Anion exchanger is present in both luminal and basolateral renal membranes

Characterization of sulphate transporters in isolated bovine articular chondrocytes

Uptake of SO(4) (2-) by articular chondrocytes is an essential step in the pathway for sulphation of glycosaminoglycans (GAGs), with mutations in SO(4) (2-) transport proteins resulting in abnormalities of skeletal growth. In the present study, the transporters mediating SO(4) (2-) transport in bovine articular chondrocytes have been characterized. Expression of candidate transporters was determined using RT-PCR, while SO(4) (2-) transport was measured in radioisotope flux experiments. RT-PCR experiments showed that bovine articular chondrocytes express three transporters known to transport SO(4) (2-): AE2 (SLC4a2), DTDST (SLC26a2), and SLC26a11. Other transporters--NaS-1 (SLC13a1), SAT-1 (SLC26a1), DRA (SLC26a3), SLC26a6 (PAT1), SLC26a7, SLC26a8 (Tat-1), and SLC26a9--were, however, not detected. In functional experiments, SO(4) (2-) uptake was temperature-sensitive, inhibited by 60% by DIDS (50 microM) and exhibited saturation kinetics, with a K(m) value of 16 mM. Uptake was also inhibited at alkaline extracellular pH. In further experiments, a K(i) value for DIDS inhibition of SO(4) (2-) efflux of 5 microM was recorded. A DIDS-sensitive component of SO(4) (2-) efflux persisted in solutions lacking Cl(-) ions. These data are interpreted as evidence for the preferential operation of carrier-mediated exchange of SO(4) (2-) for Cl(-), while an alternative SO(4) (2-)-OH(-) exchange mode is also possible.

Specificity and Regulation of Renal Sulfate Transporters

Sulfate is essential for normal cellular function. The kidney plays a major role in sulfate homeostasis. Sulfate is freely filtered and then undergoes net reabsorption in the proximal tubule. The apical membrane Na(+)/sulfate cotransporter NaS1 (SLC13A1) has a major role in mediating proximal tubule sulfate reabsorption, as demonstrated by the findings of hyposulfatemia and hypersulfaturia in Nas1-null mice. The anion exchanger SAT1 (SLC26A1), the founding member of the SLC26 sulfate transporter family, mediates sulfate exit across the basolateral membrane to complete the process of transtubular sulfate reabsorption. Another member of this family, CFEX (SLC26A6), is present at the apical membrane of proximal tubular cells. It also can transport sulfate by anion exchange, which probably mediates backflux of sulfate into the lumen. Knockout mouse studies have demonstrated a major role of CFEX as an apical membrane Cl(-)/oxalate exchanger that contributes to NaCl reabsorption in the proximal tubule. Several additional SLC26 family members mediate sulfate transport and show some level of renal expression (e.g., SLC26A2, SLC26A7, SLC26A11). Their roles in mediating renal tubular sulfate transport are presently unknown. This paper reviews current data available on the function and regulation of three sulfate transporters (NaS1, SAT1, and CFEX) and their physiological roles in the kidney.

Transport of γ-Hydroxybutyrate in Rat Kidney Membrane Vesicles: Role of Monocarboxylate Transporters

Intoxication with gamma-hydroxybutyrate (GHB) is associated with coma, seizure, and death; treatment of overdoses is symptomatic. Previous studies in our laboratory have demonstrated that L-lactate and pyruvate treatment can increase the renal clearance of GHB and increase its elimination in rats, suggesting that GHB may undergo renal reabsorption mediated by monocarboxylic acid transporters (MCTs). The goals of this study were to characterize the renal transport of GHB in rats and to determine the role of MCT in its renal transport. Brush-border membrane (BBM) and basolateral membrane (BLM) vesicles were isolated from rat kidney cortex, and the uptake of L-lactate and GHB was characterized. L-Lactate and GHB undergo both pH- and sodium-dependent transport in BBM vesicles and pH-dependent transport in BLM vesicles. A simple Michaelis-Menten equation best described the pH-dependent uptake of GHB in BBM (Km, 8.0 +/- 1.8 mM; Vmax, 838 +/- 45 pmol/mg/s) and in BLM vesicles (Km, 10.5 +/- 2.6 mM; Vmax, 806 +/- 253 pmol/mg/s). mRNA of MCT1 and MCT2 was determined in rat kidney cortex using reverse transcriptase-polymerase chain reaction; using Western blot, the protein expression of MCT1 was present mainly in BLM vesicles, with weak expression in BBM vesicles, whereas that of MCT2 was exclusively in BLM vesicles. Studies with rat MCT1 gene-transfected MDA-MB231 cells demonstrated that GHB was a substrate of MCT1. The data suggest that rat MCT1 may represent an important transporter for GHB in renal tubule cells. This investigation provides evidence for the importance of MCTs in the reabsorption of the monocarboxylic acids l-lactate and GHB in the kidney.

Electrogenic proton-regulated oxalate/chloride exchange by lobster hepatopancreatic brush-border membrane vesicles

The transport of [14C]oxalate (Ox2-) by epithelial brush-border membrane vesicles (BBMV) of lobster (Homarus americanus) hepatopancreas, formed by a magnesium precipitation technique, was stimulated by an outward Cl- gradient (in > out). By contrast, Ox2- uptake was not enhanced by an inward Na+ or K+ transmembrane gradient. Generation of an inside-positive membrane potential by K+ in the presence of valinomycin stimulated Ox2-/Cl- exchange, while an inside-negative membrane potential generated by K+ efflux in the presence of valinomycin inhibited this process. Neither Ox2-/Ox2- nor Ox2-/SO4(2-) transport exchange were affected by alterations of transmembrane potential. An inwardly directed proton gradient, or the presence of low bilateral pH, enhanced Ox2-/Cl- exchange, yet the H+ gradient alone could not stimulate Ox2) uptake in Cl(-)-equilibrated BBMV or in vesicles lacking internal Cl-. The stilbenes 4-acetamido-4'-isothiocyanotostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS) strongly inhibited Ox2-/Cl- exchange. Oxalate influx occurred by a combination of carrier-mediated transfer, exhibiting Michaelis-Menten kinetics, and nonsaturable 'apparent diffusion'. Apparent kinetic constants for Ox2-/Cl- exchange were Kt = 0.20 mmol l(-1) and Jmax = 1.03 nmol l(-1) mg(-1) protein 7 s(-1). 36Cl- influx into oxalate-loaded BBMV was stimulated by an inside-negative transmembrane potential compared with short-circuited vesicles. These results suggest that Ox2-/Cl- exchange in crustacean hepatopancreatic BBMV occurred by an electrogenic carrier mechanism exhibiting a 1:1 flux ratio that was modulated by an external proton-sensitive regulatory site.

Transport of organic anions across the basolateral membrane of proximal tubule cells

Renal proximal tubules secrete diverse organic anions (OA) including widely prescribed anionic drugs. Here, we review the molecular properties of cloned transporters involved in uptake of OA from blood into proximal tubule cells and provide extensive lists of substrates handled by these transport systems. Where tested, transporters have been immunolocalized to the basolateral cell membrane. The sulfate anion transporter 1 (sat-1) cloned from human, rat and mouse, transported oxalate and sulfate. Drugs found earlier to interact with sulfate transport in vivo have not yet been tested with sat-1. The Na(+)-dicarboxylate cotransporter 3 (NaDC-3) was cloned from human, rat, mouse and flounder, and transported three Na(+) with one divalent di- or tricarboxylate, such as citric acid cycle intermediates and the heavy metal chelator 2,3-dimercaptosuccinate (succimer). The organic anion transporter 1 (OAT1) cloned from several species was shown to exchange extracellular OA against intracellular alpha-ketoglutarate. OAT1 translocated, e.g., anti-inflammatory drugs, antiviral drugs, beta-lactam antibiotics, loop diuretics, ochratoxin A, and p-aminohippurate. Several OA, including probenecid, inhibited OAT1. Human, rat and mouse OAT2 transported selected anti-inflammatory and antiviral drugs, methotrexate, ochratoxin A, and, with high affinities, prostaglandins E(2) and F(2alpha). OAT3 cloned from human, rat and mouse showed a substrate specificity overlapping with that of OAT1. In addition, OAT3 interacted with sulfated steroid hormones such as estrone-3-sulfate. The driving forces for OAT2 and OAT3, the relative contributions of all OA transporters to, and the impact of transporter regulation by protein kinases on renal drug excretion in vivo must be determined in future experiments.

Thyroid hormone-induced sulfate absorption in Aplysia californica gut is mediated by protein synthesis

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium-sulfate symporter that is stimulated by triiodothyronine. Actinomycin D, puromycin, or cycloheximide inhibited the triiodothyronine-stimulated sulfate absorption. It appears that thyroid hormone manifests its effects on sulfate absorption in the A. californica gut through protein synthesis.

Sulfate absorption inAplysiacalifornicagut: thyroid hormone stimulation

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium/sulfate symporter. Mucosal or serosal application of triiodothyronine stimulated the absorptive activity of the sodium/sulfate symporter, whereas reverse triiodothyronine had no effect on it. It appears that thyroid hormone or its molluscan equivalent plays a role in the overall regulation of sulfate homeostasis by the A. californica gut.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Sulfate absorption in Aplysia californica gut: intracellular regulation by cyclic guanosine monophosphate

Luminal membranes of foregut epithelia of Aplysia californica contain a sodium–sulfate symporter. Cyclic guanosine monophosphate stimulated the absorptive activity of the sodium–sulfate symporter, whereas cyclic adenosine monophosphate had no effect on the sodium–sulfate symporter. It appears that guanylate cyclase plays a role in the overall regulation of sulfate homeostasis by the A. californica gut.

Sulfate transport mechanisms in epithelial systems

A novel invertebrate gastrointestinal transport mechanism has been shown to couple chloride-sulfate exchange in an electrogenic fashion. In the lobster, Homarus americanus, the hepatopancreas, or digestive gland, exists as an outpocketing of the digestive tract, representing a single cell layer separating the gut lumen and an open circulatory system composed of hemolymph. Investigations utilizing independently prepared brush border and basolateral membrane vesicles revealed discrete antiport systems which possess the capacity to bring about a transcellular secretion of sulfate. The luminal antiport system functions as a high-affinity, one-to-one chloride-sulfate exchanger that is stimulated by an increase in luminal hydrogen ion concentration. Such a system would take advantage of the high chloride concentration of ingested seawater as well as the high proton concentrations generated during digestion, which further suggests a potential regulation by resident sodium-proton exchangers. Exchange of one chloride for one divalent sulfate ion provides the driving force for electrogenic vectorial translocation. The basolateral antiport system was found to be electroneutral in nature, responsive to gradients of the dicarboxylic anion oxalate while lacking in proton stimulation. No evidence of sodium-sulfate co-transport, commonly reported for the brush border of vertebrate renal and intestinal epithelia, was observed in either membrane preparation. The two antiporters together can account for the low hemolymph to seawater sulfate levels previously described in decapod crustaceans. A secretory pathway for sulfate based upon electrogenic chloride-antiport may appear among invertebrates partly in response to digestion taking place in a seawater environment. J. Exp. Zool. 289:245-253, 2001.

The oxalate/sulfate antiporter in lobster hepatopancreas: internal and external binding constants

Utilizing a purified basolateral plasma membrane vesicle (BLMV) preparation containing a sulfate/oxalate antiporter, it was demonstrated that sulfate exhibited similar binding characteristics to the transporter whether bound internally or externally. It was also demonstrated that oxalate had similar binding characteristics to the antiporter whether it was bound internally or externally. Oxalate had a greater affinity to the transporter than did sulfate. Several organic anions affected binding and, therefore, overall transport by the antiporter. Most notably, sulfate was the only anion that stimulated oxalate uptake into BLMVs, which suggests a conservative binding specificity for the antiporter. 4-Acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS) and/or 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) inhibited the transport rate, confirming the existence of oxalate/sulfate exchange by the transporter. These results suggest that oxalate, not sulfate, regulates the transport rate because of its greater affinity to the transporter.

Competitive inhibition of p-aminohippurate transport by quinapril in rabbit renal basolateral membrane vesicles

The mechanism of quinapril's interaction with the organic anion transporter was characterized by studying its effect on the transport of p-aminohippurate (PAH) in rabbit renal basolateral membrane vesicles (BLMV). Cis-inhibition studies demonstrated that quinapril was a specific and potent inhibitor of PAH. The Ki of quinapril was about 20 microM, a value similar to that of probenecid and eight-times lower than the K(m) value of 165 microM for PAH. Even though quinapril resulted in trans-inhibition of PAH uptake during counterflow studies, kinetic studies revealed that quinapril was a competitive inhibitor of PAH transport. This latter findings suggests that quinapril and PAH share a common binding site on the transporter. Overall, the results indicate that quinapril is a high-affinity inhibitor of the organic anion transporter in renal BLMV, and that drug-drug interactions may occur with other organic anions at the basolateral membrane of proximal cells.

Effects of sulfate and chloride on three separate oxalate transporters reconstituted from rabbit renal cortex

Understanding the mechanism of sulfate-dependent, oxalate-stimulated chloride reabsorption in the mammalian proximal tubule is complicated by the presence of multiple oxalate and sulfate transport pathways. Accordingly, we developed a method of reconstituting functional oxalate transport from the rabbit renal cortex so that the individual transporters might be examined. Solubilized microvillus membrane proteins were separated by hydroxyapatite chromatography and then reconstituted into proteoliposomes. Two peaks of oxalate/oxalate exchange activity were observed. Sulfate (10 mM) cis-inhibits oxalate transport in the early peak by 93% and in the later peak by 41%. In contrast, 20 mM chloride inhibits oxalate/oxalate exchange by only 32% in the early peak but inhibits oxalate exchange by 70% in the later peak. Oxalate-stimulated sulfate uptake was observed in the early fractions but not in the later fractions. These data are consistent with the recovery of the sulfate/oxalate exchanger in the early hydroxyapatite fractions and the chloride/oxalate exchanger in the later fractions. The basolateral membrane sulfate/oxalate exchanger was also reconstituted. The reconstituted basolateral and apical membrane sulfate/oxalate exchangers demonstrate nearly identical patterns of substrate specificities. However, 98% of apical sulfate/oxalate exchange activity is lost following exposure to octylglucoside at room temperature, whereas the basolateral sulfate/oxalate exchange activity was reduced 67% (P < 0.05). In conclusion, functional reconstitution of solubilized membrane proteins demonstrates that apical membrane chloride/oxalate exchange and sulfate/oxalate exchange are mediated by different transport proteins. Apical and basolateral sulfate/oxalate exchange may also represent transport on two separate exchangers.

Cloning and functional expression of a human kidney Na+:HCO3- cotransporter

Several modes of HCO3- transport occur in the kidney, including Na+-independent Cl/HCO3- exchange (mediated by the AE family of Cl-/HCO3- exchangers), sodium-dependent Cl-/HCO3- exchange, and Na+:HCO3- cotransport. The functional similarities between the Na+-coupled HCO3- transporters and the AE isoforms (i.e. transport of HCO3- and sensitivity to inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) suggested a strategy for cloning the other transporters based on structural similarity with the AE family. An expressed sequence tag encoding part of a protein that is related to the known anion exchangers was identified in the GenBankTM expressed sequence tag data base and used to design an oligonucleotide probe. This probe was used to screen a human kidney cDNA library. Several clones were identified, isolated, and sequenced. Two overlapping cDNA clones were spliced together to form a 7.6-kilobase cDNA that contained the entire coding region of a novel protein. Based on the deduced amino acid sequence, the cDNA encodes a protein with a Mr of 116,040. The protein has 29% identity with human brain AE3. Northern blot analysis reveals that the 7.6-kilobase mRNA is highly expressed in kidney and pancreas, with detectable levels in brain. Functional studies in transiently transfected HEK-293 cells demonstrate that the cloned transporter mediates Na+:HCO3- cotransport.

Antiport-driven sulfate secretion in an invertebrate epithelium

A novel invertebrate gastrointestinal transport mechanism has been shown to couple chloride/sulfate exchange in an electrogenic fashion. In the lobster, Homarus americanus, the hepatopancreas, or digestive gland, exists as an outpocketing of the digestive tract, representing a single cell layer separating the gut lumen and an open circulatory system comprised of hemolymph. Investigations utilizing independently prepared brush-border and basolateral membrane vesicles revealed discrete antiport systems which possess the capacity to bring about a transcellular secretion of sulfate. The luminal antiport system functions as a high affinity, one-to-one chloride-sulfate exchanger that is stimulated by an increase in luminal hydrogen ion concentration. Such a system would take advantage of the high chloride concentration of ingested seawater, as well as the high proton concentrations generated during digestion, which further suggests a potential regulation by resident sodium-proton exchangers. Exchange of one chloride for one divalent sulfate ion provides the driving force for electrogenic vectorial translocation. The basolateral antiport system was found to be electroneutral in nature, responsive to gradients of the dicarboxylic anion oxalate, while lacking in proton stimulation. No evidence of sodium-sulfate cotransport, commonly reported for the brush border of vertebrate renal and intestinal epithelia, was observed in either membrane preparation. The two antiporters together can account for the low hemolymph to seawater sulfate levels previously described in decapod crustaceans. A secretory pathway for sulfate based upon electrogenic chloride-antiport may appear among invertebrates partly in response to digestion taking place in a seawater environment.

A protein with anion exchange properties found in the kidney proximal tubule

One important mechanism for reabsorption of chloride in the kidney proximal tubule involves anion exchange of chloride for a base. Anion exchange transport systems in general demonstrate sensitivity to inhibition by disulfonic stilbenes, probenecid, furosemide, and the arginyl amino group modifier phenylglyoxal. Using disulfonic stilbene affinity chromatography, we have identified and partially purified a protein with anion exchanger properties in luminal membrane vesicles isolated from rabbit kidney cortex. This protein has a molecular weight of 162 kD. The binding of the 162 kD protein to the stilbene affinity matrix is inhibited by disulfonic stilbenes, probenecid, furosemide, and phenylglyoxal. Reconstitution of the proteins eluted from the affinity matrix into liposomes demonstrates anion exchange activity as assayed by radiolabeled chloride influx. Deletion of the 162 kD protein from the eluted mixture by probenecid diminishes the anion exchanger activity in the reconstituted liposomes. Further purification of the disulfonic stilbene column eluant by Econo-Pac Q ion exchange chromatography resulted in significant enrichment in 162 kD protein abundance and also anion exchange activity in reconstituted liposomes. The results of the above experiments strongly suggest that the 162 kD protein is an anion exchanger. Insight into the functional and molecular characteristics of this protein should provide important information about the mechanism(s) of chloride reabsorption in the kidney proximal tubule.

Identification and covalent modification of a renal brush-border anion exchanger

Brush-border membrane (BBM) proteins that bind the arginine-specific reagent phenylglyoxal (PG) and interact with stilbene disulfonic derivatives were identified in canine kidney cortex. Pretreatment of BBM vesicles with PG resulted in irreversible inhibition of anion exchange as assayed by 36Cl- influx mediated via Cl-/Cl- exchange. Cl-/Cl- exchange was reversibly inhibited by the disulfonic stilbene 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS). A stilbene-affinity matrix was prepared by immobilizing DNDS in polyacrylamide beads. Elution of the BBM proteins from a disulfonic stilbene (DNDS) affinity matrix revealed two proteins at 160 and 230 kDa that were significantly enriched compared to initial material. Radiolabeling of the eluted mixture with [14C]phenylglyoxal demonstrated covalent binding to several proteins, including the 160 kDa protein. Reconstitution of the proteins eluted from the affinity matrix into phosphatidylcholine demonstrated DIDS-sensitive 36Cl(-)-influx mediated via Cl-/Cl- exchange. Pretreatment of the BBM vesicles with PG selectively blocked binding of the 160 kDa protein to the DNDS affinity matrix. Radiolabelling of the PG-pretreated, affinity-purified membrane proteins showed selective prevention of [14C]phenylglyoxal binding to the 160 kDa protein. Reconstitution of the PG-pretreated proteins eluted from the affinity matrix demonstrated significant reduction in Cl-/Cl- exchange activity. These results suggest that a 160 kDa protein is a strong candidate for anion exchange transport in kidney proximal tubules.

Heterogeneity of anion exchangers mediating chloride transport in the proximal tubule

Three distinct anion exchangers are described that directly or indirectly mediate Cl- transport across the luminal membrane of the proximal tubule cell. Studies on the intact proximal tubule indicate that the Cl(-)-formate exchanger is a major mechanism for Cl- transport under physiologic conditions. As just discussed, the physiologic importance of the Cl(-)-oxalate and SO4 = (oxalate)-CO3 = exchangers mediating Cl- transport across the luminal membrane of the proximal tubule cell is currently unknown. These three anion exchangers are part of a larger group of at least eight distinct anion transporters in the proximal tubule that share with erythrocyte Band 3 the properties of stilbene sensitivity and/or the ability to mediate anion exchange. It is tempting to speculate that these proximal tubule anion transporters are members of a family of proteins structurally related to the prototypic anion exchanger, erythrocyte Band 3. If this is true, comparing the structures of these anion transporters with each other and with Band 3 should provide important insight into the molecular basis for differences in substrate and inhibitor specificity within this family of transport proteins.

Relation between the anion exchange protein in kidney medullary collecting duct cells and red cell band 3

A membrane protein that is immunochemically similar to the red cell anion exchange protein, band 3, has been identified on the basolateral face of the outer medullary collecting duct (MCD) cells in rabbit kidney. In freshly prepared separated rabbit MCD cells, M.L. Zeidel, P. Silva and J.L. Seifter (J. Clin. Invest. 77:1682-1688, 1986) found that C1-/HCO-3 exchange was inhibited by the stilbene anion exchange inhibitor, DIDS (4,4'-diisothiocyano-2,2'-disulfonic stilbene), with a K1 similar to that for the red cell. We have measured the binding affinities of a fluorescent stilbene inhibitor, DBDS (4,4'-dibenzamido-2,2'-disulfonic stilbene), to MCD cells in 28.5 mM citrate and have characterized both a high-affinity site (Ks1 = 93 +/- 24 nM) and a lower affinity site (Ks2 = 430 +/- 260 nM), which are closely similar to values for the red cell of 110 +/- 51 nM for the high-affinity site and 980 +/- 200 nM for the lower affinity site (A.S. Verkman, J.A. Dix & A.K. Solomon, J. Gen. Physiol. 81:421-449, 1983). When Cl- replaces citrate in the buffer, the two sites collapse into a single one with Ks1 = 1500 +/- 400 nM, similar to the single Ks1 = 1200 +/- 200 nM in the red cell (J.A. Dix, A.S. Verkman & A.K. Solomon, J. Membrane Biol. 89:211-223, 1986). The kinetics of DBDS binding to MCD cells at 0.25 microM-1 are characterized by a fast process, tau = 0.14 +/- 0.03 sec, similar to tau = 0.12 +/- 0.03 sec in the red cell. These similarities show that the physical chemical characteristics of stilbene inhibitor binding to MCD cell 'band 3' closely resemble those for red cell band 3, which suggests that the molecular structure is highly conserved.

Contraluminal bicarbonate transport in the proximal tubule of the rat kidney

In order to measure the contraluminal bicarbonate flux in situ we applied the stopped flow capillary microperfusion technique and measured the influx of 14C-bicarbonate buffer into cortical tubular cells at pH 8. It was found that the influx in percent of the starting concentration is larger at 20 mmol/l bicarbonate than at 1 mmol/l, indicating a sigmoidal type influx curve. At 20 mmol/l bicarbonate the influx was inhibited by 44%, when Na+ was replaced by choline. Replacement of gluconate by chloride or sulfate did not change H14CO3- influx. At this bicarbonate concentration, influx is inhibited by 10 mmol/l 4,4'-diisothiocyanato-2,2'-stilbenedisulfonate (DIDS) (22%), 5 mmol/l of the carbonic anhydrase blocker ethoxyzolamide (40%) as well as by 5 mmol/l of the arginine reagent 4-nitrophenylglyoxal (31%). At 1 mmol/l bicarbonate starting concentration, bicarbonate influx was inhibited when chloride in the perfusate was present or when sulphate was added. Replacement of sodium by choline did not change bicarbonate influx. Addition of DIDS and 8-anilino-naphthalene-1-sulfonate (5 mmol/l each) inhibited 1 mmol/l bicarbonate influx 39 and 49%, respectively. The para-amino-hippurate transport blocker dipropylsulfamoyl-benzoate (probenecid), the chloride channel blocker 5-nitro-2'-(3-phenylpropylamino)-benzoate (NPPB), the SH group blocker 2-(3-hydroxymercuri-2-methoxypropyl)-carbamoyl-phenoxyacetate++ + (mersalyl), and formate did not inhibit bicarbonate influx, at 20 and at 1 mmol/l H14CO3- starting concentration.(ABSTRACT TRUNCATED AT 250 WORDS)