from weak acid loading was inhibited by amiloride with the Ki consistent with NHEs. NHE1 and NHE6 were expressed strongly, and expression was upregulated highly, by mineralization, in human osteoblasts. Antibody labeling of mouse bone showed NHE1 on basolateral surfaces of all osteoblasts. NHE6 occurred on basolateral surfaces of osteoblasts mainly in areas of mineralization. Conversely, elevated HCO alkalinized osteoblasts, and pH recovered in medium containing Cl^?, with or without Na⁺, in keeping with Na⁺‐independent Cl^?/HCO exchange. The exchanger AE2 also occurred on the basolateral surface of osteoblasts, consistent with Cl^?/HCO exchange for elimination of metabolic carbonate. Overexpression of NHE6 or knockdown of NHE1 in MG63 human osteosarcoma cells confirmed roles of NHE1 and NHE6 in maintaining pHi. We conclude that in mineralizing osteoblasts, slightly basic basal pHi is maintained, and external acid load is dissipated, by high‐capacity Na⁺/H⁺ exchange via NHE1 and NHE6. J. Cell. Physiol. 226: 1702–1712, 2011. © 2010 Wiley‐Liss, Inc.     

Facilitated Transport of Lactate by Rat Jejunal Enterocyte

L-lactate transport mechanism across rat jejunal enterocyte was investigated using isolated membrane vesicles. In basolateral membrane vesicles l-lactate uptake is stimulated by an inwardly directed H⁺ gradient; the effect of the pH difference is drastically reduced by FCCP, pCMBS and phloretin, while furosemide is ineffective. The pH gradient effect is strongly temperature dependent. The initial rate of the proton gradient-induced lactate uptake is saturable with respect to external lactate with a K _m of 39.2 ± 4.8 mm and a J _max of 8.9 ± 0.7 nmoles mg protein⁻¹ sec⁻¹. A very small conductive pathway for l-lactate is present in basolateral membranes. In brush border membrane vesicles both Na⁺ and H⁺ gradients exert a small stimulatory effect on lactate uptake. We conclude that rat jejunal basolateral membrane contains a H⁺-lactate cotransporter, whereas in the apical membrane both H⁺-lactate and Na⁺-lactate cotransporters are present, even if they exhibit a low transport rate. Received: 22 October 1996/Revised: 11 March 1997     

Role of Cl− in Electrogenic H+ Secretion by Cortical Distal Tubule

The presence of an electrogenic H⁺-ATPase has been described in the late distal tubule, a segment which contains intercalated cells. The present paper studies the electrogenicity of this transport mechanism, which has been demonstrated in turtle bladder and in cortical collecting duct. Transepithelial PD (V _t ) was measured by means of Ling-Gerard microelectrodes in late distal tubule of rat renal cortex during in vivo microperfusion. The tubules were perfused with electrolyte solutions to which 2 × 10⁻⁷ m bafilomycin or 4.6 × 10⁻⁸ m concanamycin were added. No significant increase in lumen-negative V _t upon perfusion with these inhibitors as compared to control, was observed as well as when 10⁻³ m amiloride, 10⁻⁵ m benzamil or 3 mm Ba²⁺ were perfused alone or in combination. The effect of an inhibition of electrogenic H⁺ secretion, i.e., increase in lumen-negative V _t by 2–4 mV, was observed only when Cl⁻ channels were blocked by 10⁻⁵ m 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). This blocker also reduced the rate of bicarbonate reabsorption in this segment from 1.21 ± 0.14 (n= 8) to 0.62 ± 0.03 (8) nmol.cm⁻².sec⁻¹ as determined by stationary microperfusion and pH measurement by ion-exchange resin microelectrodes. These results indicate that: (i) the participation of the vacuolar H⁺ ATPase in the establishment of cortical late distal tubule V _t is minor in physiological conditions, but can be demonstrated after blocking Cl⁻ channels, thus suggesting a shunting effect of this anion; and, (ii) the rate of H⁺ secretion in this segment is reduced by a Cl⁻ channel blocker, supporting coupling of H⁺-ATPase with Cl⁻ transport. Received: 6 July 1996/Revised: 27 December 1996     

Transcellular Chloride Pathways in Ambystoma Proximal Tubule

The transport mechanisms of Ambystoma proximal tubule that mediate transcellular Cl⁻ absorption linked to Na⁺ were investigated in isolated perfused tubules using Cl⁻-selective and voltage-recording microelectrodes. In control solutions intracellular activity of Cl⁻ (a _i ^Cl ) is 11.3 ± 0.5 mm, the basolateral (V ₁ ), apical (V ₂ ), and transepithelial (V ₃ ) potential differences are −68 ± 1.2 mV, +62 ± 1.2 mV and −6.4 ± 0.3 mV, respectively. When Na⁺ absorption is decreased by removal of organic substrates from the lumen, a _i ^Cl falls by 1.3 ± 0.3 mm and V ₂ hyperpolarizes by +11.4 ± 1.7 mV. Subsequent removal of Na⁺ from the lumen causes a _i ^Cl to fall further by 2.3 ± 0.4 mm and V ₂ to hyperpolarize further by +15.3 ± 2.4 mV. The contribution of transporters and channels to the observed changes of a _i ^Cl was examined using ion substitutions and inhibitors. Apical Na/Cl or Na/K/2Cl symport is excluded because bumetanide, furosemide or hydrochlorothiazide have no effect on a _i ^Cl . The effects of luminal HCO⁻ ₃ removal and/or of disulfonic stilbenes argue against the presence of apical Cl-base exchange such as Cl-HCO₃ or Cl-OH. The effects of basolateral HCO⁻ ₃ removal, of basolateral Na⁺ removal and/or of disulfonic stilbenes are compatible with presence of basolateral Na-independent Cl-base exchange and Na-driven Cl-HCO₃ exchange. Several lines of evidence favor conductive Cl⁻ transport across both the apical and basolateral membrane. Addition of the chloride-channel blocker diphenylamine-2-carboxylate to the lumen or bath, increases the a _i ^Cl by 2.4 ± 0.6 mm or 2.9 ± 1.0 mm respectively. Moreover, following inhibition by DIDS of all anion exchangers in HCO⁻ ₃-free Ringer, the equilibrium potential for Cl⁻ does not differ from the membrane potential V ₂ . Finally, the logarithmic changes in a _i ^Cl in various experimental conditions correlate well with the simultaneous changes in either basolateral or apical membrane potential. These findings strongly support the presence of Cl⁻ channels at the apical and basolateral cell membranes of the proximal tubule. Received: 14 November 1997/Revised: 6 July 1998     

H+ ATPase and Cl− Interaction in Regulation of MDCK Cell pH

MDCK cells display several acid-base transport systems found in intercalated cells, such as Na⁺-H⁺ exchange, H⁺–K⁺ ATPase and Cl⁻/HCO⁻ ₃ exchange. In this work we studied the functional activity of a vacuolar H⁺-ATPase in MDCK cells and its chloride dependence. We measured intracellular pH (pHi) in monolayers grown on glass cover slips utilizing the pH sensitive probe BCECF. To analyze the functional activity of the H⁺ transporters we observed the intracellular alkalinization in response to an acute acid load due to a 20 mm NH⁺ ₄ pulse, and calculated the initial rate of pHi recovery (dpHi/dt). The cells have a basal pHi of 7.17 ± 0.01 (n= 23) and control dpHi/dt of 0.121 ± 0.006 (n= 23) pHi units/min. This pHi recovery rate is markedly decreased when Na⁺ was removed, to 0.069 ± 0.004 (n= 16). It was further reduced to 0.042 ± 0.005 (n= 12) when concanamycin 4.6 × 10⁻⁸ m (a specific inhibitor of the vacuolar H⁺-ATPase) was added to the zero Na⁺ solution. When using a solution with zero Na⁺, low K⁺ (0.5 mm) plus concanamycin, pHi recovery fell again, significantly, to 0.023 ± 0.006 (n= 14) as expected in the presence of a H⁺–K⁺-ATPase. This result was confirmed by the use of 5 × 10⁻⁵ m Schering 28080. The Na⁺ independent pHi recovery was significantly reduced from 0.069 ± 0.004 to 0.042 ± 0.004 (n= 12) when NPPB 10⁻⁵ m (a specific blocker of Cl⁻ channels in renal tubules) was utilized. When the cells were preincubated in 0 Cl⁻/normal Na⁺ solution for 8 min. before the ammonium pulse, the pHi recovery fell from 0.069 ± 0.004 to 0.041 ± 0.007 (n= 12) in a Na⁺ and Cl⁻ free solution. From these results we conclude that: (i) MDCK cells have two Na⁺-independent mechanisms of pHi recovery, a concanamycin sensitive H⁺-ATPase and a K⁺ dependent, Schering 28080 sensitive H⁺–K⁺ ATPase; and, (ii) pHi recovery in Na⁺-free medium depends on the presence of a chloride current which can be blocked by NPPB and impaired by preincubation in Cl⁻–free medium. This finding supports a role for chloride in the function of the H⁺ ATPase, which might be electrical shunting or a biochemical interaction. Received: 24 October 1997/Revised: 19 February 1998     

Characterization of K+ Channels in the Basolateral Membrane of Rat Tracheal Epithelia

To study K⁺ channels in the basolateral membrane of chloride-secreting epithelia, rat tracheal epithelial monolayers were cultured on permeable filters and mounted into an Ussing chamber system. The mucosal membrane was permeabilized with nystatin (180 μg/ml) in the symmetrical high K⁺ (145 mm) Ringer solution. During measurement of the macroscopic K⁺ conductance properties of the basolateral membrane under a transepithelial voltage clamp, we detected at least two types of K⁺ currents: one is an inwardly rectifying K⁺ current and the other is a slowly activating outwardly rectifying K⁺ current. The inwardly rectifying K⁺ current is inhibited by Ba²⁺. The slowly activating K⁺ current was potentiated by cAMP and inhibited by clofilium, phorbol 12-myristae 13-acetate (PMA) and lowering temperature. This is consistent with the biophysical characteristics of I _SK channel. RT-PCR analysis revealed the presence of I _SK cDNA in the rat trachea epithelia. Although 0.1 mm Ba²⁺ only had minimal affect on short-circuit current (I _sc) induced by cAMP in intact epithelia, 0.1 mm clofilium strongly inhibited it. These results indicate that I _SK might be important for maintaining cAMP-induced chloride secretion in the rat trachea epithelia. Received: 1 March 1996/Revised: 5 August 1996     

Cell pH and H+ Secretion by S3 Segment of Mammalian Kidney: Role of H+-ATPase and Cl−

The role of H⁺-ATPase in proximal tubule cell pH regulation was studied by microperfusion techniques and by confocal microscopy. In a first series of experiments, proximal S3 segments of rabbit kidney were perfused ``in vitro' while their cell pH was measured by fluorescence microscopy after loading with BCECF. In Na⁺- and Cl⁻-free medium, cell pH fell by a mean of 0.37 ± 0.051 pH units, but after a few minutes started to rise again slowly. This rise was of 0.17 ± 0.022 pH units per min, and was significantly reduced by bafilomycin and by the Cl⁻ channel blocker NPPB, but not by DIDS. In a second series of experiments, subcellular vesicles of proximal tubule cells of S3 segments of mouse kidney were studied by confocal microscopy after visualization by acridine orange or by Lucifer yellow. After superfusion with low Na⁺ solution, which is expected to cause cell acidification, vesicles originally disposed in the basolateral and perinuclear cell areas, moved toward the apical area, as detected by changes in fluorescence density measured by the NIH Image program. The variation of apical to basolateral fluorescence ratios during superfusion with NaCl Ringer with time was 0.0018 ± 0.0021 min⁻¹, not significantly different from zero (P > 0.42). For superfusion with Na⁺0 Ringer, this variation was 0.081 ± 0.015 min⁻¹, P < 0.001 against 0. These slopes were markedly reduced by the Cl⁻ channel blocker NPPB, and by vanadate at a concentration that has been shown to disrupt cytoskeleton function. These data show that the delayed alkalinization of proximal tubule cells in Na⁺-free medium is probably due to a vacuolar H⁺-ATPase, whose activity is stimulated in the presence of Cl⁻, and dependent on apical insertion of subcellular vesicles. The movement of these vesicles is also dependent on Cl⁻ and on the integrity of the cytoskeleton. Received: 11 April 2000/Revised: 14 August 2000     

Effect of Dicyclohexylcarbodiimide (DCCD) on Transport Parameters in the Frog Cornea Epithelium

Dicyclohexylcarbodiimide (DCCD) is a carboxyl group modifier and it is an inhibitor of various ATPases. Present experiments, using an in vitro preparation, were designed to study whether DCCD affected the transporters of the bullfrog cornea epithelium, specifically, the Na⁺/K⁺ ATPase pump located in the basolateral membrane. For this purpose, corneas were impaled with microelectrodes and experiments were done under short-circuit current (I _sc ) conditions. Addition of DCCD to a concentration of 10⁻⁴ m to the tear solution gave a marked decrease in I _sc ; a marked depolarization of the intracellular potential, V _o ; and a significant decrease in the apical membrane fractional resistance, fR _o . There were small and variable although significant changes in the transepithelial conductance, g _t . The effects may be explained by a decrease in the basolateral membrane K⁺ conductance, in combination with a partial inhibition of the Na⁺/K⁺-ATPase pump located in the basolateral membrane. There is also evidence for an increase in the apical membrane Cl⁻ conductance. Received: 12 August 1999/Revised: 16 November 1999     

Relationship between Extra- and Intracellular Calcium in Distal Segments of the Renal Tubule. Role of the Ca2+ Receptor RaKCaR

The effect of extracellular calcium ([Ca²⁺] _e ) on cytosolic calcium ([Ca²⁺] _i ) was investigated in thick ascending limbs and collecting ducts from the rat kidney, using the fluorescent dye fura-2. In cortical collecting ducts, basolateral but not apical changes in [Ca²⁺] _e were associated with parallel changes in [Ca²⁺] _i . Basal [Ca²⁺] _i was hardly modified by nifedipine and verapamil but was decreased by 60% by basolateral La³⁺. Increasing peritubular [Ca²⁺] _e triggered Ca²⁺ release from intracellular stores. This effect was not reproduced by agonists of the renal Ca²⁺-receptor RaKCaR, e.g., Ba²⁺, Mg²⁺, Gd³⁺, and neomycin, but was reproduced by Ni²⁺. Ni²⁺-induced mobilization of intracellular Ca²⁺ was larger in the inner medullary collecting duct, a segment which poorly responds to increasing [Ca²⁺] _e . In the cortical thick ascending limb, removing basolateral Ca²⁺ hardly altered [Ca²⁺] _i but increasing [Ca²⁺] _e or adding Ba²⁺, Mg²⁺, Gd³⁺ and neomycin released intracellular calcium. These data demonstrate that (1) basolateral influx of calcium occurs in cortical collecting ducts, under basal conditions; (2) this influx occurs through nonvoltage gated channels, permeable to Ba²⁺, insensitive to verapamil and nifedipine, and blocked by La³⁺; (3) increasing [Ca²⁺] _e stimulates the influx and triggers intracellular calcium release, independently of the phospholipase C-coupled receptor RaKCaR; (4) RaKCaR is functionally expressed in thick ascending limbs; (5) another membrane receptor, sensitive to Ni²⁺ but not to Ca²⁺ is present in the collecting duct. Received: 12 July 1996/Revised: 28 October 1996     

Na+:HCO(3-) cotransporters (NBC): cloning and characterization

The sodium bicarbonate cotransporter (NBC1) is essential for bicarbonate transport across plasma membranes in epithelial and nonepithelial cells. The direction of the NaHCO₃ movement in secretory epithelia is opposite to that in reabsorptive epithelia. In secretory epithelia (such as pancreatic duct cells) NBC is responsible for the transport of bicarbonate from blood to the cell for eventual secretion at the apical membrane. In reabsorptive epithelia (such as kidney proximal tubule cells) NBC is responsible for the reabsorption of bicarbonate from cell to the blood. In nonepithelial cells this transporter is mainly involved with cell pH regulation. Recent molecular cloning experiments have identified the existence of four NBC isoforms (NBC1, 2, 3 and 4) and two NBC-related proteins AE4 and NCBE (Anion Exchanger 4 and Na-dependent Chloride-Bicarbonate Exchanger). All but AE4 are presumed to mediate the cotransport of Na⁺ and HCO₃ ⁻ under normal conditions and may be functionally altered in certain pathologic states. NBC1 shows a limited tissue expression pattern, is electrogenic and plays an important role in bicarbonate reabsorption in kidney proximal tubule. In addition to the kidney, NBC1 is expressed in pancreatic duct cells, is activated by cystic fibrosis transmembrane conductance regulator (CFTR) and plays an important role in HCO₃ ⁻ secretion. NBC2 and NBC3 have a wider tissue distribution than NBC1, are electroneutral, and are involved with cell pH regulation. The characterization of NBC4 is incomplete. The NBC-related protein called NCBE mediates Na-dependent, Cl⁻/Bicarbonate Exchange. The purpose of this review is to summarize recent advances on the cloning of NBC isoforms and related proteins and their role and regulation in physiologic and pathologic states. Received: 26 February 2001/Revised: 14 May 2001     

Cloning and Function of the Rat Colonic Epithelial K+ Channel KVLQT1

K_VLQT1 (KCNQ1) is a voltage-gated K⁺ channel essential for repolarization of the heart action potential that is defective in cardiac arrhythmia. The channel is inhibited by the chromanol 293B, a compound that blocks cAMP-dependent electrolyte secretion in rat and human colon, therefore suggesting expression of a similar type of K⁺ channel in the colonic epithelium. We now report cloning and expression of K_VLQT1 from rat colon. Overlapping clones identified by cDNA-library screening were combined to a full length cDNA that shares high sequence homology to K_VLQT1 cloned from other species. RT-PCR analysis of rat colonic musoca demonstrated expression of K_VLQT1 in crypt cells and surface epithelium. Expression of rK_VLQT1 in Xenopus oocytes induced a typical delayed activated K⁺ current, that was further activated by increase of intracellular cAMP but not Ca²⁺ and that was blocked by the chromanol 293B. The same compound blocked a basolateral cAMP-activated K⁺ conductance in the colonic mucosal epithelium and inhibited whole cell K⁺ currents in patch-clamp experiments on isolated colonic crypts. We conclude that K_VLQT1 is forming an important component of the basolateral cAMP-activated K⁺ conductance in the colonic epithelium and plays a crucial role in diseases like secretory diarrhea and cystic fibrosis. Received: 17 July 2000/Revised: 25 October 2000     

Intracellular Acidification Induces Cl/HCO3 Exchange Activity in the Basolateral Membrane of β-intercalated Cells of the Rabbit Cortical Collecting Duct

High speed video imaging microscopy and the pH-sensitive fluorophore2′,7′,-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) were used to examine acid-base functions of beta-intercalated cells of the rabbit cortical collecting duct. The presence of intercalated cells was established and the properties of apical and basolateral acid-base transporters assessed by monitoring cell pH during acid loading and luminal and basolateral ion substitutions. We showed that treatment of beta-intercalated cells with ammonium chloride (20 mm) induced a profound decrease of their intracellular pH from 6.98 ± 5.93 ± 0.08. pH recovery occurred after different lag periods ranging between 2 to 15 min (0.22 ± 0.04 dpH/dt). We demonstrated that this pH recovery mechanism was independent of basolateral Na⁺ and apical HCO⁻ ₃ and K⁺. It was also not affected by apical and basolateral addition of NEM, by basolateral DIDS and by apical application of the H-KATPase inhibitor SCH28080. The process of pH recovery was however, critically dependent on basolateral HCO⁻ ₃. These results are best explained by acid-induced insertion and/or activation of chloride-bicarbonate exchangers that are functional properties with their apical analogues. Received: 11 January 1994/Revised: 13 June 1997