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

Mechanisms of enhanced renal and hepatic erythropoietin synthesis by sodium-glucose cotransporter 2 inhibitors

Sodium-glucose cotransporter 2 (SGLT2) inhibitors reduce the risk of major heart failure events, an action that is statistically linked to enhanced erythropoiesis, suggesting that stimulation of erythropoietin and cardioprotection are related to a shared mechanism. Four hypotheses have been proposed to explain how these drugs increase erythropoietin production: (i) renal cortical reoxygenation with rejuvenation of erythropoietin-producing cells; (ii) counterregulatory distal sodium reabsorption leading to increased tubular workload and oxygen consumption, and thus, to localized hypoxia; (iii) increased iron mobilization as a stimulus of hypoxia-inducible factor-2α (HIF-2α)-mediated erythropoietin synthesis; and (iv) direct HIF-2α activation and enhanced erythropoietin gene transcription due to increased sirtuin-1 (SIRT1) signaling. The first two hypotheses assume that the source of increased erythropoietin is the interstitial fibroblast-like cells in the deep renal cortex. However, SGLT2 inhibitors do not alter regional tissue oxygen tension in the non-diabetic kidney, and renal erythropoietin synthesis is markedly impaired in patients with anemia due to chronic kidney disease, and yet, SGLT2 inhibitors produce an unattenuated erythrocytic response in these patients. This observation raises the possibility that the liver contributes to the production of erythropoietin during SGLT2 inhibition. Hypoxia-inducible factor-2α and erythropoietin are coexpressed not only in the kidney but also in hepatocytes; the liver is a major site of production when erythropoietin stimulation is maintained for prolonged periods. The ability of SGLT2 inhibitors to improve iron mobilization by derepressing hepcidin and ferritin would be expected to increase cytosolic ferrous iron, which might stimulate HIF-2α expression in both the kidney and liver through the action of iron regulatory protein 1. Alternatively, the established ability of SGLT2 inhibitors to enhance SIRT1 might be the mechanism of enhanced erythropoietin production with these drugs. In hepatic cell lines, SIRT1 can directly activate HIF-2α by deacetylation, and additionally, through an effect of SIRT in the liver, peroxisome proliferator-activated receptor-γ coactivator-1α binds to hepatic nuclear factor 4 to promote transcription of the erythropoietin gene and synthesis of erythropoietin. Since SIRT1 up-regulation exerts direct cytoprotective effects on the heart and stimulates erythropoietin, it is well-positioned to represent the shared mechanism that links erythropoiesis to cardioprotection during SGLT2 inhibition.

Hypochloraemia in Patients with Heart Failure: Causes and Consequences

Hypochloraemia is a common electrolyte abnormality in patients with heart failure (HF). It has a strong association with adverse outcome regardless of HF phenotype and independent of other prognostic markers. How hypochloraemia develops in a patient with HF and how it might influence outcome are not clear, and in this review we explore the possible mechanisms. Patients with HF and hypochloraemia almost invariably take higher doses of loop diuretic than patients with normal chloride levels. However, renal chloride and bicarbonate homeostasis are closely linked, and the latter may be influenced by neurohormonal activation: it is likely that the etiology of hypochloraemia in patients with HF is multifactorial and due to more than just diuretic-induced urinary losses. There are multiple proposed mechanisms by which low chloride concentrations may lead to an adverse outcome in patients with HF: by increasing renin release; by a stimulatory effect on the with-no-lysine kinases which might increase renal sodium-chloride co-transporter activity; and by an adverse effect on myocardial conduction and contractility. None of these proposed mechanisms are proven in humans with HF. However, if true, it might suggest that hypochloraemia is a therapeutic target that might be amenable to treatment with acetazolamide or chloride supplementation.

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.

Carbonic anhydrases and their interplay with acid/base-coupled membrane transporters

Carbonic anhydrases (CAs) have not only been identified as ubiquitous enzymes catalyzing the fast reversible hydration of carbon dioxide to generate or consume protons and bicarbonate, but also as intra- and extracellular proteins, which facilitate transport function of many acid/base transporting membrane proteins, coined 'transport metabolon'. Functional interaction between CAs and acid/base transporters, such as chloride/bicarbonate exchanger (AE), sodium-bicarbonate cotransporter (NBC) and sodium/hydrogen exchanger (NHE) has been shown to require both catalytic CA activity as well as direct binding of the enzyme to specific sites on the transporter. In contrast, functional interaction between different CA isoforms and lactate-proton-cotransporting monocarboxylate transporters (MCT) has been found to be isoform-specific and independent of CA catalytic activity, but seems to require an intramolecular proton shuttle within the enzyme. In this chapter, we review the various types of interactions between acid/base-coupled membrane carriers and different CA isoforms, as studied in vitro, in intact Xenopus oocytes, and in various mammalian cell types. Furthermore, we discuss recent findings that indicate the significance of these 'transport metabolons' for normal cell functions.

Carbonic anhydrase II increases the activity of the human electrogenic Na+/HCO3- cotransporter

Several acid/base-coupled membrane transporters, such as the electrogenic sodium-bicarbonate cotransporter (NBCe1), have been shown to bind to different carbonic anhydrase isoforms to create a "transport metabolon." We have expressed NBCe1 derived from human kidney in oocytes of Xenopus leavis and determined its transport activity by recording the membrane current in voltage clamp, and the cytosolic H(+) and Na(+) concentrations using ion-selective microelectrodes. When carbonic anhydrase isoform II (CAII) had been injected into oocytes, the membrane current and the rate of cytosolic Na(+) rise, indicative for NBCe1 activity, increased significantly with the amount of injected CAII (2-200 ng). The CAII inhibitor ethoxyzolamide reversed the effects of CAII on the NBCe1 activity. Co-expressing wild-type CAII or NH(2)-terminal mutant CAII together with NBCe1 provided similar results, whereas co-expressing the catalytically inactive CAII mutant V143Y had no effect on NBCe1 activity. Mass spectrometric analysis and the rate of cytosolic H(+) change following addition of CO(2)/HCO(3)(-) confirmed the catalytic activity of injected and expressed CAII in oocytes. Our results show that the transport capacity of NBCe1 is enhanced by the catalytic activity of CAII, in line with the notion that CAII forms a transport metabolon with NBCe1.

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.

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.

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.

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.

Regulation of the sodium bicarbonate cotransporter kNBC1 function: role of Asp(986), Asp(988) and kNBC1-carbonic anhydrase II binding

The HCO(3)(-) : Na(+) cotransport stoichiometry of the electrogenic sodium bicarbonate cotransporter kNBC1 determines the reversal potential (E(rev)) and thus the net direction of transport of these ions through the cotransporter. Previously, we showed that phosphorylation of kNBC1-Ser(982) in the carboxy-terminus of kNBC1 (kNBC1-Ct), by cAMP-protein kinase A (PKA), shifts the stoichiometry from 3 : 1 to 2 : 1 and that binding of bicarbonate to the cotransporter is electrostaticaly modulated. These results raise the possibility that phosphorylated kNBC1-Ser(982), or other nearby negatively charged residues shift the stoichiometry by blocking a bicarbonate-binding site. In the current study, we examined the role of the negative charge on Ser(982)-phosphate and three aspartate residues in a D986NDD custer in altering the stoichiometry of kNBC1. mPCT cells expressing kNBC1 mutants were grown on filters and mounted in an Ussing chamber for electrophysiological studies. Enhanced green fluorescence protein (EGFP)-tagged mutant constructs expressed in the same cells were used to determine the phosphorylation status of kNBC1-Ser(982). The data indicate that both kNBC1-Asp(986) and kNBC1-Asp(988), but not kNBC1-Asp(989), are required for the phosphorylation-induced shift in stoichiometry. A homologous motif (D887ADD) in the carboxy-terminus of the anion exchanger AE1 binds to carbonic anhydrase II (CAII). In isothermal titration calorimetry experiments, CAII was found to bind to kNBC1-Ct with a K(D) of 160 +/- 10 nM. Acetazolamide inhibited the short-circuit current through the cotransporter by 65 % when the latter operated in the 3 : 1 mode, but had no effect on the current in the 2 : 1 mode. Acetazolamide did not affect the cotransport stoichiometry or the ability of 8-Br-cAMP to shift the stoichiometry. Although CAII does not affect the transport stoichiometry, it may play an important role in enhancing the flux through the transporter when kNBC1-Ser(982) is unphosphorylated.

Inorganic anions and the renal organic cation transport system

During renal secretion, organic cations (OC) have to pass two hydrophobic membranes (basolateral and luminal) and the intervening aqueous cytoplasm. Furthermore, an uptake in intracellular endosomes may also occur. OC transport critically depends on the presence or absence of certain inorganic anions, such as Cl-, HCO3-, and others. The interaction between inorganic anions and OC may occur during the transport across the membranes or uptake by endosomes, by alterations of the transport protein or the substrate and by changes of the intracellular pH.

Dopamine inhibits renal Na+:HCO3- cotransporter in rabbits and normotensive rats but not in spontaneously hypertensive rats

BACKGROUND

Dopamine (DA) is thought to regulate renal proximal transport through the inhibition of the Na+,K+-ATPase and/or Na+/H+ exchanger. Defects in this dopaminergic system are proposed to be a pathogenic factor of genetic hypertension. However, microperfusion studies have not consistently confirmed direct tubular effects of DA.

METHODS

Isolated proximal straight tubules were perfused peritubularly with Dulbecco's modified Eagle's tissue culture medium (DMEM) containing norepinephrine (NE) to improve incubation conditions. Intracellular Na+ concentrations ([Na+]i) and cell pH (pHi) were measured with fluorescence probes.

RESULTS

When incubated in DMEM plus NE, DA increased [Na+]i in rabbit tubules. Inhibition of Na+,K+-ATPase could not explain this response, as it was not suppressed by ouabain. An analysis of pHi responses to bath HCO3- reduction revealed that DA, SKF 38393 (a DA1 agonist), and adenosine 3',5'-cyclic monophosphate (cAMP) inhibited the basolateral Na+:HCO3- cotransporter in rabbit and Wistar-Kyoto rat (WKY), if its transport stoichiometry was converted to 3 HCO3-:1 Na+ by DMEM plus NE incubation. The inhibitory effect of DA was abolished by SCH 23390, a DA1 antagonist, but not by (-)-sulpiride, a DA2 antagonist. In spontaneously hypertensive rats (SHRs), however, DA and SKF 38393 failed to inhibit the cotransporter, although the inhibitory effects of cAMP and parathyroid hormone were comparable to those in WKY.

CONCLUSION

These results indicate that DA inhibits the Na+:HCO3- cotransporter in renal proximal tubules and also suggest that dysregulation of the cotransporter, possibly through the defect in DA1 receptor signaling, could play an important role in development of hypertension in SHRs.

On the mechanism of bicarbonate exit from renal proximal tubular cells

We compare here the results of electrophysiological measurements on proximal tubular cells performed on rat kidney in vivo and on isolated rabbit and rat tubules in vitro. Based on different effects of carbonic anhydrase inhibitors in the in vivo and in vitro preparation, we conclude that NaHCO3 cotransport across the basolateral cell membrane functions as Na(+)-CO3(2-)-HCO3- cotransport in vivo, but as Na(+)-HCO3(-)-HCO3- cotransport in the classical in vitro preparation. The former, but not the latter, transport mode is characterized by generation of local disequilibrium pH/CO3(2-) concentrations that oppose fluxes if membrane-bound carbonic anhydrase is inhibited. In support of this conclusion, we find that overall transport functions with a HCO3- to Na+ stoichiometry of 3:1 in vivo (since each transported CO3(2-) eventually generates 2 HCO3- ions), but 2:1 in vitro. This has been deduced from various measurements, among them super-Nernstian and reverse nernstian, potential responses to changing ion concentrations which are characteristic of obligatorily coupled cation-anion cotransporters, but are not known in classical electrochemistry. The different transport modes in vivo and in vitro suggest that isolated proximal tubules have functional deficits compared to proximal tubules in vivo.

An electrophysiological study of angiotensin II regulation of Na-HCO3 cotransport and K conductance in renal proximal tubules. I. Effect of picomolar concentrations

The effect of picomolar concentrations of angiotensin II (AII) was investigated in isolated perfused rabbit renal proximal tubules using conventional or pH-sensitive intracellular microelectrodes. Under control conditions cell membrane potential (Vb) and cell pH (pHi) averaged -53.8 +/- 1.9 mV (mean +/- SEM, n = 49) and 7.24 +/- 0.01 (n = 10), respectively. AII (at 10(-11) mol/l), when applied from the bath (but not when applied from the lumen perfusate), produced the following effects: approximately 85% of the viable tubules responded with a small depolarization (+5.5 +/- 0.4 mV, n = 43) which was accompanied in half of the pHi measurements by a slow acidification (delta pHi = -0.03 +/- 0.01, n = 5). The remaining 15% responded with a small hyperpolarization (delta Vb = -3.1 +/- 0.4 mV, n = 6). All changes were fully reversible and repeatable. Experiments with fast changes in bath HCO3 or K concentrations, as well as measurements of the basolateral voltage divider fraction in response to transepithelial current flow, explain these observations as stimulation of a basolateral Na-HCO3 cotransporter and of a basolateral K conductance. Both counteract in their effect on Vb, but can be individuated by blocker experiments with 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and barium. Both the stimulation of Na-HCO3 cotransport and the stimulation of the K conductance may result from down-regulation of the level of cyclic adenosine monophosphate in the cell.

The Na(+)-HCO3- cotransporter operates with a coupling ratio of 2 HCO3- to 1 Na+ in isolated rabbit renal proximal tubule

All the relevant literature reports indicate that net rates of salt and water absorption and cell membrane potentials (Vb) are lower, but intracellular Na+ concentration is higher in rabbit renal proximal tubule in vitro than in rat proximal tubule in vivo. Since the different driving forces should influence basolateral Na(+)-HCO3- cotransport we have studied the operation of the cotransporter in isolated rabbit renal proximal tubule in vitro with special emphasis on the stoichiometry of flux coupling (q). Using conventional and ion-selective intracellular microelectrodes three series of experiments were performed: (a) we determined the Vb response to a 2:1 reduction of bath HCO3- or Na+ concentration, (b) we determined initial efflux rates of HCO3- or Na+ ions in response to a sudden 10:1 reduction of bath HCO3- concentration, and (c) we collapsed the tubules and determined electrochemical driving forces of Na+ and HCO3- across the basolateral cell membrane under conditions approaching zero net flux in the control state in the presence of Ba2+- and in Cl(-)-free solutions. All measurements concurrently yielded a coupling ratio of approximately two HCO3- ions to one Na+ ion (q = 2). This result contrasts with the ratio q = 3, which we have previously observed in similar experiments on rat renal proximal tubule in vivo [Yoshitomi et al. (1985) Pflügers Arch 405:360] and which was also observed on rabbit renal basolateral cell membrane vesicles in vitro [Soleimani et al. (1987) J Clin Invest 79:1276].(ABSTRACT TRUNCATED AT 250 WORDS)

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.