Kinetics of cerebral uptake processes in vitro of L-glutamine, branched-chain L-amino acids, and L-phenylalanine: effects of ouabain

Characterization of cerebral glutamine uptake from blood in the mouse brain: implications for metabolic modeling of 13C NMR data

(13)C Nuclear Magnetic Resonance (NMR) studies of rodent and human brain using [1-(13)C]/[1,6-(13)C2]glucose as labeled substrate have consistently found a lower enrichment (∼25% to 30%) of glutamine-C4 compared with glutamate-C4 at isotopic steady state. The source of this isotope dilution has not been established experimentally but may potentially arise either from blood/brain exchange of glutamine or from metabolism of unlabeled substrates in astrocytes, where glutamine synthesis occurs. In this study, the contribution of the former was evaluated ex vivo using (1)H-[(13)C]-NMR spectroscopy together with intravenous infusion of [U-(13)C5]glutamine for 3, 15, 30, and 60 minutes in mice. (13)C labeling of brain glutamine was found to be saturated at plasma glutamine levels >1.0 mmol/L. Fitting a blood-astrocyte-neuron metabolic model to the (13)C enrichment time courses of glutamate and glutamine yielded the value of glutamine influx, VGln(in), 0.036±0.002 μmol/g per minute for plasma glutamine of 1.8 mmol/L. For physiologic plasma glutamine level (∼0.6 mmol/L), VGln(in) would be ∼0.010 μmol/g per minute, which corresponds to ∼6% of the glutamine synthesis rate and rises to ∼11% for saturating blood glutamine concentrations. Thus, glutamine influx from blood contributes at most ∼20% to the dilution of astroglial glutamine-C4 consistently seen in metabolic studies using [1-(13)C]glucose.

Glutamine transport in cerebellar granule cells in culture

In the present study, uptake of glutamine by rat cerebellar granule cells, a predominantly glutamatergic nerve cell population, has been investigated. Glutamine is taken up by granule cells via at least three transport systems, A, ASC and L. The L-type low affinity system (K(m) = 2.6 mM) is the major transport system in the absence of Na+. The systems A and ASC represent the Na(+)-dependent transport routes, both with almost identical high affinity for glutamine (K(m) = 0.26 mM). Similar transport systems for glutamine are also found in cerebral cortical neurons, a predominantly GABAergic nerve cell population, and cerebral cortical astrocytes. The glutamine transport properties in granule cells, however, show a series of differences from that of cortical neurons and astrocytes: (1) uptake of glutamine by granule cells is primarily mediated by system A (54%), while contributions by system A in cortical neurons and astrocytes are less than 30%; (2) granule cells exhibit strikingly higher transport efficiency for glutamine (V(max)/K(m) = 20 min(-1) for system A as compared to the V(max)/K(m) ratio of 5 min(-1) in cortical neurons and astrocytes), and (3) the initial uptake rates and the steady-state accumulation levels of glutamine are two- to threefold higher in granule cells than that of cortical neurons and astrocytes. These results taken together suggest that in accordance with the important need to replenish the neurotransmitter pool of glutamate, glutamatergic neurons exhibit highly efficient transport systems to accumulate glutamine, one of the major precursors of glutamate.

Cerebral alanine transport and alanine aminotransferase reaction: alanine as a source of neuronal glutamate

Alanine transport and the role of alanine amino-transferase in the synthesis and consumption of glutamate were investigated in the preparation of rat brain synaptosomes. Alanine was accumulated rapidly via both the high- and low-affinity uptake systems. The high-affinity transport was dependent on the sodium concentration gradient and membrane electrical potential, which suggests a cotransport with Na+. Rapid accumulation of the Na(+)-alanine complex by synaptosomes stimulated activity of the Na+/K+ pump and increased energy utilization; this, in turn, activated the ATP-producing pathways, glycolysis and oxidative phosphorylation. Accumulation of Na+ also caused a small depolarization of the plasma membrane, a rise in [Ca2+]i, and a release of glutamate. Intra-synaptosomal metabolism of alanine via alanine amino-transferase, as estimated from measurements of N fluxes from labeled precursors, was much slower than the rate of alanine uptake, even in the presence of added oxoacids. The velocity of [15N]alanine formation from [15N]glutamine was seven to eight times higher than the rate of [15N]-glutamate generation from [15N]alanine. It is concluded that (a) overloading of nerve endings with alanine could be deleterious to neuronal function because it increases release of glutamate; (b) the activity of synaptosomal alanine aminotransferase is much slower than that of glutaminase and hence unlikely to play a major role in maintaining [glutamate] during neuronal activity; and (c) alanine amino-transferase might serve as a source of glutamate during recovery from ischemia/hypoxia when the alanine concentration rises and that of glutamate falls.

Decreased glutamate transport by the brain and spinal cord in amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a chronic degenerative neurologic disorder characterized by the death of motor neurons in the cerebral cortex and spinal cord. Recent studies have suggested that the metabolism of glutamate, a potentially neurotoxic amino acid, is abnormal in patients with ALS. We hypothesized that the high-affinity glutamate transporter is the site of the defect.

METHODS

We measured high-affinity, sodium-dependent glutamate transport in synaptosomes from neural tissue obtained from 13 patients with ALS, 17 patients with no neurologic disease, and 27 patients with other neuro-degenerative diseases (Alzheimer's disease in 15 patients and Huntington's disease in 12 patients). The groups were comparable with respect to age and the interval between death and autopsy. Synaptosomes were prepared from spinal cord, motor cortex, sensory cortex, visual cortex, striatum, and hippocampus. We also measured sodium-dependent transport of gamma-aminobutyric acid and phenylalanine in the synaptosomal preparations.

RESULTS

In patients with ALS, there was a marked decrease in the maximal velocity of transport for high-affinity glutamate uptake in synaptosomes from spinal cord (-59 percent, P less than 0.001), motor cortex (-70 percent, P less than 0.001), and somatosensory cortex (-39 percent, P less than 0.05), but not in those from visual cortex, striatum, or hippocampus. The affinity of the transporter for glutamate was not altered. No abnormalities in glutamate transport were found in synaptosomes from patients with other chronic neurodegenerative disorders. The transport of gamma-aminobutyric acid and phenylalanine was normal in patients with ALS.

CONCLUSIONS

ALS is associated with a defect in high-affinity glutamate transport that has disease, region, and chemical specificity. Defects in the clearance of extracellular glutamate because of a faulty transporter could lead to neurotoxic levels of extracellular glutamate and thus be pathogenic in ALS.

Lactate utilization by isolated cells from early neonatal rat brain

The utilization of lactate, glucose, 3-hydroxybutyrate, and glutamine has been studied in isolated brain cells from early newborn rats. Isolated brain cells actively utilized these substrates, showing saturation at concentrations near physiological levels during the perinatal period. The rate of lactate utilization was 2.5-fold greater than that observed for glucose, 3-hydroxybutyrate, or glutamine, suggesting that lactate is the main metabolic substrate for the brain immediately after birth. The apparent Km for glucose utilization suggested that this process is limited by the activity of hexokinase. However, lactate, 3-hydroxybutyrate, and glutamine utilization seems to be limited by their transport through the plasma membrane. The presence of fatty acid-free bovine serum albumin (BSA) in the incubation medium significantly increased the rate of lipogenesis from lactate or 3-hydroxybutyrate, although this was balanced by the decrease in their rates of oxidation in the same circumstances. BSA did not affect the rate of glucose utilization. The effect of BSA was due not to the removal of free fatty acid, but possibly to the binding of long-chain acyl-CoA, resulting in the disinhibition of acetyl-CoA carboxylase and citrate carrier.

Transport of asparagine by rat brain synaptosomes: an approach to evaluate glutamine accumulation

Isolated rat brain synaptosomes accumulated L-asparagine with a Km value of 348 microM and a Vmax value of 3.7 nmol/mg of protein/min at 28 degrees C. Uptake of L-asparagine was inhibited by the presence of L-glutamine, whereas transport of L-glutamine was blocked by L-asparagine. Alanine, serine, cysteine, threonine, and, in particular, leucine were also inhibitory whereas alpha-(methylamino)isobutyrate, ornithine, lysine, arginine, and glutamate were much less effective blockers. Transport of L-asparagine had a substantial sodium-dependent component, whereas that of the D-stereoisomer was almost unaffected by the presence or absence of the cation. L-Asparagine was accumulated to a maximal gradient, [L-Asn]i/[L-Asn]o, of 20-30, and this value was reduced to 5-6 by withdrawal of sodium or addition of high [KCI]. A plot of log [Na+]o/[Na+]i against the log [L-Asn]i/[L-Asn]o had a slope close to I, which indicates that a single sodium ion is transported inward with each asparagine molecule. It is postulated that uptake of L-asparagine occurs, to a large extent, in cotransport with Na+ and that it utilizes the sodium chemical gradient and the membrane electrical potential as the source of energy. The similarity between the L-asparagine and L-glutamine transport systems and the reciprocal inhibition of influx of the two amino acids suggest that the same mechanism is responsible for glutamine accumulation. This could explain the high [Gln]i maintained by the brain in vivo.

Neuronal glutamine utilization: glutamine/glutamate homeostasis in synaptosomes

The synaptosomal metabolism of glutamine was studied under in vitro conditions that simulate depolarization in vivo. With [2-15N]glutamine as precursor, the [glutamine]i was diminished in the presence of veratridine or 50 mM KCl, but the total amounts of [15N]glutamate and [15N]aspartate formed were either equal to those of control incubations (veratridine) or higher (50 mM [KCl]). This suggests that depolarization decreases glutamine uptake and independently augments glutaminase activity. Omission of sodium from the medium was associated with low internal levels of glutamine which indicates that influx occurs as a charged Na(+)-amino acid complex. It is postulated that a reduction in membrane potential and a collapse of the Na+ gradient decrease the driving forces for glutamine accumulation and thus inhibit its uptake and enhance its release under depolarizing conditions. Inorganic phosphate stimulated glutaminase activity, particularly in the presence of calcium. At 2 mM or lower [phosphate] in the medium, calcium inhibited glutamine utilization and the production of glutamate, aspartate, and ammonia from glutamine. At a high (10 mM) medium [phosphate], calcium stimulated glutamine catabolism. It is suggested that a veratridine-induced increase in intrasynaptosomal inorganic phosphate is responsible for the enhancement of flux through glutaminase; calcium affects glutaminase indirectly by modulating the level of free intramitochondrial [phosphate]. Because phosphate also lowers the Km of glutaminase for glutamine, augmentation of the amino acid breakdown may occur even when depolarization lowers [glutamine]i. Reducing the intrasynaptosomal glutamate to 26 nmol/mg of protein had little effect on glutamine catabolism, but raising the pH to 7.9 markedly increased formation of glutamate and aspartate. It is concluded that phosphate and H+ are the major physiologic regulators of glutaminase activity.

High-affinity transport of L-glutamine by a plasma membrane preparation from rat brain

Plasma membrane vesicles prepared from rat brain contain a saturable, high-affinity transport system for L-glutamine that exhibits the following characteristics: (1) The rate of L-glutamine transport is linear up to 200 micrograms/mL membrane protein. (2) Transport of [3H]-L-glutamine is linear with time for at least 10 min, is significantly reduced by lowering the assay temperature to 4 degrees C, and is essentially abolished by the addition of excess unlabeled L-glutamine. (3) The transport rate is optimal in the range of pH 7.4-8.2. (4) The system exhibits a Km for L-glutamine of approximately 1.7 microM and a Vmax of approximately 46 pmol/(min.mg of protein). (5) The system is not highly dependent upon the addition of monovalent or divalent cations. (6) Inhibitor studies reveal that the amino acid amides exhibit the highest affinity for the system and that there is a high specificity for the L-isomers.

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.

The uptake and metabolism of L-glutamate by tissue slices of the cestode Hymenolepis diminuta

The in vitro uptake of L-[3H]glutamate by tissue slices of the cestode Hymenolepis diminuta, denuded of tegument, was investigated. Two sodium concentration-dependent mechanisms, one of high affinity (Kt 1.8 X 10(-5) M; Vmax 4.76 pMoles/min/mg wet weight) and another of low affinity (Kt 2.2 X 10(-4) M; Vmax 50.7 pMoles/min/mg wet weight), were identified, in addition to a sodium insensitive component. Exchange of preloaded [3H]glutamate did not occur in tissue slices incubated in dilute unlabelled glutamate. Acidic amino acids, imipramine and fluoxetine were effective inhibitors of high and low affinity uptake, while glutamate receptor ligands, neurotransmitters and some antihelminthics generally were not. The concentrations present in, and the metabolism of glutamate by, tissue slices was examined by HPLC. The significance of the three modes of glutamate uptake and their possible role in the physiology of H. diminuta are discussed.

Comparative aspects of the apparent Michaelis constant for neutral amino acid transport in several animal tissues

The apparent Michaelis constant, Km, for transport of a number of neutral amino acids has been compared between intestine, heart, brain and erythrocytes among a variety of animals using values available in the literature. Neutral amino acids with side chains containing 3, 4, 7 and 9 carbon atoms had approximately equal mean Km values when tested for intestinal transport among a variety of species; alanine appeared to have a mean Km value that was larger than those found for the first group, and glycine had a significantly greater mean Km than all of the other compounds tested. Km values for phenylalanine and tryptophan measured in rat heart were found to be close to the means measured for these substrates in intestine. The mean Km values measured in mammalian brain for each of the neutral amino acid substrates were found not be significantly different from each other. When the means of Km values for the neutral amino acids tested were compared between intestine and brain, only the glycine means were shown to differ significantly between the organs. Based on data for several mammalian species, brain appears to have a greater average apparent affinity for glycine than does intestine. In the human erythrocytes and in a few other mammalian species, Km values for all neutral amino acids tested with exception of glycine were found to be similar in magnitude to each other and to the Km averages of neutral amino acids found in intestine for the series containing 3-9 carbon atoms. The Km value for glycine in the human erythrocyte was noted to be substantially lower in value than the averages for glycine in brain or intestine. Avian red blood cells appear to have high apparent affinity for neutral amino acid transport when compared with red cells of several mammalian species.

High-affinity transport of glutamate in rat brain microvessels

The maintenance of low extracellular concentrations of glutamate in the brain is a complex process in which the role of capillary transport is poorly understood. We examined the kinetics and substrate specificity of glutamate uptake by isolated rat brain microvessels. We showed that these microvessels take up glutamate by an energy- and temperature-dependent, concentrative, high-affinity active transport system with Km of about 2 microM. The presence of this active transport system, coupled with the known slow inward transport of glutamate across the blood-brain barrier, allows us to suggest that this capillary transport system may function in vivo in the unidirectional outward transport of glutamate from brain to blood.

Glutamine oxidation by dissociated cells and homogenates of rat brain: kinetics and inhibitor studies

The rates of [U-14C]glutamine oxidation to 14CO2 were determined under a variety of experimental conditions using whole homogenates and dissociated cells from rat brain. The pattern of glutamine oxidation by homogenates differed from that by dissociated brain cells in several respects. The rates of glutamine oxidation by dissociated brain cells showed saturation kinetics with an apparent Km of 0.30 mM. Lineweaver-Burk plots of glutamine oxidation by homogenates revealed two linear segments with two apparent Km values (0.58 mM and 3.0 mM). In the presence of aminooxyacetate, however, the Lineweaver-Burk plots for homogenates were linear with a single Km of 0.47 mM. The oxidation of glutamine by homogenates was inhibited by both rotenone and antimycin A (80-85%), as were glutamate and glucose oxidation, suggesting that a significant amount of glutamine is oxidized via the tricarboxylic acid cycle. In the presence of aminooxyacetate, glutamine oxidation was inhibited less than 40%, whereas the oxidation of glutamate was inhibited 75%; in contrast, glucose oxidation was enhanced 50%. The rates of glutamine oxidation by homogenates were highest in the presence of high levels of potassium (50 mM) and low levels of sodium (2.5 mM). Varying ionic composition, however, had little or no effect on the rates of glutamine oxidation by dissociated brain cells. Measurements of glutamine oxidation by homogenates prepared from 2-, 10-, 15-, 25-, and 90-day-old rats revealed little or no age-dependent difference. In contrast, the oxidation by dissociated brain cells from 2-day-old animals was significantly less than that obtained for animals 10 days or older (7.76 vs. 15.6 nmol/h/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake of L-glutamine into synaptosomes. Is the gamma-glutamyl cycle involved?

Transport of L-glutamine into rat cortical synaptosomes has been investigated by (14C)L-glutamine uptake experiments. This amino acid enters synaptosomes both by an active carrier mediated system, which may be the result of gamma-glutamyl cycle activity and by a Na+-dependent transport system. This view is supported by the following observations: a) as demonstrated previously (10), glutamine inside synaptosomes reaches concentrations higher than those of the incubation medium, and initial rates of uptake approach saturation kinetics; b) the uptake of glutamine is inhibited by uncouplers; c) the uptake is inhibited by methionine sulfoximine, a suicide-inhibitor of an enzyme of the gamma-glutamyl cycle; d) the initial rate of uptake is lowered by decreasing the Na+-level of the incubation medium or by adding ouabain. The validity of this hypothesis is discussed.

Effects of sodium fluxes and ouabain on brain-slice tryptophan transport

Efflux of preloaded [3H]tryptophan from rat cerebral cortex slices has been monitored into superfusion media that were altered in their sodium content. Total replacement of sodium with choline greatly increased the release of tryptophan. This release could be cancelled by re-introducing sodium into the slices. A brief exposure to ouabain, an efficient inhibitor of Na+, K+-ATPase activity, only slightly increased tryptophan efflux at the concentration of 0.1 mM, whereas at 1.0 mM it produced a similar effect as the sodium-free medium. Accordingly, when the slices were superfused in the presence of ouabain and sodium, the change of medium to sodium-free caused much greater relative enhancement of tryptophan efflux with 0.1 than 1.0 mM ouabain. Tryptophan efflux was modified by changes in sodium fluxes also in slices initially depleted of sodium ions and treated with ouabain. The results suggest that the sodium-free medium and ouabain have a similar mechanism of action in modifying the tryptophan transport, and that the cation gradients across the cell membranes are more crucial for normal amino acid transport than the functional Na+, K+-ATPase.

Glutamine inhibits the accumulation and hydroxylation of tryptophan in rat striatal synaptosomes

L-Glutamine at the concentration present in cerebrospinal fluid decreases the steady-state accumulation of the aromatic amino acids tryptophan, tyrosine and dihydroxyphenylalanine (DOPA) in rat striatal synaptosomes. Glutamine significantly inhibits synaptosomal tryptophan hydroxylase activity; it has less marked effects on tyrosine hydroxylase and DOPA decarboxylase activities. Thus, interaction between glutamine and tryptophan transport into nerve terminals may be one of the factors regulating the rate of serotonin synthesis in vivo.

The activities in different neural cell types of certain enzymes associated with the metabolic compartmentation glutamate

The cellular distribution of certain enzymes associated with the metabolic compartmentation of glutamate was estimated in ultrastructurally preserved and metabolically competent perikarya fractions that were enriched in astrocytes, granule cells and Purkinje cells and derived from 8-day-old rat cerebellum, and in monolayer cultures (14 days in vitro) composed principally of interneurones or astrocytes. The neuronal activities of glutamine synthetase and glutamate dehydrogenase were respectively about 4- to 8-fold and 2- to 5-fold lower than in astrocytes, depending upon the class of neurone and the type of preparation used for comparison. By contrast glutaminase activity was about 3- to 12-fold higher in neuronal than in astroglial preparations. Estimations of the specific activity of succinate dehydrogenase differed less between cell types, indicating that the differences in glutamate dehydrogenase and glutaminase were not simply related to variations in the concentration of mitochondria relative to the other cellular constituents. The findings presented provide direct evidence in support of our model assigning the 'small' glutamate compartment, where most of the labelled glutamine is synthesized, to glial cells, and the 'large' compartment to neurones, and also underline the metabolic interaction between these two cell types in the brain.

Studies on metabolism of branched chain amino acids in brain and other tissues of rat with special reference to leucine

Leucine aminotransferase (EC 2.6.1.6) and 2-oxoisocaproate dehydrogenase (EC 1.2.4.3) were studied in rat cerebral cortex, cerebellum, brain stem, liver, and muscle in normal and animals starved for 48 hours. In the brain, leucine aminotransferase, valine aminotransferase, and 2-oxoisocaproate dehydrogenase showed a significant increase in starvation only in cerebellum while there was increase in 2-oxoisocaproate dehydrogenase in cerebral cortex only. A significantly high increase in the activity of 2-oxoisocaproate dehydrogenase was observed in muscle in starvation. A significant decrease in the activity of leucine aminotransferase was observed in liver in starvation. The increase in the activity of 2-oxoisocaproate dehydrogenase in muscle and a decrease in the activity of leucine aminotransferase in liver in starvation indicate that the leucine is predominantly metabolized in extra hepatic tissues particularly in muscle. As a result of intraperitoneal administration of 2 ml of leucine (5 mM), a significant increase in 2-oxoisocaproate dehydrogenase occurred in cerebral cortex, liver, and muscle while a profound increase in the activity of glutamate dehydrogenase (EC 1.4.1.2) was observed in all the brain regions and liver under these conditions. A significant increase in the content of glutamic acid, alanine, and GABA was observed in all the three regions of the brain after the administration of leucine. A significant increase in the content of glutamine was observed only in the cerebellum and cerebral cortex after leucine administration. These results indicate that leucine in brain might contribute to the formation of glutamate, not only by transamination, but also by promoting glutamate dehydrogenase activity. Thus, there is a change in the metabolism of glutamate family of amino acids and energy depletion. These results are discussed in relation to the brain function.

Acetylcholine synthesis in synaptosomes: mode of transfer of mitochondrial acetyl coenzyme A

Labeled acetylcholine derived from labeled pyruvate in a synaptosomal preparation from rat brain, incubated with nicotinamide adenine dinucleotide as well as coenzyme A, is stimulated by calcium ions in the absence but not in the presence of Triton X-100. Whereas citrate is taken up by cholinergic synaptosomes because it suppresses the formation of acetylcholine from pyruvate, it is not itself converted into acetylcholine. The evidence suggests that there is a calcium-dependent transfer of mitochondrial acetyl coenzyme A into the cholinergic synaptoplasm, which is apparently devoid of the citrate cleavage enzyme, and is there converted into acetylcholine. The permeability of the inner mitochondrial membrane to coenzyme A and acetyl coenzyme A seems to be enhanced by calcium ions, and this effect may be mediated by mitochondrial phospholipase A2.

Control of glutaminase activity in rat brain cortex in vitro: influence of glutamate, phosphate, ammonium, calcium and hydrogen ions

Studies of the kinetics of product inhibitions of glutaminase activity of rat brain cortical homogenates incubated in Krebs-Ringer medium in presence of the activating anion phosphate show that whereas ammonium acts competitively with respect to L-glutamine, L-glutamate acts non-competitively. Calcium ions are stimulatory with maximum stimulation occurring at 1 mM. This effect is blocked by EGTA. The optimum pH for glutamine hydrolysis is 8.2 with maximum stimulation by Ca2+ occurring at pH 7.8. Similar observations have been made with a crude synaptosomal fraction of brain cortex and also with brain cortex slices where the rates of hydrolyses are about one order of magnitude lower than those with brain homogenates. Ammonium (2 mM) diminishes glutamate and aspartate, and enhances glutamine and ammonium levels of brain cortex slices incubated in a physiological glucose-saline medium. It is suggested that stimulation by Ca2+ ions may occur in part by enhanced phosphate, and by diminished glutamate and H+ ion concentrations of mitochondria and that mutual regulation of glutaminase and glutamine synthetase may exist in glia but not in nerve terminals. It is proposed that calcium plays a role in maintaining normal electrical activities of certain neurons in part by its effects on glutamate regeneration in nerve terminals from glial-derived glutamine and that when brain cell ammonium is maintained at levels above the normal, suppressions of glutaminase activity in nerve terminals could affect the rate of formation of glutamine-derived neurotransmitters.