Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models

Optimizing SGLT inhibitor treatment for diabetes with chronic kidney diseases

Diabetes induces glomerular hyperfiltration, affects kidney function, and may lead to chronic kidney diseases. A novel therapeutic treatment for diabetic patients targets the sodium-glucose cotransporter isoform 2 (SGLT2) in the kidney. SGLT2 inhibitors enhance urinary glucose, [Formula: see text] and fluid excretion and lower hyperglycemia in diabetes by inhibiting [Formula: see text] and glucose reabsorption along the proximal convoluted tubule. A goal of this study is to predict the effects of SGLT2 inhibitors in diabetic patients with and without chronic kidney diseases. To that end, we applied computational rat kidney models to assess how SGLT2 inhibition affects renal solute transport and metabolism when nephron population are normal or reduced (the latter simulates chronic kidney disease). The model predicts that SGLT2 inhibition induces glucosuria and natriuresis, with those effects enhanced in a remnant kidney. The model also predicts that the [Formula: see text] transport load and thus oxygen consumption of the S3 segment are increased under SGLT2 inhibition, a consequence that may increase the risk of hypoxia for that segment. To protect the vulnerable S3 segment, we explore dual SGLT2/SGLT1 inhibition and seek to determine the optimal combination that would yield sufficient urinary glucose excretion while limiting the metabolic load on the S3 segment. The model predicts that the optimal combination of SGLT2/SGLT1 inhibition lowers the oxygen requirements of key tubular segments, but decreases urine flow and [Formula: see text] excretion; the latter effect may limit the cardiovascular protection of the treatment.

A computational model of epithelial solute and water transport along a human nephron

We have developed the first computational model of solute and water transport from Bowman space to the papillary tip of the nephron of a human kidney. The nephron is represented as a tubule lined by a layer of epithelial cells, with apical and basolateral transporters that vary according to cell type. The model is formulated for steady state, and consists of a large system of coupled ordinary differential equations and algebraic equations. Model solution describes luminal fluid flow, hydrostatic pressure, luminal fluid solute concentrations, cytosolic solute concentrations, epithelial membrane potential, and transcellular and paracellular fluxes. We found that if we assume that the transporter density and permeabilities are taken to be the same between the human and rat nephrons (with the exception of a glucose transporter along the proximal tubule and the H⁺-pump along the collecting duct), the model yields segmental deliveries and urinary excretion of volume and key solutes that are consistent with human data. The model predicted that the human nephron exhibits glomerulotubular balance, such that proximal tubular Na⁺ reabsorption varies proportionally to the single-nephron glomerular filtration rate. To simulate the action of a novel diabetic treatment, we inhibited the Na⁺-glucose cotransporter 2 (SGLT2) along the proximal convoluted tubule. Simulation results predicted that the segment’s Na⁺ reabsorption decreased significantly, resulting in natriuresis and osmotic diuresis.

In addition to its well-known function of waste removal from the body, the kidney is also responsible for the critical regulation of the body’s salt, potassium, acid content, and blood pressure. The kidneys perform these life-sustaining task by filtering and returning to blood stream about 200 quarts of blood every 24 hours. What isn’t returned to blood stream is excreted as urine. The production of urine involves highly complex steps of secretion and reabsorption. To study these processes without employing invasive experimental procedures, we developed the first computational model of the human nephron (which is the functional unit of a kidney). The model contains detailed representation of the transport processes that take place in the epithelial cells that form the walls of the nephron. Using that model, we conducted simulations to predict how much filtered solutes and and water is transported along each individual and functionally distinct nephron segment. We conducted these simulations under normal physiological conditions, and under pharmacological conditions. The nephron model can be used as an essential component in an integrated model of kidney function in humans.

Predicted consequences of diabetes and SGLT inhibition on transport and oxygen consumption along a rat nephron

Diabetes increases the reabsorption of Na(+) (TNa) and glucose via the sodium-glucose cotransporter SGLT2 in the early proximal tubule (S1-S2 segments) of the renal cortex. SGLT2 inhibitors enhance glucose excretion and lower hyperglycemia in diabetes. We aimed to investigate how diabetes and SGLT2 inhibition affect TNa and sodium transport-dependent oxygen consumption [Formula: see text] along the whole nephron. To do so, we developed a mathematical model of water and solute transport from the Bowman space to the papillary tip of a superficial nephron of the rat kidney. Model simulations indicate that, in the nondiabetic kidney, acute and chronic SGLT2 inhibition enhances active TNa in all nephron segments, thereby raising [Formula: see text] by 5-12% in the cortex and medulla. Diabetes increases overall TNa and [Formula: see text] by ∼50 and 100%, mainly because it enhances glomerular filtration rate (GFR) and transport load. In diabetes, acute and chronic SGLT2 inhibition lowers [Formula: see text] in the cortex by ∼30%, due to GFR reduction that lowers proximal tubule active TNa, but raises [Formula: see text] in the medulla by ∼7%. In the medulla specifically, chronic SGLT2 inhibition is predicted to increase [Formula: see text] by 26% in late proximal tubules (S3 segments), by 2% in medullary thick ascending limbs (mTAL), and by 9 and 21% in outer and inner medullary collecting ducts (OMCD and IMCD), respectively. Additional blockade of SGLT1 in S3 segments enhances glucose excretion, reduces [Formula: see text] by 33% in S3 segments, and raises [Formula: see text] by <1% in mTAL, OMCD, and IMCD. In summary, the model predicts that SGLT2 blockade in diabetes lowers cortical [Formula: see text] and raises medullary [Formula: see text], particularly in S3 segments.

Modeling oxygen consumption in the proximal tubule: effects of NHE and SGLT2 inhibition

The objective of this study was to investigate how physiological, pharmacological, and pathological conditions that alter sodium reabsorption (TNa) in the proximal tubule affect oxygen consumption (QO2 ) and Na(+) transport efficiency (TNa/QO2 ). To do so, we expanded a mathematical model of solute transport in the proximal tubule of the rat kidney. The model represents compliant S1, S2, and S3 segments and accounts for their specific apical and basolateral transporters. Sodium is reabsorbed transcellularly, via apical Na(+)/H(+) exchangers (NHE) and Na(+)-glucose (SGLT) cotransporters, and paracellularly. Our results suggest that TNa/QO2 is 80% higher in S3 than in S1-S2 segments, due to the greater contribution of the passive paracellular pathway to TNa in the former segment. Inhibition of NHE or Na-K-ATPase reduced TNa and QO2 , as well as Na(+) transport efficiency. SGLT2 inhibition also reduced proximal tubular TNa but increased QO2 ; these effects were relatively more pronounced in the S3 vs. the S1-S2 segments. Diabetes increased TNa and QO2 and reduced TNa/QO2 , owing mostly to hyperfiltration. Since SGLT2 inhibition lowers diabetic hyperfiltration, the net effect on TNa, QO2 , and Na(+) transport efficiency in the proximal tubule will largely depend on the individual extent to which glomerular filtration rate is lowered.

Trypanosoma brucei eflornithine transporter AAT6 is a low-affinity low-selective transporter for neutral amino acids

Amino acid transporters are crucial for parasite survival since the cellular metabolism of parasitic protozoa depends on the up-take of exogenous amino acids. Amino acid transporters are also of high pharmacological relevance because they may mediate uptake of toxic amino acid analogues. In the present study we show that the eflornithine transporter AAT6 from Trypanosoma brucei (TbAAT6) mediates growth on neutral amino acids when expressed in Saccharomyces cerevisiae mutants. The transport was electrogenic and further analysed in Xenopus laevis oocytes. Neutral amino acids, proline analogues, eflornithine and acivicin induced inward currents. For proline, glycine and tryptophan the apparent affinities and maximal transport rates increased with more negative membrane potentials. Proline-induced currents were dependent on pH, but not on sodium. Although proline represents the primary energy source of T. brucei in the tsetse fly, down-regulation of TbAAT6-expression by RNAi showed that in culture TbAAT6 is not essential for growth of procyclic form trypanosomes in the presence of glucose or proline as energy source. TbAAT6-RNAi lines of both bloodstream and procyclic form trypanosomes showed reduced susceptibility to eflornithine, whereas the sensitivity to acivicin remained unchanged, indicating that acivicin enters the cell by more than one transporter.

Mechanism of proton/substrate coupling in the heptahelical lysosomal transporter cystinosin

Secondary active transporters use electrochemical gradients provided by primary ion pumps to translocate metabolites or drugs "uphill" across membranes. Here we report the ion-coupling mechanism of cystinosin, an unusual eukaryotic, proton-driven transporter distantly related to the proton pump bacteriorhodopsin. In humans, cystinosin exports the proteolysis-derived dimeric amino acid cystine from lysosomes and is impaired in cystinosis. Using voltage-dependence analysis of steady-state and transient currents elicited by cystine and neutralization-scanning mutagenesis of conserved protonatable residues, we show that cystine binding is coupled to protonation of a clinically relevant aspartate buried in the membrane. Deuterium isotope substitution experiments are consistent with an access of this aspartate from the lysosomal lumen through a deep proton channel. This aspartate lies in one of the two PQ-loop motifs shared by cystinosin with a set of eukaryotic membrane proteins of unknown function and is conserved in about half of them, thus suggesting that other PQ-loop proteins may translocate protons.

SLC4A transporters

SLC4A gene family proteins include bicarbonate transporters that move HCO(3)(-) across the plasma membrane and regulate intracellular pH and transepithelial movement of acid-base equivalents. These transporters are Cl/HCO(3) exchangers, electrogenic Na/HCO(3) cotransporters, electroneutral Na/HCO(3) cotransporters, and Na(+)-driven Cl/HCO(3) exchanger. Studies of the bicarbonate transporters in vitro and in vivo have demonstrated their physiological importance for acid-base homeostasis at the cellular and systemic levels. Recent advances in structure/function analysis have also provided valuable information on domains or motifs critical for regulation, ion translocation, and protein topology. This chapter focuses on the molecular mechanisms of ion transport along with associated structural aspects from mutagenesis of particular residues and from chimeric constructs. Structure/function studies have helped to understand the mechanism by which ion substrates are moved via the transporters. This chapter also describes some insights into the structure of SLC4A1 (AE1) and SLC4A4 (NBCe1) transporters. Finally, as some SLC4A transporters exist in concert with other proteins in the cells, the structural features associated with protein-protein interactions are briefly discussed.

The C-terminal domain of the neutral amino acid transporter SNAT2 regulates transport activity through voltage-dependent processes

SNAT (sodium-coupled neutral amino acid transporter) 2 belongs to the SLC38 (solute carrier 38) family of solute transporters. Transport of one amino acid molecule into the cell is driven by the co-transport of one Na(+) ion. The functional significance of the C-terminus of SNAT2, which is predicted to be located in the extracellular space, is currently unknown. In the present paper, we removed 13 amino acid residues from the SNAT2 C-terminus and studied the effect of this deletion on transporter function. The truncation abolished amino acid transport currents at negative membrane potentials (<0 mV), as well as substrate uptake. However, transport currents were observed at positive membrane potentials demonstrating that transport was accelerated while the driving force decreased. Membrane expression levels were normal in the truncated transporter. SNAT2(Del C-ter) (13 residues deleted from the C-terminus) showed 3-fold higher apparent affinity for alanine, and 2-fold higher Na(+) affinity compared with wild-type SNAT2, suggesting that the C-terminus is not required for high-affinity substrate and Na(+) interaction with SNAT2. The pH sensitivity of amino acid transport was retained partially after the truncation. In contrast with the truncation after TM (transmembrane domain) 11, the deletion of TM11 resulted in an inactive transporter, most probably due to a defect in cell surface expression. Taken together, the results demonstrate that the C-terminal domain of SNAT2 is an important voltage regulator that is required for a normal amino acid translocation process at physiological membrane potentials. However, the C-terminus appears not to be involved in the regulation of membrane expression.

Highly conserved asparagine 82 controls the interaction of Na+ with the sodium-coupled neutral amino acid transporter SNAT2

The neutral amino acid transporter 2 (SNAT2), which belongs to the SLC38 family of solute transporters, couples the transport of amino acid to the cotransport of one Na(+) ion into the cell. Several polar amino acids are highly conserved within the SLC38 family. Here, we mutated three of these conserved amino acids, Asn(82) in the predicted transmembrane domain 1 (TMD1), Tyr(337) in TMD7, and Arg(374) in TMD8; and we studied the functional consequences of these modifications. The mutation of N82A virtually eliminated the alanine-induced transport current, as well as amino acid uptake by SNAT2. In contrast, the mutations Y337A and R374Q did not abolish amino acid transport. The K(m) of SNAT2 for its interaction with Na(+), K(Na(+)), was dramatically reduced by the N82A mutation, whereas the more conservative mutation N82S resulted in a K(Na(+)) that was in between SNAT2(N82A) and SNAT2(WT). These results were interpreted as a reduction of Na(+) affinity caused by the Asn(82) mutations, suggesting that these mutations interfere with the interaction of SNAT2 with the sodium ion. As a consequence of this dramatic reduction in Na(+) affinity, the apparent K(m) of SNAT2(N82A) for alanine was increased 27-fold compared with that of SNAT2(WT). Our results demonstrate a direct or indirect involvement of Asn(82) in Na(+) coordination by SNAT2. Therefore, we predict that TMD1 is crucial for the function of SLC38 transporters and that of related families.

Transport model of the human Na+-coupled L-ascorbic acid (vitamin C) transporter SVCT1

Vitamin C (L-ascorbic acid) is an essential micronutrient that serves as an antioxidant and as a cofactor in many enzymatic reactions. Intestinal absorption and renal reabsorption of the vitamin is mediated by the epithelial apical L-ascorbic acid cotransporter SVCT1 (SLC23A1). We explored the molecular mechanisms of SVCT1-mediated L-ascorbic acid transport using radiotracer and voltage-clamp techniques in RNA-injected Xenopus oocytes. L-ascorbic acid transport was saturable (K(0.5) approximately 70 microM), temperature dependent (Q(10) approximately 5), and energized by the Na(+) electrochemical potential gradient. We obtained a Na(+)-L-ascorbic acid coupling ratio of 2:1 from simultaneous measurement of currents and fluxes. L-ascorbic acid and Na(+) saturation kinetics as a function of cosubstrate concentrations revealed a simultaneous transport mechanism in which binding is ordered Na(+), L-ascorbic acid, Na(+). In the absence of L-ascorbic acid, SVCT1 mediated pre-steady-state currents that decayed with time constants 3-15 ms. Transients were described by single Boltzmann distributions. At 100 mM Na(+), maximal charge translocation (Q(max)) was approximately 25 nC, around a midpoint (V(0.5)) at -9 mV, and with apparent valence approximately -1. Q(max) was conserved upon progressive removal of Na(+), whereas V(0.5) shifted to more hyperpolarized potentials. Model simulation predicted that the pre-steady-state current predominantly results from an ion-well effect on binding of the first Na(+) partway within the membrane electric field. We present a transport model for SVCT1 that will provide a framework for investigating the impact of specific mutations and polymorphisms in SLC23A1 and help us better understand the contribution of SVCT1 to vitamin C metabolism in health and disease.

The Inner Interhelix Loop 4–5 of the Melibiose Permease from Escherichia coli Takes Part in Conformational Changes after Sugar Binding

Cytoplasmic loop 4-5 of the melibiose permease from Escherichia coli is essential for the process of Na+-sugar translocation (Abdel-Dayem, M., Basquin, C., Pourcher, T., Cordat, E., and Leblanc, G. (2003) J. Biol. Chem. 278, 1518-1524). In the present report, we analyze functional consequences of mutating each of the three acidic amino acids in this loop into cysteines. Among the mutants, only the E142C substitution impairs selectively Na+-sugar translocation. Because R141C has a similar defect, we investigated these two mutants in more detail. Liposomes containing purified mutated melibiose permease were adsorbed onto a solid supported lipid membrane, and transient electrical currents resulting from different substrate concentration jumps were recorded. The currents evoked by a melibiose concentration jump in the presence of Na+, previously assigned to an electrogenic conformational transition (Meyer-Lipp, K., Ganea, C., Pourcher, T., Leblanc, G., and Fendler, K. (2004) Biochemistry 43, 12606-12613), were much smaller for the two mutants than the corresponding signals in cysteineless MelB. Furthermore, in R141C the stimulating effect of melibiose on Na+ affinity was lost. Finally, whereas tryptophan fluorescence spectroscopy revealed impaired conformational changes upon melibiose binding in the mutants, fluorescence resonance energy transfer measurements indicated that the mutants still show cooperative modification of their sugar binding sites by Na+. These data suggest that: 1) loop 4-5 contributes to the coordinated interactions between the ion and sugar binding sites; 2) it participates in an electrogenic conformational transition after melibiose binding that is essential for the subsequent obligatory coupled translocation of substrates. A two-step mechanism for substrate translocation in the melibiose permease is suggested.

Kinetics of bidirectional H+ and substrate transport by the proton-dependent amino acid symporter PAT1

PAT1 is a recently identified member of the PAT family of proton/amino acid co-transporters with predominant expression in the plasma membrane of enterocytes and in lysosomal membranes of neurons. Previous studies in Xenopus oocytes expressing PAT1 established proton/substrate co-transport associated with positive inward currents for a variety of small neutral amino acids. Here we provide a detailed analysis of the transport mode of the murine PAT1 in oocytes using the two-electrode voltage-clamp technique to measure steady-state and pre-steady-state currents. The GPC (giant patch clamp) technique and efflux studies were employed to characterize the reversed transport mode. Kinetic parameters [K(m) (Michaelis constant) and I(max) (maximum current)] for transport of various substrates revealed a dependence on membrane potential: hyperpolarization increases the substrate affinity and maximal transport velocity. Proton affinity for interaction with PAT1 is almost 100 nM, corresponding to a pH of 7.0 and is independent of substrate. Kinetic analysis revealed that binding of proton most likely occurs before substrate binding and that the proton and substrate are translocated in a simultaneous step. No evidence for a substrate-uncoupled proton shunt was observed. As shown by efflux studies and current measurements by the GPC technique, PAT1 allows bidirectional amino acid transport. Surprisingly, PAT1 exhibits no pre-steady-state currents in the absence of substrate, even at low temperatures, and therefore PAT1 takes an exceptional position among the ion-coupled co-transporters.

Divalent metal-ion transporter DMT1 mediates both H+ -coupled Fe2+ transport and uncoupled fluxes

The H(+) -coupled divalent metal-ion transporter DMT1 serves as both the primary entry point for iron into the body (intestinal brush-border uptake) and the route by which transferrin-associated iron is mobilized from endosomes to cytosol in erythroid precursors and other cells. Elucidating the molecular mechanisms of DMT1 will therefore increase our understanding of iron metabolism and the etiology of iron overload disorders. We expressed wild type and mutant DMT1 in Xenopus oocytes and monitored metal-ion uptake, currents and intracellular pH. DMT1 was activated in the presence of an inwardly directed H(+) electrochemical gradient. At low extracellular pH (pH(o)), H(+) binding preceded binding of Fe(2+) and its simultaneous translocation. However, DMT1 did not behave like a typical ion-coupled transporter at higher pH(o), and at pH(o) 7.4 we observed Fe(2+) transport that was not associated with H(+) influx. His(272) --> Ala substitution uncoupled the Fe(2+) and H(+) fluxes. At low pH(o), H272A mediated H(+) uniport that was inhibited by Fe(2+). Meanwhile H272A-mediated Fe(2+) transport was independent of pH(o). Our data indicate (i) that H(+) coupling in DMT1 serves to increase affinity for Fe(2+) and provide a thermodynamic driving force for Fe(2+) transport and (ii) that His-272 is critical in transducing the effects of H(+) coupling. Notably, our data also indicate that DMT1 can mediate facilitative Fe(2+) transport in the absence of a H(+) gradient. Since plasma membrane expression of DMT1 is upregulated in liver of hemochromatosis patients, this H(+) -uncoupled facilitative Fe(2+) transport via DMT1 can account for the uptake of nontransferrin-bound plasma iron characteristic of iron overload disorders.

The Broadly Selective Human Na+/Nucleoside Cotransporter(hCNT3) Exhibits Novel Cation-coupled Nucleoside TransportCharacteristics

The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.

Substrate specificity and transport mode of the proton-dependent amino acid transporter mPAT2

The PAT2 transporter has been shown to act as an electrogenic proton/amino acid symporter. The PAT2 cDNA has been cloned from various human, mouse and rat tissues and belongs to a group of four genes (pat1 to pat4) with PAT3 and PAT4 still resembling orphan transporters. The first immunolocalization studies demonstrated that the PAT2 protein is found in the murine central nervous system in neuronal cells with a proposed role in the intra and/or intercellular amino acid transport. Here we provide a detailed analysis of the transport mode and substrate specificity of the murine PAT2 transporter after expression in Xenopus laevis oocytes, by electrophysiological techniques and flux studies. The structural requirements to the PAT2 substrates - when considering both low and high affinity type substrates - are similar to those reported for the PAT1 protein with the essential features of a free carboxy group and a small side chain. For high affinity binding, however, PAT2 requires the amino group to be located in an alpha-position, tolerates only one methyl function attached to the amino group and is highly selective for the L-enantiomers. Electrophysiological analysis revealed pronounced effects of membrane potential on proton binding affinity, but substrate affinities and maximal transport currents only modestly respond to changes in membrane voltage. Whereas substrate affinity is dependent on extracellular pH, proton binding affinity to PAT2 is substrate-independent, favouring a sequential binding of proton followed by substrate. Maximal transport currents are substrate-dependent which suggests that the translocation of the loaded carrier to the internal side is the rate-limiting step.

Electrophysiological characterization of a recombinant human Na+-coupled nucleoside transporter (hCNT1) produced in Xenopus oocytes

Human concentrative nucleoside transporter 1 (hCNT1) mediates active transport of nucleosides and anticancer and antiviral nucleoside drugs across cell membranes by coupling influx to the movement of Na(+) down its electrochemical gradient. The two-microelectrode voltage-clamp technique was used to measure steady-state and presteady-state currents of recombinant hCNT1 produced in Xenopus oocytes. Transport was electrogenic, phloridzin sensitive and specific for pyrimidine nucleosides and adenosine. Nucleoside analogues that induced inwardly directed Na(+) currents included the anticancer drugs 5-fluorouridine, 5-fluoro-2'-deoxyuridine, cladribine and cytarabine, the antiviral drugs zidovudine and zalcitabine, and the novel thymidine mimics 1-(2-deoxy-beta-d-ribofuranosyl)-2,4-difluoro-5-methylbenzene and 1-(2-deoxy-beta-d-ribofuranosyl)-2,4-difluoro-5-iodobenzene. Apparent K(m) values for 5-fluorouridine, 5-fluoro-2'-deoxyuridine and zidovudine were 18, 15 and 450 microm, respectively. hCNT1 was Na(+) specific, and the kinetics of steady-state uridine-evoked Na(+) currents were consistent with an ordered simultaneous transport model in which Na(+) binds first followed by uridine. Membrane potential influenced both ion binding and carrier translocation. The Na(+)-nucleoside coupling stoichiometry, determined directly by comparing the uridine-induced inward charge movement to [(14)C]uridine uptake was 1: 1. hCNT1 presteady-state currents were used to determine the fraction of the membrane field sensed by Na(+) (61%), the valency of the movable charge (-0.81) and the average number of transporters present in the oocyte plasma membrane (6.8 x 10(10) per cell). The hCNT1 turnover rate at -50 mV was 9.6 molecules of uridine transported per second.

Functional properties and cellular distribution of the system A glutamine transporter SNAT1 support specialized roles in central neurons

Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K0.5 approximately 0.3 mm), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na+ electrochemical gradient. The voltage-dependent binding of Na+ precedes that of the amino acid in a simultaneous transport mechanism. Li+ (but not H+) can substitute for Na+ but results in reduced Vmax. In the absence of amino acid, SNAT1 mediates Na+-dependent presteady-state currents (Qmax approximately 9 nC) and a nonsaturable cation leak with selectivity Na+, Li+ >> H+, K+. Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na+ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via alpha-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

PEPT1 as a paradigm for membrane carriers that mediate electrogenic bidirectional transport of anionic, cationic, and neutral substrates

The capability for electrogenic inward transport of substrates that carry different net charge is a phenomenon observed in a variety of membrane-solute transporters but is not yet understood. We employed the two-electrode voltage clamp technique combined with intracellular pH recordings and the giant patch technique to assess the selectivity for bidirectional transport and the underlying stoichiometries in proton to substrate flux coupling for electrogenic transfer of selected anionic, cationic, and neutral dipeptides by the intestinal peptide transporter PEPT1. Anionic dipeptides such as Gly-Asp and Asp-Gly are transported in their neutral and negatively charged forms with high and low affinities, respectively. The positive transport current obtained with monoanionic substrates results from the cotransport of two protons. Cationic dipeptides can be transported in neutral and positively charged form, resulting in an excess transport current as compared with neutral substrates. However, binding and transport of cationic dipeptides shows a pronounced selectivity for the position of charged side chains demonstrating that the binding domain of PEPT1 is asymmetric, both in its inward and outward facing conformation. The simultaneous presence of identically charged substrates on both membrane surfaces generates outward and, unexpectedly, enhanced inward transport currents probably by increasing the turnover rate.

GAT1 (GABA:Na+:Cl-) cotransport function. Steady state studies in giant Xenopus oocyte membrane patches

Neurotransmitter transporters are reported to mediate transmembrane ion movements that are poorly coupled to neurotransmitter transport and to exhibit complex "channel-like" behaviors that challenge the classical "alternating access" transport model. To test alternative models, and to develop an improved model for the Na+- and Cl--dependent gamma-aminobutyric acid (GABA) transporter, GAT1, we expressed GAT1 in Xenopus oocytes and analyzed its function in detail in giant membrane patches. We detected no Na+- or Cl--dependent currents in the absence of GABA, nor did we detect activating effects of substrates added to the trans side. Outward GAT1 current ("reverse" transport mode) requires the presence of all three substrates on the cytoplasmic side. Inward GAT1 current ("forward" transport mode) can be partially activated by GABA and Na+ on the extracellular (pipette) side in the nominal absence of Cl-. With all three substrates on both membrane sides, reversal potentials defined with specific GAT1 inhibitors are consistent with the proposed stoichiometry of 1GABA:2Na+:1Cl-. As predicted for the "alternating access" model, addition of a substrate to the trans side (120 mM extracellular Na+) decreases the half-maximal concentration for activation of current by a substrate on the cis side (cytoplasmic GABA). In the presence of extracellular Na+, the half-maximal cytoplasmic GABA concentration is increased by decreasing cytoplasmic Cl-. In the absence of extracellular Na+, half-maximal cytoplasmic substrate concentrations (8 mM Cl-, 2 mM GABA, 60 mM Na+) do not change when cosubstrate concentrations are reduced, with the exception that reducing cytoplasmic Cl- increases the half-maximal cytoplasmic Na+ concentration. The forward GAT1 current (i.e., inward current with all extracellular substrates present) is inhibited monotonically by cytoplasmic Cl- (Ki, 8 mM); cytoplasmic Na+ and cytoplasmic GABA are without effect in the absence of cytoplasmic Cl-. In the absence of extracellular Na+, current-voltage relations for reverse transport current (i.e., outward current with all cytoplasmic substrates present) can be approximated by shallow exponential functions whose slopes are consistent with rate-limiting steps moving 0.15-0.3 equivalent charges. The slopes of current-voltage relations change only little when current is reduced four- to eightfold by lowering each cosubstrate concentration; they increase twofold upon addition of 100 mM Na+ to the extracellular (pipette) side.

Voltage and cosubstrate dependence of the Na-HCO3 cotransporter kinetics in renal proximal tubule cells

The voltage dependence of the kinetics of the sodium bicarbonate cotransporter was studied in proximal tubule cells. This electrogenic cotransporter transports one Na+, three HCO3-, and two negative charges. Cells were grown to confluence on a permeable support, mounted on a Ussing-type chamber, and permeabilized apically to small monovalent ions with amphotericin B. The steady-state, di-nitro-stilbene-di-sulfonate-sensitive current was shown to be sodium and bicarbonate dependent and therefore was taken as flux through the cotransporter. Voltage-current relations were measured as a function of Na+ and HCO3- concentrations between -160 and +160 mV under zero-trans and symmetrical conditions. The kinetics could be described by a Michaelis-Menten behavior with a Hill coefficient of 3 for HCO3- and 1 for Na+. The data were fitted to six-state ordered binding models without restrictions with respect to the rate-limiting step. All ordered models could quantitatively account for the observed current-voltage relationships and the transinhibition by high bicarbonate concentration. The models indicate that 1) the unloaded transporter carries a positive charge; 2) the binding of cytosolic bicarbonate to the transporter "senses" 12% of the electric field in the membrane, whereas its translocation across the membrane "senses" 88% of the field; 3) the binding of Na+ to the cotransporter is voltage independent.

Thyroid Na+/I- symporter. Mechanism, stoichiometry, and specificity

The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ >> Li+ >> H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Boltzmann relation. The voltage for half-maximal charge translocation (V0.5) was -15 +/- 3 mV, and the apparent valence of the movable charge was 1. Total charge was insensitive to [Na+]o, but V0.5 shifted to more negative potentials as [Na+]o was reduced. NIS charge movements are attributed to the conformational changes of the empty transporter within the membrane electric field. The turnover rate of NIS was >/=22 s-1 in the Na+ uniport mode and >/=36 s-1 in the Na+/I- cotransport mode. Transporter density in the plasma membrane was determined using freeze-fracture electron microscopy. Expression of NIS in oocytes led to a approximately 2. 5-fold increase in the density of plasma membrane protoplasmic face intramembrane particles. On the basis of the kinetic results, we propose an ordered simultaneous transport mechanism in which the binding of Na+ to NIS occurs first.

Cloning and characterization of a mammalian proton-coupled metal-ion transporter

Metal ions are essential cofactors for a wealth of biological processes, including oxidative phosphorylation, gene regulation and free-radical homeostasis. Failure to maintain appropriate levels of metal ions in humans is a feature of hereditary haemochromatosis, disorders of metal-ion deficiency, and certain neurodegenerative diseases. Despite their pivotal physiological roles, however, there is no molecular information on how metal ions are actively absorbed by mammalian cells. We have now identified a new metal-ion transporter in the rat, DCT1, which has an unusually broad substrate range that includes Fe2+, Zn2+, Mn2+, Co2+, Cd2+, Cu2+, Ni2+ and Pb2+. DCT1 mediates active transport that is proton-coupled and depends on the cell membrane potential. It is a 561-amino-acid protein with 12 putative membrane-spanning domains and is ubiquitously expressed, most notably in the proximal duodenum. DCT1 is upregulated by dietary iron deficiency, and may represent a key mediator of intestinal iron absorption. DCT1 is a member of the 'natural-resistance-associated macrophage protein' (Nramp) family and thus its properties provide insight into how these proteins confer resistance to pathogens.

Specificity and stoichiometry of the Arabidopsis H+/amino acid transporter AAP5

The H+-dependent AAP5 amino acid transporter from Arabidopsis thaliana was expressed in Xenopus oocytes, and we used radiotracer flux and electrophysiology methods to investigate its substrate specificity and stoichiometry. Inward currents of up to 9 microA were induced by a broad spectrum of amino acids, including anionic, cationic, and neutral amino acids. The apparent affinity of AAP5 for amino acids was influenced by the position of side chain branches, bulky ring structures, and charged groups. The maximal current was dependent on amino acid charge, but was relatively independent of amino acid structure. A detailed kinetic analysis of AAP5 using lysine, alanine, glutamate, and histidine revealed H+-dependent differences in the apparent affinity constants for each substrate. The differences were correlated to the effect of H+ concentration on the net charge of each amino acid and suggested that AAP5 transports only the neutral species of histidine and glutamate. Stoichiometry experiments, whereby the uptake of 3H-labeled amino acid and net inward charge were simultaneously measured in voltage-clamped oocytes, showed that the charge:amino acid stoichiometry was 2:1 for lysine and 1:1 for alanine, glutamate, and histidine. The results confirm that histidine is transported in its neutral form and show that the positive charge on lysine contributes to the magnitude of its inward current. Thus, the transport stoichiometry of AAP5 is 1 H+:1 amino acid irrespective of the net charge on the transported substrate. Structural features of amino acid molecules that are involved in substrate recognition by AAP5 are discussed.

Biophysical characteristics of the pig kidney Na+/glucose cotransporter SGLT2 reveal a common mechanism for SGLT1 and SGLT2

The Na+-dependent, low affinity glucose transporter SGLT2 cloned from pig kidney is 76% identical (at the amino acid level) to its high affinity homologue SGLT1. Using two-microelectrode voltage clamp, we have characterized the presteady-state and steady-state kinetics of SGLT2 expressed in Xenopus oocytes. The kinetic properties of the steady-state sugar-evoked currents as a function of external Na+ and alpha-methyl-D-glucopyranoside (alphaMG) concentrations were consistent with an ordered, simultaneous transport model in which Na+ binds first. Na+ binding was voltage-dependent and saturated with hyperpolarizing voltages. Phlorizin was a potent inhibitor of the sugar-evoked currents (KiPz approximately 10 microM) and blocked an inward Na+ current in the absence of sugar. SGLT2 exhibited Na+-dependent presteady-state currents with time constants 3-7 ms. Charge movements were described by Boltzmann relations with apparent valence approximately 1 and maximal charge transfer approximately 11 nC, and were reduced by the addition of sugar or phlorizin. The differences between SGLT1 and SGLT2 were that (i) the apparent affinity constant (K0.5) for alphaMG (approximately 3 mM) was an order of magnitude higher for SGLT2; (ii) SGLT2 excluded galactose, suggesting discrete sugar binding; (iii) K0.5 for Na+ was lower in SGLT2; and (iv) the Hill coefficient for Na+ was 1 for SGLT2 but 2 for SGLT1. Simulations of the six-state kinetic model previously proposed for SGLT1 indicated that many of the kinetic properties observed in SGLT2 are expected by simply reducing the Na+/glucose coupling from 2 to 1.

Transport mechanism of the cloned potato H+/sucrose cotransporter StSUT1

The transport mechanism of the potato StSUT1 H+/sucrose cotransporter expressed in Xenopus oocytes was investigated using the 2-electrode voltage clamp and radiotracer flux methods. Sucrose induced inward currents through the transporter that were dependent on the extracellular sucrose and H+ concentrations and the membrane voltage. The activation of StSUT1 by H+ and sucrose displayed Michaelis-Menten-type kinetics suggestive of a 1:1 H+:sucrose stoichiometry. This was confirmed by simultaneously measuring inward currents and sucrose flux in voltage-clamped oocytes. The apparent affinities K0.5 for H+ and sucrose were voltage-dependent. At -150 mV Ksuc0.5 was 0.5 +/- 0.07 mM at 10 microM H+o, and KH0.5 was 0.1 +/- 0.05 microM at 20 mM sucroseo. StSUT1 exhibited presteady-state transient currents, which relaxed with time constants between <1 and 4 ms and fitted to the Boltzmann equation: maximum charge transfer Qmax approximately 1.8 nanocoulombs; apparent valence z approximately 1; potential for 50% charge transfer V0.5 approximately -15 mV at 0.032 microM H+o and -45 mV at 10 microM H+o. The steady-state data were used to formulate a kinetic model for sucrose transport, and computer simulations were performed to obtain rate constants for the partial reaction steps. Our model is consistent with protons binding to StSUT1 before sucrose with both ligands transported simultaneously across the membrane.

Kinetics and stoichiometry of a proton/myo-inositol cotransporter

Voltage clamp recording was used to measure steady-state and presteady-state currents mediated by a myo-inositol transporter cloned from Leishmania donovani and expressed in Xenopus oocytes. Application of myo-inositol resulted in inward currents, which did not require external sodium and which were increased by increasing the extracellular proton concentration and by membrane hyperpolarization. Alkalinization of the extracellular space occurred concomitantly with myo-inositol influx. Correlation of membrane currents with radiolabeled myo-inositol flux revealed that one positive charge is translocated with each molecule of myo-inositol, consistent with cotransport of one proton. The transport concentration dependence on both species suggested ordered binding of a proton followed by a molecule of myo-inositol. In the absence of myo-inositol, a voltage-dependent capacitance was observed that correlated with the transporter expression level. This charge movement obeyed a Boltzmann function, which was used to estimate a turnover of 0.70 +/- 0.06 s-1 at -60 mV. The pH and voltage dependence of the charge movements were simulated with a model involving alternating access of internal and external protons to sites within an occluded pore.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Mechanisms of the human intestinal H+-coupled oligopeptide transporter hPEPT1

The hPEPT1 cDNA cloned from human intestine (Liang, R., Fei, Y.-J., Prasad, P. D., Ramamoorthy, S., Han, H., Yang-Feng, T. L., Hediger, M. A., Ganapathy, V., and Leibach, F. H. (1995) J. Biol. Chem. 270, 6456-6463) encodes a H+/oligopeptide cotransporter. Using two-microelectrode voltage-clamp in Xenopus oocytes expressing hPEPT1, we have investigated the transport mechanisms of hPEPT1 with regard to voltage dependence, steady-state kinetics, and transient charge movements. The currents evoked by 20 mM glycyl-sarcosine (Gly-Sar) at pH 5.0 were dependent upon membrane potential (Vm) between -150 mV and +50 mV. Gly-Sar-evoked currents increased hyperbolically with increasing extracellular [H+], with Hill coefficient approximately 1, and the apparent affinity constant (K0.5H) for H+ was in the range of 0.05 1 microM. K0.5 for Gly-Sar (K0.5GS) was dependent upon Vm and pH; at -50 mV, K0.5H was minimal (approximately 0.7 mM) at pH 6.0. Following step-changes in Vm, in the absence of Gly-Sar, hPEPT1 exhibited H+-dependent transient currents with characteristics similar to those of Na+-coupled transporters. These charge movements (which relaxed with time constants of 2-10 ms) were fitted to Boltzmann relations with maximal charge (Qmax) of up to 12 nC; the apparent valence was determined to be approximately 1. Qmax is an index of the level of transporter expression which for hPEPT1 was in the order of 1011/oocyte. In general our data are consistent with an ordered, simultaneous transport model for hPEPT1 in which H+ binds first.

Kinetics and specificity of a H+/amino acid transporter from Arabidopsis thaliana

The amino acid transporter AAP1/NAT2 recently cloned from Arabidopsis thaliana was expressed in Xenopus oocytes, and we used electrophysiological, radiotracer flux, and electron microscopic methods to characterize the biophysical properties, kinetics, and specificity of the transporter. Uptake of alanine was H(+)-dependent increasing from 14 pmol/oocyte/h at 0.032 microM H+ to 370 pmol/oocyte/h at 10 microM H+. AAP1 was electrogenic; there was an amino acid-induced depolarization of the oocyte plasma membrane and net inward currents through the transporter due to the transport of amino acids favoring neutral amino acids with shortside chains. The maximal current (imax) for alanine, proline, glutamine, histidine, and glutamate was voltage and [H+]o-dependent. Similarly, the imaxH was voltage and [amino acid]o-dependent. The imax for both H+ and amino acid were dependent on the concentrations of their respective cosubstrates, suggesting that both ligands bind randomly to the transporter. The K0.5 of the transporter for amino acids decreased as [H+]o increased and was lower at negative membrane potentials. The K0.5 for H+ was relatively voltage-independent and decreased as [amino acid]o increased. This positive cooperativity suggests that the transporter operates via a simultaneous mechanism. The Hill coefficients n for amino acids and H+ were > 1, suggesting that the transporter has more than one binding site for both H+ and amino acid. Freeze-fracture electron microscopy was used to estimate the number of transporters expressed in the plasma membrane of oocytes. The density of particles on the protoplasmic face of the plasma membrane of oocytes expressing AAP1 increased approximately 5-fold above water-injected controls and corresponded to a turnover number 350 to 800 s-1.

A model for the kinetic mechanism of sodium-coupled L-alanine transport in LLC-PK1 cells

The kinetics of sodium-dependent L-alanine transport were characterized in ATP-depleted LLC-PK1 cells, which allows experimental imposition of an interior negative diffusion potential across the plasma membrane. Under these conditions a wide range of sodium concentrations can be studied without altering the membrane potential. When Na+ is the variable substrate, the apparent maximal velocity (V max) for transport changes nearly fourfold for the five different alanine concentrations studied (0.05-2.0 mM). In contrast, at five different sodium concentrations, ranging from 10 to 135 mM, the apparent V max with variable alanine remains nearly constant at 5.3 +/- 1.2 nmol.min-1.mg cell protein-1. The ratio of the two primary kinetic parameters, Michaelis constant (Km)/V max, varies markedly no matter which solute is treated as the variable illustrate. These data are consistent with a simultaneous ordered transport mechanism in which sodium binds before alanine to the transport protein at the extracellular surface of the membrane. Alanine-dependent 22Na+ influx is more than five times faster if unlabeled intracellular sodium is present than in its absence. Sodium-dependent influx of [14C]alanine is more rapid than net alanine flux only if unlabeled Na+ and alanine are both present intracellularly. These results indicate that the cotransporter can function more rapidly in an exchange mode than when it catalyzes net solute uptake and that Na+ is the first solute to be released at the intracellular side of the membrane. A model is presented that can be used for further quantitative analysis of the kinetic and functional properties of the cotransport system.

Sodium-alanine cotransport in renal proximal tubule cells investigated by whole-cell current recording

Sodium-alanine cotransport was investigated in single isolated proximal tubule cells from rabbit kidney with the whole-cell current recording technique. Addition of L-alanine at the extracellular side induced an inward-directed sodium current and a cell depolarization. The sodium-alanine cotransport current was stereospecific and sodium dependent. Competition experiments suggested a common cotransport system for L-alanine and L-phenylalanine. Sodium-alanine cotransport current followed simple Michaelis-Menten kinetics, with an apparent Km of 6.6 mM alanine and 11.6 mM sodium and a maximal cotransport current of 0.98 pA/pF at -60 mV clamp potential. Hill plots of cotransport current suggested a potential-independent coupling ratio of one sodium and one alanine. The apparent Km for sodium and the maximal cotransport current were potential dependent, whereas the apparent Km for L-alanine was not affected by transmembrane potential. The increase in Km for alanine with decreasing inward-directed sodium gradients suggested a simultaneous transport mechanism. These results are consistent with a cotransport model with potential-dependent binding or unbinding of sodium (high-field access channel) and a potential-dependent translocation step.

Voltage-clamp studies of the Na+/glucose cotransporter cloned from rabbit small intestine

Inward Na+ currents associated with the cloned intestinal Na+/glucose cotransporter expressed in Xenopus oocytes have been studied using the two-microelectrode voltage-clamp method. The steady-state current/voltage relations showed voltage-dependent (Vm from +20 to -75 mV) and relatively voltage-independent (Vm from -75 to -150 mV) regions. The apparent Imax for Na+ and glucose increased with negative membrane potentials, and the apparent K0.5 for glucose (K(Glc)0.5) depended on Vm and [Na]o. Increasing [Na]o from 7 to 110 mmol/l had the same effect in decreasing K(Glc)0.5 from 0.44 to 0.03 mmol/l as increasing the Vm from -40 to -150 mV. The I/V curves under saturating conditions (20 mmol/l external sugars and 110 mmol/l [Na]o) were identical for D-glucose, D-galactose, alpha-methyl D-glucopyranoside and 3-O-methyl D-glucoside. The specificity of the cotransporter for sugars was: D-glucose, D-galactose, alpha-methyl D-glucopyranoside greater than 3-O-methyl D-glucoside much greater than D-xylose greater than D-allose much greater than D-mannose. Ki for phlorizin (approximately 10 mumol/l) was independent of Vm at saturating [Na]o. We conclude that a variety of sugars are transported by the cloned Na+/glucose cotransporter at the same maximal rate and that membrane potential affects both the maximal current and the apparent K0.5 of the cotransporter for Na+ and glucose.

Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus oocytes, and transmembrane currents associated with this transporter were monitored using a two-electrode voltage clamp. Addition of D-glucose to a Na(+)-containing solution bathing these oocytes generated a current which was blocked by phlorizin. Water-injected control oocytes did not exhibit any currents under these conditions. The magnitude and shape of the currents were dependent on the extracellular glucose and Na+ concentrations and the membrane potential. At Vhold = -50 mV, the Km values for glucose and Na+ were 14 +/- 2 (N = 4) microM and 17 +/- 1 (N = 3) mM, respectively. These Km values and imax exhibited voltage dependence: increasing the membrane potential from -30 to -150 mV increased KGlcm and imax threefold and decreased KNam eightfold. The reversal potential (VR) of the phlorizin-sensitive, glucose-dependent current varied with log Nao+ (slope 46 +/- 6 [N = 9] mV). In the absence of sugar, a Na(+)-dependent, phlorizin-sensitive (Ki = 3 +/- 0.5 microM) current was detected only in RNA-injected oocytes. The amplitude of this current at -50 mV was 6 +/- 1% (N = 13) of the maximum current measured in the presence of D-glucose. The VR of this sugar-independent current varied with log Nao+ (slope 63 +/- 1 [N = 4] mV), indicating that the cotransporter may carry Na+ in the absence of sugar. We conclude that the Na+/glucose cotransporter is electrogenic and that investigations of currents associated with its operation can yield valuable insights into the mechanisms of solute translocation.

New techniques for the study of lens electrophysiology

The development of the gigohm seal patch clamp has extended the use of the voltage clamp technique to cells too small to study with more conventional approaches. We report the application of this technique, in its many configurations, to the study of lens epithelial cells. The technique allows measurement of membrane current at the microscopic (single channel) level or at the macroscopic (whole cell) level. At the single channel level, three configurations (on-cell, inside-out, and outside-out) are standard. The advantages and disadvantages of each are discussed. A more novel configuration, the on-cell whole-cell configuration, has several advantages over these more traditional patch configurations. Measurement of current from the entire cell membrane under voltage clamp can be achieved with the whole cell variant of the patch clamp technique. Two less invasive versions of this procedure (the perforated patch technique and droop analysis) are presented. Finally, the extension of the whole cell technique to a pair of dissociated cells is shown to allow the measurement of currents flowing through gap junctions. For each case, data is presented from lens epithelial cells (from frog, chick, rabbit and human).

Transport of potassium in Chara australis: II. Kinetics of a symport with sodium

An electrogenic K(+)-Na+ symport with a high affinity for K+ has been found in Chara (Smith & Walker, 1989). Under voltage-clamp conditions, the symport shows up as a change in membrane current upon adding either K+ or Na+ to the bathing medium in the presence of the other. Estimation of kinetic parameters for this transport has been difficult when using intact cells, since K(+)-Na+ current changes show a rapid falling off with time at K+ concentrations above 50 microM. Cytoplasm-enriched cell fragments are used to overcome this difficulty, since they do not show the rapid falling off of current change seen with intact cells. Current-voltage curves for the membrane in the absence or presence of either K+ or Na+ are obtained, yielding difference current-voltage curves which isolate the symport currents from other transport processes. The kinetic parameters describing this transport are found to be voltage dependent, with Km for K+ ranging from 30 down to 2 microM as membrane potential varies from -140 to -400 mV, and Km for Na+ ranging between 470 and 700 microM over a membrane potential range of -140 to -310 mV. Two different models for this transport system have been investigated. One of these involves the simultaneous transport of both the driver and substrate ions across the membrane, while the other allows for the possibility of the two ions being transported consecutively in two distinct reaction steps. The experimental results are shown to be consistent with either of these cotransport models, but they do suggest that binding of K+ occurs before that of Na+, and that movement of charge across the membrane (the voltage-dependent step) occurs when the transport protein has neither K+ nor Na+ bound to it.

Whole cell recording of sugar-induced currents in LLC-PK1 cells

Gigaohm-seal whole cell recording techniques were used to monitor function of the Na(+)-coupled sugar transport system in LLC-PK1 cells. The currents coupled to sugar transport were identified as those that are induced by the presence of 10 mM alpha-methylglucoside (AMG) in either the extracellular or intracellular compartment and were inhibited by addition of 320-800 microM phlorizin to the extracellular bathing medium. The sugar-induced currents are small, 15-20 pA, but of the expected magnitude as determined from the known kinetic parameters for Na(+)-coupled sugar transport in LLC-PK1 cells. The phlorizin-sensitive currents are Na+ dependent and can be studied under conditions in which the net Na+ and sugar flux (and consequently the Na+ electrical current) is in either the inward or outward direction. The reversal potential of the sugar-induced currents measured under conditions with high Na+ and AMG concentrations inside the cell is close to values predicted from thermodynamic principles, assuming a coupling stoichiometry of 2 Na+: 1 sugar for the transport system. The reversal potential of the sugar-induced currents with high extracellular Na+ and AMG is not equal to the predicted value, but it is of the polarity expected for inward-imposed solute gradients. Reasons for the observed discrepancy between observed and calculated values are discussed.

Analysis of the H+/sugar symport in yeast under conditions of depolarized plasma membrane

H+/sugar symport in the obligatory aerobic yeast Rhodotorula glutinis was analyzed under conditions where the plasma membrane was selectively depolarized by the lipophilic cation tetraphenylphosphonium (TPP+). Control experiments showed that this treatment did not impair the transmembrane delta pH, the cell energy charge, and the function of plasma membrane H+-ATPase. The kinetic data were fitted to elementary functions derived from a model constructed on the basis of some simplifying premises for ordered (either C + H+ + S or C + S + H+) and random reaction mechanisms. In addition, the comparison of the kinetic parameters in fully energized and depolarized cells provided information about the free carrier charge. It was concluded that the binding sequence of formation of the ternary carrier/H+/substrate complex follows a random mechanism and that the carrier bears a negative charge.

Amino acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells

The method of tight-seal whole-cell recording was used to study the amino-acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells. Single cells or small clusters of electrically coupled cells were obtained by enzymatic dissociation of mouse pancreas. Inward currents were measured under 'zero-trans' conditions, i.e., at finite concentrations of Na+ and amino acid at the extracellular side and vanishing concentrations at the cytoplasmic side. The cotransporter, which corresponds to 'system A', as previously defined in the literature, was found to exhibit a wide tolerance to neutral amino acids (L-cysteine, L-serine, L-alanine, glycine, L-phenylalanine). Competition experiments with 2-methylaminoisobutyric acid (MeAIB) indicate that for glycine a second electrogenic transport system exists in pancreatic acinar cells.

Na+-coupled sugar transport: membrane potential-dependent Km and Ki for Na+

Kinetic analysis of the characteristics of phlorizin binding and of the Na+, sugar, and potential dependence of alpha-methylglucoside (alpha-MG) influx into isolated avian intestinal cells has pointed toward two alternative models for the transport mechanism (D. Restrepo and G. A. Kimmich, J. Membr. Biol. 89: 269-280, 1986). One of these models envisions a potential-dependent Na+ binding event (Na+ well concept) as a part of the molecular mechanism. The data reported here show that the apparent Km for Na+ for sugar transport is sharply dependent on the magnitude of the membrane potential. When intracellular Na+ is absent, the maximal velocity (Vmax) achieved for sugar influx is the same with or without a potential, although Vmax is obtained at a lower Na+ concentration when a potential is imposed (interior negative). Intracellular Na+ severely inhibits the influx of sugar in the absence of a potential, but this effect is largely overcome when a potential is present. The Vmax obtained when intracellular Na+ is present is a function of the potential. These results are consistent with a transport model in which Na+ binding to the Na+-dependent sugar carrier at the extracellular surface of the membrane and debinding at the inner surface of the membrane are both potential-dependent events.

Kinetics of the Na+/alanine cotransporter in pancreatic acinar cells

Electric currents associated with Na+-coupled alanine transport in pancreatic acinar cells were investigated by the technique of tight-seal whole-cell recordings. In a previous study the observed concentration dependence of alanine-dependent currents was found to be consistent with a 'simultaneous' transport mechanism with 1:1 stoichiometry. In the present work the sidedness of the cotransporter was investigated by comparing inward (I") and outward currents (I') measured under mirror-symmetrical conditions. I' and I" were found to be nearly equal (within a factor of approx. 2) in a wide range of Na+ and alanine concentrations. The transport model was further tested by 'infinite-cis' experiments with fixed, saturating concentrations of Na+ and L-alanine on one side of the membrane and variable concentrations on the other. By measuring transmembrane currents as a function of Na+ and alanine concentrations, numerical values of the equilibrium dissociation constants of both substrates could be estimated.

Voltage dependence of sodium-calcium exchange: predictions from kinetic models

Voltage effects on the Na-Ca exchange system are analyzed on the basis of two kinetic models, a "consecutive" and a "simultaneous" reaction scheme. The voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from movement of an ion along the transport pathway, from displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of ion fluxes is described by a set of coefficients reflecting the dielectric distances over which charge is translocated in the individual reaction steps. Depending on the charge of the ligand system and on the values of the dielectric coefficients, the flux-voltage curve can assume a variety of different shapes. When part of the transmembrane voltage drops between aqueous solution and binding site, the equilibrium constant of ion binding becomes a function of membrane potential. By studying the voltage dependence of ion fluxes in a wide range of sodium and calcium concentrations, detailed information on the microscopic properties of the transport system may be obtained.