Evidence of chloride/bicarbonate exchange mediating bicarbonate efflux from S3 segments of rabbit renal proximal tubule

Chloride transport

Chloride transport along the nephron is one of the key actions of the kidney that regulates extracellular volume and blood pressure. To maintain steady state, the kidney needs to reabsorb the vast majority of the filtered load of chloride. This is accomplished by the integrated function of sequential chloride transport activities along the nephron. The detailed mechanisms of transport in each segment generate unique patterns of interactions between chloride and numerous other individual components that are transported by the kidney. Consequently, chloride transport is inextricably intertwined with that of sodium, potassium, protons, calcium, and water. These interactions not only allow for exquisitely precise regulation but also determine the particular patterns in which the system can fail in disease states.

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

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

Interaction of Membrane Skeletal Protein, Protein 4.1B and p55, and Sodium Bicarbonate Cotransporter1 in Mouse Renal S1-S2 Proximal Tubules

Our recent studies demonstrated the localization of protein 4.1B, a member of the 4.1 skeletal membrane proteins, to the basolateral membranes of the S1-S2 renal proximal tubules. In the present studies, we investigated the presence of binding partners that could form a molecular complex with the 4.1B protein. Immunohistochemistry revealed the localization of p55, a membrane-associated guanylate kinase, and the sodium bicarbonate cotransporter1 (NBC1), to the basolateral membrane domain of S1-S2 in mouse renal proximal tubules. Using immunoprecipitation of kidney lysates with anti-p55 antibody, a positive band was blotted with anti-4.1B antibody. GST fusion proteins including the NBC1 and 4.1B regions were confirmed to bind with each other by electrophoresis after mixing. Both NBC1- and 4.1B-specific bands were detected in renal protein mixtures immunoprecipated by either anti-4.1B- or NBC1-specific antibodies. It is likely that NBC1, 4.1B, and p55 form a molecular complex in the basolateral membrane of the kidney S1-S2 proximal tubules. We propose that the 4.1B-containing membrane skeleton may play a role in regulating the Na+ and HCO3- reabsorption in S1-S2 proximal tubules.

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.

Kidneys sans glomeruli

The evolution of the vertebrate kidney records three occasions, each separated by about 50 million years, when fish have abandoned glomeruli to produce urine by tubular mechanisms. The recurring dismissal of glomeruli suggests a mechanism of aglomerular urine formation intrinsic to renal tubules. Indeed, the transepithelial secretion of organic solutes and of inorganic solutes such as sulfate, phosphate, and magnesium can all drive secretory water flow in renal proximal tubules of fish. However, the secretion of NaCl via secondary active transport of Cl is the primary mover of secretory water flow in, surprisingly, proximal tubules of both glomerular and aglomerular fish. In filtering kidneys, the tubular secretion of solute and water is overshadowed by reabsorptive transport activities, but secretion progressively comes to light as glomerular filtration decreases. Thus the difference between glomerular and aglomerular urine formation is more a difference of degree than of kind. At low rates of glomerular filtration in seawater fish, NaCl-coupled water secretion serves to increase the renal excretory capacity by increasing the luminal volume into which waste, excess, and toxic solutes can be secreted. The reabsorption of NaCl and water in the distal nephron and urinary bladder concentrates unwanted solutes for excretion while minimizing renal water loss. In aglomerular fish, NaCl-coupled water secretion across proximal tubules replaces glomerular filtration to increase renal excretory capacity. A review of the literature suggests that tubular secretion of NaCl and water is an early function of the vertebrate proximal tubule that has been retained throughout evolution. Active transepithelial Cl secretion takes place in gall bladders studied as models of the mammalian proximal tubule and in proximal tubules of amphibians and apparently also of mammals. The tubular secretion of Cl is also observed in mammalian distal tubules. The evidence consistent with and for Cl secretion in, respectively, proximal and distal tubules of the mammalian kidney calls for a reexamination of basic assumptions in renal physiology that may lead to new opportunities for managing some forms of renal disease.

Defective D1-like receptor-mediated inhibition of the Cl-/HCO3- exchanger in immortalized SHR proximal tubular epithelial cells

The sensitivity of the Cl(-)/HCO(3)(-) exchanger to dopamine D(1)- and D(2)-like receptor stimulation in immortalized renal proximal tubular epithelial cells from the spontaneous hypertensive rat (SHR) and Wistar-Kyoto rat (WKY) was examined. The activity of the Cl(-)/HCO(3)(-) exchanger (in pH U/s) in SHR cells (0.00191) was greater than in WKY cells (0.00126). The activity of Cl(-)/HCO(3)(-) exchanger was exclusively observed at the apical cell side and probably occurs through the SLC26A6 anion transporter that is expressed in both WKY and SHR cells. Stimulation of D(1)-like receptors with SKF-38393 markedly attenuated the HCO(3)(-)-dependent intracellular pH recovery in WKY cells but not in SHR cells. Stimulation of D(2)-like receptors with quinerolane did not alter Cl(-)/HCO(3)(-) exchanger activity in both WKY and SHR cells. The selective D(1)-like receptor antagonist SKF-83566 prevented the effect of SKF-38393. Both WKY and SHR cells responded to dibutyryl-cAMP (DBcAMP) with inhibition of the Cl(-)/HCO(3)(-) exchanger, and downregulation of PKA (overnight exposure to DBcAMP) abolished the inhibitory effect of both DBcAMP and SKF-38393 in WKY cells. Both SHR and WKY cells responded to forskolin with increases in the formation of cAMP. However, only WKY responded to SKF-38393 with increases in the formation of cAMP that was prevented by SKF-83566. It is concluded that WKY cells respond to D(1)-like dopamine receptor stimulation with inhibition of the apical Cl(-)/HCO(3)(-) (SLC26A6) exchanger and SHR cells have a defective D(1)-like dopamine response.

NH4Cl activates AE2 anion exchanger in Xenopus oocytes at acidic pHi

In the course of experiments to define regulation by intracellular pH (pHi) of the AE2 anion exchanger expressed in Xenopus oocytes, we discovered an unexpected regulation of AE2 by NH4+. Intracellular acidification produced by extracellular acidification or produced by equimolar substitution of NaCl with sodium acetate each inhibited AE2 activity. In contrast, intracellular acidification by equimolar substitution of NaCl with NH4Cl activated AE2-associated, trans-anion-dependent, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive 36Cl- influx and efflux. Regulation by NH4+ was isoform specific, since neither erythroid nor kidney AE1 was activated. AE2 activation was maximal at <5 mM NH4Cl; was not mimicked by extracellular KCl, chloroquine, or polyamines; and was insensitive to amiloride, bumetanide, barium, and gadolinium. Whether NH4Cl acts directly on AE2 or on another target remains to be determined. Activation of AE2 by NH4+ may serve to sustain Cl-/HCO3- exchange activity in the presence of acidic pH in renal medulla, colon, abscesses, and other AE2-expressing acidic locales exposed to elevated NH4+ concentration.

Is resting state HCO3- secretion in frog gastric fundus mucosa mediated by apical Cl(-)-HCO3- exchange?

1. We have tested the widely accepted hypothesis that resting-state bicarbonate secretion of gastric fundus mucosa is mediated by Cl(-)-HCO3- exchange in the apical membrane of surface epithelial cells (SECs). To this end, SECs of isolated fundus mucosa of Rana esculenta were punctured with double-barrelled microelectrodes to measure intracellular pH (pHi). 2. No significant pHi changes were observed in response to changing luminal HCO3- and/or Cl- concentrations. The change in pHi (delta pHi) in response to luminal chloride substitution averaged 0.00 +/- 0.01 pH units (mean +/- S.E.M.; n = 48), and did not change after blocking putative basolateral acid/base transporters which could have masked the pHi response. 3. On the other hand, pHi responded readily and reversibly to luminal perfusion with either low-pH (pH 2.5) solution (delta pHi = -0.36 +/- 0.05; n = 4; P < 0.01) or CO2-free HCO3- Ringer solution (delta pHi = +0.10 +/- 0.01; n = 29; P < 0.001). These observations demonstrate that the solution change was effective and complete within 1 min and show that the apical membrane of SECs is permeable to CO2. 4. The apical membrane of frog SECs could not be stained with an antibody against the C-terminal end of the mouse Cl(-)-HCO3- exchanger isoform AE2, although this antibody readily stained the basolateral membrane of the oxyntopeptic cells (OCs). 5. In conclusion, the presence of a Cl(-)-HCO3- exchanger in the apical membrane of SECs of frog gastric fundus mucosa in the resting state could not be confirmed, but other models of HCO3- secretion cannot be fully excluded. Observations from electrical measurements, favouring a model of conductive HCO3- secretion, point to the OCs rather than the SECs as a site of origin of HCO3- secretion.

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.

Activation of the basolateral Cl- conductance by cAMP in rabbit renal proximal tubule S3 segments

The regulatory mechanism of basolateral Cl- conductance in rabbit renal proximal tubule S3 segments was investigated with conventional and Cl- sensitive microelectrodes. After the basolateral Cl-/HCO3- exchanger was blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) we increased the bath K+ concentration from 5 mmol/l to 20 mmol/l, which depolarized the cells and thereby increased intracellular Cl- activity ([Cl-]i). This [Cl-]i response was enhanced by +63% in the presence of forskolin (20 mumol/l), by +40% in the presence of dibutyryl adenosine 3',5'-cyclic monophosphate (db-cAMP) (1 mmol/l) and by +44% in the presence of parathyroid hormone (PTH, 10 nmol/l), whereas it was inhibited by a Cl- channel blocker, indanyl-oxyacetic acid (IAA-94, 0.3 mmol/l). In addition, forskolin, PTH and chlorophenylthio-cAMP enhanced the electrogenic response to removal of bath Cl- after the blockade of K+ conductance, and this activation was also sensitive to IAA-94. On the other hand, 2 mumol/l ionomycin and 0.5 mumol/l phorbol myristate failed to activate the [Cl-]i response to elevation of bath K+ concentration and the electrogenic response to Cl- removal, and ionomycin had no effect even in the absence of DIDS. These results indicate that this basolateral Cl- conductance can be activated by cAMP, while neither the increase in cytosolic Ca2+ nor the activation of protein kinase C has direct effects on this conductance.

The effect of acute metabolic alkalosis on bicarbonate transport along the loop of Henle. The role of active transport processes and passive paracellular backflux

The loop of Henle (LOH) reabsorbs approximately 15% of filtered HCO3- via a luminal Na(+)-H+ exchanger and H+ATPase. During acute metabolic alkalosis (AMA) induced by i.v. HCO3- infusion, we have observed previously inhibition of LOH net HCO3- reabsorption (JHCO3-), which contributes to urinary elimination of the HCO3- load and correction of the systemic alkalosis. To determine whether the activities of the Na(+)-H+ exchanger and/or H(+)-ATPase are reduced during AMA, two inhibitors believed to be sufficiently specific for each transporter were delivered by in vivo LOH microperfusion during AMA. AMA reduced LOH JHCO3- from 205.0 +/- 10.8 to 96.2 +/- 11.8 pmol.min-1 (P < 0.001). Luminal perfusion with bafilomycin A1 (10(-4) mol.l-1) caused a further reduction in JHCO3- by 83% and ethylisopropylamiloride (EIPA; 5.10(-4) mol.l-1) completely abolished net HCO3- reabsorption. The combination of bafilomycin A1 and EIPA in the luminal perfusate was additive, resulting in net HCO3- secretion (-66.6 +/- 20.8 pmol.min-1; P < 0.001) and abolished net fluid reabsorption (from 5.0 +/- 0.6 during AMA to 0.2 +/- 1.1 nl.min-1; P < 0.001). To establish whether HCO3- secretion via luminal stilbene-sensitive transport mechanism participates in LOH adaptation to AMA, we added diisothiocyanato-2,2'-stilbenedisulphonate (DIDS; 10(-4) mol.l-1) to the perfusate. No effect was found.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of acetazolamide and amlodipine on the intracellular sodium content of rat proximal tubular cells

1. This investigation set out to use 23Na n.m.r. spectroscopy to measure changes in intracellular levels of sodium in isolated suspensions of rat proximal tubules. The effects of temperature, an inhibitor of the sodium pump and known natriuretic drugs on intracellular sodium content of such tubular preparations were measured and compared with calcium channel antagonists where action at this level is unclear. 2. Rat kidneys were perfused with collagenase, roughly chopped, subjected to mechanical dispersion and washed to remove all traces of the enzyme. The proximal tubules were then purified and concentrated by Percoll density gradient centrifugation and then resuspended in buffer containing dysprosium tripolyphosphate shift reagent. 3. Distinct peaks corresponding to intracellular and extracellular sodium signals were observed when the tubules were placed into the n.m.r. spectrometer. As the temperature of the suspension rose to 37 degrees C, there was an exponential decrease in sodium content, with a decay constant of 0.15 +/- 0.02 min-1, which reached a stable level within 20 to 25 min. Addition of ouabain, 10(-3) M, resulted in a significant (P < 0.01) 30% increase in intracellular sodium content within 5 min which peaked at 70% 20 min later. Although acetazolamide (10(-3) M) significantly (P < 0.01) increased intracellular sodium content by 45%, amlodipine (10(-4) M) had no effect. 4. These data show that changes in the activity of the Na+/K+/ATPase have a considerable influence on the intracellular levels of sodium in proximal tubule cells. Inhibition of carbonic anhydrase activity resulted in a rise in intracellular sodium content which is compatible with its action to reduce the turnover rate of the Na+/(HCO3-)3 symporter. The lack of effect of amlodipine was consistent with the suggestion that it does not have a direct action on the sodium handling processes at the level of the proximal tubule.

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.

Acetazolamide inhibition of basolateral base exit in rabbit renal proximal tubule S2 segment

The influence of the carbonic anhydrase inhibitor acetazolamide (ACZ) was investigated on HCO3- transport mechanisms in the basolateral cell membrane of rabbit renal proximal tubule. Experiments were performed on isolated S2 segments using double-barrelled microelectrodes to measure cell membrane potential (Vb) and cell pH (pH(i)) during step changes in bath perfusate ion concentrations. Peritubular application of ACZ (1 mmol/l) reduced the initial Vb response to 10:1 reduction of bath HCO3- concentration only slightly, from +53.8 +/- 4.2 mV to +49.1 +/- 0.3 mV (n = 5), but caused an intermittent overshooting repolarization in the secondary Vb response. In conjunction with these effects it left the initial pH(i)) response virtually unchanged but induced a secondary slow acidification. These observation indicate that--under the present experimental conditions--ACZ does not block the Na(+)-HCO3- cotransporter but acts via inhibition of cytosolic carbonic anhydrase. This was confirmed by studying the effect of elevated intracellular HCO3- concentrations under reduced flux conditions and by comparing the concentration dependence of the Vb response with the inhibition kinetics of cytosolic carbonic anhydrase. In contrast, peritubular ACZ inhibited Na(+)-independent Cl-/HCO3- exchange in the basolateral cell membrane of S2 segments directly in a similar way to that described in the preceding publication for S3 segments.

Effect of calcitonin on the regulation of intracellular pH in primary cultures of rabbit early distal tubule

To examine the intracellular pH (pHi) regulation in primary cultures of rabbit distal convoluted tubules (DCTb) we used the pH-sensitive dye 2,7-bis-carboxyethyl-5(6)-carboxyfluorescein (BCECF/AM) and a video-microscopy technique. DCTb segments were microdissected from rabbit kidney cortex and cultured in a hormonally defined medium. The culture epithelia were grown on semi-transparent permeable supports. Before pHi measurement, DCTb primary cultures were maintained for 48-96 h in growth-factor-free medium to obtain quiescent cells. We had previously shown that two mechanisms are involved in the regulation of intracellular pH: a basolateral Na+/H+ exchanger and an apical Cl-/HCO3- exchanger. The pHi of DCTb cells was significantly decreased by the addition of 60 nM human calcitonin (from 7.30 +/- 0.04 to 7.08 +/- 0.04). This response to calcitonin was dose-dependent and mimicked by both forskolin and permeant cyclic AMP derivatives. An initial acidification (of 0.25 pH unit in 7-8 min) was observed after the addition of basolateral amiloride (1 mM). The persistence of the effect induced by human calcitonin in these conditions, suggests that the Na+/H+ exchanger is not involved in the response. However, the acidification response was blocked in both the absence of chloride at the apical side and by the apical addition of 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). These experiments suggest that the target for the human calcitonin effect on pHi is the Cl-/HCO3- exchanger. This study confirms the importance of this transporter in pHi regulation within the physiological pHi range and the influence of calcitonin in the regulation of DCTb cell function.

Cl/HCO3 exchange in the basolateral membrane domain of rat jejunal enterocyte

Basolateral membrane vesicles isolated from rat jejunal enterocyte and well purified from brush border contamination were tested to examine Cl and HCO3 movements. Uptake experiments provided no evidence for a coupling between Na and HCO3 fluxes; K-HCO3 and K-Cl cotransports also could be excluded. Transport studies revealed the presence of a Cl/HCO3 exchanger accepting other anions and inhibitable by the disulfonic stilbenes SITS and DIDS. We can exclude that the evidenced HCO3-dependent Cl uptake is due to brush border contamination, since in jejunal brush border membranes this mechanism, if present, has a very low transport rate. Besides the Cl/HCO3 antiporter, a Cl-conductive pathway seems to exist in jejunal basolateral membranes.

Basolateral electrogenic Na/HCO3 symport in the amphibian distal tubule

This study was carried out to assess whether the amphibian distal tubule possesses a basolateral Na/(HCO3)n greater than 1 cotransport. The experiments were performed in the kidney of Necturus maculosus in vivo, by means of double-barreled selective microelectrodes. Basolateral membrane potential (Vm), intracellular pH (pHi), intracellular sodium activity (alpha Nai) and intracellular chloride activity (alpha Cli), were recorded during selected disturbances of peritubular fluid composition. The following results were obtained. (a) A sudden decrease of [HCO3]o leads to Vm depolarization, intracellular acidification and decrease of alpha Nai. (b) A rapid fall of [Na]o elicits Vm depolarization and decreases pHi: these patterns are not substantially altered in the presence of millimolar amiloride concentrations or in the nominal absence of peritubular Cl. (c) An acute decrease of [Na]o does not alter alpha Cli. (d) In the functional absence of CO2/HCO3 buffer (nominally CO2-free solution plus methazolamide), the reduction of [Na]o has no effect on Vm and/or pHi. We conclude that the distal tubule basolateral cell membrane is endowed with an electrogenic chloride-independent Na/base carrier, mediating Na and base efflux. The blockade of this carrier by carbonic anhydrase inhibitor indicates that the cotransported base is HCO3 or a related species.

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

Isolated microperfused S3 segments of rabbit renal proximal tubule were investigated with pH-sensitive double-barrelled intracellular microelectrodes to determine whether the Cl-/base exchanger, which we have previously identified in the basolateral cell membrane of this segment requires HCO3- or can also work in CO2/HCO3- free conditions. Cell pH (pHi) was measured in response to sudden substitution of bath Cl- by gluconate. In control solutions containing 25 mmol/l HCO3 pHi increased initially by 5.0 +/- 0.3 x 10(-3) unit/s but after perfusion with CO2/HCO3(-)-free solutions pHi of the same cells increased only by 1.3 +/- 0.2 x 10(-3) unit/s in response to Cl- substitution. From measurements of the cellular buffering power it was calculated that the control base flux had fallen drastically from 3.7 +/- 0.3 to 0.3 +/- 0.1 x 10(-12) mol/s.cm tubule length. To test whether the remaining flux might have resulted from metabolic CO2, oxidative metabolism was poisoned with cyanide (5 mmol/l). This abolished the pH change (delta pHi) in CO2/HCO3(-)-free solutions, but did not affect the pH shift in the presence of HCO3-. The data indicate that basolateral Cl-/base exchange in S3 segment requires HCO3- to operate. A model in which HCO3- absorption proceeds in form of OH- and CO2 can be largely excluded.