Dibasic amino acid interactions with Na+-independent transport system asc in horse erythrocytes. Kinetic evidence of functional and structural homology with Na+-dependent system ASC

Trans-inhibition of glutamate transport prevents excitatory amino acid-induced glycolysis in astrocytes

Previous studies have demonstrated that activation of glutamate transporters promotes glycolysis in astrocytes. Current evidence indicates that compounds such as threo-beta-hydroxyaspartate (THA) are both competitive inhibitors and substrates for glutamate transporters. In this study, we have analyzed the effect of THA on excitatory amino acid (EAA) transport and on EAA-induced glycolysis in mouse primary astrocyte cultures. In agreement with previous studies in rat astrocytes, THA competitively inhibited 3H-D-aspartate (3H-D-Asp) uptake with an IC50 of 319 microM (Ki = 36.6 microM). In contrast, it did not prevent D-aspartate-induced 3H-2-deoxyglucose (2DG) uptake in these conditions. Preexposure of cells to THA for at least 15 min revealed another form of glutamate transport inhibition. This effect was concentration-dependent with an apparent IC50 of 47.7 microM and showed kinetic characteristics consistent with a mechanism of trans-inhibition. Preincubation with THA also inhibited D-aspartate-induced 3H-2DG uptake in a concentration-dependent manner with an apparent IC50 of 59.8 microM. Comparison with other transportable analogues reveals that they share with THA the ability to cause trans-inhibition of glutamate transport and to prevent glutamate-stimulated glycolysis; THA, however, is unique in that it has no effect alone on glucose utilization after preexposure. These data indicate that trans-inhibition of glutamate transport may be a mechanism by which certain glutamate transport inhibitors can prevent the stimulation of aerobic glycolysis by glutamate in astrocytes.

Transporters for cationic amino acids in animal cells: discovery, structure, and function

The structure and function of the four cationic amino acid transporters identified in animal cells are discussed. The systems differ in specificity, cation dependence, and physiological role. One of them, system y+, is selective for cationic amino acids, whereas the others (B[0,+], b[0,+], and y+ L) also accept neutral amino acids. In recent years, cDNA clones related to these activities have been isolated. Thus two families of proteins have been identified: 1) CAT or cationic amino acid transporters and 2) BAT or broad-scope transport proteins. In the CAT family, three genes encode for four different isoforms [CAT-1, CAT-2A, CAT-2(B) and CAT-3]; these are approximately 70-kDa proteins with multiple transmembrane segments (12-14), and despite their structural similarity, they differ in tissue distribution, kinetics, and regulatory properties. System y+ is the expression of the activity of CAT transporters. The BAT family includes two isoforms (rBAT and 4F2hc); these are 59- to 78-kDa proteins with one to four membrane-spanning segments, and it has been proposed that these proteins act as transport regulators. The expression of rBAT and 4F2hc induces system b[0,+] and system y+ L activity in Xenopus laevis oocytes, respectively. The roles of these transporters in nutrition, endocrinology, nitric oxide biology, and immunology, as well as in the genetic diseases cystinuria and lysinuric protein intolerance, are reviewed. Experimental strategies, which can be used in the kinetic characterization of coexpressed transporters, are also discussed.

Functional characterization of a recombinant sodium-dependent nucleoside transporter with selectivity for pyrimidine nucleosides (cNT1rat) by transient expression in cultured mammalian cells

We have demonstrated that monkey kidney (COS-1) cells have a single type of nucleoside transport process, which, because it was equilibrative, sodium-independent and could be inhibited by nitrobenzylthioinosine (NBMPR), was identified as the 'equilibrative sensitive' or 'es' transporter. Using NBMPR or dilazep to inhibit the endogenous nucleoside transport activity, we have transiently expressed a cDNA that encodes an inhibitor-insensitive, concentrative nucleoside transporter protein (cNT1rat) of rat intestine in COS-1 cells. The production of recombinant cNT1rat was examined by immunoblotting using an epitope-tagged construct and by analysis of inward fluxes of 3H-labelled nucleosides. Recombinant cNT1rat was sodium-dependent and selective for pyrimidine nucleosides, with approximately Km values of 21 microM, 12.5 microM and 15 microM for uridine, thymidine and adenosine, respectively. Although adenosine exhibited high affinity for the recombinant transporter, its Vmax value was low. A variety of anti-viral and anti-cancer nucleoside drugs inhibited cNT1rat-mediated uptake of uridine by transfected COS-1 cells although to different extents (Floxidine > Idoxuridine > Zidovudine > Zalcitabine > Cytarabine > Gemcitabine), suggesting that the concentrative pyrimidine-selective nucleoside transporters, of which cNT1rat is a representative, may play a role in cellular uptake of these drugs. The cNT1rat/COS-1 expression system is a useful tool for analysis of cNT1rat-mediated transport processes.

A pump-pore model for transmembrane transport of hydrophilic solutes

Transmembrane transport of a hydrophilic solute is presumed to begin when hydrated ligand adheres in Velcro-like fashion to hydrated membrane surface. Asymmetric physical forces cause rolling movements of ligand over membrane surface until contact occurs with appropriate transport machinery, consisting of a pump (Pu) to which is tethered a ligand (Li)-specific perm-selective pore (Po). The Po is in the open form when the Li is attached to an external high-affinity allosteric site on it. The active form of the Pu is stabilized by attachment of the Li to high-affinity internal or low-affinity external allosteric sites. The active form of the Pu induces closure of the Po, even when ligand is bound to it; the inactive conformation of the Pu permits Po opening. Attachment of Li to either one of two binding sites on the active Pu and irreversible envelopment by it in Venus fly-trap fashion trigger transmembrane transport of Li. Multistep attachment of Li is rate-limiting in the transport process. Application of a simple equation derived from relevant kinetic considerations relating velocity of transport (V) to concentration of Li (L), V = k1(L)1/2, gives V-L curves approximating transport data obtained in a variety of biological systems. This model is congruent with the ability of cells to concentrate substances from extremely dilute solutions and with the adaptive informational value to cells of rates of transport.

Cation and harmaline interactions with Na(+)-independent dibasic amino acid transport system y+ in human erythrocytes and in erythrocytes from a primitive vertebrate the pacific hagfish (Eptatretus stouti)

Transport systems y+, asc and ASC exhibit dual interactions with dibasic and neutral amino acids. For conventional Na(+)-dependent neutral amino acid system ASC, side chain amino and guanido groups bind to the Na+ site on the transporter. The topographically equivalent recognition site on related system asc binds harmaline (a Na(+)-site inhibitor) with the same affinity as asc (apparent Ki range 1-4 mM), but exhibits no detectable affinity for Ha. Although also classified as Na(+)-independent, dibasic amino acid transport system y+ accepts neutral amino acids when Na+ or another acceptable cation is also present. This latter observation implies that the y+ translocation site binds Na+ and suggests possible functional and structural similarities with ASC/asc. In the present series of experiments with human erythrocytes, system y(+)-mediated lysine uptake (5 microM, 20 degrees C) was found to be 3-fold higher in isotonic sucrose medium than in normal 150 mM NaCl medium. This difference was not a secondary consequence of changes in membrane potential, but resulted from Na+ functioning as a competitive inhibitor of transport. Apparent Km and Vmax values for lysine transport at 20 degrees C were 15.2 microM and 183 mumol/l cells per h, respectively, in sucrose medium and 59.4 microM and 228 mumol/l cells per h in Na+ medium. Similar results were obtained with y+ in erythrocytes of a primitive vertebrate, the Pacific hagfish (Eptatretus stouti), indicating that Na(+)-inhibition is a general property of this class of amino acid transporter. At a permeant concentration of 5 microM, the IC50 value for Na(+)-inhibition of lysine uptake by human erythrocytes was 27 mM. Other inorganic and organic cations, including K+ and guanidinium+, also inhibited transport. In parallel with its actions on ASC/asc harmaline competitively inhibited lysine uptake by human cells in sucrose medium. As predicted from mutually competitive binding to the y+ translocation site, the presence of 150 mM Na+ increased the harmaline inhibition constant (Ki) from 0.23 mM in sucrose medium to 0.75 mM in NaCl medium. We interpret these observations as further evidence that y+, asc and ASC represent a family of closely related transporters with a common evolutionary origin.

Cis-inhibition and trans-stimulation of cationic amino acid transport in the perfused rat pancreas

Transport of cationic amino acids in the isolated perfused rat pancreas was studied using dual-isotope dilution techniques. At 50 microM substrate concentration, unidirectional tracer uptakes for L-arginine (56 +/- 1%), L-lysine (49 +/- 2%), and L-ornithine (44 +/- 3%) were followed by rapid tracer efflux. In the presence of Na+, influx of L-arginine [Michaelis constant (Km) = 1.74 +/- 0.15 mM, maximum velocity (Vmax) = 1.97 +/- 0.07 mumol.min-1.g-1] and L-lysine (Km = 2.48 +/- 0.17 mM, Vmax = 2.42 +/- 0.08 mumol.min-1.g-1) was mediated by a common transport system, sensitive to cis-inhibition by L-ornithine, 2,4-L-diaminobutyric acid, D-lysine, and D-arginine. Substrates for system A [alpha-(methylamino)isobutyric acid] and an anionic carrier (L-aspartate) were poor cis-inhibitors of L-arginine entry. Removal of Na+ resulted in a 40% reduction in cationic amino acid influx. After cell loading (20 min), L-[3H]-lysine cleared predominantly from a slowly exchanging pool with a rate constant of 5.97 +/- 0.67 min. An influx/efflux permeability ratio of 14.5 +/- 1.6 was determined, and efflux of L-lysine was trans-stimulated by vascular challenges with cationic or large neutral amino acids. The specificity, relative Na+ independence, and exchange properties of this saturable cationic amino acid transporter in the pancreatic epithelium resemble those reported for system y+ in cultured fibroblasts and hepatocytes.

Paradoxical effects of endogenous and exogenous insulin on amino acid transport activity in the isolated rat pancreas: somatostatin-14 inhibits insulin action

Regulatory effects of insulin, somatostatin and cholecystokinin on amino acid transport in the isolated perfused rat pancreas have been studied using a rapid dual isotope dilution technique. Uni-directional L-serine transport (15 s) was quantified relative to an extracellular tracer D-mannitol over a wide range of substrate concentrations. In pancreata perfused with 2.5 mmol/l D-glucose, a weighted nonlinear regression analysis of overall transport indicated an apparent Km = 14.4 +/- 1.6 mmol/l and Vmax = 25.9 +/- 1.4 mumol.min-1.g-1 (n = 6). Although L-serine transport was stimulated during perfusion with 100 microU/ml bovine insulin, endogenous insulin (7-25 ng.min-1.g-1) released during continuous perfusion with either 8.8 mmol/l or 16.8 mmol/l D-glucose had no such effect. Exogenous somatostatin-14 (250 pg/ml) or cholecystokinin octapeptide (CCK-8, 3 x 10(-11) mol/l) appeared to increase only the Km for transport. Only CCK-8 evoked a notable protein output (2.9 +/- 0.3 mg.30 min-1.g-1) and juice flow (68 +/- 10 microliters.30 min-1.g-1, n = 3) from the exocrine pancreas. When pancreata were perfused with bovine insulin (100 microU/ml) and somatostatin-14 (250 pg/ml), the stimulatory action of exogenous insulin on L-serine transport was abolished. If endogenous insulin and somatostatin, released concurrently in response to 16.8 mmol/l D-glucose, were conveyed to the exocrine epithelium via an islet-acinar portal axis, it is conceivable that somatostatin modulates the stimulatory action of insulin on basolateral amino acid transport in the exocrine pancreas.

Ethanol-induced alterations in exocrine pancreatic amino acid transport and secretion

The effects of ethanol on exocrine pancreatic amino acid transport and secretion were investigated during perfusion of the isolated rat pancreas with ethanol concentrations ranging from 0.06% to 4.1%. Amino acid transport was quantitated using a rapid dual isotope dilution technique in which unidirectional substrate uptake (15-20 sec) is assessed relative to an extracellular tracer. Pancreatic secretion evoked by 0.3 microM carbachol was abolished during perfusion with 0.32% ethanol. Influx of L-lysine, L-serine and methylaminoisobutyric acid (MeAIB) was marginally increased by 0.32% ethanol but significantly inhibited during subsequent perfusion with 1.28-4.1% ethanol. Pancreatic oxygen consumption and effluent PCO2 levels decreased with increasing ethanol concentration, and the control venous pH (7.21 +/- 0.01, n = 8) gradually approached arterial pH values (7.46 +/- 0.02, n = 9). These results indicate that low concentrations of ethanol readily inhibit secretagogue-induced pancreatic secretion. Amino acid transport at the basolateral membrane of the exocrine pancreatic epithelium appears only to be inhibited after acute exposure to high ethanol concentrations.