Bicarbonate transport by isolated perfused rabbit proximal convoluted tubules

Analysis of the binding moiety mediating the interaction between monocarboxylate transporters and carbonic anhydrase II

Proton-coupled monocarboxylate transporters (MCTs) mediate the exchange of high energy metabolites like lactate between different cells and tissues. We have reported previously that carbonic anhydrase II augments transport activity of MCT1 and MCT4 by a noncatalytic mechanism, while leaving transport activity of MCT2 unaltered. In the present study, we combined electrophysiological measurements in Xenopus oocytes and pulldown experiments to analyze the direct interaction between carbonic anhydrase II (CAII) and MCT1, MCT2, and MCT4, respectively. Transport activity of MCT2-WT, which lacks a putative CAII-binding site, is not augmented by CAII. However, introduction of a CAII-binding site into the C terminus of MCT2 resulted in CAII-mediated facilitation of MCT2 transport activity. Interestingly, introduction of three glutamic acid residues alone was not sufficient to establish a direct interaction between MCT2 and CAII, but the cluster had to be arranged in a fashion that allowed access to the binding moiety in CAII. We further demonstrate that functional interaction between MCT4 and CAII requires direct binding of the enzyme to the acidic cluster (431)EEE in the C terminus of MCT4 in a similar fashion as previously shown for binding of CAII to the cluster (489)EEE in the C terminus of MCT1. In CAII, binding to MCT1 and MCT4 is mediated by a histidine residue at position 64. Taken together, our results suggest that facilitation of MCT transport activity by CAII requires direct binding between histidine 64 in CAII and a cluster of glutamic acid residues in the C terminus of the transporter that has to be positioned in surroundings that allow access to CAII.

Renal defects in KCNE1 knockout mice are mimicked by chromanol 293B in vivo: identification of a KCNE1-regulated K+ conductance in the proximal tubule

Non-technical summary

The kidney plays a critical role in regulating body fluid volume and blood pressure by conserving ions, solutes and water. Knowing the processes that underpin the handling of ions, solutes and water by the kidney is essential to our understanding of fluid and blood pressure regulation. Movement of ions is mediated by specific transport proteins found in the membranes of kidney cells. These proteins are regulated by additional proteins, called accessory proteins. In the current study, we have examined the role of the accessory protein KCNE1 in regulating a channel, KCNQ1, which is important in kidney function. We have observed that in the absence of KCNE1 the kidney has difficulty conserving sodium, chloride and water. However, by using specific inhibitors of these proteins we have also determined that although KCNE1 has a role in kidney function, the mechanism of its action is unlikely to be by regulating the protein KCNQ1.

Abstract

KCNE1 is a protein of low molecular mass that is known to regulate the chromanol 293B and clofilium-sensitive K⁺ channel, KCNQ1, in a number of tissues. Previous work on the kidney of KCNE1 and KCNQ1 knockout mice has revealed that these animals have different renal phenotypes, suggesting that KCNE1 may not regulate KCNQ1 in the renal system. In the current study, in vivo clearance approaches and whole cell voltage-clamp recordings from isolated renal proximal tubules were used to examine the physiological role of KCNE1. Data from wild-type mice were compared to those from KCNE1 knockout mice. In clearance studies the KCNE1 knockout mice had an increased fractional excretion of Na⁺, Cl⁻, HCO₃⁻ and water. This profile was mimicked in wild-type mice by infusion of chromanol 293B, while chromanol was without effect in KCNE1 knockout animals. Clofilium also increased the fractional excretion of Na⁺, Cl⁻ and water, but this was observed in both wild-type and knockout mice, suggesting that KCNE1 was regulating a chromanol-sensitive but clofilium-insensitive pathway. In whole cell voltage clamp recordings from proximal tubules, a chromanol-sensitive, K⁺-selective conductance was identified that was absent in tubules from knockout animals. The properties of this conductance were not consistent with its being mediated by KCNQ1, suggesting that KCNE1 regulates another K⁺ channel in the renal proximal tubule. Taken together these data suggest that KCNE1 regulates a K⁺-selective conductance in the renal proximal tubule that plays a relatively minor role in driving the transport of Na⁺, Cl⁻ and HCO₃⁻.

SLC4 base (HCO3 -, CO3 2-) transporters: classification, function, structure, genetic diseases, and knockout models

In prokaryotic and eukaryotic organisms, biochemical and physiological processes are sensitive to changes in H(+) activity. For these processes to function optimally, a variety of proteins have evolved that transport H(+)/base equivalents across cell and organelle membranes, thereby maintaining the pH of various intracellular and extracellular compartments within specific limits. The SLC4 family of base (HCO(3)(-), CO(3)(2(-))) transport proteins plays an essential role in mediating Na(+)- and/or Cl(-)-dependent base transport in various tissues and cell types in mammals. In addition to pH regulation, specific members of this family also contribute to vectorial transepithelial base transport in several organ systems including the kidney, pancreas, and eye. The importance of these transporters in mammalian cell biology is highlighted by the phenotypic abnormalities resulting from spontaneous SLC4 mutations in humans and targeted deletions in murine knockout models. This review focuses on recent advances in our understanding of the molecular organization and functional properties of SLC4 transporters and their role in disease.

Effects of angiotensin II on the CO2dependence of HCO3−reabsorption by the rabbit S2 renal proximal tubule

Previous authors showed that, at low doses, both basolateral and luminal ANG II increase the proximal tubule's HCO3−reabsorption rate ( JHCO3). Using out-of-equilibrium CO2/HCO3−solutions, we demonstrated that basolateral CO2increases JHCO3. Here, we examine interactions between ANG II and CO2in isolated, perfused rabbit S2 segments. We first used equilibrated 5% CO2/22 mM HCO3−/pH 7.40 in bath and lumen. At 10−11M, basolateral (BL) ANG II increased JHCO3by 41%, and luminal ANG II increased JHCO3by 35%. At 10−9M, basolateral ANG II decreased JHCO3by 43%, whereas luminal ANG II was without effect. Second, we varied [CO2]BLfrom 0 to 20% at fixed [HCO3−]BLand pHBL. Fractional stimulation produced by BL 10−11M ANG II falls when [CO2]BLexceeds 5%. Fractional inhibition produced by BL 10−9M ANG II tends to rise when [CO2]BLexceeds 5%. Regarding luminal ANG II, fractional stimulation produced by 10−11M ANG II fell monotonically as [CO2]BLrose from 0 to 20%. Fractional inhibition produced by 10−9M ANG II rose monotonically with increasing [CO2]BL. Viewed differently, ANG II at 10−11M tended to reduce stimulation by CO2, and at 10−9M, produced an even greater reduction. In conclusion, the mutual effects of 1) ANG II on the JHCO3response to basolateral CO2and 2) basolateral CO2on the JHCO3responses to ANG II suggest that the signal-transduction pathways for ANG II and basolateral CO2intersect or merge.

Stimulation of renal sulfate secretion by metabolic acidosis requires Na+/H+exchange induction and carbonic anhydrase

The acute effect of metabolic acidosis on SO42−secretion by the marine teleost renal proximal tubule was examined. Metabolic acidosis was mimicked in primary cultures of winter flounder renal proximal tubule epithelium (fPTCs) mounted in Ussing chambers by reducing interstitial pH to 7.1 (normally 7.7). fPTCs with metabolic acidosis secreted SO42−at a net rate that was 40% higher than in paired isohydric controls (pH 7.7 on interstitium). The stimulation was completely blocked by the carbonic anhydrase inhibitor methazolamide (100 μM). Although Na+/H+exchange (NHE) isoforms 1, 2, and 3 were identified in fPTCs by immunoblotting, administering EIPA (20 μM) to the interstitial and luminal bath solutions had no effect on net SO42−secretion by fPTCs with a normal interstitial pH of 7.7. However, EIPA (20 μM) blocked most of the stimulation caused by acidosis when applied to the lumen but not interstitium, demonstrating that induction of brush-border NHE activity is important. In the intact flounder, serum pH dropped 0.4 pH units (pH 7.7 to 7.3, at 2–3 h) when environmental pH was lowered from 7.8 to ∼4.3. Whereas serum [SO42−] was not altered by acidosis, renal tubular SO42−secretion rate was elevated 200%. Thus metabolic acidosis strongly stimulates renal sulfate excretion most likely by a direct effect on active renal proximal tubule SO42−secretion. This stimulation appears to be dependent on inducible brush-border NHE activity.

Molecular mechanism of kNBC1-carbonic anhydrase II interaction in proximal tubule cells

We have recently shown that carbonic anhydrase II (CAII) binds in vitro to the C-terminus of the electrogenic sodium bicarbonate cotransporter kNBC1 (kNBC1-ct). In the present study we determined the molecular mechanisms for the interaction between the two proteins and whether kNBC1 and CAII form a transport metabolon in vivo wherein bicarbonate is transferred from CAII directly to the cotransporter. Various residues in the C-terminus of kNBC1 were mutated and the effect of these mutations on both the magnitude of CAII binding and the function of kNBC1 expressed in mPCT cells was determined. Two clusters of acidic amino acids, L(958)DDV and D(986)NDD in the wild-type kNBC1-ct involved in CAII binding were identified. In both acidic clusters, the first aspartate residue played a more important role in CAII binding than others. A significant correlation between the magnitude of CAII binding and kNBC1-mediated flux was shown. The results indicated that CAII activity enhances flux through the cotransporter when the enzyme is bound to kNBC1. These data are the first direct evidence that a complex of an electrogenic sodium bicarbonate cotransporter with CAII functions as a transport metabolon.

Activation of the apical Na+/H+ exchanger NHE3 by formate: a basis of enhanced fluid and electrolyte reabsorption by formate in the kidney

Formate stimulates sodium chloride and fluid reabsorption in kidney proximal tubule; however, the exact cellular mechanism of this effect remains unknown. We hypothesized that the primary target of formate is the apical Na(+)/H(+) exchanger. Here, we demonstrate that formate directly enhances the apical Na(+)/H(+) exchanger (NHE3) activity in mouse kidney proximal tubule. In the absence of CO(2)/HCO(3)(-), addition of formate (500 microM) to the bath and lumen of microperfused mouse kidney proximal tubule caused significant intracellular alkalinization, with intracellular pH (pH(i)) increasing from baseline levels 7.17 +/- 0.01 to 7.55 +/- 0.01 (P < 0.001, n = 14), with a Delta pH of 0.38 +/- 0.02. Removal of luminal chloride did not block cell pH alkalinization by formate (baseline pH of 7.26 +/- 0.01 to 7.53 +/- 0.01 with formate, P < 0.001, n = 10), indicating that the apical Cl(-)/OH(-) exchanger was not the primary mediator of the effect of formate on cell pH. However, removal of sodium from the lumen or addition of EIPA completely prevented cell pH alkalinization. Addition of formate to the lumen and bath in the outer medullary collecting duct, which does not express any apical Na(+)/H(+) exchanger, did not cause any cell pH alkalinization. At lower concentrations (50 microM), formate caused significant pH(i) alkalinization in proximal tubule cells, with pH(i) increasing from baseline levels 7.15 +/- 0.02 to 7.36 +/- 0.02 (P < 0.02, n = 11). Acetate, at 50 microM, had no effect on pH(i). Formate's effect was observed both in the absence and presence of CO(2)/HCO(3)(-) in the media. We conclude that formate stimulates the apical Na(+)/H(+) exchanger NHE3 in the kidney proximal tubule. We propose that formate stimulation of chloride reabsorption in the proximal tubule is indirect and is secondary to the activation of apical Na(+)/H(+) exchanger NHE3, which then leads to the stimulation of the apical chloride/base exchanger.

Identification of an apical Cl-/HCO-3 exchanger in rat kidney proximal tubule

SLC26A6 (or putative anion transporter 1, PAT1) is located on the apical membrane of mouse kidney proximal tubule and mediates Cl-/HCO3- exchange in in vitro expression systems. We hypothesized that PAT1 along with a Cl-/HCO3- exchange is present in apical membranes of rat kidney proximal tubules. Northern hybridizations indicated the exclusive expression of SLC26A6 (PAT1 or CFEX) in rat kidney cortex, and immunocytochemical staining localized SLC26A6 on the apical membrane of proximal tubules, with complete prevention of the labeling with the preadsorbed serum. To examine the functional presence of apical Cl-/HCO3- exchanger, proximal tubules were isolated, microperfused, loaded with the pH-sensitive dye BCPCF-AM, and examined by digital ratiometric imaging. The pH of the perfusate and bath was kept at 7.4. Buffering capacity was measured, and transport rates were calculated as equivalent base flux. The results showed that in the presence of basolateral DIDS (to inhibit Na+-HCO3- cotransporter 1) and apical EIPA (to inhibit Na+/H+ exchanger 3), the magnitude of cell acidification in response to addition of luminal Cl- was approximately 5.0-fold higher in the presence than in the absence of CO2/HCO3-. The Cl--dependent base transport was inhibited by approximately 61% in the presence of 0.5 mM luminal DIDS. The presence of physiological concentrations of oxalate in the lumen (200 microM) did not affect the Cl-/HCO3- exchange activity. These results are consistent with the presence of SLC26A6 (PAT1) and Cl-/HCO3- exchanger activity in the apical membrane of rat kidney proximal tubule. We propose that SLC26A6 is likely responsible for the apical Cl-/HCO3- (and Cl-/OH-) exchanger activities in kidney proximal tubule.

Regulation of the apical Cl-/HCO-3 exchanger pendrin in rat cortical collecting duct in metabolic acidosis

Pendrin is an apical Cl(-)/OH(-)/HCO(3)(-) exchanger in beta-intercalated cells (beta-ICs) of rat and mouse cortical collecting duct (CCD). However, little is known about its regulation in acid-base disorders. Here, we examined the regulation of pendrin in metabolic acidosis, a condition known to decrease HCO(3)(-) secretion in CCD. Rats were subjected to NH(4)Cl loading for 4 days, which resulted in metabolic acidosis. Apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs was determined as amplitude and rate of intracellular pH change when Cl was removed in isolated, microperfused CCDs. Intracellular pH was measured by single-cell digital ratiometric imaging using fluorescent pH-sensitive dye 2',7'-bis-(3-carboxypropyl)-5-(and-6)-carboxyfluorescein-AM. Pendrin mRNA expression in kidney cortex was examined by Northern blot hybridizations. Expression of pendrin protein was assessed by indirect immunofluorescence. Microperfused CCDs isolated from acidotic rats demonstrated approximately 60% reduction in apical Cl(-)/HCO(3)(-) exchanger activity in beta-ICs (P < 0.001 vs. control). Northern blot hybridizations indicated that the mRNA expression of pendrin in kidney cortex decreased by 68% in acidotic animals (P < 0.02 vs. control). Immunofluorescence labeling demonstrated significant reduction in pendrin expression in CCDs of acidotic rats. We conclude that metabolic acidosis decreases the activity of the apical Cl(-)/HCO(3)(-) exchanger in beta-ICs of the rat CCD by reducing the expression of pendrin. Adaptive downregulation of pendrin in metabolic acidosis indicates the important role of this exchanger in acid-base regulation in the CCD.

Apical H(+)/base transporters mediating bicarbonate absorption and pH(i) regulation in the OMCD

The outer medullary collecting duct (OMCD) plays an important role in mediating transepithelial HCO transport [J(HCO(3)(-))] and urinary acidification. HCO absorption by type A intercalated cells in the OMCD inner stripe (OMCD(is)) segment is thought to by mediated by an apical vacuolar H(+)-ATPase and H(+)-K(+)-ATPase coupled to a basolateral Cl(-)-HCO exchanger (AE1). Besides these Na(+)-independent transporters, previous studies have shown that OMCD(is) type A intercalated cells have an apical electroneutral EIPA-sensitive, DIDS-insensitive Na(+)-HCO cotransporter (NBC3); a basolateral Na(+)/H(+) antiporter; and a basolateral Na(+)-K(+)-ATPase. In this study, we reexamined the Na(+) dependence of transepithelial Na(+) transport in the OMCD(is) and determined the role of apical NBC3 in intracellular (pH(i)) regulation in OMCD(is) type A intercalated cells. Control tubules absorbed HCO at a rate of approximately 13 pmol. min(-1). mm(-1). Lowering luminal Na(+) from 140 to 40 mM decreased [J(HCO(3)(-))] by approximately 15% without a change in transepithelial potential (V(te)). Furthermore, 50 microM EIPA (lumen) also decreased [J(HCO(3)(-))] by approximately 13% without a change in V(te). The effect of lowering luminal Na(+) and adding EIPA were not additive. These results demonstrate that [J(HCO(3)(-))] in the OMCD(is) is in part Na(+) dependent. In separate experiments, the pH(i) recovery rate after an NH prepulse was monitored in single type A intercalated cells with confocal fluorescence microscopy. The pH(i) recovery rate was approximately 0.21 pH/min in Na(+)-containing solutions and decreased to approximately 0.16 pH/min with EIPA (50 microM, lumen). In tubules perfused/bathed without Na(+), luminal Na(+) addition resulted in a pH(i) recovery rate of approximately 0.36 pH/min, whereas the Na(+)-independent recovery rate was approximately 0.16 pH/min. EIPA (50 microM, lumen) decreased the Na(+)-dependent pH(i) recovery rate to approximately 0.07 pH/min. The Na(+)-independent recovery rate was decreased to approximately 0.06 pH/min by bafilomycin (10 nM, lumen) and to approximately 0.10 pH/min using Schering 28080 (10 microM, lumen). These findings indicate that NBC3 contributes to pH(i) regulation in OMCD(is) type A intercalated cells and plays only a minor role in mediating [J(HCO(3)(-))] in the OMCD(is).

Novel Schering and ouabain-insensitive potassium-dependent proton secretion in the mouse cortical collecting duct

The intercalated (IC) cells of the cortical collecting duct (CCD) are important to acid-base homeostasis by secreting acid and reabsorbing bicarbonate. Acid secretion is mediated predominantly by apical membrane Schering (SCH-28080)-sensitive H(+)-K(+)- ATPase (HKA) and bafilomycin-sensitive H(+)-ATPase. The SCH-28080-sensitive HKA is believed to be the gastric HKA (HKAg). Here we examined apical membrane potassium-dependent proton secretion in IC cells of wild-type HKAg (+/+) and HKAg knockout (-/-) mice to determine relative contribution of HKAg to luminal proton secretion. The results demonstrated that HKAg (-/-) and wild-type mice had comparable rates of potassium-dependent proton secretion, with HKAg (-/-) mice having 100% of K(+)-dependent H(+) secretion vs. wild-type mice. Potassium-dependent proton secretion was resistant to ouabain and SCH-28080 in HKAg knockout mice but was sensitive to SCH-28080 in wild-type animals. Northern hybridizations did not demonstrate any upregulation of colonic HKA in HKAg knockout mice. These data indicate the presence of a previously unrecognized K(+)-dependent SCH-28080 and ouabain-insensitive proton secretory mechanism in the cortical collecting tubule that may play an important role in acid-base homeostasis.

Role of NHE isoforms in mediating bicarbonate reabsorption along the nephron

This study assessed the functional role of Na(+)/H(+) exchanger (NHE) isoforms NHE3 and NHE2 in the proximal tubule, loop of Henle, and distal convoluted tubule of the rat kidney by comparing sensitivity of transport to inhibition by Hoe-694 (an agent known to inhibit NHE2 but not NHE3) and S-3226 (an agent with much higher affinity for NHE3 than NHE2). Rates of transport of fluid (J(v)) and HCO(3)(-) (J(HCO3)) were studied by in situ microperfusion. In the proximal tubule, addition of ethylisopropylamiloride or S-3226 significantly reduced J(v) and J(HCO3), but addition of Hoe-694 caused no significant inhibition. In the loop of Henle, J(HCO3) was also inhibited by S-3226 and not by Hoe-694, although much higher concentrations of S-3226 were required than what was necessary to inhibit transport in the proximal tubule. In contrast, in the distal convoluted tubule, J(HCO3) was inhibited by Hoe-694 but not by S-3226. These results are consistent with the conclusion that NHE2 rather than NHE3 is the predominant isoform responsible for apical membrane Na(+)/H(+) exchange in the distal convoluted tubule, whereas NHE3 is the predominant apical isoform in the proximal tubule and possibly also in the loop of Henle.

Role of basolateral carbonic anhydrase in proximal tubular fluid and bicarbonate absorption

Membrane-bound carbonic anhydrase (CA) is critical to renal acidification. The role of CA activity on the basolateral membrane of the proximal tubule has not been defined clearly. To investigate this issue in microperfused rabbit proximal straight tubules in vitro, we measured fluid and HCO(3)(-) absorption and cell pH before and after the extracellular CA inhibitor p-fluorobenzyl-aminobenzolamide was applied in the bath to inhibit only basolateral CA. This inhibitor was 1% as permeant as acetazolamide. Neutral dextran (2 g/dl, molecular mass 70,000) was used as a colloid to support fluid absorption because albumin could affect CO(2) diffusion and rheogenic HCO(3)(-) efflux. Indeed, dextran in the bath stimulated fluid absorption by 55% over albumin. Basolateral CA inhibition reduced fluid absorption ( approximately 30%) and markedly decreased HCO(3)(-) absorption ( approximately 60%), both reversible when CA was added to the bathing solution. In the presence of luminal CA inhibition, which reduced fluid ( approximately 16%) and HCO(3)(-) ( approximately 66%) absorption, inhibition of basolateral CA further decreased the absorption of fluid (to 74% of baseline) and HCO(3)(-) (to 22% of baseline). CA inhibition also alkalinized cell pH by approximately 0.2 units, suggesting the presence of an alkaline disequilibrium pH in the interspace, which would secondarily block HCO(3)(-) exit from the cell and thereby decrease luminal proton secretion (HCO(3)(-) absorption). These data clearly indicate that basolateral CA has an important role in mediating fluid and especially HCO(3)(-) absorption in the proximal straight tubule.

Metabolic acidosis stimulates H+ secretion in the rabbit outer medullary collecting duct (inner stripe) of the kidney

The outer medullary collecting duct (OMCD) absorbs HCO3- at high rates, but it is not clear if it responds to metabolic acidosis to increase H+ secretion. We measured net HCO3- transport in isolated perfused OMCDs taken from deep in the inner stripes of kidneys from control and acidotic (NH4Cl-fed for 3 d) rabbits. We used specific inhibitors to characterize the mechanisms of HCO3- transport: 10 microM Sch 28080 or luminal K+ removal to inhibit P-type H+,K+-ATPase activity, and 5-10 nM bafilomycin A1 or 1-10 nM concanamycin A to inhibit H+-ATPase activity. The results were comparable using either of each pair of inhibitors, and allowed us to show in control rabbits that 65% of net HCO3- absorption depended on H+-ATPase (H flux), and 35% depended on H+,K+-ATPase (H,K flux). Tubules from acidotic rabbits showed higher rates of HCO3- absorption (16.8+/-0.3 vs. 12.8+/-0.2 pmol/min per mm, P < 0.01). There was no difference in the H,K flux (5.9+/-0.2 vs. 5.8+/-0.2 pmol/min per mm), whereas there was a 61% higher H flux in segments from acidotic rabbits (11.3+/-0.2 vs. 7.0+/-0.2 pmol/min per mm, P < 0.01). Transport was then measured in other OMCDs before and after incubation for 1 h at pH 6.8, followed by 2 h at pH 7.4 (in vitro metabolic acidosis). Acid incubation in vitro stimulated HCO3- absorption (12.3+/-0.3 to 16.2+/-0.3 pmol/min per mm, P < 0.01), while incubation at pH 7.4 for 3 h did not change basal rate (11.8+/-0.4 to 11.7+/-0.4 pmol/min per mm). After acid incubation the H,K flux did not change, (4.7+/-0.4 to 4.6+/-0.4 pmol/min per mm), however, there was a 60% increase in H flux (6.6+/-0.3 to 10.8+/-0.3 pmol/min per mm, P < 0.01). In OMCDs from acidotic animals, and in OMCDs incubated in acid in vitro, there was a higher basal rate and a further increase in HCO3- absorption (16.7+/-0.4 to 21.3+/-0.3 pmol/min per mm, P < 0.01) because of increased H flux (11.5+/-0.3 to 15.7+/-0.2 pmol/min per mm, P < 0.01) without any change in H,K flux (5.4+/-0.3 to 5.6+/-0.3 pmol/min per mm). These data indicate that HCO3- absorption (H+ secretion) in OMCD is stimulated by metabolic acidosis in vivo and in vitro by an increase in H+-ATPase-sensitive HCO3- absorption. The mechanism of adaptation may involve increased synthesis and exocytosis to the apical membrane of proton pumps. This adaptation helps maintain homeostasis during metabolic acidosis.

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.

Cellular remodeling of HCO3(-)-secreting cells in rabbit renal collecting duct in response to an acidic environment

The renal cortical collecting duct (CCD) consists of principal and intercalated cells. Two forms of intercalated cells, those cells involved in H+/HCO3- transport, have recently been described. H+-secreting cells are capable of apical endocytosis and have H+ATPase on the apical membrane and a basolateral Cl-/HCO3- exchanger. HCO3(-)-secreting cells bind peanut agglutinin (PNA) to apical membrane receptors and have diffuse or basolateral distribution of H+ATPase; their Cl-/HCO3- exchanger is on the apical membrane. We found that 20 h after acid feeding of rabbits, there was a fourfold increase in number of cells showing apical endocytosis and a numerically similar reduction of cells binding PNA. Incubation of CCDs at pH 7.1 for 3-5 h in vitro led to similar, albeit less pronounced, changes. Evidence to suggest internalization and degradation of the PNA binding sites included a reduction in apical binding of PNA, decrease in pH in the environment of PNA binding, and incorporation of electron-dense PNA into cytoplasmic vesicles. Such remodeling was dependent on protein synthesis. There was also functional evidence for loss of apical Cl-/HCO3- exchange on PNA-labeled cells. Finally, net HCO3- flux converted from secretion to absorption after incubation at low pH. Thus, exposure of CCDs to low pH stimulates the removal/inactivation of apical Cl-/HCO3- exchangers and the internalization of other apical membrane components. Remodeling of PNA-labeled cells may mediate the change in polarity of HCO3- flux observed in response to acid treatment.

Effects of acetazolamide on Na+-HCO-3 cotransport in basolateral membrane vesicles isolated from rabbit renal cortex

We evaluated the effects of acetazolamide on Na+-HCO3- cotransport in basolateral membrane vesicles isolated from the rabbit renal cortex. Na+ uptake stimulated by an imposed inward HCO3- gradient was not significantly reduced by 1.2 mM acetazolamide, indicating that acetazolamide does not directly inhibit Na+-HCO3- cotransport. 4,4'-Diisothiocyanostilbene-2,2'-disulfonate (DIDS)-sensitive Na+-base cotransport was found to be absolutely CO2/HCO3--dependent. We therefore tested whether acetazolamide-sensitive availability of HCO3- at the basolateral membrane could be rate-limiting for Na+-base cotransport under some conditions. In the presence of a CO2/HCO3- buffer system but absence of an initial HCO3- gradient, Na+ influx was stimulated fivefold by an outward NH4+ gradient. This stimulation of Na+ influx by an outward NH4+ gradient was inhibited greater than 75% by 0.6 mM acetazolamide, suggesting that acetazolamide blocked the ability of the NH4+ gradient to generate an inward HCO3- gradient. In the presence of an inward HCO3- gradient, Na+ influx was inhibited greater than 70% by an inward NH4+ gradient. This inhibition of Na+ influx was reduced to only 35% by 0.6 mM acetazolamide, suggesting that acetazolamide blocked the ability of NH4+ to collapse the inward HCO3- gradient. Similarly, Na+ influx in the presence of an inward HCO3- gradient was inhibited greater than 80% by an outward acetate gradient, and this inhibition was reduced to only 50% by acetazolamide. Thus, acetazolamide caused either inhibition or stimulation of Na+ uptake depending on the conditions with respect to pH and HCO3- gradients. The indirect interaction of acetazolamide with the basolateral membrane Na+-HCO3- cotransport system may be an important mechanism underlying inhibition of proximal tubule acid secretion by this agent.

Mechanisms of adaptation to chronic respiratory acidosis in the rabbit proximal tubule

The hyperbicarbonatemia of chronic respiratory acidosis is maintained by enhanced bicarbonate reabsorption in the proximal tubule. To investigate the cellular mechanisms involved in this adaptation, cell and luminal pH were measured microfluorometrically using (2",7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein in isolated, microperfused S2 proximal convoluted tubules from control and acidotic rabbits. Chronic respiratory acidosis was induced by exposure to 10% CO2 for 52-56 h. Tubules from acidotic rabbits had a significantly lower luminal pH after 1-mm perfused length (7.03 +/- 0.09 vs. 7.26 +/- 0.06 in controls, perfusion rate = 10 nl/min). Chronic respiratory acidosis increased the initial rate of cell acidification (dpHi/dt) in response to luminal sodium removal by 63% and in response to lowering luminal pH (7.4-6.8) by 69%. Chronic respiratory acidosis also increased dpHi/dt in response to peritubular sodium removal by 63% and in response to lowering peritubular pH by 73%. In conclusion, chronic respiratory acidosis induces a parallel increase in the rates of the luminal Na/H antiporter and the basolateral Na/(HCO3)3 cotransporter. Therefore, the enhanced proximal tubule reabsorption of bicarbonate in chronic respiratory acidosis may be, at least in part, mediated by a parallel adaptation of these transporters.

Relation of ATPases in rat renal brush-border membranes to ATP-driven H+ secretion

In the presence of inhibitors for mitochondrial H+-ATPase, (Na+ + K+)- and Ca2+-ATPases, and alkaline phosphatase, sealed brush-border membrane vesicles hydrolyse externally added ATP demonstrating the existence of ATPases at the outside of the membrane ("ecto-ATPases"). These ATPases accept several nucleotides, are stimulated by Ca2+ and Mg2+, and are inhibited by N.N'-dicyclohexylcarbodiimide (DCCD), but not by N-ethylmaleimide (NEM). They occur in both brush-border and basolateral membranes. Opening of brush-border membrane vesicles with Triton X-100 exposes ATPases located at the inside (cytosolic side) of the membrane. These detergent-exposed ATPases prefer ATP, are activated by Mg2+ and Mn2+, but not by Ca2+, and are inhibited by DCCD as well as by NEM. They are present in brush-border, but not in basolateral membranes. As measured by an intravesicularly trapped pH indicator. ATP-loaded brush-border membrane vesicles extrude protons by a DCCD- and NEM-sensitive pump. ATP-driven H+ secretion is electrogenic and requires either exit of a permeant anion (Cl-) or entry of a cation, e.g., Na+ via electrogenic Na+/D-glucose and Na+/L-phenylalanine uptake. In the presence of Na+, ATP-driven H+ efflux is stimulated by blocking the Na+/H+ exchanger with amiloride. These data prove the coexistence of Na+-coupled substrate transporters, Na+/H+ exchanger, and an ATP-driven H+ pump in brush-border membrane vesicles. Similar location and inhibitor sensitivity reveal the identity of ATP-driven H+ pumps with (a part of) the DCCD- and NEM- sensitive ATPases at the cytosolic side of the brush-border membrane.

Chronic metabolic acidosis causes an adaptation in the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter in the rat proximal convoluted tubule

The effect of chronic dietary acid on the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter was examined in the in vivo microperfused rat proximal tubule. Transporter activity was assayed with the epifluorescent measurement of cell pH using the intracellular, pH-sensitive fluorescent dye, (2'7')-bis(carboxyethyl)-(5,6)-carboxy-fluorescein (BCECF). BCECF was calibrated intracellularly, demonstrating similar pH-sensitivity of the dye in control and acidotic animals. In subsequent studies, lumen and peritubular capillaries were perfused to examine Na/H and Na(HCO3)3 transporter activity in the absence of contact with native fluid. The initial rate of change in cell pH (dpHi/dt) was 97, 50, and 44% faster in tubules from acidotic animals when peritubular [HCO3] was changed from 25 to 10 mM in the presence or absence of chloride, or peritubular [Na] was changed from 147 to 50 mM, respectively. dpHi/dt was 57% faster in tubules from acidotic animals when luminal [Na] was changed from 152 to 0 mM. Buffer capacities, measured using NH3/NH+4 addition, were similar in the two groups. The results demonstrate that chronic metabolic acidosis causes an adaptation in the intrinsic properties of both the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter.

Electrogenicity of phosphate transport by renal brush-border membranes

Phosphate uptake by rat renal brush-border membrane vesicles was studied under experimental conditions where transmembrane electrical potential (delta psi) could be manipulated. Experiments were performed under initial rate conditions to avoid complications associated with the dissipation of ion gradients. First, phosphate uptake was shown to be strongly affected by the nature of Na+ co-anions, the highest rates of uptake being observed with 100 mM-NaSCN (1.010 +/- 0.086 pmol/5 s per micrograms of protein) and the lowest with 50 mM-Na2SO4 (0.331 +/- 0.046 pmol/5 s per micrograms of protein). Anion substitution studies showed that potency of the effect of the co-anions was in the order thiocyanate greater than nitrate greater than chloride greater than isethionate greater than gluconate greater than sulphate, which correlates with the known permeability of the membrane to these anions and thus to the generation of transmembrane electrical potentials of decreasing magnitude (inside negative). The stimulation by ion-diffusion-induced potential was observed from pH 6.5 to 8.5, indicating that the transport of both monovalent and divalent phosphate was affected. In addition, inside-negative membrane potentials were generated by valinomycin-induced diffusion of K+ from K+-loaded vesicles and showed a 57% stimulation of phosphate uptake, at pH 7.5. Similar experiments with H+-loaded vesicles, in the presence of carbonyl cyanide m-chlorophenylhydrazone gave a 50% stimulation compared with controls. Inside-positive membrane potentials were also induced by reversal of the K+ gradient (outside greater than inside) in the presence of valinomycin and gave 58% inhibition of phosphate uptake. The membrane-potential dependency of phosphate uptake was finally analysed under thermodynamic equilibrium, and a stimulation by inside-negative potential was observed. The transport of phosphate was thus driven against a concentration gradient by a membrane potential, implicating the net transfer of a positive charge during the translocation process. These results indicate a major contribution of electrical potential to phosphate uptake in renal brush-border membranes.

Regulation of cell pH by ambient bicarbonate, carbon dioxide tension, and pH in the rabbit proximal convoluted tubule

To study the regulation of cell pH by ambient pH, carbon dioxide tension (PCO2), and bicarbonate (HCO3), cell pH was measured in the isolated, in vitro microperfused rabbit proximal convoluted tubule using the fluorescent dye (2',7')-bis-(carboxyethyl)-(5,6)-carboxyfluorescein. For the same changes in external pH, changes in [HCO3] and PCO2 affected cell pH similarly ([HCO3]: pHi/pHe = 0.67, PCO2: pHi/pHe = 0.64, NS). Isohydric changes in extracellular [HCO3] and PCO2 did not change cell pH significantly. Changes in peritubular [HCO3] elicited larger changes in cell pH than changes in luminal [HCO3], which were enhanced by peritubular 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS). The cell pH defense against acute increases and decreases in PCO2 was inhibited by sodium, but not by chloride removal. Peritubular SITS inhibited the cell pH defense against increases and decreases of PCO2, whereas luminal amiloride inhibited cell pH defense against increases in PCO2.

CONCLUSIONS

(a) Steady-state cell pH changes in response to changes in extracellular [HCO3] and PCO2 are quantitatively similar for a given change in extracellular pH; (b) the rate of the basolateral Na/(HCO3)3 cotransporter is a more important determinant of cell pH than the rate of the apical membrane mechanism(s); (c) cell pH defense against acute changes in PCO2 depends on the basolateral Na/(HCO3)3 cotransporter (acid and alkaline loads) and the luminal Na/H antiporter (acid loads).

Role of the Na+/H+ antiporter in rat proximal tubule bicarbonate absorption

Amiloride and the more potent amiloride analog, 5-(N-t-butyl) amiloride (t-butylamiloride), were used to examine the role of the Na+/H+ antiporter in bicarbonate absorption in the in vivo microperfused rat proximal convoluted tubule. Bicarbonate absorption was inhibited 29, 46, and 47% by 0.9 mM or 4.3 mM amiloride, or 1 mM t-butylamiloride, respectively. Sensitivity of the Na+/H+ antiporter to these compounds in vivo was examined using fluorescent measurements of intracellular pH with (2', 7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Amiloride and t-butylamiloride were shown to be as potent against the antiporter in vivo as in brush border membrane vesicles. A model of proximal tubule bicarbonate absorption was used to correct for changes in the luminal profiles for pH and inhibitor concentration, and for changes in luminal flow rate in the various series. We conclude that the majority of apical membrane proton secretion involved in transepithelial bicarbonate absorption is mediated by the Na+-dependent, amiloride-sensitive Na+H+ antiporter. However, a second mechanism of proton secretion contributes significantly to bicarbonate absorption. This mechanism is Na+-independent and amiloride-insensitive.

Presence of an extramitochondrial anion-stimulated ATPase in the rabbit kidney: localization along the nephron and effect of corticosteroids

To determine whether kidney membrane fractions contain an extramitochondrial anion-stimulated ATPase, we compared the pharmacological and kinetic properties of HCO3-ATPase activities in mitochondrial and microsomal fractions prepared from rabbit kidney cortex and outer medulla. The results indicated that this activity differed markedly in each type of fraction. Microsomal HCO3-ATPase was less sensitive than mitochondrial ATPase to azide, oligomycin, DCCD and thiocyanate, but was more sensitive to filipin and displayed different dependency towards ATP, magnesium and pH. Microsomal ATPase activity was stimulated by sulfite much more strongly than by bicarbonate, whereas mitochondrial activity was stimulated by both these anions to a similar extent. These results demonstrate the presence of an extramitochondrial HCO3-ATPase in kidney membrane fractions. HCO3-ATPase was also measured in single microdissected segments of the rabbit nephron using a radiochemical microassay previously developed for tubular Na, K-ATPase activity. An enzyme with the pharmacological and kinetic properties of the microsomal enzyme was detected in both proximal tubule, distal convoluted tubule and collecting duct, but the thick ascending limb was devoid of any detectable activity. Long-term DOCA administration markedly increased HCO3-ATPase activity in the distal convoluted and collecting tubule. The insensitivity of microsomal HCO3-ATPase to vanadate indicates that it belongs to the F0-F1 class of ATPases, and might therefore be involved in proton transport. This hypothesis is also supported by the localization of tubular HCO3-ATPase activity at the sites of urinary acidification.

Cell pH in the rat proximal convoluted tubule. Regulation by luminal and peritubular pH and sodium concentration

To examine the relative roles of apical and basolateral membrane transport mechanisms in the regulation of cell pH in the proximal convoluted tubule, cell pH was measured in the in vivo microperfused rat tubule using fluorescence. Decreasing luminal pH by 0.7 pH units caused cell pH to decrease by 0.08 pH units, whereas a similar decrease in peritubular pH caused cell pH to decrease by 0.32 pH units. Inhibition of basolateral membrane bicarbonate transport with peritubular 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS) enhanced the response to luminal fluid acidification. Removal of luminal sodium caused a small transient acidification which was followed by a late alkalinization. Peritubular SITS increased the magnitude of the transient acidification, and eliminated the late alkalinization. The acidification was partially inhibited by luminal amiloride. The results demonstrate sodium-coupled processes on both the apical (Na/H antiport) and basolateral (Na/HCO3 symport) membranes. Basolateral membrane transporters are more important determinants of cell pH.

Proximal tubular HCO3-, H+ and fluid transport during maleate-induced acidification defect

The mechanism of tubular acidification was studied in proximal tubular acidification defect induced in rats by acute parenteral infusion of maleate (200 mg/kg), which causes diuresis and bicarbonaturia. Proximal tubular bicarbonate reabsorption and H+ ion secretion were determined by stopped-flow microperfusion and measurement of luminal pH by Sb microelectrodes. Stationary pH increased in proximal tubule from 6.78 to 7.25 and bicarbonate reabsorption decreased from 1.32 to 0.51 nmol/cm2 X s. In these segments, mean cell PD fell from -66.6 to -20.2 mV, while Jv as estimated by the Gertz technique fell to 15% of controls. A similar impairment of acidification was observed during luminal and capillary perfusion with phosphate Ringer's. Since H+-ion efflux from the lumen was not significantly increased and both acidification and alkalinization half-times (t/2) were increased, no evidence for an increase in passive permeability for H+/HCO3- was obtained. The increased t/2 found during luminal perfusion with acid phosphate indicates, according to an electrical analog model, a reduction in pump series conductance. These results show that maleate affects both proximal Na+ and H+ transport; this effect may be ascribed to impairment of sodium-dependent transport systems in the brush-border membrane.

Internephron heterogeneity for carbonic anhydrase-independent bicarbonate reabsorption in the rat

The present experiments were designed to localize the sites of carbonic anhydrase-independent bicarbonate reabsorption in the rat kidney and to examine some of its mechanisms. Young Munich-Wistar rats were studied using standard cortical and papillary free-flow micropuncture techniques. Total CO2 (tCO2) was determined using microcalorimetry. In control rats both superficial and juxtamedullary proximal nephrons reabsorbed approximately 95% of the filtered load of bicarbonate. The administration of acetazolamide (20 mg/kg body weight [bw]/h) decreased proximal reabsorption to 65.6% of the filtered load in superficial nephrons (32% was reabsorbed by the proximal convoluted tubule while 31.7% was reabsorbed by the loop segment), and to 38.4% in juxtamedullary nephrons. Absolute reabsorption of bicarbonate was also significantly higher in superficial than in juxtamedullary nephrons after administration of acetazolamide (727 +/- 82 vs. 346 +/- 126 pmol/min; P less than 0.05). The infusion of amiloride (2.5 mg/kg bw/h) to acetazolamide-treated rats increased the fractional excretion of bicarbonate as compared with animals treated with acetazolamide alone (34.9 +/- 1.9 vs. 42.9 +/- 2.1%; P less than 0.01), and induced net addition of bicarbonate between the superficial early distal tubule and the final urine (34.8 +/- 3.0 vs. 42.9 +/- 2.1%; P less than 0.05). Amiloride at this dose did not affect proximal water or bicarbonate transport; our studies localize its site of action to the terminal nephron. Vasa recta (VR) plasma and loop of Henle (LH) tubular fluid tCO2 were determined in control and acetazolamide-treated rats in order to identify possible driving forces for carbonic anhydrase-independent bicarbonate reabsorption in the rat papilla. Control animals showed a tCO2 gradient favoring secretion (LH tCO2, 7.4 +/- 1.7 mM vs. VR tCO2, 19.1 +/- 2.3 mM; P less than 0.005). Acetazolamide administration reversed this chemical concentration gradient, inducing a driving force favoring reabsorption of bicarbonate (LH tCO2, 27.0 +/- 1.4 mM vs. VR tCO2, 20.4 +/- 1.0 mM; P less than 0.005). Our study shows that in addition to the superficial proximal convoluted tubule, the loop segment and the collecting duct show acetazolamide-insensitive bicarbonate reabsorption. No internephron heterogeneity for bicarbonate transport was found in controls. The infusion of acetazolamide, however, induced significant internephron heterogeneity for bicarbonate reabsorption, with superficial nephrons reabsorbing a higher fractional and absolute load of bicarbonate than juxtamedullary nephrons. We think that the net addition of bicarbonate induced by amiloride is secondary to inhibition of voltage-dependent, carbonic anhydrase-independent bicarbonate reabsorption at the level of the collecting duct, which uncovers a greater delivery of carbonate from deeper nephrons to the collecting duct. Finally, our results suggest that carbonic anhydrase-independent bicarbonate reabsorption is partly passive, driven by favorable chemical gradients in the papillary tubular structures, and partly voltage-dependent, in the collecting duct.

Na+-H+ exchange activity in renal brush border membrane vesicles in response to metabolic acidosis: The role of glucocorticoids

Amiloride-sensitive Na+ -H+ exchange activity in brush border membrane vesicles isolated from rat proximal tubule was increased in metabolic acidosis. The enhancement of exchange activity required an intact adrenal gland or glucocorticoid supplements. Ammonium and phosphate excretions were increased during acidosis and these were also largely dependent on an intact adrenal gland or glucocorticoid supplements. Amiloride-insensitive Na+ uptake and passive H+ permeability were not altered by acidosis or the glucocorticoid status of the animal. These findings are consistent with glucocorticoids having an important regulatory role in the kidney by orchestrating the proximal tubular adaptation to metabolic acidosis.

Effects of extracellular fluid volume and plasma bicarbonate concentration on proximal acidification in the rat

The effects of systemic bicarbonate concentration and extracellular fluid volume status on proximal tubular bicarbonate absorption, independent of changes in luminal composition and flow rate, were examined with in vivo luminal microperfusion of rat superficial proximal convoluted tubules. Net bicarbonate absorption and bicarbonate permeability were measured using microcalorimetry. From these data, net bicarbonate absorption was divided into two parallel components: proton secretion and passive bicarbonate diffusion. The rate of net bicarbonate absorption was similar in hydropenic and volume-expanded rats when tubules were perfused with 24 mM bicarbonate, but was inhibited in volume-expanded rats when tubules were perfused with 5 mM bicarbonate. Volume expansion caused a 50% increase in bicarbonate permeability, which totally accounted for the above inhibition. The rate of proton secretion was unaffected by volume expansion in both studies. The rate of net bicarbonate absorption was markedly inhibited in alkalotic expansion as compared with isohydric expansion. Bicarbonate permeabilities were not different in these two conditions, and the calculated rates of proton secretion were decreased by greater than 50% in alkalosis. Net bicarbonate absorption was stimulated in acidotic rats compared to hydropenic rats. This stimulation was attributable to a 25% increase in the rate of proton secretion. We conclude that (a) proton secretion is stimulated in acidosis, inhibited in alkalosis, and is not altered by volume status; (b) bicarbonate permeability is increased by volume expansion but is not altered by increases in plasma bicarbonate concentration; (c) when luminal bicarbonate concentrations are similar to those of plasma, net bicarbonate absorption is dominated by proton secretion and is thus sensitive to peritubular bicarbonate concentrations, and insensitive to extracellular fluid volume; (d) when luminal bicarbonate concentrations are low and proton secretion is slowed, bicarbonate permeability and thus extracellular fluid volume have a greater influence on net bicarbonate absorption.

Effect of carbonic anhydrase inhibition on superficial and deep nephron bicarbonate reabsorption in the rat

The nephron segment responsible for the acetazolamide-insensitive fraction of renal bicarbonate reabsorption has not been clearly delineated. This study compares superficial and deep nephron bicarbonate reabsorption before and after acetazolamide at two dose levels (20 and 50 mg/kg per h) in mutant Munich-Wistar rats employing both cortical and papillary micropuncture and microcalorimetry. Systemic acid-base balance and right whole kidney glomerular filtration rate were similar in all groups examined. The effects of the two doses of acetazolamide were indistinguishable and resulted in a significant increase in whole kidney bicarbonate excretion that compared favorably with the fraction delivered out of the left papillary tip. Acetazolamide inhibited superficial proximal bicarbonate reabsorption by 80.0%, whereas reabsorption up to the deep loop of Henle was decreased by only 52% (P less than 0.001). Bicarbonate reabsorption that was insensitive to acetazolamide occurred in the superficial and deep loop of Henle and between the distal tubule and base collecting duct. Because water reabsorption in these segments could serve to generate transepithelial bicarbonate concentration gradients favorable for reabsorption, we attempted to minimize water abstraction by combined administration of mannitol and acetazolamide. During this condition a significant increase in bicarbonate delivery up to the deep loop of Henle was noted (52 vs. 65%), whereas superficial nephron reabsorption was not altered. Furthermore, an outwardly directed bicarbonate concentration gradient from the deep loop of Henle to vasa recta was demonstrated during acetazolamide (delta tCO2 = 20.9 +/- 3.3 mM), but was abolished during combined mannitol and acetazolamide administration (delta tCO2 = 3.5 +/- 0.9 mM). It is concluded that carbonic anhydrase inhibition results in a disparate effect on nephron bicarbonate reabsorption when juxtamedullary and superficial nephron segments are compared. Our findings suggest that a mechanism for residual bicarbonate reabsorption during acetazolamide administration may be passive reabsorption driven by favorable transepithelial concentration gradients.

Evidence for coupled sodium/hydrogen exchange in the rat superficial proximal convoluted tubule

Recent in vitro studies from the rat and rabbit have suggested a tightly coupled sodium/hydrogen ion exchanger on the luminal membrane of proximal tubules. The steep sodium gradient from the lumen to cell supplies indirect energy for hydrogen ions to be pumped from the cell to the lumen. However, a proton translocating pump has been demonstrated in other epithelia, which is independent of sodium transport and directly driven by ATP. To examine the role that sodium might play in the process of acidification, rat proximal convoluted tubules and their surrounding peritubular capillaries were perfused in vivo with artificial ultrafiltrate-like perfusion solutions. Total CO2 absorption was measured by microcalorimetry during alterations in sodium transport by replacement of the sodium with an impermeant cation, choline, or by inhibition of the (Na+ + K+)-ATPase by removing potassium from both perfusion solutions. Under control conditions the absolute rate of total CO2 absorption was 140 pmol/mm X min. In the choline substitution and potassium removal experiments, absolute total CO2 absorption fell to 23 and 28 pmol/mm X min, respectively. The data suggest that: 1) in the rat superficial proximal convoluted tubule approximately 80% of the bicarbonate absorption is tightly coupled to sodium transport; 2) this process is driven indirectly by the (Na+ + K+)-ATPase system; and 3) the residual 20% of acidification appears to be mediated by another mechanism or may be a consequence of technical limitations.

Influence of peritubular protein on solute absorption in the rabbit proximal tubule. A specific effect on NaCl transport

The effect of removal of peritubular protein on the reabsorption of various solutes and water was examined in isolated rabbit proximal convoluted tubules (PCT) perfused in vitro. In 22 PCT perfused with ultrafiltrate (UF) and bathed in serum, volume absorption (Jv) was 1.44 nl/mm per min and potential difference (PD) was -3.6 mV. When these same PCT were bathed in a protein-free UF, Jv was reduced 38% without a change in PD. Simultaneous measurements of total CO2 net flux (JTCO2) and glucose efflux (JG) showed that less than 2% of the decrease in JV could be accounted for by a reduction in JTCO2 and JG, suggesting that removal of peritubular protein inhibited sodium chloride transport (JNaCl). Therefore, in eight additional PCT, JNaCl was measured, in addition to PD, Jv, JG, and JTCO2. In these PCT, the decrease in total solute transport induced by removal of bath protein was 201.7 +/- 37.5 posmol/mm per min. JG decreased slightly (9.1 +/- 3.9 posmol/mm per min); NaHCO3 transport did not change (9.2 +/- 6.6 posmol/mm per min); but JNaCl decreased markedly (160.6 +/- 35.7 posmol/mm per min). 80% of the decrease in Jv could be accounted for by a decrease in JNaCl. In 13 additional PCT perfused with simple NaCl solutions, a comparable decrease in Jv and JNaCl was observed when peritubular protein was removed without an increase in TCO2 backleak. In summary, removal of peritubular protein reduced Jv and JNacl, but did not significantly alter PD, JG, JTCO2, or TCO2 backleak. The failure to inhibit JG and JTCO2, known sodium-coupled transport processes, indicates that protein removal does not primarily affect the Na-K ATPase pump system. Furthermore, since PD and TCO2 backleak were not influenced, it is unlikely that protein removal increased the permeability of the paracellular pathway. We conclude that protein removal specifically inhibits active transcellular or passive paracellular NaCl transport.

Water reabsorption capacity of the proximal convoluted tubule: a microperfusion study on rat kidney

1. The differences in the water reabsorption capacity observed from one proximal tubule to another were investigated in vivo by continuous microperfusion. 2. Two to seven loops were punctured along the same tubule. The [3H]inulin, 22Na, [14C]glucose, sodium, chloride and magnesium concentrations as well as the osmolality of the collected samples were studied as a function of the perfused length. 3. With Ringer bicarbonate solution perfused in Saclay Wistar rats, the water reabsorption capacity ranged from 0 to 3 nl . min-1 . mm-1 depending on the tubule. This reabsorption rate was closely correlated with the unidirectional reabsorption flux of sodium, and with the rise in tubular chloride and magnesium concentrations. 4. In Munich Wistar rats with glomeruli accessible at the kidney surface, tubule perfusion with a Ringer bicarbonate solution showed that the highest water reabsorption rates per mm of tubule were found for the perfusion sites closest to the glomerulus; water fluxes were also positively correlated with glucose transport. 5. In a second series of experiments on Saclay rats, perfusion of a Ringer solution containing a high chloride concentration (137 m-equiv/l.) was unable to increase the water reabsorption rate compared to the control perfusion; here again, water fluxes were positively correlated with glucose transport.

Chemical kinetic and diffusional limitations on bicarbonate reabsorption by the proximal tubule

It is accepted that bicarbonate reabsorption in the proximal tubule is mediated by H+ secretion, but several aspects of this process have remained controversial. To examine some of these issues, we have developed a model that allows for spatial variations in the concentrations of CO2, HCO3-, and H2CO3 within the tubule lumen and cell cytoplasm, passive transport of these substances across cell membranes, carbonic anhydrase-catalyzed interconversion of HCO3- and CO2 within the cell and at the luminal membrane surface, and the corresponding uncatalyzed reactions in lumen and cell. Most of the required kinetic and transport parameters were estimated from physicochemical data in the literature, whereas intracellular pH and HCO3- permeability at the basal cell membrane, found to be the most significant parameters under normal conditions, were adjusted to yield reabsorption rates of "total CO2" (tCO2, the sum of CO2, HCO3- and H2CO3) comparable to measured values in the rat. Our results suggest that for normal carbonic anhydrase activity, almost all tCO2 leaves the lumen as CO2, yet the transepithelial differences in CO2 partial pressure does not exceed approximately 2 mm Hg. Electrochemical potential gradients favor substantial passive backleak of HCO3- from cell to lumen. Gradients in CO2 partial pressure remain small during simulated inhibition of carbonic anhydrase, with approximately 70% of tCO2 leaving the lumen as H2CO3 in this case, and the remainder as CO2. Predicted tCO2 reabsorption rates for carbonic anhydrase inhibition are approximately of normal, in good agreement with recent measurements in the rat, indicating that the concept of "carbonic acid recycling" is viable.

Evidence for a bicarbonate leak in the proximal tubule of the rat kidney

To elucidate the mechanism responsible for the establishment of steady state pH at zero net flux (pH infinity) in proximal convoluted tubules, luminal pH was recorded continuously with antimony microelectrodes under three experimental conditions. First: luminal pH in stationary droplets was allowed to reach pH infinity (6.76 +/- 0.07) and then carbonic anhydrase inhibitor benzolamide (3 x 10(-3) mol/l) was superfused on the kidney surface. Following application of benzolamide, luminal pH decreased within seconds (delta pH = -0.27 +/- 0.03 SEM). Second: tubule segments were perfused continuously with MES-buffer containing solution set to a pH of 6.1. Some 1--2 mm distal to the perfusion pipette luminal pH was recorded and was 6.5 +/- 0.04. After superfusion of benzolamide (3 x 10(-3) mol/l) pH decreased (delta pH = -0.15 +/- 0.03). Third: pH in stationary droplets was again allowed to reach PH infinity (6.69 +/- 0.01) and bicarbonate and CO2- free solution (5 mmol/l phosphate set to a pH of 7.4) was microinfused into the adjacent peritubular capillary. Luminal pH again decreased almost immediately (delta pH = -0.23 +/- 0.02). The data are interpreted as evidence for a bicarbonate leak. In a fourth series of experiments, segments of proximal tubules were perfused under benzolamide (0.4 x 10(-6) mol/min) with solutions initially free of bicarbonate or other buffers. In the collected fluid, bicarbonate was determined by a micro-Astrup method. A significant increase of luminal bicarbonate concentration (r = 0.88) indicates a permeability of 0.98 +/- 0.14 x 10(-6) cm2/s of the tubular wall for bicarbonate. Since bicarbonate eventually increases more than 3-fold the equilibrium concentration, collected bicarbonate could not have been formed by H2CO3 or CO2. Bicarbonate enters the luminal fluid and reacts with secreted hydrogen ions to forms carbonic acid. It, therefore, buffers secreted hydrogen ions and increases luminal pH at or below steady state. Inhibition of carbonic anhydrase and lowering of peritubular bicarbonate thus lower pH infinity.

Loop of Henle bicarbonate accumulation in vivo in the rat

We have carried out perfusion studies on hydropenic and bicarbonate-loaded rats to provide direct in vivo observations on bicarbonate accumulation in the short loops of Henle. Analysis of early distal tubular fluid was made during bicarbonate-free saline perfusion from the end proximal to the early distal site, documenting accumulation of "new" bicarbonate. During perfusion in hydropenic rats, steady-state bicarbonate concentrations were suggested by early distal values of approximately equal to mM, which were independent of perfusion rate and virtually indistinguishable from bicarbonate concentration measured during free flow when filtered bicarbonate was allowed to enter the loop. Thus, loop bicarconate accumulation was apparently sufficient to allow new bicarbonate to enter at a rate comparable to that delivered to the early distal site during free flow, recognizing of course that free-flow delivery rates are the result of complex components of filtration and bidirectional fluxes. In bicarbonate-loaded rats, however, bicarbonate accumulation rates although higher than in hydropenia, were much lower than free-flow delivery rates. Furthermore, early distal bicarbonate concentrations during bicarbonate loading fell as perfusion rate increased, presumably because of a limitation to increasing ionic bicarbonate entry.

Sodium, phosphate, glucose, bicarbonate, and alanine interactions in the isolated proximal convoluted tubule of the rabbit kidney

Interactions among the transport systems involved with sodium, bicarbonate, glucose, phosphate, and alanine absorption in isolated segments of the rabbit proximal convoluted tubule were examined with radioisotopic techniques to measure glucose, phosphate, and fluid absorption rates. The composition of the perfusate and bath varied from normal, physiological fluids to fluids deficient in a single solute. The deletion of glucose from the perfusate increased the lumen-to-bath flux of phosphate from 5.51 +/- 1.15 to 8.32 +/- 1.34 pmol/mm-min (P less than 0.01). Similar changes occurred when glucose transport was inhibited by phlorizin 10 micron in the perfusate, The deletion of alanine from the perfusate increased the lumen-to-bath flux of phosphate from 6.55 +/- 1.08 to 9.00 +/- 1.30 pmol/mm-min (P less than 0.01) but did not affect glucose transport significantly, 80.1 +/- 10.1 vs. 72.5 +/- 5.4 pmol/mm-min. Replacement of intraluminal sodium with choline, elimination of potassium from the bath, and removal of bicarbonate from the lumen and bath each reduced glucose, phosphate, and fluid absorption. These data indicate that the proximal absorptive processes for glucose and for phosphate include elements that are dependent upon some function of sodium transport. Additionally, the effects on phosphate transport of deleting glucose or alanine occur independent of any changes in net sodium transport and are opposite the effects of deleting bicarbonate. These differences may relate to the observations that the transport of glucose and alanine is electrogenic while that of bicarbonate is not. Regardless of possible mechanisms, the data demonstrate that important changes in the absorption rates of different solutes handled significantly by the proximal convoluted tubule may occur in response to changes in specific components of proximal sodium transport.