The role of potassium and chloride ions on the Na+/acidic amino acid cotransport system in rat intestinal brush-border membrane vesicles

Effects of pH changes on systems ASC and B in rabbit ileum

Influx of D-aspartate (D-Asp), L-glutamate (L-Glu), and serine (Ser) across the brush-border membrane of the intact mucosa from rabbit ileum has been examined. L-Glu influx is chloride independent and completely sodium dependent. D-Asp and L-Glu share a transport system with a maximum transport rate of 1 micromol. cm-2. h-1 and an apparent affinity constant (K1/2) of approximately 0.3 mM. The function of this transport system is pH insensitive between pH 5.65 and 8.2, and bipolar amino acids do not affect the way in which the transport system handles D-Asp and L-Glu. The characteristics of this transport system match those of system X-AG. L-Glu and Ser share a transporter for which the inhibitor constant (Ki) of L-Glu against Ser decreases from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum rate of transport remains unaffected at approximately 10 micromol. cm-2. h-1. The Ki values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive contribution to Ser influx (0.8 micromol. cm-2. h-1 at 1 mM Ser) are the same at both pH values. The L-Glu-sensitive transport of Ser together with the contribution of system bo,+ account for approximately 50% of Ser influx at pH 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by system B is reduced by >95% at pH 5.65. At pH 7. 2 Ki of Ser against transport of leucine (Leu) by system B is 18 mM and Ki of Leu against transport of Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitive transport of Ser are performed by an equivalent of system ASC. Supplementary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small intestine.

Sequential ordered mechanism for the sodium-glutamate transport in intestinal brush border membrane vesicles

The glutamate/aspartate carrier localized in the brush-border membrane vesicles from enterocytes is known as a transport system catalyzing a sodium-substrate cotransport driven by the sodium gradient across the membrane. The kinetics of this transport system is studied by analogy with an enzymatic bi-substrate reaction. The results of this approach can be summarized as follows: (1) The dependence of the L-glutamate transport rate on the sodium concentration is sigmoidal, and the stoichiometry of the transport is 2 Na+/1 glutamate/1 carrier molecule. (2) The mechanism is sequential ordered, with L-glutamate binding after both the sodium cations. In addition, there is a very high degree of cooperativity between the two sodium binding sites.

The transport of acidic amino acids and their analogues across monolayers of human intestinal absorptive (Caco-2) cells in vitro

The X-AG system, a sodium-dependent, acidic amino-acid transport system has been implicated in the transport of L-aspartate and L-glutamate across monolayers of human Caco-2 cells, an in vitro model of intestinal absorption. This system, which shares many properties with the L-glutamate carrier present in the human jejunum, is highly saturable (> 95% at 50 microM), vectorial (apical-to-basolateral >> basolateral-to-apical) and sodium-, pH- and temperature-dependent. L-Aspartate was also transported against a 10-fold reverse concentration gradient. These data are consistent with a major (saturable) carrier-mediated pathway superimposed onto a minor non-saturable (diffusional) pathway. The carrier has an absolute sodium-dependence and the Michaelis constants for the sodium-dependent transport component (Km) for L-aspartate and L-glutamate were 56 +/- 3 microM and 65 +/- 6 microM, respectively. Cross-inhibition studies showed that strong interaction with the carrier was limited to close analogues of the natural substrates. Potent inhibitors included L-aspartate, D-aspartate (Ki, 70 microM), L-glutamate (Ki 180 microM) and threo-beta-hydroxy-DL-aspartate (Ki, 55 microM), while partial inhibitors included alpha-methyl-DL-aspartate, D-glutamate, L-asparagine, L-proline and L-alanine. Replacement of the side-chain -COO- group (aspartate) with -SO-3 (L-cysteate, Ki, 65 microM) or -(H)P(O)O- (DL-3-(hydroxyphosphoryl)alanine, Ki, 60 microM) maintained strong interaction with the carrier while -As(O)(OH)O- (DL-3-arsonoalanine, Ki, 1100 microM) and -P(O)(OH)O- (DL-3-phosphonoalanine, Ki, 3270 microM) were much more weakly bound, with the larger, but probably less ionised, arsono analogue being more tightly bound than the phosphono compound. The corresponding analogues of glutamate (homologous extension of the methylene chain) showed negligible interaction. We conclude that Caco-2 monolayers are a relevant experimental model for the study of the transport of acidic amino acids and their analogues in man.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Potassium activation of Na(+)-dependent leucine transport in brush-border membrane vesicles from rat jejunum

Na(+)-dependent leucine uptake was greater in potassium loaded brush-border membrane vesicles compared with controls. This effect was not mediated by an electrical potential difference, since it was still present in voltage-clamped conditions. Inhibition experiments indicate the same Na(+)-dependent leucine transport activity in the presence or in the absence of potassium. The affinity of sodium for the cotransporter was identical at 10 or 100 mM potassium. Leucine kinetics at different potassium concentrations showed a maximum 2.4-fold increase in Vmax, while Km was unaffected. The secondary plots of the kinetic results were not linear. This kinetic behavior suggests that K+ acts as a non-essential activator of Na(+)-dependent leucine cotransport. A charge compensation of sodium-leucine influx is most probably a component of the potassium effect in the presence of valinomycin.

L-alanine transport in small intestine brush-border membrane vesicles of obese rats

Membrane vesicles from the small intestine brush border were obtained and used to determine the possible effects of genetic or nutritional obesity on L-alanine uptake. Membrane vesicles from Zucker fa/fa obese rats and cafeteria diet-fed Zucker Fa/? rats showed the same characteristics as those of standard diet-fed lean animals. All preparations showed sodium-dependent transport as the main pathway for L-alanine uptake within the substrate concentration range tested. The apparent substrate affinity constant (Km) values and the pattern of inhibition of Na(+)-dependent L-alanine uptake by other amino acids (L-leucine and L-glutamine), suggests that system B involved in the transport of dipolar amino acids (formerly named Neutral Brush Border System) participates in the Na(+)-dependent transport of L-alanine. The affinity constant (Km) for L-alanine was essentially the same for all the groups studied (in the range of 10 mM). However, there was a higher (P < 0.05) maximal capacity (Vmax) in preparations from diet-induced obese animals (cafeteria diet) than that of genetically obese rats. These results indicate that either nutritional or genetic obesity may modify the capacity but not the affinity of transport systems for L-alanine uptake in the brush border of rat small intestine.

Age-related modifications of leucine uptake in brush-border membrane vesicles from rat jejunum

Leucine uptake in brush-border membrane vesicles purified from rat jejunum is sodium-dependent, sensitive to the membrane electrical potential difference and enhanced by the intravesicular presence of potassium. This last effect is not mediated by the genesis of an electrical potential difference, since potassium activation and electrical potential effects are additive. Sodium-dependent leucine Vmax (1568 +/- 91 pmol/mg per 3 s, is higher in young rats than in adult and old animals. The diffusion component of leucine transport decreases with increasing age. Preloading the vesicles with 100 mM KCl increases leucine Vmax 200% in young animals, 100% in adult and 44% in old animals. The potassium activation is a saturation function of the cation concentration. Leucine uptake in brush border membrane from old animals is less sensitive to the electrical potential difference than in membranes from adult and young animals.

Both Na+ and Cl- gradients energize NaCl/L-glutamate cotransport in lobster hepatopancreatic brush border membrane vesicles

Previous work with L-[3H]glutamate transport by lobster (Homarus americanus) hepatopancreatic brush border membrane vesicles (BBMV) indicated that the transport of this amino acid was stimulated by the presence of both Na+ and Cl- ions in the external medium, however, the specific catalytic or energetic role of each monovalent ion in amino acid transfer was not established (Ahearn and Clay (1987) J. Exp. Biol. 130, 175-191). The present study employs a variety of experimental treatments with this membrane preparation to clarify the nature of the ion dependency in the cotransport process. A zero-trans time course experiment using inwardly-directed transmembrane Na+ or Cl- gradients led to similar transient accumulations of the amino acid above equilibrium values in the presence of equilibrated concentrations of the respective counterions. The uptake overshoots observed in the presence of single ion gradients were significantly increased when gradients of both Na+ and Cl- were used simultaneously. When vesicles were pre-equilibrated with L-[3H]glutamate and either of the monovalent ions, an inwardly-directed gradient of each counterion led to the transient accumulation of additional labelled amino acid above its equilibrium concentration, indicating that either ion gradient was capable of energizing the net flow of L-glutamate. A cotransport stoichiometry of 1 Na+/1 Cl-/1 L-glutamate was established using the Static Head analysis where a balance of ion and amino acid driving forces were attained with a 7:1 Na+ or Cl- gradient (o greater than i) against a 7:1 L-glutamate gradient (i greater than o).

The renal transport of taurine and the regulation of renal sodium-chloride-dependent transporter activity

A model for the beta-amino acid taurine transport is presented to help define the ionic, pH, and voltage requirements for the movement of taurine into the rat proximal tubule brush border membrane vesicle (BBMV). Sodium-(Na+)-taurine symport across the apical surface of the proximal tubule has a highly specific requirement for Cl- and Br-. Active taurine transport operates with a 2 Na+:1 Cl-:1 taurine-carrier complex. Complexes like the one required for maximal taurine transport may be pertinent for many other amino acids whose uptake is Na(+)-dependent. Renal epithelial cell lines LLC-PK and MDCK were used to define the nature of taurine uptake; they express Na(+)-Cl(-)-taurine cotransport that is inhibited by beta-alanine. The cell lines up- or down-regulate taurine transport in response to changes in the taurine concentration of the medium in a manner similar to that seen in BBMV. The adaptation is present by 12 h and depends on new protein synthesis and protein import to the cell membrane. The role of trafficking in the adaptive response was also explored in brush border vesicles. During dietary surfeit, transporter could be down-regulated and transporters could be shifted back into the microtubule system, resulting in taurinuria. Use of continuous renal cell lines allowed a more mechanistic exploration of intracellular trafficking in the up- and down-expression of the Na(+)-Cl(-)-taurine cotransporter. Colchicine appeared to be a more potent inhibitor of the rapid (over hours) adaptive response to a reduction in media and, therefore, intracellular taurine content.(ABSTRACT TRUNCATED AT 250 WORDS)

Anionic amino acid uptake by microvillous membrane vesicles from human placenta

The transport mechanisms for anionic amino acids in trophoblast microvillous (maternal facing) membrane were investigated by characterization of L-[3H]aspartate and L-[3H]glutamate uptake in membrane vesicles. Uptake of the anionic amino acids was by a single high-affinity Na+-dependent K+-stimulated cotransporter that is pH sensitive and electrogenic. A second Na+-dependent transporter could not be discriminated, and there was no observable Na+-independent uptake. An outwardly directed K+ gradient (100 mM KCl inside) resulted in a 5- to 10-fold stimulation in glutamate uptake in the presence of Na+. Intravesicular KCl had no effect on transporter affinity but increased transporter velocity in a concentration-dependent manner. Inhibition of Na+-K+-dependent uptake of L-aspartate and L-glutamate (20 mM, 30 s) by 2 mM unlabeled amino acids demonstrated stereoselectivity for L-glutamate but not for L-aspartate. The neutral amino acids (L-alanine, L-threonine, L-serine, L-cysteine, L-phenylalanine) were not effective inhibitors. These data are consistent with an anionic amino acid transporter in the microvillous membrane of the trophoblast, which has characteristics qualitatively similar to the X-AG system found in other epithelia. This system may mediate the concentrative placental uptake of anionic amino acids from maternal blood in utero.

Characterization of amino-acid transport systems in guinea-pig intestinal brush-border membrane

The amino-acid transport systems have been characterized in brush-border membrane vesicles prepared from guinea-pig small intestine. Uptake of all amino acids tested was measured at the initial velocity for 5 s. L-Proline, alpha-(methylamino)isobutyrate, glycine, L-alanine and L-methionine were transported dependent solely on an Na+ gradient from the outside to the inside of the vesicles, and L-cysteine, L-phenylalanine and L-leucine were transported dependent largely on the Na+ gradient with a small fraction of Na+-independent transport. The transport of L-aspartic acid and L-lysine was independent of the Na+ gradient and L-lysine transport was somewhat inhibited by the presence of cations, including Na+, K+ and Li+. A cross-inhibition study of the uptake of these amino acids in the brush border of guinea-pig intestine revealed the presence of at least three Na+-dependent and three Na+-independent carrier-mediated systems. One Na+-dependent system interacted mainly with imino acid. Another Na+-dependent system interacted with neutral amino acids, while a third system was selective for glycine. One Na+-independent system is for acidic amino acids, another is responsible for neutral amino acids and a third for cationic amino acids. These transport systems of amino acids in guinea-pig small intestine are compared with those in rabbit and mouse intestine.

Ionic requirements for amino acid transport

The reabsorption of amino acids by the proximal tubule is remarkably efficient. Current evidence indicates that this process occurs by Na+-amino acid cotransport or symport. The energy for amino acid entry is derived from the chemical and voltage gradient for Na+ entry across the apical surface of the renal cell maintained by pumping Na+ out of the cell by Na+-K+-adenosine triphosphatase (ATPase) activity at the basolateral membrane. We chose the beta-amino acid taurine to study the anionic requirements as well as voltage- and pH-dependence of Na+-taurine symport into rat proximal tubule brush border membrane vesicles. Maximal uptake was found when Cl- or Br- were the anions. The addition of various ionophores (amiloride, carbonyl cyanide-n chlorophenyl-hydrazone, and valinomycin) under pH-equilibrated conditions did not change taurine entry into the vesicle. Hill equation analysis of the initial rate of taurine uptake into vesicles indicates that transport operates by means of a 2 Na+:1 Cl-:1 taurine-carrier complex. Because taurine is a zwitterion, this complex has a net positive charge. Its entry into the vesicle is favored by the imposition of an outwardly directed K+ gradient in the presence of valinomycin. The movement of a quaternary complex of this type across the apical surface of the proximal tubular cell would assure that the movement of both Cl- and the amino acid is energized by the Na+ gradient. Because most amino acids are zwitterions at physiologic pH this complex would be positively charged, favoring entry into the voltage negative renal cell interior.

Taurine transport by rabbit kidney brush-border membranes: coupling to sodium, chloride, and the membrane potential

Ion dependence and electrogenicity of taurine uptake were studied in rabbit renal outer cortical brush-border membrane vesicles isolated by differential precipitation. Na+-D-glucose cotransport was followed in parallel to monitor changes in the membrane potential. Concentrative taurine flux was dependent on a chemical and/or an electrical Na+ gradient (K+ diffusion potential) and could be completely inhibited by other beta-amino acids. It displayed a specific anion requirement (Cl- greater than or equal to Br- much greater than SCN- greater than I- greater than NO-3). At chemical Na+ equilibrium, Cl- gradients, depending on their orientation, stimulated or inhibited taurine uptake more than could be attributed solely to electrical anion effects, although a Cl- gradient alone could not energize an overshoot. Furthermore, taurine tracer exchange was significantly stimulated by Cl- as well as Br-. The Cl- stoichiometry was found to be one, whereas taurine transport, in the presence of Cl-, was sigmoidally related to the Na+ concentration, resulting in a coupling ratio of 2 to 3 Na+: 1 taurine. Upon Cl- replacement with gluconate, taurine uptake showed a reduced potential sensitivity and was no longer detectably affected by the Na+ concentration (up to 150 mM). These results suggest a 2 to 3 Na+ :1 Cl- :1 taurine cotransport mechanism driven mainly by the Na+ gradient, which is sensitive to the membrane potential due to a negatively charged empty carrier. Cl- appears to stimulate taurine flux primarily by facilitating the formation of the translocated solute-carrier complex.

Taurocholate transport by human ileal brush border membrane vesicles

Human ileal brush border membrane vesicles were prepared from intestines obtained from cadaveric renal allograft donors. The energetics and kinetics of taurocholate transport were studied. Fifty-five percent of equilibrium uptake (picomoles per mg protein) resulted from binding to the vesicle surface or incorporation into an osmotically insensitive compartment. The initial rate of transport was stimulated fourfold by an inwardly directed Na+ gradient when compared with a K+ gradient, and cation gradient-dependent differences persisted throughout the initial 5 min of incubation (p less than 0.05). Taurocholate uptake was half-maximally stimulated by a Na+ concentration of 23 +/- 4 mM. A Hill transformation of this plot gave a slope (n) of 0.97, indicating a 1:1 (mol/mol) Na+-taurocholate coupling ratio. Generation of a negative inside diffusion potential by anion substitution or valinomycin-induced K+ diffusion potential failed to alter bile salt uptake, suggesting an electroneutral transport mechanism. When Na+-dependent uptake velocity (10 s) was examined over a range of taurocholate concentration (0.036-0.9 mM), the plot described a rectangular hyperbola. The mean apparent Michaelis constant was 0.037 +/- 0.007 mM and maximum velocity was 1093 +/- 329 pmol taurocholate per milligram protein per 10 s. These data confirm and extend animal studies of ileal bile salt transport. Taurocholate uptake by the human ileal brush border occurs by a Na+-dependent, carrier-mediated electroneutral mechanism. According to this model, a single Na ion is coupled with a single taurocholate anion and transported across the brush border by a carrier mechanism that is driven by a transmembrane Na+ gradient.

Transport characteristics of L-glutamate in human jejunal brush-border membrane vesicles

Previous work using human jejunal brush-border membrane vesicles has demonstrated the existence of a distinct transport system in man for acidic amino acids. This system is energized by an inwardly directed Na+ gradient and an outwardly directed K+ gradient. These studies further characterize the transport of L-glutamate in the human jejunal brush-border membrane vesicles. Efflux studies were performed by loading the brush-border membrane vesicles with radiolabeled L-glutamate and sodium chloride. Extravesicular K+ accelerated the efflux of L-glutamate when compared to extravesicular Na+ or choline, indicating that potassium serves to recycle the carrier. Unlabeled extravesicular L-glutamate (but not D-glutamate) also enhanced the efflux of radiolabeled L-glutamate demonstrating that there is a bidirectional similarity to the transport system. The effect of pH on the transport system was also investigated by varying the intravesicular and extravesicular pH from 5.5 to 9. A pH environment of 6.5 produced the highest initial uptake rates as well as the greatest overshoots for transport of L-glutamate into brush-border membrane vesicles. The imposition of an inwardly directed pH gradient (5.5 outside, 7.5 inside) accelerated both the influx and efflux of L-glutamate. These results demonstrate that the L-glutamate carrier system in human jejunum appears to have similar energizing characteristics in either direction across the brush-border membrane. In addition, the system operates at an optimal pH of 6.5 and protonation of the system may enhance its mobility.

Chloride dependence of the sodium-dependent glycine transport in pig kidney cortex brush-border membrane vesicles

The Na+-dependent glycine uptake in pig kidney cortex brush-border membrane vesicles is specifically enhanced by the presence of Cl-. The Na+-independent glycine uptake is not affected by Cl-. Various anions tested could not substitute Cl- in the activation of the Na+-dependent glycine transport. Cl- is specifically required on the outer membrane side. The Na+-dependent glycine uptake is higher in the presence of an inwardly directed Cl- gradient than the one measured in the presence of equilibrated Cl-. The Na+-dependent glycine uptake depends on, and is saturable at increasing Cl- concentrations. By studying the activation of glycine uptake by Na+ in the presence and in the absence of Cl-, evidence was found that two different Na+-dependent glycine transport pathways are present in pig kidney cortex brush-border membrane vesicles. The kinetics of the glycine uptake measured in the presence of an inwardly directed NaCl gradient show the presence of two glycine transport systems, a low-affinity, high-capacity one and a high-affinity, low capacity one. In the absence of Cl- the high-affinity, low-capacity transport is almost suppressed, thus indicating the presence of a high-affinity glycine transport system simultaneously dependent on both Na+ and Cl- ions.

Acidic amino acid transport in animal cells and tissues

1. The occurrence and characterization of acidic amino acid transport in the plasma membrane of a variety of cells and tissues of a number of organisms is reviewed. 2. Several cell types, especially in brain, possess both high- and low-affinity transport systems for acidic amino acids. 3. High-affinity systems in brain may function to remove neurotransmitter amino acid from the extracellular environment. 4. Many cell systems for acidic amino acid transport are energized by an inwardly directed Na+ gradient. Moreover, certain cell types, such as rat brain neurons, human placental trophoblast and rabbit and rat kidney cortex epithelium, respond to an outwardly directed K+ gradient as an additional source of energization. This simultaneous action may account for the high accumulation ratios seen with acidic amino acids. 5. Rabbit kidney has been found to have a glutamate-H+ co-transport system which is subject to stimulation by protons in the medium. 6. Acidic amino acid transport in rat brain neurons occurs with a stoichiometric coupling of 1 mol of amino acid to 2 mol of Na+. For rabbit intestine, one Na+ is predicted to migrate for each mol of amino acid. 7. Uptake in rat kidney cortex and in high-K+ dog erythrocytes is electrogenic. However, uptake in rabbit and newt kidney and in rat and rabbit intestine is electroneutral. 8. Na+-independent acidic amino acid transport systems have been described in the mouse lymphocyte, the human fibroblast, the mouse Ehrlich cell and in rat hepatoma cells. 9. In a number of cell systems, D-acidic amino acids have substantial affinity for transport; D-glutamate, in a number of systems, however, appears to have little reactivity. 10. Acidic amino acid transport in some cell systems appears to occur via the "classical" routes (Christensen, Adv. Enzymol. Relat. Areas Mol. Biol. 49, 41-101, 1979). For example, uptake in the Ehrlich cell is partitioned between the Na+-dependent A system (which transports a wide spectrum of neutral amino acids), the Na+-dependent ASC system (which transports alanine, serine, threonine, homoserine, etc.), and the Na+-independent L system (which shows reactivity centering around neutral amino acids such as leucine and phenylalanine). Also, a minor component of uptake in mouse lymphocytes occurs by a route resembling the A system. 11. Human fibroblasts possess a Na+-independent adaptive transport system for cystine and glutamate that is enhanced in activity by cystine starvation.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane potential dependency of glutamic acid transport in rabbit jejunal brush-border membrane vesicles: K+ and H+ effects

We have applied our recently developed approach for quantitative generation and estimation of membrane potential differences (Berteloot, A. (1986) Biochim. Biophys. Acta 857, 180-188) to the reevaluation of glutamic acid transport rheogenicity in rabbit jejunal brush-border membrane vesicles. Membrane diffusion-potentials were created by altering iodide concentrations in the intra- and extravesicular compartments while keeping isosmolarity, isotonicity and ionic strength constant by chloride replacement. The known value of ion permeabilities relative to sodium in this preparation also allows calculation of membrane potential differences using the Goldman-Hodgkin-Katz equation. This strategy appears superior to more classical methods involving ionophore-induced membrane diffusion-potentials of protons or potassium as both cations have been shown to participate in the transport mechanism. In this paper, we demonstrate that this approach is perfectly suitable for the investigation of membrane potential dependency of glutamic acid transport as our results showed that chloride replacement by iodide did not affect uptake in vesicles with membrane potential clamped to zero by gramicidin D (sodium conditions) or by gramicidin D plus valimonycin (sodium + potassium conditions). The method thus allows to dissociate membrane potential effects from possible effects that might be introduced by altering the anion species. In these conditions, our studies clearly demonstrate that glutamic acid uptake, whether analyzed over a 1 min time scale or under initial rate conditions, was sensitive to membrane potential differences. However, our results also show that the electrogenicity of the transport system varied depending upon the intravesicular presence or absence of potassium, its presence stimulating the membrane potential dependency of uptake. This effect is modulated by the internal pH and it is concluded that inside H+ and K+ are not equivalent as countertransported cations. The external pH also seems to modulate the response to potential by acting on the fully loaded form(s) of the transporter. The possibility that outside H+ competes for (an) external Na+ binding site(s) and/or precludes the attachment of (an) extra sodium ion(s) should be considered.

Beta-alanine transport in synaptic plasma membrane vesicles from rat brain. Efflux, exchange and stoichiometry

The efflux and exchange of beta-alanine were studied in synaptic plasma membrane vesicles from rat brain. The mechanism of beta-alanine translocation has been probed by comparing the ion dependence of net efflux to that of exchange. Dilution-induced efflux requires the simultaneous presence of internal sodium and chloride ions while influx is dependent on the presence of these two ions on the outside [Zafra, F., Aragón, M. C., Valdivieso, F. and Giménez, C. (1984) Neurochem Res. 9, 695-707]. These data show that the release of beta-alanine occurs via the carrier system and that it is cotransported with sodium and chloride ions. beta-Alanine efflux from the membrane vesicles is stimulated by external beta-alanine. This exchange does not require external sodium and chloride but it is dependent on the external concentration of beta-alanine. Half-maximal stimulation is obtained at a beta-alanine concentration similar to the Km for beta-alanine influx. Results of the direct measurements of the coupling of sodium and chloride to the transport of beta-alanine by using a kinetic approach allow us to propose a stoichiometry for the translocation cycle catalyzed by the beta-alanine transporter of three sodium ions and one chloride ion per beta-alanine zwitterion. To account for all the observed effects of external ions, beta-alanine concentrations and membrane potential on beta-alanine influx and efflux, a kinetic model of the Na+/Cl-/beta-alanine cotransport system is discussed.

Transport of acidic amino acids by the bovine pigment epithelium

The regulation of acidic amino-acid transport across the retinal pigment epithelium is of particular interest since glutamate and possibly aspartate have been identified as putative neurotransmitters in the retina, at the level of the photoreceptor cell. The present study, designed to measure the rate of acidic amino-acid transport across the mammalian pigment epithelium (PE), shows that there is a net transport of both glutamate and aspartate in the retina to choroid direction (R-C), with the R-C unidirectional flux of glutamate being substantially larger than the corresponding aspartate flux. The R-C and C-R fluxes of glutamate were found to be inhibited by ouabain. Further investigations utilizing aspartate revealed that the fluxes in both directions were inhibited when ouabain was present on the retinal side of the tissue preparation. The R-C flux of glutamate was significantly reduced by lowered concentrations of Na+, K+ and Ca2+, whereas the C-R flux was diminished only by the reduced concentration Ca2+. The changes in K+ concentration which markedly altered the R-C flux of glutamate were within the range of light-induced changes of K+ which has been observed in the extracellular space of the photoreceptor cells. The transporting system appears to be relatively specific for the acidic amino acids; for aspartate was an effective competitive inhibitor of glutamate transport whereas basic (lysine) and neutral (leucine) amino acids were not. The directionality, ouabain sensitivity, ionic dependence and substrate specificity of the transmembrane fluxes tend to support the concept of active transport as a mechanism of acidic amino-acid removal from the neural retina.

Origin and voltage dependence of asparagine-induced depolarization in intestinal cells of Xenopus embryo

The kinetics and voltage dependence of asparagine (Asn)-induced depolarization in endoderm cells from Xenopus laevis embryos were analysed using current-clamp techniques. The depolarization is assumed to reflect the activation of an amino acid membrane carrier; it is accompanied by a slight increase in membrane resistance and cannot be explained by only the electrogenic character of the Asn carrier. It is proposed that the Asn depolarization arises, at least in part, from the decrease of the permeability ratio PK/PNa indirectly associated with the Na-coupled amino acid uptake. At room temperature (20-23 degrees C) the Asn response develops according to a single exponential function whose time constant is correlated with the final level of depolarization. Both amplitude and rise time of the depolarization are sensitive to variations of membrane potential and changes in Asn or Na external concentrations. Lowering the temperature decreases the amplitude of the Asn depolarization and increases its rise time with a Q10 factor of two; the kinetics remain of the Michaelis-Menten type, with a marked decrease in delta Emax and no change in Km. When the holding potential is altered by depolarizing and hyperpolarizing currents, the Asn response varies according to a bell-shaped characteristic presenting an optimum near the normal resting level. Membrane depolarizations induced by Na/K-pump inhibitors or high external K concentrations reduce the size of the Asn response; repolarizing the cell by current injection does not reverse the inhibitory effect of external K ions. Hyperpolarizing the membrane with a K-free Ringer solution increases the amplitude of the Asn response. In all these cases a decrease in delta Emax accounts for the apparent voltage sensitivity of the carrier mechanism. When induced by alterations of [K]o, an additional change in Km is observed, suggesting a K/Na-competitive inhibition of the Asn carrier. The results are discussed in terms of the amino acid carrier and passive membrane properties. It is suggested that the outward K-electrochemical gradient contributes an additional source of energy to the Na-dependent Asn uptake.

Factors affecting the transport of beta-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the beta-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na+ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN- and NO3-) or less permeant (SO4(2-)), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl- stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl- in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other beta-amino acids and in a competitive fashion. D-Glucose and p-aminohippurate at high concentrations (greater than 10(-3) M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of D-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal beta-amino acid transport system in brush-border vesicles and indicate a role for external Cl- in this uptake system.