Hydrogen ion cotransport by the renal brush border glutamate transporter

The ion-coupling mechanism of human excitatory amino acid transporters

Excitatory amino acid transporters (EAATs) maintain glutamate gradients in the brain essential for neurotransmission and to prevent neuronal death. They use ionic gradients as energy source and co-transport transmitter into the cytoplasm with Na+ and H+ , while counter-transporting K+ to re-initiate the transport cycle. However, the molecular mechanisms underlying ion-coupled transport remain incompletely understood. Here, we present 3D X-ray crystallographic and cryo-EM structures, as well as thermodynamic analysis of human EAAT1 in different ion bound conformations, including elusive counter-transport ion bound states. Binding energies of Na+ and H+ , and unexpectedly Ca2+ , are coupled to neurotransmitter binding. Ca2+ competes for a conserved Na+ site, suggesting a regulatory role for Ca2+ in glutamate transport at the synapse, while H+ binds to a conserved glutamate residue stabilizing substrate occlusion. The counter-transported ion binding site overlaps with that of glutamate, revealing the K+ -based mechanism to exclude the transmitter during the transport cycle and to prevent its neurotoxic release on the extracellular side.

Cysteine scanning mutagenesis of transmembrane helix 3 of a brain glutamate transporter reveals two conformationally sensitive positions

Glutamate transporters in the brain remove the neurotransmitter from the synapse by cotransport with three sodium ions into the surrounding cells. Recent structural work on an archaeal homolog suggests that, during substrate translocation, the transport domain, including the peripheral transmembrane helix 3 (TM3), moves relative to the trimerization domain in an elevator-like process. Moreover, two TM3 residues have been proposed to form part of a transient Na3' site, and another, Tyr-124, appears close to both Na3' and Na1. To obtain independent evidence for the role of TM3 in glutamate transport, each of its 31 amino acid residues from the glial GLT-1 transporter was individually mutated to cysteine. Except for six mutants, substantial transport activity was detected. Aqueous accessibility of the introduced cysteines was probed with membrane-permeant and membrane-impermeant sulfhydryl reagents under a variety of conditions. Transport of six single cysteine mutants, all located on the intracellular side of TM3, was affected by membrane-permeant sulfhydryl reagents. However, only at two positions could ligands modulate the reactivity. A120C reactivity was diminished under conditions expected to favor the outward-facing conformation of the transporter. Sulfhydryl modification of Y124C by 2-aminoethyl methanethiosulfonate, but not by N-ethylmaleimide, was fully protected in the presence of sodium. Our data are consistent with the idea that TM3 moves during transport. Moreover, computational modeling indicated that electrostatic repulsion between the positive charge introduced at position 124 and the sodium ions bound at Na3' and Na1 underlies the protection by sodium.

Conserved asparagine residue located in binding pocket controls cation selectivity and substrate interactions in neuronal glutamate transporter

Transporters of the major excitatory neurotransmitter glutamate play a crucial role in glutamatergic neurotransmission by removing their substrate from the synaptic cleft. The transport mechanism involves co-transport of glutamic acid with three Na(+) ions followed by countertransport of one K(+) ion. Structural work on the archeal homologue Glt(Ph) indicates a role of a conserved asparagine in substrate binding. According to a recent proposal, this residue may also participate in a novel Na(+) binding site. In this study, we characterize mutants of this residue from the neuronal transporter EAAC1, Asn-451. None of the mutants, except for N451S, were able to exhibit transport. However, the K(m) of this mutant for l-aspartate was increased ∼30-fold. Remarkably, the increase for d-aspartate and l-glutamate was 250- and 400-fold, respectively. Moreover, the cation specificity of N451S was altered because sodium but not lithium could support transport. A similar change in cation specificity was observed with a mutant of a conserved threonine residue, T370S, also implicated to participate in the novel Na(+) site together with the bound substrate. In further contrast to the wild type transporter, only l-aspartate was able to activate the uncoupled anion conductance by N451S, but with an almost 1000-fold reduction in apparent affinity. Our results not only provide experimental support for the Na(+) site but also suggest a distinct orientation of the substrate in the binding pocket during the activation of the anion conductance.

A conserved aspartate residue located at the extracellular end of the binding pocket controls cation interactions in brain glutamate transporters

In the brain, transporters of the major excitatory neurotransmitter glutamate remove their substrate from the synaptic cleft to allow optimal glutamatergic neurotransmission. Their transport cycle consists of two sequential translocation steps, namely cotransport of glutamic acid with three Na(+) ions, followed by countertransport of K(+). Recent studies, based on several crystal structures of the archeal homologue Glt(Ph), indicate that glutamate translocation occurs by an elevator-like mechanism. The resolution of these structures was not sufficiently high to unambiguously identify the sites of Na(+) binding, but functional and computational studies suggest some candidate sites. In the Glt(Ph) structure, a conserved aspartate residue (Asp-390) is located adjacent to a conserved tyrosine residue, previously shown to be a molecular determinant of ion selectivity in the brain glutamate transporter GLT-1. In this study, we characterize mutants of Asp-440 of the neuronal transporter EAAC1, which is the counterpart of Asp-390 of Glt(Ph). Except for substitution by glutamate, this residue is functionally irreplaceable. Using biochemical and electrophysiological approaches, we conclude that although D440E is intrinsically capable of net flux, this mutant behaves as an exchanger under physiological conditions, due to increased and decreased apparent affinities for Na(+) and K(+), respectively. Our present and previous data are compatible with the idea that the conserved tyrosine and aspartate residues, located at the external end of the binding pocket, may serve as a transient or stable cation binding site in the glutamate transporters.

Cotransporters as molecular water pumps

Molecular water pumps are membrane proteins of the cotransport type in which a flux of water is coupled to substrate fluxes by a mechanism within the protein. Free energy can be exchanged between the fluxes. Accordingly, the flux of water may be relatively independent of the external water chemical potential and can even proceed uphill. In short, water is being cotransported. The evidence for water cotransport is reviewed with particular emphasis on electrogenic cotransporters expressed in Xenopus oocytes under voltage clamped conditions. Phenomena such as uphill water transport, tight coupling between water transport and clamp current, cotransport of small hydrophilic molecules, and shifts in reversal potentials with osmolarity are discussed with examples from the Na+/glutamate and Na+/glucose cotransporters. Unstirred layers and electrode artifacts as alternative explanations for such cotransport can be ruled out for both experimental and theoretical reasons. Indeed, substrate fluxes mediated by channels or ionophores generate much smaller water fluxes than those observed with cotransporters. Theoretical models, using reasonable values for the intracellular diffusion coefficient, indicate the presence of only small unstirred layers in the membranes studied.

Passive water and urea permeability of a human Na(+)-glutamate cotransporter expressed in Xenopus oocytes

The human Na(+)-glutamate transporter (EAAT1) was expressed in Xenopus laevis oocytes. The passive water permeability, L(p), was derived from volume changes of the oocyte induced by changes in the external osmolarity. Oocytes were subjected to two-electrode voltage clamp. In the presence of Na(+), the EAAT1-specific (defined in Discussion) L(p) increased linearly with positive clamp potentials, the L(p) being around 23 % larger at +50 mV than at -50 mV. L-Glutamate increased the EAAT1-specific L(p) by up to 40 %. The K(0.5) for the glutamate-dependent increase was 20 +/- 6 microM, which is similar to the K(0.5) value for glutamate activation of transport. The specific inhibitor DL-threo-beta-benzyloxyaspartate (TBOA) reduced the EAAT1-specific L(p) to 72 %. EAAT1 supported passive fluxes of [(14)C]urea and [(14)C]glycerol. The [(14)C]urea flux was increased in the presence of glutamate. The data suggest that the permeability depends on the conformational equilibrium of the EAAT1. At positive potentials and in the presence of Na(+) and glutamate, the pore is enlarged and water and urea penetrate more readily. The L(p) was larger when measured with urea or glycerol as osmolytes as compared with mannitol. Apparently, the properties of the pore are not uniform along its length. The outer section may accommodate urea and glycerol in an osmotically active form, giving rise to larger water fluxes. The physiological role of EAAT1 for water homeostasis in the central nervous system is discussed.

Early intermediates in the transport cycle of the neuronal excitatory amino acid carrier EAAC1

Electrogenic glutamate transport by the excitatory amino acid carrier 1 (EAAC1) is associated with multiple charge movements across the membrane that take place on time scales ranging from microseconds to milliseconds. The molecular nature of these charge movements is poorly understood at present and, therefore, was studied in this report in detail by using the technique of laser-pulse photolysis of caged glutamate providing a 100-micros time resolution. In the inward transport mode, the deactivation of the transient component of the glutamate-induced coupled transport current exhibits two exponential components. Similar results were obtained when restricting EAAC1 to Na(+) translocation steps by removing potassium, thus, demonstrating (1) that substrate translocation of EAAC1 is coupled to inward movement of positive charge and, therefore, electrogenic; and (2) the existence of at least two distinct intermediates in the Na(+)-binding and glutamate translocation limb of the EAAC1 transport cycle. Together with the determination of the sodium ion concentration and voltage dependence of the two-exponential charge movement and of the steady-state EAAC1 properties, we developed a kinetic model that is based on sequential binding of Na(+) and glutamate to their extracellular binding sites on EAAC1 explaining our results. In this model, at least one Na(+) ion and thereafter glutamate rapidly bind to the transporter initiating a slower, electroneutral structural change that makes EAAC1 competent for further, voltage-dependent binding of additional sodium ion(s). Once the fully loaded EAAC1 complex is formed, it can undergo a much slower, electrogenic translocation reaction to expose the substrate and ion binding sites to the cytoplasm.

Water transport by the human Na+-coupled glutamate cotransporter expressed in Xenopus oocytes

The water transport properties of the human Na+-coupled glutamate cotransporter (EAAT1) were investigated. The protein was expressed in Xenopus laevis oocytes and electrogenic glutamate transport was recorded by two-electrode voltage clamp, while the concurrent water transport was monitored as oocyte volume changes. Water transport by EAAT1 was bimodal. Water was cotransported along with glutamate and Na+ by a mechanism within the protein. The transporter also sustained passive water transport in response to osmotic challenges. The two modes could be separated and could proceed in parallel. The cotransport modality was characterized in solutions of low Cl- concentration. Addition of glutamate promptly initiated an influx of 436 +/- 55 water molecules per unit charge, irrespective of the clamp potential. The cotransport of water occurred in the presence of adverse osmotic gradients. In accordance with the Gibbs equation, energy was transferred within the protein primarily from the downhill fluxes of Na+ to the uphill fluxes of water. Experiments using the cation-selective ionophore gramicidin showed no unstirred layer effects. Na+ currents in the ionophore did not lead to any significant initial water movements. In the absence of glutamate, EAAT1 contributed a passive water permeability (Lp) of (11.3 +/- 2.0) x 10(-6) cm s(-1) (osmol l(-1))(-1). In the presence of glutamate, Lp was about 50 % higher for both high and low Cl- concentrations. The physiological role of EAAT1 as a molecular water pump is discussed in relation to cellular volume homeostasis in the nervous system.

On the mechanism of proton transport by the neuronal excitatory amino acid carrier 1

Uptake of glutamate from the synaptic cleft is mediated by high affinity transporters and is driven by Na(+), K(+), and H(+) concentration gradients across the membrane. Here, we characterize the molecular mechanism of the intracellular pH change associated with glutamate transport by combining current recordings from excitatory amino acid carrier 1 (EAAC1)-expressing HEK293 cells with a rapid kinetic technique with a 100-micros time resolution. Under conditions of steady state transport, the affinity of EAAC1 for glutamate in both the forward and reverse modes is strongly dependent on the pH on the cis-side of the membrane, whereas the currents at saturating glutamate concentrations are hardly affected by the pH. Consistent with this, the kinetics of the pre-steady state currents, measured after saturating glutamate concentration jumps, are not a function of the pH. In addition, we determined the deuterium isotope effect on EAAC1 kinetics, which is in agreement with proton cotransport but not OH(-) countertransport. The results can be quantitatively explained with an ordered binding model that includes a rapid proton binding step to the empty transporter followed by glutamate binding and translocation of the proton-glutamate-transporter complex. The apparent pK of the extracellular proton binding site is approximately 8. This value is shifted to approximately 6.5 when the substrate binding site is exposed to the cytoplasm.

Glutamate transporters in kidney and brain

Glutamate transporters play important roles in the termination of excitatory neurotransmission and in providing cells with glutamate for metabolic purposes. In the kidney, glutamate transporters are involved in reabsorption of filtered acidic amino acids, regulation of ammonia and bicarbonate production, and protection of cells against osmotic stress.

Involvement of the glutamate transporter and the sodium-calcium exchanger in the hypoxia-induced increase in intracellular Ca2+ in rat hippocampal slices

Hippocampal slices prepared from adult rats were loaded with fura-2 and the intracellular free Ca2+ concentration ([Ca2+]i) in the CA1 pyramidal cell layer was measured. Hypoxia (oxygen-glucose deprivation) elicited a gradual increase in [Ca2+]i in normal Krebs solution. At high extracellular sodium concentrations ([Na+]o), the hypoxia-induced response was attenuated. In contrast, hypoxia in low [Na+]o elicited a significantly enhanced response. This exaggerated response to hypoxia at a low [Na+]o was reversed by pre-incubation of the slice at a low [Na+]o prior to the hypoxic insult. The attenuation of the response to hypoxia by high [Na+]o was no longer observed in the presence of antagonist to glutamate transporter. However, antagonist to Na+-Ca2+ exchanger only slightly influenced the effects of high [Na+]o. These observations suggest that disturbance of the transmembrane gradient of Na+ concentrations is an important factor in hypoxia-induced neuronal damage and corroborates the participation of the glutamate transporter in hypoxia-induced neuronal injury. In addition, the excess release of glutamate during hypoxia is due to a reversal of Na+-dependent glutamate transporter rather than an exocytotic process.

Patch-clamp, ion-sensing, and glutamate-sensing techniques to study glutamate transport in isolated retinal glial cells

We have described how a combination of electrical, ion-sensing, and glutamate-sensing techniques has advanced our understanding of glutamate uptake into isolated salamander retinal glial cells. The next steps in understanding glutamate transport will inevitably depend strongly on molecular biological methods, as described elsewhere in this book, but will also require more detailed study of transporters in their normal environment, perhaps by using patch-clamping or imaging techniques to study cells in situ.

Macroscopic and microscopic properties of a cloned glutamate transporter/chloride channel

The behavior of a Cl- channel associated with a glutamate transporter was studied using intracellular and patch recording techniques in Xenopus oocytes injected with human EAAT1 cRNA. Channels could be activated by application of glutamate to either face of excised membrane patches. The channel exhibited strong selectivity for amphipathic anions and had a minimum pore diameter of approximately 5A. Glutamate flux exhibited a much greater temperature dependence than Cl- flux. Stationary and nonstationary noise analysis was consistent with a sub-femtosiemen Cl- conductance and a maximum channel Po << 1. The glutamate binding rate was similar to estimates for receptor binding. After glutamate binding, channels activated rapidly followed by a relaxation phase. Differences in the macroscopic kinetics of channels activated by concentration jumps of L-glutamate or D-aspartate were correlated with differences in uptake kinetics, indicating a close correspondence of channel gating to state transitions in the transporter cycle.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Glutamate transport and cellular glutamine metabolism: regulation in LLC-PK1 vs. LLC-PK1-F+ cell lines

The glutamate (Glu) transporter may modulate cellular glutamine (Gln) metabolism by regulating both the rates of hydrolysis and subsequent conversion of Glu to alpha-ketoglutarate and NH+4. By delivering Glu, a competitive inhibitor of Gln for the phosphate-dependent glutaminase (PDG) as well as an acid-load activator of glutamate dehydrogenase (GDH) flux, the transporter may effectively substitute extracellularly generated Glu from the gamma-glutamyltransferase for that derived intracellularly from Gln. We tested this hypothesis in two closely related porcine kidney cell lines, LLC-PK1 and LLC-PK1-F+, the latter selected to grow in the absence of glucose, relying on Gln as their sole energy source. Both cell lines exhibited PDG suppression as the result of Glu uptake while disrupting the extracellular L-Glu uptake, with D-aspartate-accelerated intracellular Glu formation coupled primarily to the ammoniagenic pathway (GDH). Conversely, enhancing the extracellular Glu formation with p-aminohippurate and Glu uptake suppressed intracellular Gln hydrolysis while NH+4 formation from Glu increased. Thus these results are consistent with the transporter's dual role in modulating both PDG and GDH flux. Interestingly, PDG flux was actually higher in the Gln-adapted LLC-PK1-F+ cell line because of a two- to threefold enhancement in Gln uptake despite greater Glu uptake than in the parental LLC-PK1 cells, revealing the importance of both Glu and Gln transport in the modulation of PDG flux. Nevertheless, when studied at physiological Gln concentration, PDG flux falls under tight Glu transporter control as Gln uptake decreases, suggesting that cellular Gln metabolism may indeed be under Glu transporter control in vivo.

Bioenergetics of neurotransmitter transport

Neurotransmitter transporters are essential components in the recycling of neurotransmitters released during neuronal activity. These transporters are the targets for important drugs affecting mood and behavior. They fall into at least four gene families, two encoding proteins in the plasma membrane and two in the synaptic vesicle membrane, although the known vesicular transporters have not all been cloned. Each of these transporters works by coupling the downhill movement of small ions such as Na+, Cl-, K+, and H+ to the uphill transport of neurotransmitter. Plasma membrane transporters move the transmitter into the cytoplasm by cotransport with Na+. Many transporters also couple Cl- cotransport to transmitter influx and these all belong to the NaCl-coupled family, although within the family the coupling stoichiometry can vary. Transporters for glutamate couple influx of this excitatory amino acid to Na+ and H+ influx and K+ efflux. Transporters in synaptic vesicles couple H+ efflux to neurotransmitter transport from the cytoplasm to the vesicle lumen.

Currents evoked in Bergmann glial cells by parallel fibre stimulation in rat cerebellar slices

1. Whole-cell recordings were obtained from Bergmann glial cells in rat cerebellar slices. 2. The cells had low input resistances (70 +/- 38 M omega; n = 13) and a mean resting potential of -82 +/- 6 mV (n = 12) with a potassium-based internal solution. Electrical and dye coupling between Bergmann glia were confirmed. 3. Stimulation of parallel fibres induced a complex, mostly inward current which could be decomposed pharmacologically. 4. The ionotropic glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM), but not DL-2-amino-5-phosphonopentanoic acid (DL-APV; 100 microM) consistently blocked an early inward current component that may reflect synaptic activation of AMPA/kainate receptors in Bergmann glia. 5. Addition of cadmium ions (100 microM) to inhibit transmitter release blocked most of the CNQX-APV-insensitive current. This component probably reflects electrogenic uptake of the synaptically released glutamate. 6. Tetrodotoxin (TTX; 1 microM) blocked the remaining inward current: a slow component, possibly produced by the potassium ion efflux during action potential propagation in parallel fibres. An initial triphasic component of the response was also TTX sensitive and reflected passage of the parallel fibre action potential volley. 7. The putative glutamate uptake current was further characterized; it was blocked by the competitive uptake blockers D-aspartate (0.5 mM) and L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC; 0.5 mM), and by replacement of sodium with lithium. Monitoring the triphasic TTX-sensitive component showed that this inhibition did not result from changes of action potential excitation and propagation. 8. Intracellular nitrate ions increased the putative uptake current, consistent with the effect of this anion on glutamate transporters. 9. The putative uptake current was reduced by depolarization, consistent with the voltage dependence of glutamate uptake. 10. It is concluded that a large fraction of the current induced by parallel fibre stimulation reflects the uptake of synaptically released glutamate. The uptake current activated rapidly, with a 20-80% rise time of 2.3 +/- 0.7 ms (n = 10), and decayed with a principal time constant of 25 +/- 6 ms (n = 10).

Mutation of an amino acid residue influencing potassium coupling in the glutamate transporter GLT-1 induces obligate exchange

Glutamate transporters maintain low synaptic concentrations of neurotransmitter by coupling uptake to flux of other ions. After cotransport of glutamic acid with Na+, the cycle is completed by countertransport of K+. We have identified an amino acid residue (glutamate 404) influencing ion coupling in a domain of the transporter implicated previously in kainate binding. Mutation of this residue to aspartate (E404D) prevents both forward and reverse transport induced by K+. Sodium-dependent transmitter exchange and a transporter-mediated chloride conductance are unaffected by the mutation, indicating that this residue selectively influences potassium flux coupling. The results support a kinetic model in which sodium and potassium are translocated in distinct steps and suggest that this highly conserved region of the transporter is intimately associated with the ion permeation pathway.

Anion conductance behavior of the glutamate uptake carrier in salamander retinal glial cells

Glutamate uptake is driven by the cotransport of Na+ ions, the countertransport of K+ ions, and either the countertransport of OH- or the cotransport of H+ ions. In addition, activating glutamate uptake carriers has been shown to lead to activation of an anion conductance present in the carrier structure. Here we characterize the ion selectivity and gating of this anion conductance. The conductance is small with Cl- as the permeant anion, but it is large with NO3- or ClO4- present, undermining the earlier use of NO3- and ClO4- to suggest that OH- countertransport rather than H+ cotransport helps drive uptake. Activation of the anion conductance can be evoked by extra- or intracellular glutamate and can occur even when glutamate transport is inhibited. By running the carrier backward and detecting glutamate release with AMPA receptors in neurons placed near the glial cells, we show that anion flux is not coupled thermodynamically to glutamate movement, but OH-/H+ transport is. The possibility that cell excitability is modulated by the anion conductance associated with glutamate uptake suggests a target for therapeutic drugs to reduce glutamate release in conditions like epilepsy.

Mercuric chloride uncouples glutamate uptake from the countertransport of hydroxyl equivalents

The cotransport of sodium and glutamate by system X(AG)- is believed to be coupled to the countertransport of potassium and hydroxyl ion equivalents. Accordingly, the uptake of glutamate or D-aspartate in astrocytes is accompanied by an intracellular acidification. Here, we report that HgCl2 blocks the glutamate-induced acidification with an approximate 50% inhibitor concentration (IC50) of 55 nM, an order of magnitude below its IC50 for inhibition of glutamate uptake. At 100 nM HgCl2, glutamate-induced acidification was abolished, whereas glutamate uptake was unaffected. D-Aspartate-induced acidification was equally sensitive to HgCl2, indicating that HgCl2 blocked a transporter-mediated, rather than a receptor-mediated, acidification. Unaltered responses to acute acid and alkaline loads showed that HgCl2 was not acting indirectly via a change in pH regulation. We conclude that HgCl2 acted directly on the glutamate transporter to uncouple the uptake of glutamate from the export of hydroxyl equivalents. In contrast, two other sulfhydryl reagents, p-chloromercuribenzensulfonate and N-ethylmaleimide, failed to discriminate between glutamate-induced acidification and glutamate uptake. An additional effect of > or = 100 nM HgCl2, in this case shared by p-chlormercuribenzenesulfonate, was transient intracellular acidification. There is evidence that glutamate transport is regulated by intracellular pH. Mercuric mercury may disrupt the regulation of glutamate transport at lower concentrations than those that block transport.

Flux coupling in a neuronal glutamate transporter

Synaptic transmission is commonly terminated by diffusion and reuptake of neurotransmitter from the synaptic cleft. Glutamate reuptake prevents neurotoxicity and sets the lower limit for the concentration of extracellular glutamate, so it is important to understand the thermodynamics of this process. Here we use voltage clamping with a pH-sensitive fluorescent dye to monitor electrical currents and pH changes associated with flux of glutamate mediated by the human neuronal glutamate transporter EAAT3. In contrast to a previous model, we find that three sodium ions and one proton are cotransported with each glutamate ion into the cell, while one potassium ion is transported out of the cell. This coupling can support a transmembrane glutamate concentration gradient ([Glu]in/[Glu]out) exceeding 10(6) under equilibrium conditions, and would allow the transporter to continue removing glutamate over a wide range of ionic conditions.

Interaction of L-cysteine with a human excitatory amino acid transporter

1. The interaction of L-cysteine with three excitatory amino acid transporter subtypes cloned from human brain (EAAT1-3) was examined by measuring transporter-mediated electrical currents and radiolabelled amino acid flux in voltage-clamped Xenopus oocytes expressing the transporters. 2. L-Cysteine was transported by the neuronal subtype EAAT3 (EAAC1) with an affinity constant of 190 microM and a maximal rate of flux similar to that of L-glutamate; the relative efficacies (Vmax/K(m)) of the EAAT1 and EAAT2 subtypes for transporting L-cysteine were 10- to 20-fold lower. 3. Changing the ionization state of L-cysteine by raising the external pH did not significantly change the apparent affinity, transport rate, or magnitude of currents induced by L-cysteine, suggesting that both the neutral zwitterionic and anionic forms of the amino acid are transported with the same net charge stoichiometry. 4. In addition to competing with L-glutamate for uptake by the neuronal carrier, L-cysteine caused transporter-mediated release of transmitter by heteroexchange; both actions would elevate extracellular glutamate concentrations and may thus contribute to the known excitotoxic actions of L-cysteine in the brain. 5. Because the EAAT3 transporter is also expressed in tissues including kidney and intestine, the results suggest the possibility of a heretofore unrecognized mechanism of L-cysteine uptake in peripheral tissues as well as in brain.

Modulation of non-vesicular glutamate release by pH

Glutamate uptake into glial cells helps to keep the brain extracellular glutamate concentration, [glu]o, below levels that kill neurons. Uptake is powered by the transmembrane gradients of Na+, K+ and pH. When the extracellular [K+] rises in brain ischaemia, uptake reverses, releasing glutamate into the extracellular space. Here we show, by monitoring glutamate transport electrically and detecting released glutamate with ion channels in neurons placed outside glial cells, that a raised [H+] inhibits both forward and reversed glutamate uptake. No electroneutral reversed uptake was detected, contradicting the idea that forward and reversed uptake differ fundamentally. Suppression of reversed uptake by the low pH occurring in ischaemia will slow the Ca(2+)-independent release of glutamate with can raise [glu]o to a neurotoxic level, and will thus protect the brain during a transient loss of blood supply.

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).

Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina

The effects of excitatory amino acids on the membrane current of isolated retinal glial cells (Müller cells) were investigated using whole-cell patch clamping. 2. L-Glutamate evoked an inward current at membrane potentials between -140 and +50 mV. The current was larger at more negative potentials. 3. The glutamate-evoked current was activated by external cations with relative efficacies: Na+ much greater than Li+ greater than K+ greater than Cs+, choline. It was activated by internal cations with relative efficacies K+ greater than Rb+ greater than Cs+ much greater than choline. Chloride and divalent cations did not affect the glutamate-evoked current. 4. Raising the intracellular sodium or glutamate concentrations, or raising the extracellular potassium concentration, reduced the current evoked by external glutamate. The suppressive effect of internal glutamate was larger when the internal sodium concentration was high. 5. Some analogues of glutamate also evoked an inward current. Responses to L-aspartate resembled those to glutamate, but for aspartate the apparent affinity was higher and the voltage dependence of the current was steeper. In the physiological potential range the current evoked by a saturating dose of aspartate was less than that evoked by a saturating dose of glutamate. 6. The uptake blocker threo-3-hydroxy-DL-aspartate (30 microM) reduced the glutamate-evoked current, but also generated a current itself. Dihydrokainate (510 microMs) weakly inhibited the glutamate-evoked current without generating a current itself. 7. The commonly used blockers of glutamate-gated ion channels, 2-amino-5-phosphonovalerate (APV; 100 microMs), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microMs), and kynurenate (1mM) had no effect on the glutamate-evoked current. 8. The voltage dependence, cation dependence and pharmacological profile of the current evoked by excitatory amino acids indicate that it is caused by activation of the high-affinity glutamate uptake carrier. This carrier appears to transport one glutamate anion into the cell, one K+ ion out of the cell, and two or more Na+ ions into the cell, on each carrier cycle. At the inner membrane surface some or all of the transported Na+ dissociates from the carrier after the transported glutamate has dissociated. 9. In addition to glutamate, the uptake carrier can also transport aspartate and threo-3-hydroxy-DL-aspartate, but not dihydrokainate.

Presteady-state kinetics and carrier-mediated transport: a theoretical analysis

Kinetic studies of cotransport mechanisms have so far been limited to the conventional steady-state approach which does not allow in general to resolve either isomerization or rate-limiting steps and to determine the values of the individual rate constants for the elementary reactions involved along a given transport pathway. Such questions can only be answered using presteady-state or relaxation experiments which, for technical reasons, have not yet been introduced into the field of cotransport kinetics. However, since two recent reports seem compatible with the observation of such transient kinetics, it would appear that theoretical studies are needed to evaluate the validity of such claims and to critically evaluate the expectations from a presteady-state approach. We thus report such a study which was performed on a simple four-state mechanism of carrier-mediated transport. The time-dependent equation for zero-trans substrate uptake was thus derived and then extended to models with p intermediary steps. It is concluded that (p-1) exponential terms will describe the approach to the steady state but that such equations have low analytical value since the parameters of the flux equation cannot be expressed in terms of the individual rate constants of the elementary reactions for models with p greater than 5. We thus propose realistic simplifications based on the time-scale separation hypothesis which allows replacement of the rate constants of the rapid steps by their equilibrium constants, thereby reducing the complexity of the kinetic system. Assuming that only one relaxation can be observed, this treatment generates approximate models for which analytical expressions can easily be derived and simulated through computer modeling. When performed on the four-state mechanism of carrier-mediated transport, the simulations demonstrate the validity of the approximate solutions derived according to this hypothesis. Moreover, our approach clearly shows that presteady-state kinetics, should they become applicable to (co)transport kinetics, could be invaluable in determining more precise transport mechanisms.

Glutamate uptake in mammalian retinal glia is voltage- and potassium-dependent

Glutamate evokes an inward membrane current in glial cells from the rabbit retina, by activating high-affinity glutamate uptake. Uptake is strongly inhibited by depolarization. It is also inhibited by removing extracellular sodium or intracellular potassium and by raising the extracellular potassium concentration, suggesting that the uptake carrier transports sodium ions into and potassium ions out of the cell. The voltage- and potassium-dependence of glutamate uptake may have clinical implications: during anoxia, when [K+]0 rises, uptake will be inhibited and the extracellular glutamate concentration may then rise to neurotoxic levels.

Sodium-coupled amino acid transport in renal tubule

Amino acids are reabsorbed from the tubular lumen by a saturable, carrier-mediated, concentrative transport mechanism driven by a Na+ electrochemical gradient across the luminal membrane. This process is followed by efflux mainly via carrier-mediated, Na+-independent facilitated diffusion across the basolateral membrane. Individual amino acids may have two or more Na+-dependent transport systems with different kinetic characteristics along the luminal membrane of the proximal tubule, thereby enabling very efficient amino acid reabsorption. Dual Na+-coupled transport pathways for some amino acids located in both the luminal and the peritubular membranes may operate in concert to provide the tubular epithelial cell with essential nutrients. One or more Na+ ions, H+, Cl- and in the case of acidic amino acids, K+ ion, may be involved in the translocation of the carrier complex. For most amino acids this process is electrogenic positive, favored by a negative cell interior. At least seven distinct, but largely interacting, Na+-dependent amino acid transport systems have been identified in the brush border membrane. A diet-induced adaptation in Na+-coupled taurine transport and acidosis-induced adaptive response in Na+-dependent glutamine transport are expressed at the luminal and the basolateral membrane surfaces, respectively. The aminoaciduria of early life may be related to a rapid dissipation of the Na+ electrochemical gradient necessary for amino acid reabsorption.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Low-affinity intestinal L-aspartate transport with 2:1 coupling stoichiometry for Na+/Asp

Epithelial cells isolated from chick small intestine were used to define the ionic and electrical characteristics of a low-affinity (Km = 4.1 mM) L-aspartate transport system. L-Glutamate and D-aspartate, but not D-glutamate, were found to inhibit L-aspartate influx, suggesting that this uptake system has a substrate specificity similar to that previously described for a high-affinity (Km = 16 microM) acidic amino acid transporter in the same cells. Low-affinity uptake is Na+ dependent with a Hill coefficient (n) of 1.4. Intracellular K+ moderately enhances but is not required for aspartate influx, and this response is modulated by changes in intracellular pH. The Na+-dependent uptake of aspartate is electroneutral, as evidenced by insensitivity to pronounced changes in delta psi induced by anion gradients or valinomycin in the presence of K+ gradients. Because the above characteristics can be consistent with several transport models, direct measurement of delta Na+-delta Asp coupling stoichiometry were performed. The coupling ratio was determined to be approximately 2.0. A model for intestinal Na+-dependent L-Asp transport is suggested in which each transport cycle involves inward transfer of 2Na+:1Asp+ and outward transfer of K+ or H+ in a net electroneutral set of events.

A proton gradient, not a sodium gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles

An inward-directed H+ gradient markedly stimulated lactate uptake in rabbit intestinal brush-border membrane vesicles, and uphill transport against a concentration gradient could be demonstrated under these conditions. Uptake of lactate was many-fold greater in the presence of a H+ gradient than in the presence of a Na+ gradient. Moreover, there was no evidence for uphill transport of lactate in the presence of a Na+ gradient. The H+-gradient-dependent stimulation of lactate uptake was not due to the effect of a H+-diffusion potential. The uptake process in the presence of a H+ gradient was saturable [Kt (concn. giving half-maximal transport) for lactate 12.7 +/- 4.5 mM] and was inhibited by many monocarboxylates. It is concluded that a H+ gradient, not a Na+ gradient, is the driving force for active transport of lactate in rabbit intestinal brush-border membrane vesicles.

Modulation of sodium-cotransport systems by other ions

After a brief review of Na+-cotransport systems which also accept other ions as co-ions or modifiers, modulation of the Na+-L-glutamate transport system in rabbit renal brush border membranes by K+ and H+ is discussed in more detail. Intravesicular K+ increases the initial uptake rate and electrogenicity of the cotransport. This effect of K+ is attributed to the formation of a K+-carrier complex that moves much more rapidly than do the other complexes. The resulting shift in rate limitancy (relative increase in overall rate over the relative increase in rate of step under consideration) from an electroneutral towards a charge-translocating pathway unmasks the electrogenicity of the initial L-glutamate uptake. A positive correlation between relative rate limitancy of the electrogenic pathway and electrogenicity is demonstrated supporting this model. Protons, in addition to acting as co-ions, modify Na+-glutamate cotransport by increasing both the initial rate and the electrogenicity of uptake. This phenomenon is assumed to represent a transition of the transport system from a carrier-like to an open channel-like translocation mode. Thus, the intrinsic properties of Na+-cotransport systems may vary under the influence of other ions. This holds true in particular for the electrogenicity of the initial transport rate which may change independently of alterations in charge stoichiometry.

Electrogenicity of sodium/L-glutamate cotransport in rabbit renal brush-border membranes: a reevaluation

In order to clarify contradictory reports on the electrogenicity of sodium/L-glutamate cotransport, this cotransport was studied using brush-border membrane vesicles isolated from rabbit renal cortex. Beforehand, the claim that the symport of L-glutamate with Na+ is linked to simultaneous antiport with K+ has been confirmed by the demonstration that equilibrium exchange of L-glutamate is inhibited by potassium. Concerning the electrogenicity of the system, the following results are reported: net uptake of sodium-dependent L-glutamate uptake was stimulated when the transmembranal electrical potential difference was increased by replacing a sodium sulfate gradient by a sodium nitrate gradient. At 100 mM Na+ the 'relative electrogenicity' of the initial uptake in the presence of intravesicular potassium was 2-times higher than in its absence. At a sodium concentration of 20 mM, when overall uptake was reduced, the relative electrogenicity in the presence of K+ was even 3-fold higher than in K+-free media. The relative electrogenicity of sodium/D-glucose cotransport measured under the same experimental conditions was not affected by K+. These results are discussed in terms of a model where the apparent electrogenicity of a cotransport system is dependent on the extent to which the charge translocating step is rate limiting ('rate limitancy'). It is proposed that potassium antiport, while decreasing charge stoichiometry of Na+/glutamate transport, increases the relative rate limitancy of the transport step translocating three cations (probably two Na+, one H+) together with one glutamate. Thereby the positive electrogenicity of glutamate uptake increases, in complete contrast to what would be expected from simple considerations of charge stoichiometry.

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.

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)

Mechanism of transport and storage of neurotransmitters

This review will focus on the bioenergetics, mechanism, and molecular basis of neurotransmitter transport. As indicated in the next section, these processes play an important role in the overall process of synaptic transmission. During the last few years, direct evidence has been obtained that these processes are coupled chemiosmotically, i.e., the accumulation of neurotransmitters is driven by ion gradients. Two types of neurotransmitter transport systems have been identified: sodium-coupled systems located in the synaptic plasma membrane of nerves (and sometimes in the plasma membrane of glial cells) and proton-coupled systems which are part of the membrane of intracellular storage organelles. From a bioenergetic point of view, the sodium-coupled systems are especially interesting, since it has recently been discovered that many systems require other ions in addition to sodium. It has now been demonstrated in several cases that, besides sodium ions, these additional ions, such as chloride and potassium, serve as additional coupling ions. These systems will be reviewed here in considerable detail with emphasis on the role of the additional ions. In the second part of the review we shall focus on neurotransmitter transport into storage organelles. Although both sodium and proton coupled systems have been reviewed in the past, there has been a shift from a kinetic and thermodynamic to a biochemical approach. In fact, a few transporters have been identified and functionally reconstituted. These developments have of course been incorporated in this review.

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.

Highly permeant anions and glucose uptake as an alternative for quantitative generation and estimation of membrane potential differences in brush-border membrane vesicles

We have analyzed the combined utilization of highly permeant anions to induce membrane diffusion potentials and glucose uptake to probe the created potentials as a new approach to quantitative generation and estimation of membrane potential differences in vesicle studies. Rabbit jejunal brush-border membrane vesicles were used in our experiments so that membrane potential differences can be calculated from the Goldman-Hodgkin-Katz equation with the relative ion permeabilities recently reported for this preparation (Gunther, R.D., Schell, R.E. and Wright, E.M. (1984) J. Membrane Biol. 78, 119-127) or approximated by the Nernst potential for the anion. Iodide was selected as the highly permeant anion after showing its absence of effect on glucose uptake with equal concentrations of Na+ inside and outside the vesicles and the membrane potential clamped to zero with gramicidin D. Membrane potential was varied by altering the intra- and extravesicular iodide concentrations while keeping isosmolarity and isotonicity constant by chloride replacement. In these conditions, glucose uptake was sensitive and correlated to the expected membrane potentials. Moreover, a linear relationship between the log initial rate of glucose transport and membrane potential differences could be established. This linear relationship was quite insensitive to inside replacement of choline by potassium and to pH variations in the incubation medium, thus showing the reproducibility and the versatility of the method and the adequacy of glucose uptake as a probe for membrane potentials. However, no information can be gained on the stoichiometry of the Na+-glucose transporter as the slope of the straight line depends on both the charge carried by the fully loaded carrier and the point in the electric field at which the transition state of the carrier from cis to trans occurs. This new approach was compared with the more conventional one using valinomycin-induced K+-diffusion potentials and the Nernst potential for potassium as means for creating and estimating membrane potential differences. Both techniques were not equivalent, as linear relationships showing smaller slopes and sensitivity to pH were recorded with the latter. These differences are compatible with a potassium permeability in the presence of valinomycin that is lower than generally assumed, at least when compared to the permeability of the other ions present in the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Effectors of amino acid transport processes in animal cell membranes

Various effectors, which act upon ion gradients, protein synthesis, membrane components or cellular functional groups, have been employed to provide insights into the nature of amino acid-membrane transport processes in animal cells. Such effectors, for example, include ions, hormones, metabolites and various organic reagents and their judicious use has allowed the following list of conclusions. Sodium ion has been found to stimulate amino acid transport in a wide variety of cell systems, although depending on the tissue and/or substrate, this ion may have no effect on such transport, or even inhibit it. Amino acid transport can be stimulated in some cell systems by other ions such as K+, Li+, H+ or Cl-. Both H+ and K+ have been found to be inhibitory in other systems. Amino acid transport is dependent in many cell systems upon an inwardly directed Na+ gradient and is stimulated by a membrane potential (negative cell interior). In some cell systems an inwardly directed Cl- and H+ gradient or an outwardly directed K+ gradient can energize transport. Structurally dissimilar effectors such as ouabain, Clostridium enterotoxin, aspirin and amiloride inhibit amino acid transport presumably through dissipation of the Na+ gradient. Inhibition by certain sugars or metabolic intermediates of the tricarboxylic acid cycle may compete with the substrate for the energy of the Na+ gradient or interact with the substrate at the carrier level either allosterically or at a common site. Stimulation of transport by other sugars or intermediates may result from their catabolism to furnish energy for transport. Insulin and glucagon stimulate transport of amino acids in a variety of cell systems by a mechanism which involves protein synthesis. Microtubules may be involved in the regulation of transport by insulin or glucagon. Some reports also suggest that insulin has a direct effect on membranes. In addition, a number of growth hormones and factors have stimulatory effects on amino acid transport which are also mediated by protein synthesis. Steroid hormones have been noted to enhance or diminish transport of amino acids depending on the nature of the hormone. These agents appear to function at the level of protein synthesis. While stimulation may involve increased carrier synthesis, inhibition probably involves synthesis of a labile protein which either decreases the rate of synthesis or increases the rate of degradation of a component of the transport system.(ABSTRACT TRUNCATED AT 400 WORDS)