Differential effects of transmembrane potential on two Na+-dependent transport systems for neutral amino acids

Voltage-dependent processes in the electroneutral amino acid exchanger ASCT2

Neutral amino acid exchange by the alanine serine cysteine transporter (ASCT)2 was reported to be electroneutral and coupled to the cotransport of one Na⁺ ion. The cotransported sodium ion carries positive charge. Therefore, it is possible that amino acid exchange is voltage dependent. However, little information is available on the electrical properties of the ASCT2 amino acid transport process. Here, we have used a combination of experimental and computational approaches to determine the details of the amino acid exchange mechanism of ASCT2. The [Na⁺] dependence of ASCT2-associated currents indicates that the Na⁺/amino acid stoichiometry is at least 2:1, with at least one sodium ion binding to the amino acid–free apo form of the transporter. When the substrate and two Na⁺ ions are bound, the valence of the transport domain is +0.81. Consistently, voltage steps applied to ASCT2 in the fully loaded configuration elicit transient currents that decay on a millisecond time scale. Alanine concentration jumps at the extracellular side of the membrane are followed by inwardly directed transient currents, indicative of translocation of net positive charge during exchange. Molecular dynamics simulations are consistent with these results and point to a sequential binding process in which one or two modulatory Na⁺ ions bind with high affinity to the empty transporter, followed by binding of the amino acid substrate and the subsequent binding of a final Na⁺ ion. Overall, our results are consistent with voltage-dependent amino acid exchange occurring on a millisecond time scale, the kinetics of which we predict with simulations. Despite some differences, transport mechanism and interaction with Na⁺ appear to be highly conserved between ASCT2 and the other members of the solute carrier 1 family, which transport acidic amino acids.

Electrogenic Na(+)-dependent L-alanine transport in the lizard duodenum. Involvement of systems A and ASC

L-Alanine transport across the isolated duodenal mucosa of the lizard Gallotia galloti has been studied in Ussing chambers under short-circuit conditions. Net L-alanine fluxes, transepithelial potential difference (PD), and short-circuit current (Isc) showed concentration-dependent relationships. Na(+)-dependent L-alanine transport was substantially inhibited by the analog alpha-methyl aminoisobutyric acid (MeAIB). Likewise, MeAIB fluxes were completely inhibited by L-alanine, indicating the presence of system A for neutral amino acid transport. System A transport activity was electrogenic and exhibited hyperbolic relationships for net MeAIB fluxes, PD, and Isc, which displayed similar apparent K(m) values. Na(+)-dependent L-alanine transport, but not MeAIB transport, was partially inhibited by L-serine and L-cysteine, indicating the participation of system ASC. This transport activity represents the major pathway for L-alanine absorption and seemed to operate in an electroneutral mode with a negligible contribution to the L-alanine-induced electrogenicity. It is concluded from the present study that the active Na(+)-dependent L-alanine transport across the isolated duodenal mucosa of Gallotia galloti results from the independent activity of systems A and ASC for neutral amino acid transport.

Plasma membrane NADH-oxidoreductase system: a critical review of the structural and functional data

The observation in the early 1970s that ferricyanide can replace transferrin as a growth factor highlighted the major role plasma membrane proteins can play within a mammalian cell. Ferricyanide, being impermeant to the cell, was assumed to act at the level of the plasma membrane. Since that time, several enzymes isolated from the plasma membrane have been described, which, using NADH as the intracellular electron donor, are capable of reducing ferricyanide. However, their exact modes of action, and their physiological substrates and functions have not been solved to date. Numerous hypotheses have been proposed for the role of such redox enzymes within the plasma membrane. Examples include the regulation of cell signaling, cell growth, apoptosis, proton pumping, and ion channels. All of these roles may be a result of the function of these enzymes as cellular redox sensors. The emergence of many diverse roles for ferricyanide utilizing redox enzymes present in the plasma membrane might also, in part, be due to the numerous redox enzymes present within the membrane; the poor molecular characterization of the enzymes may be the reason for some of the diverging results reported in the literature as various researchers may be working on different enzymes. Here we review the diverse proposals given for structure and function to the plasma membrane NADH-oxidoreductase system(s) with a specific focus on those enzyme activities which can couple ferricyanide and NADH. Although they are still ill-defined enzymes, evidence is rising that they are of utmost significance for cellular regulation.

ASCT-1 is a neutral amino acid exchanger with chloride channel activity

The ubiquitous transport activity known as system ASC is characterized by a preference for small neutral amino acids including alanine, serine, and cysteine. ASCT-1 and ASCT-2, recently cloned transporters exhibiting system ASC-like selectivity, are members of a major amino acid transporter family that includes a number of glutamate transporters. Here we show that ASCT1 functions as an electroneutral exchanger that mediates negligible net amino acid flux. The electrical currents previously shown to be associated with ASCT1-mediated transport result from activation of a thermodynamically uncoupled chloride conductance with permeation properties similar to those described for the glutamate transporter subfamily. Like glutamate transporters, ASCT1 activity requires extracellular Na+. However, unlike glutamate transporters, which mediate net flux and complete a transport cycle by countertransport of K+, ASCT-1 mediates only homo- and heteroexchange of amino acids and is insensitive to K+. The properties of ASCT-1 suggest that it may function to equilibrate different pools of neutral amino acids and provide a mechanism to link amino acid concentration gradients.

Regulation of taurine transport in Ehrlich ascites tumor cells

Taurine influx is inhibited and taurine efflux accelerated when the cell membrane of Ehrlich ascites tumor cells is depolarized. Taurine influx is inhibited at acid pH partly due to the concomitant depolarization of the cell membrane partly due to a reduced availability of negatively charged free carrier. These results are in agreement with a 2Na,1C1,1taurine cotransport system which is sensitive to the membrane potential due to a negatively charged empty carrier. Taurine efflux from Ehrlich cells is stimulated by addition of LTD4 and by swelling in hypotonic medium. Cell swelling in hypotonic medium is known to result in stimulation of the leukotriene synthesis and depolarization of the cell membrane. The taurine efflux, activated by cell swelling, is dramatically reduced when the phospholipase A2 is inhibited indirectly by addition of the anti-calmodulin drug pimozide, or directly by addition of RO 31-4639. The inhibition is in both cases lifted by addition of LTD4. The swelling-induced taurine efflux is also inhibited by addition of the 5-lipoxygenase inhibitors ETH 615-139 and NDGA. It is concluded that the swelling-induced activation of the taurine leak pathway involves a release of arachidonic acid from the membrane phospholipids and an increased oxidation of arachidonic acid into leukotrienes via the 5-lipoxygenase pathway. LTD4 seems to act as a second messenger for the swelling induced activation of the taurine leak pathway either directly or indirectly via its activation of the Cl- channels, i.e., via a depolarization of the cell membrane.

Membrane potential, anion and cation conductances in Ehrlich ascites tumor cells

The fluorescence intensity of the dye 1,1'-dipropylox-adicarbocyanine (DiOC3-(5] has been measured in suspensions of Ehrlich ascites tumor cells in an attempt to monitor their membrane potential (Vm) under different ionic conditions, after treatment with cation ionophores and after hypotonic cell swelling. Calibration is performed with gramicidin in Na+-free K-/choline-media, i.e., standard medium in which NaCl is replaced by KCl and cholineCl and where the sum of potassium and choline is kept constant at 155 mM. Calibration by the valinomycin "null point" procedure described by Laris et al. (Laris, P.C., Pershadsingh, A., Johnstone, R.M., 1976, Biochim, Biophys. Acta 436:475-488) is shown to be valid only in the presence of the Cl- -channel blocker indacrinone (MK196). Distribution of the lipophilic anion SCN- as an indirect estimation of the membrane potential is found not to be applicable for the fast changes in Vm reported in this paper. Incubation with DiOC3-(5) for 5 min is demonstrated to reduce the Cl permeability by 26 +/- 5% and the NO3- permeability by 15 +/- 2%, while no significant effect of the probe could be demonstrated on the K+ permeability. Values for Vm, corrected for the inhibitory effect of the dye on the anion conductance, are estimated at -61 +/- 1 mV in isotonic standard NaCl medium, -78 +/- 3 mV in isotonic Na+-free choline medium and -46 +/- 1 mV in isotonic NaNO3 medium. The cell membrane is depolarized by addition of the K+ channel inhibitor quinine and it is hyperpolarized when the cells are suspended in Na+-free choline medium, indicating that Vm is generated partly by potassium and partly by sodium diffusion. Ehrlich cells have previously been shown to be more permeable to nitrate than to chloride. Substituting NO3- for all cellular and extracellular Cl- leads to a depolarization of the membrane, demonstrating that Vm is also generated by the anions and that anions are above equilibrium. Taking the previously demonstrated single-file behavior of the K+ channels into consideration, the membrane conductances in Ehrlich cells are estimated at 10.4 microS/cm2 for K+, 3.0 microS/cm2 for Na+, 0.6 microS/cm2 for Cl- and 8.7 microS/cm2 for NO3-. Addition of the Ca2+-ionophore A23187 results in net loss of KCl and a hyperpolarization of the membrane, indicating that the K+ permeability exceeds the Cl- permeability also after the addition of A23187. The K+ and Cl- conductances in A23187-treated Ehrlich cells are estimated at 134 and 30 microS/cm2, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Validation of the use of the lipophilic thiocyanate anion for the determination of membrane potential in Ehrlich ascites tumor cells

The utility of the lipophilic anion thiocyanate (SCN-) as a probe for the indirect estimation of the cell membrane potential (Vm) in Ehrlich ascites tumor cells has been evaluated by comparison to direct electrophysiological measurements. SCN accumulation is consistent with first-order uptake into a single, kinetically-identifiable cellular compartment, achieving steady-state distribution in 20-30 min at 22 degrees C. The steady state distribution ratio ([SCN-]e/[SCN-]e) in physiological saline is 0.44 +/- 0.02. Treatment of the cells with propranolol (0.13 mM), an activator of Ca2+ dependent K+ channels, reduces the steady-state distribution ratio to 0.19 +/- 0.02. Conversely, treatment with BaCl2 (10 mM), an antagonist of the pathway, increases the SCN- distribution ratio to 0.62 +/- 0.01. The equilibrium potentials (VSCN) calculated under these conditions are virtually identical to direct electrophysiological measurements of the Vm made under the same conditions. The effect of varying extracellular [K+] ([K+]e) in the presence of constant [Na+]e = 100 mM has also been tested. In control cells, elevation of [K+]e from 6 to 60 mM reduces VSCN from -20.6 +/- 1.0 to -13.2 +/- 1.2 mV. Again, microelectrode measurements give excellent quantitative agreement. Propranolol increases the sensitivity of the cells to varying [K+]e, so that a 10-fold elevation reduces VSCN by approximately 31 mV. BaCl2 greatly reduces this response: a 10-fold elevation in [K+]e yielding only a 4-mV reduction in VSCN. It is concluded that the membrane potential of Ehrlich cells can be estimated accurately from SCN- distribution measurements.

Secretagogue-induced changes in system A amino acid transport in the rat exocrine pancreas: stimulation of 2-methylaminoisobutyric acid efflux by carbachol

Secretagogue-induced changes in exocrine pancreatic amino acid transport are poorly understood. In this study uptake of the specific non-metabolized System A amino acid analogue 2-methylaminoisobutyric acid (2-MeAIB) was measured in the isolated perfused rat pancreas during 60 min loading with D-[3H]mannitol (extracellular tracer) and 2-[14C]MeAIB. Tracer 2-MeAIB reached a maximal uptake of 37 +/- 4% (n = 4) after 3 min of loading and gradually decreased to a steady-state uptake of 13 +/- 1%. Infusion of carbachol (3.10(-7) M) during the tracer loading period abolished net tracer 2-MeAIB uptake, and reperfusion in the absence of carbachol restored net uptake to the prestimulus value. Less than 41% of the arterial 2-[14C]MeAIB or D-[3H]mannitol activity appeared in the basal pancreatic secretion. Carbachol evoked a 4.8-fold increase in pancreatic juice flow and appeared to reduce the activity of both tracers in the exocrine secretion. During washout of the pancreas with an isotope-free medium 2-[14C]MeAIB cleared from a rapidly exchanging pool with a time constant (tau 1) of 1.4 +/- 0.3 min (n = 4) and a more slowly exchanging pool with a time constant (tau 2) of 20.7 +/- 1.1 min. Carbachol accelerated efflux of 2-[14C]MeAIB from the epithelium but had no effect on the slow phase of D-[3H]mannitol washout. Our findings suggest that activation of cholinergic receptors modifies Na+-dependent System A amino acid transport in the basolateral membrane of the exocrine pancreatic epithelium.

Energetic mechanism of system A amino acid transport in normal and transformed mouse fibroblasts

Ouabain treatment (0.4 mM) of normal and transformed C3H-10T1/2 cells caused a progressive increase in 2-aminoisobutyrate (AIB) transport reaching a maximum after 16 to 18 h exposure. There was a virtually complete blockage of this stimulated rate when 3 microM cycloheximide (CHX) was added together with ouabain at T = 0. In the transformed cell, addition of CHX after 14 h had no effect; in the normal cell, it inhibited (ca. 50%) the final AIB transport rate achieved after 24 h. The t1/2 for reaching maximal activity (insensitive to CHX exposure) was thus shifted from 8 h in the transformed cell to 15 h in the normal cell. Since the rate of achieving maximal activity in the absence of CHX was about the same in the two cells, the shift in t1/2 in the presence of CHX suggests that the rate of degradation is more rapid in the normal cell. Following ouabain treatment, the apparent Km for Na+ was decreased in both cells. The Km returned to the basal level 1 h after ouabain removal in the normal cell, but remained low in the transformed cell during this time period. The stimulation of AIB transport following ouabain removal was largely abolished by a proton ionophore (1799), a lipophilic cation (tetraphenyl-phosphonium), or ouabain. These results suggest that, under the conditions of ouabain stress, there is a switch in the bioenergetic mechanism. The Na+/K+ pump and System A transporter appear to be linked and the membrane potential generated by the Na+/K+ pump activity becomes a major driving force for AIB uptake.

Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings

Electrical currents associated with sodium-coupled alanine transport in mouse pancreatic acinar cells were studied using the method of whole-cell recording with patch pipettes. Single cells or small clusters of (electrically coupled) cells were isolated by collagenase treatment. The composition of the intracellular solution could be controlled by internal perfusion of the patch pipette. In this way both inward and outward currents could be measured under "zero-trans" conditions, i.e., with finite concentrations of sodium and L-alanine on one side and zero concentrations on the other. Inward and outward currents for equal but opposite concentration gradients were found to be of similar magnitude, meaning that the cotransporter is functionally nearly symmetric. The dependence of current on the concentrations of sodium and L-alanine exhibited a Michaelis-Menten behavior. From the sodium-concentration dependence of current as well as from the reversal potential of the current in the presence of an alanine-concentration gradient, a sodium/alanine stoichiometric ratio of 1:1 can be inferred. The finding that N-methylated amino acids may substitute for L-alanine, as well as the observed pH dependence of currents indicate that the pancreatic alanine transport system is similar to (or identical with) the "A-system" which is widespread in animal cells. The transport system is tightly coupled with respect to Na+; alanine-coupled inward flow of Na+ is at least 30 times higher than uncoupled Na+ flow mediated by the cotransporter. The current-voltage characteristic of the cotransporter could be (approximately) determined from the difference of transmembrane current in the presence and in the absence of L-alanine. The sodium-concentration dependence of the current-voltage characteristic indicates that a Na+ ion approaching the binding site from the extracellular medium has to cross part of the transmembrane electric field.