The chloride/base exchanger in the basolateral cell membrane of rabbit renal proximal tubule S3 segment requires bicarbonate to operate

A pure chloride channel mutant of CLC-5 causes Dent's disease via insufficient V-ATPase activation

Dent's disease is characterized by defective endocytosis in renal proximal tubules (PTs) and caused by mutations in the 2Cl(-)/H(+) exchanger, CLC-5. However, the pathological role of endosomal acidification in endocytosis has recently come into question. To clarify the mechanism of pathogenesis for Dent's disease, we examined the effects of a novel gating glutamate mutation, E211Q, on CLC-5 functions and endosomal acidification. In Xenopus oocytes, wild-type (WT) CLC-5 showed outward-rectifying currents that were inhibited by extracellular acidosis, but E211Q and an artificial pure Cl(-) channel mutant, E211A, showed linear currents that were insensitive to extracellular acidosis. Moreover, depolarizing pulse trains induced a robust reduction in the surface pH of oocytes expressing WT CLC-5 but not E211Q or E211A, indicating that the E211Q mutant functions as a pure Cl(-) channel similar to E211A. In HEK293 cells, E211A and E211Q stimulated endosomal acidification and hypotonicity-inducible vacuolar-type H(+)-ATPase (V-ATPase) activation at the plasma membrane. However, the stimulatory effects of these mutants were reduced compared with WT CLC-5. Furthermore, gene silencing experiments confirmed the functional coupling between V-ATPase and CLC-5 at the plasma membrane of isolated mouse PTs. These results reveal for the first time that the conversion of CLC-5 from a 2Cl(-)/H(+) exchanger into a Cl(-) channel induces Dent's disease in humans. In addition, defective endosomal acidification as a result of insufficient V-ATPase activation may still be important in the pathogenesis of Dent's disease.

Acid-base transport by the renal proximal tubule

One of the major tasks of the renal proximal tubule is to secrete acid into the tubule lumen, thereby reabsorbing approximately 80% of the filtered HCO3- as well as generating new HCO3- for regulating blood pH. This review summarizes the cellular and molecular events that underlie four major processes in HCO3- reabsorption. The first is CO2 entry across the apical membrane, which in large part occurs via a gas channel (aquaporin 1) and acidifies the cell. The second process is apical H+ secretion via Na-H exchange and H+ pumping, processes that can be studied using the NH4+ prepulse technique. The third process is the basolateral exit of HCO3- via the electrogenic Na/HCO3 co-transporter, which is the subject of at least 10 mutations that cause severe proximal renal tubule acidosis in humans. The final process is the regulation of overall HCO3- reabsorption by CO2 and HCO3- sensors at the basolateral membrane. Together, these processes ensure that the proximal tubule responds appropriately to acute acid-base disturbances and thereby contributes to the regulation of blood pH.

Chloride transport in the renal proximal tubule

The renal proximal tubule is responsible for most of the renal sodium, chloride, and bicarbonate reabsorption. Micropuncture studies and electrophysiological techniques have furnished the bulk of our knowledge about the physiology of this tubular segment. As a consequence of the leakiness of this epithelium, paracellular ionic transport--in particular that of Cl(-)--is of considerable importance in this first part of the nephron. It was long accepted that proximal Cl(-) reabsorption proceeds solely paracellularly, but it is now known that transcellular Cl(-) transport also exists. Cl(-) channels and Cl(-)-coupled transporters are involved in transcellular Cl(-) transport. In the apical membrane, Cl(-)/anion (formate, oxalate and bicarbonate) exchangers represent the first step in transcellular Cl(-) reabsorption. A basolateral Cl(-)/HCO(3)(-) exchanger, involved in HCO(3)(-) reclamation, participates in the rise of intracellular Cl(-) activity above its equilibrium value, and thus also contributes to the creation of an outwardly directed electrochemical Cl(-) gradient across the cell membranes. This driving force favours Cl(-) diffusion from the cell to the lumen and to the interstitium. In the basolateral membrane, the main mechanism for transcellular Cl(-) reabsorption is a Cl(-) conductance, but a Na(+)-driven Cl(-)/HCO(3)(-) exchanger may also participate in Cl(-) reabsorption.

Axial heterogeneity of sodium-bicarbonate cotransporter expression in the rabbit proximal tubule

It is generally accepted that Na(HCO3)n cotransport is the most important mechanism mediating basolateral bicarbonate efflux in the early proximal tubule. The presence of basolateral Na(HCO3)n cotransport in the late proximal tubule (S3 segment) and in the juxtamedullary S1 and S2 segments has been controversial. The renal sodium-bicarbonate cotransporter (NBC) has been recently cloned from rat (M. F. Romero, M. A. Hediger, E. L. Boulpaep, and W. F. Boron. J. Am. Soc. Nephrol. 7: 1259, 1996), salamander (M. F. Romero, M. A. Hediger, E. L. Boulpaep, and W. F. Boron. Nature 387: 409-413, 1997), and human (C. E. Burnham, H. Amlal, Z. Wang, G. E. Shull, and M. Soleimani. J. Biol. Chem. 272: 19111-19114, 1997). The localization of NBC in the kidney is unknown. The present study was designed to localize NBC mRNA expression in the rabbit proximal tubule. In situ hybridization studies were combined with functional studies of basolateral Na(HCO3)n cotransport in superficial and juxtamedullary S1, S2, and S3 segments of the rabbit proximal tubule. The results demonstrate that NBC mRNA is localized predominantly to the cortex, with less expression in the outer medulla. NBC expression was not detected in the inner medulla. The highest level of NBC mRNA is in the S1 proximal tubule. NBC is expressed at a low levels in the S3 segment, with intermediate expression in the S2 segment. In bicarbonate-buffered solutions, the rate of base efflux mediated by Na(HCO3)n cotransport followed a similar pattern in superficial and juxtamedullary proximal tubule segments, i.e., S1 > S2 > S3. The juxtamedullary S1 segment had the greatest rate of basolateral Na(HCO3)n cotransport and the highest level of NBC expression in the proximal tubule.

Bicarbonate absorption in eel intestine: evidence for the presence of membrane-bound carbonic anhydrase on the brush border membranes of the enterocyte

Bicarbonate absorptive fluxes through the isolated intestine of the European eel (Anguilla anguilla) were evaluated by the pH-stat method under short-circuited conditions. It was found that bicarbonate absorptive flux was dependent on the luminal Na+ and was inhibited by luminal 4-acetamido-4' stilbene-2-2' disulfonic acid (SITS; 2.5 x 10(-4) M) and luminal acetazolamide (10(-4) M), while luminal amiloride (1 mM) was without effect. Furthermore, by using brush border membrane vesicles (BBMV) isolated from eel intestine, the existence of two carbonic anhydrase (CA) isoforms, one tightly associated to the brush border membrane (BBM) and the other soluble in the cytosol, was demonstrated. The membrane-bound CA differs from the cytoplasmic isoform in that 1) it is relatively resistant to treatment with 0.045% lauryl sulfate sodium salt (SDS); 2) it is less inhibitable by ethoxzolamide and sulfanilamide; and 3) its Kmapp is significantly lower than that of the cytoplasmic isoform. These results suggest that a BBM-bound CA isozyme would play an important role in bicarbonate absorption from the lumen, facilitating the HCO3- transfer through the luminal membrane of the eel enterocyte most likely via a Na+ (HCO3-) or (OH-) cotransport system.

Mechanism of apical and basolateral Na(+)-independent Cl-/base exchange in the rabbit superficial proximal straight tubule

The present study was undertaken to determine the magnitude and mechanism of base transport via the apical and basolateral Na(+)-independent Cl-/base exchangers in rabbit isolated perfused superficial S2 proximal tubules. The results demonstrate that there is an apical Na(+)-independent Cl-/base exchanger on both membranes. HCO3- fails to stimulate apical Cl-/base exchange in contrast to the basolateral exchanger. Inhibition of endogenous HCO3- production does not alter the rate of apical Cl-/base exchange in Hepes-buffered solutions. Both exchangers are inhibited by H2DIDS and furosemide; however, the basolateral anion exchanger is more sensitive to these inhibitors. The results indicate that the apical and basolateral Cl-/base exchangers differ in their transport properties and are able to transport base equivalents in the absence of formate. The formate concentration in rabbit arterial serum is approximately 6 microM and in vitro tubule formate production is < 0.6 pmol/min per mm. Formate in the micromolar range stimulates Jv in a dose-dependent manner in the absence of a transepithelial Na+ and Cl- gradient and without a measurable effect on Cl(-)-induced equivalent base flux. Apical formic acid recycling cannot be an important component of any cell model, which accounts for formic acid stimulation of transcellular NaCl transport in the rabbit superficial S2 proximal tubule. We propose that transcellular NaCl transport in this nephron segment is mediated by an apical Na+/H+ exchanger in parallel with a Cl-/OH- exchanger and that the secreted H+ and OH- ions form H2O in the tubule lumen.

Chloride transport in brush-border membrane vesicles from chick jejunum

This study sought to characterize the mechanism(s) of Cl- transport across brush-border membrane vesicles (BBMV) isolated from chick jejunum. An inwardly directed proton gradient stimulated chloride (36Cl-) uptake. This uptake was inhibited by SITS and H2-DIDS. pH-gradient-stimulated Cl- uptake was electroneutral, since it was only slightly decreased by voltage clamping the BBMV with K+ and valinomycin. An outwardly directed HCO3- gradient significantly increased chloride uptake in the presence of a pH gradient. pH-driven chloride uptake was reduced by the presence of several anions in the uptake buffer. The rank order of potency for inhibition of pH-driven Cl- uptake was Cl- > SCN- > HCO3- > I- > Glu- > HPO4(2-). In the absence of a pH gradient, chloride was less concentrated inside the vesicles than outside. Chloride uptake under these conditions was stimulated by a positive electrical potential inside the vesicles. This stimulation was inhibited by the addition of several anions outside the vesicles. The order of inhibitory potency was SCN- > I- > Cl- > HCO3- > Glu- > HPO4(2-). The results are consistent with the presence of a Cl-/base exchanger and a chloride conductance pathway in the brush-border membrane of chick small intestine.

Evidence for conductive Cl- pathway in the basolateral membrane of rabbit renal proximal tubule S3 segment

The mechanism of Cl- exit was examined in the basolateral membrane of rabbit renal proximal tubule S3 segment with double-barreled, ion-selective microelectrodes. After the basolateral Cl-/HCO3- exchanger was blocked by 2'-disulfonic acid, a bath K+ step from 5 to 20 mM induced 26.6 mV depolarization and 7.7 mM increase in intracellular Cl- activities ([Cl(-)]i). K+ channel blockers, Ba2+, and quinine strongly suppressed both the response in cell membrane potentials (Vb) and in (Cl-)i to the bath K+ step, while Cl- channel blockers, A9C (1 mM) and IAA-94 (0.3 mM) inhibited only the latter response by 49 and 74%, respectively. By contrast, an inhibitor of K(+)-Cl- cotransporter, H74, had no effect on the increase in (Cl-)i to the bath K+ step. Furosemide and the removal of bath Na+ were also ineffective, suggesting that (Cl-)i are sensitive to the cell potential changes. Bath Cl- removal in the presence of quinine induced a depolarization of more than 10 mV and a decrease in (Cl-)i, and IAA-94 inhibited these responses similarly in the bath K+ step experiments. These results indicate that a significant Cl- conductance exists in the basolateral membrane of this segment and functions as a Cl- exit mechanism.

Effect of parathyroid hormone on acid/base transport in rabbit renal proximal tubule S3 segment

The effect of parathyroid hormone (PTH) on acid/base transport in isolated rabbit renal proximal tubule S3 segment was investigated with double-barreled and conventional microelectrodes. PTH (10 nM) induced a small depolarization and enhanced the initial rates of cell pH (pHi) increase and cell Cl- ([Cl-]i) decrease in response to bath Cl- removal by 28.0 +/- 2.1% and 31.0 +/- 6.4% respectively. The calculated initial HCO3- influx to bath Cl- removal was also enhanced by 28%. On the other hand, PTH reduced the initial rate of pHi decrease to luminal Na+ removal in the absence of HCO3-/CO2 by 20.4 +/- 3.9%. The PTH-induced depolarization was not accompanied with changes in steady-state pHi or [Cl-]i levels, but was greatly attenuated in the presence of ouabain (0.1 mM). Either dibutyryl-cAMP (0.1 mM) plus theophylline (1 mM) or forskolin (10 microM) alone could reproduce all the effects of PTH. These results indicate that (a) PTH inhibits the luminal Na+/H+ exchanger but stimulates the basolateral Cl-/HCO3- exchanger in the S3 segment; (b) the PTH-induced depolarization largely results from inhibition of Na+/K(+)-ATPase and (c) all these effects are at least partly mediated by a cAMP-dependent mechanism.

Acetazolamide inhibition of basolateral Cl-/HCO3- exchange in rabbit renal proximal tubule S3 segment

Cell pH (pH(i)) and cell membrane potential (Vb) were measured in isolated S3 segments of rabbit renal proximal tubule with double-barrelled microelectrodes to search for a possible effect of the carbonic anhydrase inhibitor, acetazolamide (ACZ), on Cl-/HCO3- exchange in the basolateral cell membrane. ACZ was found to retard and reduce the pH(i) response to bath Cl- removal reversibly with half-maximal inhibition at 0.42 mmol/l and a rather flat concentration dependence (Hill coefficient approximately 0.36). To determine whether the retardation resulted from inhibition of cytoplasmic carbonic anhydrase, which might have delayed the attainment of HCO3-/CO2 equilibrium, we have measured the response of pH(i) to step changes in PCO2 in the presence and absence of ACZ. ACZ greatly retarded the pH(i) response to CO2 steps; however, the concentration dependence differed (half-maximal inhibition at 18 mumol/l) and even at maximal ACZ concentrations the response to CO2 steps was more than twice as fast as the response to Cl- replacement. Since, in addition, the ACZ inhibition of Cl-/HCO3- exchange could not be overcome by increasing PCO2 we conclude that the ACZ effect on Cl-/HCO3- exchange in rabbit proximal tubule S3 segments does not result from inhibition of cytosolic or membrane-bound carbonic anhydrase, but from a direct interaction with the exchanger molecule.

Cl-/base exchange and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Cl-/base exchange activity have been examined in isolated chicken enterocytes, both in the presence and absence of 25 mM HCO3-/5% CO2. Intracellular pH was measured with BCECF, a pH-sensitive carboxyfluorescein derivative. Under resting conditions pHi was 7.17 in Hepes and 7.12 in HCO3(-)-buffered solutions. Cells became more alkaline upon withdrawal of Cl-. Cells depleted of Cl- acidified upon reinstatement of Cl-. These changes were faster in the presence of HCO3- than in its absence. After an alkaline load (removal of HCO3- from the medium) pHi decreases towards base line in the presence of Cl-, but not in its absence. The Cl(-)-dependent pHi changes were prevented by H2DIDS and were unaffected by Na+. The Cl(-)-induced recovery from an alkaline load exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Cl- and maximum velocity of approximately 0.20 pH units min-1. The Cl-/base exchange is functional under resting conditions, as shown by cell alkalinization on exposure to 0.5 mM H2DIDS, both in the presence and in the absence of HCO3-. It is concluded that Cl-/base exchange participates in setting the resting intracellular pH in isolated chicken enterocytes and helps recover from alkaline loads. The exchange operates both in the presence and in the absence of bicarbonate.