The uptake and metabolism of 5-hydroxytryptamine by tissue slices of the cestode Hymenolepis diminuta

The in vitro uptake of [3H]5HT was investigated in tissue slices of the cestode Hymenolepis diminuta. A concentrative, sodium sensitive, high affinity uptake mechanism (Km 1.43 X 10(-6) M; Vmax 222 fmoles/mg wet wt/min), together with a sodium insensitive component (linear up to 5 X 10(-6) M) were present. In the presence of 2-nitroimipramine the sodium sensitive component was significantly suppressed (Vmax 33 fmoles/mg/wet wt/min) although the Km (1.37 X 10(-6) M) was not affected. Nitroimipramine showed an IC50 of approximately 2 X 10(-6) M. The sodium insensitive component was not affected by nitroimipramine. Biogenic amines and related indoleamines were weak inhibitors of the sodium sensitive and sodium insensitive components of 5HT uptake. The tricyclic antidepressants and fluoxetine were effective inhibitors of the sodium sensitive component of 5-HT uptake; receptor ligands were weak inhibitors or without effect. The metabolism of [3H]5HT in tissue slices of H. diminuta was examined by HPLC. The role of the sodium sensitive uptake and metabolism of 5HT in terms of inactivation and recycling of neurally released 5HT and the possible importance of exogenous recruitment of 5HT are discussed.     

Osmotically regulated transport of proline by Lactobacillus acidophilus IFO 3532.

We reported previously that, when exposed to high osmotic pressure, Lactobacillus acidophilus IFO 3532 cells accumulated N,N,N-trimethylglycine (glycine betaine), which serves as a compatible intracellular solute. When grown in medium with high osmotic pressure, these cells also accumulated one amino acid, proline. The uptake of [3H]proline by resting, glucose-energized cells was stimulated by increasing the osmotic pressure of the assay medium with 0.5 to 1.0 M KCl, 1.0 M NaCl, or 0.5 M sucrose. The accumulated [3H]proline was not metabolized further. In contrast, there was no osmotic stimulation of [3H]leucine uptake. The uptake of proline was activated rather than induced by exposure of the cells to high osmotic pressure. Only one proline transport system could be discerned from kinetics plots. The affinity of the carrier for proline remained constant over a range of osmotic pressures from 650 to 1,910 mosM (Kt, 7.8 to 15.5 mM). The Vmax, however, increased from 15 nmol/min/mg of dry weight in 0.5 M sucrose to 27 and 40 nmol/min/mg of dry weight in 0.5 M KCl and in 1.0 M KCl or NaCl, respectively. The efflux of proline from preloaded cells occurred rapidly when the osmotic pressure of the suspending buffer was lowered.     

Active Uptake of Substance P Carboxy-Terminal Heptapeptide (5–11) into Rat Brain and Rabbit Spinal Cord Slices

We previously reported that nerve terminals and glial cells lack an active uptake system capable of terminating transmitter action of substance P (SP). In the present study, we demonstrated the existence of an active uptake system for SP carboxy-terminal heptapeptide, (5-11)SP. When the slices from either rat brain or rabbit spinal cord were incubated with [3H](5-11)SP, the uptake of (5-11)SP into slices was observed. The uptake system has the properties of an active transport mechanism: it is dependent on temperature and sensitive to hypoosmotic treatment and is inhibited by ouabain and dinitrophenol (DNP). In the brain, (5-11)SP was accumulated by means of a high-affinity and a low-affinity uptake system. The Km and the Vmax values for the high-affinity system were 4.20 x 10(-8) M and 7.59 fmol/10 mg wet weight/min, respectively, whereas these values for the low-affinity system were 1.00 x 10(-6) M and 100 fmol/10 mg wet weight/min, respectively. In the spinal cord, there was only one uptake system, with a Km value of 2.16 x 10(-7) M and Vmax value of 26.2 fmol/10 mg wet weight/min. These results suggest that when SP is released from nerve terminals, it is hydrolysed into (5-11)SP before or after acting as a neurotransmitter, which is in turn accumulated into nerve terminals. Therefore, the uptake system may represent a possible mechanism for the inactivation of SP.     

Uptake of GABA by neuronal and nonneuronal cells in dispersed cell cultures of postnatal rat cerebellum.

A study was made of the time course and kinetics of [3H]GABA uptake by dispersed cell cultures of postnatal rat cerebellum with and without neuronal cells. The properties of GABA neurons were calculated from the biochemical difference between the two types of cultures. It was found that for any given concentration of [3H]GABA, or any time up to 20 min, GABA neurons in cultures 21 days in vitro had an average velocity of uptake several orders of magnitude greater than that of nonneuronal cells. In addition, the apparent Kmvalues for GABA neurons for high and low affinity uptake were 0.33 X 10(-6) M and 41.8 X 10(-4) M, respectively. For nonneuronal cells, the apparent Km for high affinity uptake was 0.29 X 10(-6) M. The apparent Vmax values for GABA neurons for high and low affinity uptake were 28.7 X 10(-6) mol/g DNA/min and 151.5 mmol/g DNA/min, respectively. For nonneuronal cells, the apparent Vmax for high affinity uptake was 0.06 X 10(-6) mol/g DNA/min. No low affinity uptake system for nonneuronal cells could be detected after correcting the data for binding and diffusion. By substituting the apparent kinetic constants in the Michaelis-Menten equation, it was determined that for GABA concentrations of 5 X 10(-9) M to 1 mM or higher over 99% of the GABA should be accumulated by GABA neurons, given equal access of all cells to the label. In addition, high affinity uptake of [3H]GABA by GABA neurons was completely blocked by treatment with 0.2 mM ouabain, whereas that by noneuronal cells was only slightly decreased. Most (75-85%) of the [3H]GABA (4.4 X 10(-6) M) uptake by both GABA neurons and nonneuronal cells was sodium and temperature dependent.     

Osmotically regulated transport of proline by Lactobacillus acidophilus IFO 3532.

We reported previously that, when exposed to high osmotic pressure, Lactobacillus acidophilus IFO 3532 cells accumulated N,N,N-trimethylglycine (glycine betaine), which serves as a compatible intracellular solute. When grown in medium with high osmotic pressure, these cells also accumulated one amino acid, proline. The uptake of [3H]proline by resting, glucose-energized cells was stimulated by increasing the osmotic pressure of the assay medium with 0.5 to 1.0 M KCl, 1.0 M NaCl, or 0.5 M sucrose. The accumulated [3H]proline was not metabolized further. In contrast, there was no osmotic stimulation of [3H]leucine uptake. The uptake of proline was activated rather than induced by exposure of the cells to high osmotic pressure. Only one proline transport system could be discerned from kinetics plots. The affinity of the carrier for proline remained constant over a range of osmotic pressures from 650 to 1,910 mosM (Kt, 7.8 to 15.5 mM). The Vmax, however, increased from 15 nmol/min/mg of dry weight in 0.5 M sucrose to 27 and 40 nmol/min/mg of dry weight in 0.5 M KCl and in 1.0 M KCl or NaCl, respectively. The efflux of proline from preloaded cells occurred rapidly when the osmotic pressure of the suspending buffer was lowered.     

The characterization of a sodium-dependent high affinity choline uptake system unassociated with acetylcholine biosynthesis

The cardiac ganglion of the horseshoe crab, Limulus polyphemus, was incubated in Chao's solution containing 0.01 microM [3H]choline at room temperature (25 +/- 2 degrees C) and the ganglion readily accumulated the radiolabel. The ganglion uptake of [3H]choline was linear over 60 min. Kinetic analysis revealed dual choline uptake systems within the cardiac ganglion, a high affinity uptake system (Km = 2.2 microM, Vmax = 0.16 pmoles/mg/min) and a low affinity system (Km = 92.3 microM, Vmax = 3.08 pmoles/mg/min). The high affinity uptake system was sodium-dependent and inhibited by micromolar concentrations of hemicholinium-3. A 15 min pre-exposure of the ganglion to Chao's solution containing 90 mM potassium stimulated a significant increase in choline uptake. There was no detectable synthesis of [3H]acetylcholine from the [3H]choline taken up by the cardiac ganglion. The major portion of the extractable label appeared in a fraction which co-electrophoresed with phosphorylcholine. These results suggest that the sodium-dependent high affinity [3H]choline uptake system of the cardiac ganglion subserves a specific requirement for choline which is unrelated to a cholinergic function.     

Thyrotropin-releasing hormone (TRH) and its analog (DN-1417): interaction with pentobarbital in choline uptake and acetylcholine synthesis of rat brain slices

TRH or its analog DN-1417 (gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-proliamide) given 15 min after intravenous (i.v.) administration of pentobarbital (30 mg/kg) markedly shortened the pentobarbital-induced sleeping time in rats. This effect was almost completely abolished by intracerebroventricular pretreatment with atropine methylbromide (20 micrograms/rat), thereby suggesting the involvement of cholinergic mechanism. The action mechanism was investigated using rat brain slices. TRH (10(-6)-10(-4)M) or DN-1417 (10(-7)-10(-5)M) caused significant increases in the uptake of [3H]-choline into striatal slices. TRH(10(-4)M) or DN-1417(10(-5)M) also stimulated the conversion of [3H]-choline to [3H]-acetylcholine in striatal slices. A 30% reduction of acetylcholine synthesis from [3H]-choline in hippocampal slices and a 40% reduction of [3H]-choline uptake in slices of cerebral cortex, hippocampus and hypothalamus were observed in rats pretreated with pentobarbital (60 mg/kg, i.v.). TRH or DN-1417 (20 mg/kg, i.v.) given 15 min after the administration of pentobarbital markedly reversed both of the pentobarbital effects. Direct application of pentobarbital (5 X 10(-4)M) to slices in vitro also caused a 20-40% reduction of [3H]-choline uptake of cerebral cortex, hippocampus and diencephalon. A concomitant application of TRH(10(-4)M) or DN-1417(10(-5)M) and pentobarbital abolished the pentobarbital effect. These results provide neurochemical evidence that the antagonistic effects of TRH and DN-1417 on pentobarbital-induced narcosis are closely related to alterations in the rat brain choline uptake and acetylcholine synthesis, which are considered to be measures of the activity of cholinergic neurons.     

Release of exogenously supplied [3H]glutamate and endogenous glutamate from tissue slices of the cestode Hymenolepis diminuta

The effect of high potassium depolarization on the release of exogenously supplied [3H]glutamate and endogenous glutamate from tissue slices of the cestode Hymenolepis diminuta was examined. Increasing concentrations of potassium stimulated the release of radiolabel from tissues preloaded with [3H]glutamate. This release was by a partially calcium-dependent, magnesium-antagonized process. In the presence of tetrodotoxin, or absence of sodium, release of radiolabel was depressed, presumably by blockade of sodium-dependent neuronal potentials. The release of glutamate of both exogenous and endogenous origin was specifically and significantly elevated by high potassium; glutamate release was significantly depressed in calcium-free saline. The release of other amino acids of endogenous origin, including aspartate, was not elevated by high potassium. Collectively the data provide strong evidence for glutamate to be viewed as the only acidic amino acid neurotransmitter candidate in the cestodes.     

Hymenolepis diminuta (Cestoda): uptake of cycloleucine by metacestodes

Cycloleucine uptake by metacestodes of H. diminuta of various ages was investigated. Absorption occurs by active mediated transport, mean Kt = 0.28 mM. Vmax values are age-related, and can be correlated to developmental changes. Cycloleucine uptake in the metacestode is very similar to that in the adult worm and the implications of this are discussed.     

Prevention of the Nigrostriatal Toxicity of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine by Inhibitors of 3,4-Dihydroxyphenylethylamine Transport

The 3,4-dihydroxyphenylethylamine (DA, dopamine) uptake inhibitors GBR 13,069, amfonelic acid, WIN-35,065-2, WIN-35,428, nomifensine, mazindol, cocaine, McN-5908, McN-5847, and McN-5292 were effective in preventing [3H]DA and [3H]1-methyl-4-phenylpyridinium (MPP+) uptake in rat and mouse neostriatal tissue slices. These DA uptake inhibitors also were effective in attenuating the MPP+-induced release of [3H]DA in vitro. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration to mice (6 X 25 mg/kg i.p.) resulted in a large (70-80%) decrement in neostriatal DA. WIN-35,428 (5 mg/kg), GBR 13,069 (10 mg/kg), McN-5292 (5 mg/kg), McN-5908 (2 mg/kg), and amfonelic acid (2 mg/kg), when administered intraperitoneally 30 min prior to each MPTP injection, fully protected against MPTP-induced neostriatal damage. Other DA uptake inhibitors showed partial protection in vivo at the doses selected. Desmethylimipramine did not prevent [3H]MPP+ uptake or MPP+-induced release of [3H]DA in vitro, and did not protect against MPTP neurotoxicity in vivo. These results support the hypothesis put forth previously by others that the active uptake of MPP+ by dopaminergic neurons is necessary for toxicity.     

Depolarization Increases Chloride-Dependent Glutamate Sequestration in Synaptic Membranes of Rat Cerebral Cortex

To assess the functions of Cl- -dependent glutamate "binding" (Cl- -dependent glutamate uptake) in synaptic membranes, possible effects of depolarization on the uptake were examined. When rat cerebral cortical slices were preincubated with depolarizing agents such as veratrine (7 micrograms/ml), 10 microM aconitine, 56 mM K+, and 50 microM monensin, [3H]glutamate uptake by the crude synaptic membranes, which were subsequently prepared from the pretreated slices, was increased by 60-85%. Stimulation of the glutamate uptake by predepolarization was dependent on Na+ but not on Ca2+. The bindings of gamma-[3H]aminobutyric acid and 5-[3H]hydroxytryptamine were not significantly affected by the predepolarization. Veratrine pretreatment increased the maximal density of the glutamate uptake sites without affecting the affinity for glutamate. Several characteristics of the uptake sites increased by the veratrine pretreatment coincided with those of Cl- -dependent glutamate uptake sites. Na+-dependent glutamate binding (Na+-dependent glutamate uptake) to the membranes was not affected by pretreatment with veratrine. The content of endogenous glutamate and the noninulin space in the membrane fractions were not changed by the predepolarization. The increase in the glutamate uptake induced by pretreatment with high K+ was reversible: it returned to the control level after a second incubation of the slices in control medium. These results suggest that the Cl- -dependent glutamate sequestration system in synaptic membranes is regulated by the membrane potential.     

Kinetic and pharmacological analysis of L-[35S]cystine transport into rat brain synaptosomes

The synaptosomal transport of L-[35S]cystine occurs by three mechanisms that are distinguishable on the basis of their ionic dependence, kinetics of transport and the specificity of inhibitors. They are (a) low affinity sodium-dependent transport (Km 463 +/- 86 microM, Vmax 185 +/- 20 nmol mg protein-1 min-1), (b) high affinity sodium-independent transport (Km 6.90 +/- 2.1 microM, Vmax 0.485 +/- 0.060 nmol mg protein(-1) min(-1)) and (c) low affinity sodium-independent transport (Km 327 +/- 29 microM, Vmax 4.18 +/- 0.25 nmol mg protein(-1) min(-1)). The sodium-dependent transport of L-cystine was mediated by the X(AG)- family of glutamate transporters, and accounted for almost 90% of the total quantity of L-[35S]cystine accumulated into synaptosomes. L-glutamate (Ki 11.2 +/- 1.3 microM) was a non-competitive inhibitor of this transporter, and at 100 microM L-glutamate, the Vmax for L-[35S]cystine transport was reduced to 10% of control. L-cystine did not inhibit the high-affinity sodium-dependent transport of D-[3H]aspartate into synaptosomes. L-histidine and glutathione were the most potent inhibitors of the low affinity sodium-independent transport of L-[35S]cystine. L-homocysteate, L-cysteine sulphinate and L-homocysteine sulphinate were also effective inhibitors. 1 mM L-glutamate reduced the sodium-independent transport of L-cystine to 63% of control. These results suggest that the vast majority of the L-cystine transported into synaptosomes occurs by the high-affinity glutamate transporters, but that L-cystine may bind to a site that is distinct from that to which L-glutamate binds. The uptake of L-cystine by this mechanism is sensitive to inhibition by increased extracellular concentrations of L-glutamate. The importance of these results for understanding the mechanism of glutamate-mediated neurotoxicity is discussed.     

ATP-regulated neuronal catecholamine uptake: a new mechanism

Uptake of the catecholamines (CA), dopamine (DA) and norepinephrine (NE) into synaptosomes prepared from rat and bovine brains was potentiated by ATP (from 0.1 to 5.0 mM) in a dose-dependent manner. Other nucleotides, particularly the nonhydrolyzable ATP analogs beta,gamma-imidoadenosine-5'-triphosphate (AMP-PNP) and beta,gamma-methyladenosine-5'-triphosphate (AMP-PCP) also potentiated [3H]DA and [3H]NE uptake. Several endogenous 5'-nucleotide triphosphates (e.g. GTP, UTP and CTP) potentiated [3H]CA uptake, but were less effective than ATP. Among the ATP metabolites, only ADP potentiated uptake whereas AMP and adenosine did not. [3H]Dopamine uptake measured in Krebs bicarbonate buffer had a Km of 2.1 microM and a Vmax of 163.9 pmol/mg prot./min. In presence of ATP, [3H]DA uptake had much higher affinity (Km = 0.56 microM) and larger capacity (Vmax = 333 pmol/mg prot./min) than uptake in absence of added ATP. Furthermore, [3H]DA uptake in presence of ATP had faster rate of uptake, and was independent of temperature while in absence of added ATP it was temperature-dependent. This ATP-dependent [3H]DA uptake was retained by synaptosomal ghosts that were obtained after lysing the striatal synaptosomes and removing their contents of synaptic vesicles and mitochondria. It is proposed that, in addition to the carrier-mediated (neuronal) uptake of CA, there is neuronal uptake that is regulated by ATP and inhibited by cocaine, which may be more relevant for terminating the synaptic action of CA because of its faster rate of uptake and larger capacity.     

Basic and neutral amino acid transport in Aspergillus nidulans.

Arginine and methionine transport by Aspergillus nidulans mycelium was investigated. A single uptake system is responsible for the transport of arginine, lysine and ornithine. Transport is energy-dependent and specific for these basic amino acids. The Km value for arginine is 1 X 10(-5) M, and Vmax is 2-8 nmol/mg dry wt/min; Km for lysine is 8 X 10(-6) M; Kt for lysine as inhibitor of arginine uptake is 12 muM, and Ki for ornithine is mM. On minimal medium, methionine is transported with a Km of 0-I mM and Vmax about I nmol/mg dry wt/min; transport is inhibited by azide. Neutral amnio acids such as serine, phenylalanine and leucine are probably transported by the same system, as indicated by their inhibition of methionine uptake and the existence of a mutant specifically impaired in their transport. The recessive mutant nap3, unable to transport neutral amino acids, was isolated as resistant to selenomethionine and p-fluorophenylanine. This mutant has unchanged transport of methionine by general and specific sulphur-regulated permeases.     

Transport and phosphorylation of 2-deoxy-D-glucose by amphibian retina. Effects of light and darkness

We studied the uptake of 2-deoxy-D-glucose (2DG) and the synthesis of its phosphorylated product 2DG-6-phosphate (2DG-6P) by the retinas of the clawed frog (Xenopus laevis) and the bullfrog (Rana catesbeiana). Autoradiographs showed that most of the retinal 2DG uptake is by the photoreceptor layer. The 2DG accumulation by isolated Xenopus retinas was time and concentration dependent. The Kt for transport was 5.05 mM; Vmax was 6.99 X 10(-10) mol . mg-1 tissue wet weight min-1. The Km for 2DG-6P formation was estimated to be 2-3 mM and Vmax to be approximately 4 x 10(-9) mol . mg-1 min-1. 2DG uptake was inhibited competitively by glucose with a Ki of 2.29 mM. Exposure to light reduced 2DG uptake by no more than 10% as compared with dark uptake. Low sodium or ouabain (10(-4)-10(-7) M) treatment did not significantly alter 2DG uptake as compared with control retinas. In experiments upon intact, anesthetized bullfrogs, light reduced both the total amount of radioactivity acquired by the retina and the fraction of 2DG-6P present. The results are discussed in terms of the fraction of energy consumed by the retina required to maintain the photoreceptor dark current.     

Uptake of Kynurenine into Rat Brain Slices

The transport of [3H]kynurenine ([3H]KYN) into slices from rat tissue was examined in vitro. Brain accumulated KYN seven to eight times more effectively than any of several peripheral organs. Of all the organs tested, only the brain exhibited a sodium-dependent component of the uptake process. After an incubation period of 1 h, sodium-dependent transport amounted to 60% of total uptake. Both processes were abolished by prior sonication of the tissue and significantly inhibited by inclusion of metabolic blockers in the incubation medium. Time resolution showed that the sodium-independent uptake occurred rapidly and reached saturation within 30 min. In contrast, sodium-dependent transport was linear for at least 2 h of incubation. Brain regional analysis revealed a sevenfold difference between the areas of highest (cortex) and lowest (cerebellum) uptake. With the exception of cerebellar tissue, the ratio between sodium-dependent and sodium-independent processes was consistent among brain regions. Kinetic analyses were performed on striatal slices and revealed a Km of 927 microM and a Vmax of 18 nmol/h/mg of protein for the sodium-dependent process, and a Km of 3.8 mM and a Vmax of 38 nmol/10 min/mg of protein for the sodium-independent transport. The transporters were equally amenable to inhibition by KYN and tryptophan, indicating that KYN entry into the cell may be mediated by neutral amino acid uptake sites. No strict stereoselectivity existed, but L enantiomers were clearly more active than the D forms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Active uptake of MPP+, a metabolite of MPTP, by brain synaptosomes

Mouse brain synaptosomal preparations were used to study uptake of N-methyl-4-phenylpyridine (MPP+), a metabolite of the neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). The uptake of [3H]-MPP+ by striatal synaptosomes was approximately 25 X greater than that of [3H]-MPTP, with a KM of 0.48 microM and a Vmax of 5.3 nmoles/g tissue/min. Uptake was Na+ dependent and inhibited by ouabain, cocaine and dopamine (Ki 0.12 microM). Synaptosomes prepared from the corpus striatum accumulated [3H]-MPP+ at a rate 5-10 times higher than preparations from other brain regions. This selective uptake of MPP+ may contribute to the specificity of the toxic effects of MPTP on nigrostriatal dopaminergic neurons.     

Study on the effect of hypergravity stress on L-glutamate uptake by rat brain nerve endings

Using rat brain synaptosomes, we have investigated the effect of hypergravity on the kinetic parameters of Na(+)-dependent, high-affinity L-glutamate transport activity. The time-course of L-[14C]-glutamate uptake and dependence of L-[14C]-glutamate uptake velocity on glutamate concentrations were analyzed. K(m) and Vmax of this process have been determined. The hypergravity stress was created by centrifugation of rats for 1 hour at 10 g. We observed no differences in K(m) values between the control rats (10.7 +/- 2.5 microM) and animals exposed to hypergravity (6.7 +/- 1.5 microM). The similarity of this parameter for the two studied groups of animals showed that affinity of glutamate transporter to substrate was not sensitive to hypergravity stress. In contrast, the maximal velocity of glutamate uptake changed in hypergravity conditions. Vmax reduced from 12.5 +/- +/- 3.2 nmol/min per 1 mg of protein (control group) to 5.6 +/- 0.9 nmol/min per 1 mg of protein (animals, exposed to hypergravity stress). The possible mechanisms of attenuation of the glutamate transporter activity without modifying K(m) of glutamate uptake were discussed.     

Astroglia Contain a Specific Transport Mechanism for N-Acetyl-l-Aspartate

N-Acetylaspartate (NAA) is the second most abundant amino acid in the adult brain. It is located and synthesized in neurons and probably degraded in the glia compartment, but the transport mechanisms are unknown. Rat primary neuron and astrocyte cell cultures were exposed to the L isomer of [3H]NAA and demonstrated concentration-dependent uptake of [3H]NAA with a Km approximately 80 microM. However, Vmax was 23+/-6.4 pmol/mg of protein/min in astrocytes but only 1.13+/-0.4 pmol/mg of protein/min in neurons. The fact that neuron cultures contain 3-5% astrocytes suggests that the uptake mechanism is expressed only in glial cells. The astrocyte uptake was temperature and sodium chloride dependent and specific for L-NAA. The affinity for structural analogues was (IC50 in mM) as follows: L-NAA (0.12) > N-acetylaspartylglutamate (0.4) > N-acetylglutamate (0.42) > L-aspartate (>1) > L-glutamate (>1) > or = DL-threo-beta-hydroxyaspartate > N-acetyl-L-histidine. The naturally occurring amino acids showed no inhibitory effect at 1 mM. The glutamate transport blocker trans-pyrrolidine-2,4-dicarboxylate exhibited an IC50 of 0.57 mM, whereas another specific glutamate transport inhibitor, DL-threo-beta-hydroxyaspartate, had an IC50 of >1 mM. The experiments suggest that NAA transport in brain parenchyma occurs by a novel type of sodium-dependent carrier that is present only in glial cells.     

Basolateral amino acid transport systems in the perfused exocrine pancreas: sodium-dependency and kinetic interactions between influx and efflux mechanisms

Basolateral amino acid transport systems have been characterized in the perfused exocrine pancreas using a high-resolution paired-tracer dilution technique. Significant epithelial uptakes were measured for L-alanine, L-serine, alpha-methylaminoisobutyric acid, glycine, methionine, leucine, phenylalanine, tyrosine and L-arginine, whereas L-tryptophan and L-aspartate had low uptakes. alpha-Methylaminoisobutyric acid transport was highly sodium dependent (81 +/- 3%), while uptake of L-serine, L-leucine and L-phenylalanine was relatively insensitive to perfusion with a sodium-free solution. Cross-inhibition experiments of L-alanine and L-phenylalanine transport by twelve unlabelled amino acids indicated overlapping specificities. Unidirectional L-phenylalanine transport was saturable (Kt = 16 +/- 1 mM, Vmax = 12.3 +/- 0.4 mumol/min per g), and weighted non-linear regression analysis indicated that influx was best described by a single Michaelis-Menten equation. The Vmax/Kt ratio (0.75) for L-phenylalanine remained unchanged in the presence of 10 mM L-serine. Although extremely difficult to fit, L-serine transport appeared to be mediated by two saturable carriers (Kt1 = 5.2 mM, Vmax1 = 7.56 mumol/min per g; Kt2 = 32.8 mM, Vmax2 = 22.9 mumol/min per g). In the presence of 10 mM L-phenylalanine the Vmax/Kt ratio for the two L-serine carriers was reduced, respectively, by 79% and 50%. Efflux of transported L-[3H]phenylalanine or L-[3H]serine was accelerated by increasing perfusate concentrations of, respectively, L-phenylalanine and L-serine, and trans-stimulated by other amino acids. In the pancreas neutral amino acid transport appears to be mediated by Na+-dependent Systems A and ASC, the classical Na+-independent System L and another Na+-independent System asc recently identified in erythrocytes. The interactions in amino acid influx and efflux may provide one of the mechanisms by which the supply of extracellular amino acids for pancreatic protein synthesis is regulated.