QUANTITATIVE ASPECTS OF CO<sub>2</sub> FIXATION IN MAMMALIAN BRAIN <i>IN VIVO</i>*

Waelsch, Heinrich; Berl, S.; Rossi, C. A.; Clarke, Donald D.; Purpura, D.P.

doi:10.1111/j.1471-4159.1964.tb06117.x

Cited by 147 publications

(61 citation statements)

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“…The finding that radiolabeled bicarbonate is primarily metabolized by neurons when injected intracerebrally (this study) and by astrocytes when injected intravenously (Waelsch et al, 1964) is probably explained by the mode of administration. When injected intravenously, the bicarbonate enters the brain by passing through the astrocytic end feet that surround brain capillaries, thereby being exposed primarily to the astrocytic pyruvate-carboxylating enzymes, whereas the intracerebral administration bypasses this astrocytic interphase.…”

Section: Discussionmentioning

confidence: 48%

“…In the brain the only truly anaplerotic reaction is carboxylation of pyruvate to the TCA cycle intermediates malate or oxaloacetate. This process has been thought to occur in astrocytes and not in neurons, because pyruvate carboxylase, one of the pyruvatecarboxylating enzymes in the brain (Patel, 1974), is only expressed in astrocytes (Yu et al, 1983;Shank et al, 1985;Cesar and Hamprecht, 1995), and because intravenous administration of radiolabeled bicarbonate to rats leads to stronger labeling of glutamine than of glutamate (Waelsch et al, 1964), a sign of astrocytic metabolism (Van den Berg et al, 1969;Hassel et al, 1992). The resulting conclusion has been that astrocytes maintain a net export of glutamine to glutamatergic neurons and that these neurons are completely dependent on astrocytes for neurotransmission.…”

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confidence: 99%

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Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Hassel

Bråthe

2000

J. Neurosci.

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Release of transmitter glutamate implies a drain of ␣-ketoglutarate from neurons, because glutamate, which is formed from ␣-ketoglutarate, is taken up by astrocytes. It is generally believed that this drain is compensated by uptake of glutamine from astrocytes, because neurons are considered incapable of de novo synthesis of tricarboxylic acid cycle intermediates, which requires pyruvate carboxylation.

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Section: Discussionmentioning

confidence: 48%

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confidence: 99%

Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Hassel

Bråthe

2000

J. Neurosci.

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“…2 to [' 4C I glutamate plus '4C] g!utamine was higher when NH 3 levels were higher (Berl et a!., 1962;Waelsch et a!., 1964). Recently, by using more elegant techniques of ' 3C NMR spectroscopy to study hyperammonemic rats, the same conclusion was drawn (Gopher and Lapidot, 1991).…”

Section: C0mentioning

confidence: 87%

“…These pathways usually involve CO2 fixation. CO2 fixation in the brain has been extensively investigated since 1962 (Berl et al, 1962;Waelsch et al, 1964), and it is recognized that most of the C02-fixing, anaplerotic activity in the brain is catalyzed by pyruvate carboxy!ase (Patel, 1989). Pyruvate carboxylase in the brain (Yu et al, 1983;Shank et al, 1985), along with g!utamine synthetase (Martinez-Hernandez et al, 1977;Norenberg, 1979), is found exclusively in the astrocytes as indicated in Scheme 1.…”

mentioning

confidence: 99%

Role of Pyruvate Carboxylase in Facilitation of Synthesis of Glutamate and Glutamine in Cultured Astrocytes

Gamberino

Berkich

Lynch

et al. 1997

Journal of Neurochemistry

101

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CO 2 fixation was measured in cultured astrocytes isolated from neonatal rat brain to test the hypothesis that the activity of pyruvate carboxylase influences the rate of de novo glutamate and glutamine synthesis in astrocytes. Astrocytes were incubated with 14002 and the incorporation of 140 into medium or cell extract products was determined. After chromatographic separation of 140-Iabelled products, the fractions of 140 cycled back to pyruvate, incorporated into citric acid cycle intermediates, and converted to the amino acids glutamate and glutamine were determined as a function of increasing pyruvate carboxylase flux. The consequences of increasing pyruvate, bicarbonate, and ammonia were investigated. Increasing extracellular pyruvate from 0 to 5 mM increased pyruvate carboxylase flux as observed by increases in the 140 incorporated into pyruvate and citric acid cycle intermediates, but incorporation into glutamate and glutamine, although relatively high at low pyruvate levels, did not increase as pyruvate carboxylase flux increased. Increasing added bicarbonate from 15 to 25 mM almost doubled 002 fixation. When 25 mM bicarbonate plus 0.5 mM pyruvate increased pyruvate carboxylase flux to approximately the same extent as 15 mM bicarbonate pIus 5 mM pyruvate, the rate of appearance of [14C]glutamate and glutamine was higher with the lower level of pyruvate. The conclusion was drawn that, in addition to stimulating pyruvate carboxylase, added pyruvate (but not added bicarbonate) increases alanine aminotransferase flux in the direction of glutamate utilization, thereby decreasing glutamate as pyruvate + glutamate -~a-ketoglutarate + alanine. In contrast to previous in vivo studies, the addition of ammonia (0.1 and 5 mM) had no effect on net 14002 fixation, but did alter the distribution of 14C-labelled products by decreasing glutamate and increasing glutamine. Rather unexpectedly, ammonia did not increase the sum of glutamate plus glutamine (mass amounts or 140 incorporation). Low rates of conversion of cr- [140]ketoglutarate to [14C]glutamate,even in the presence of excess added ammonia, suggested that reductive amination of a-ketoglutarate is inactive under conditions studied in these cultured astrocytes. We conclude that pyruvate carboxylase is required for de novo synthesis of glutamate plus glutamine, but that conversion of aketoglutarate to glutamate may frequently be the ratelimiting step in this process of glutamate synthesis.

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“…The present study yielded maximum rates of glutamine synthesis under normal and hyperammonemic conditions of Ϸ0.21 mol͞min per g and Ϸ0.43 mol͞min per g, respectively, giving a ⌬V gln of Ϸ0.22 mol͞min per g. These rates were determined assuming that the precursor glutamate pool for glutamine synthesis had the same fractional enrichment as the large neuronal pool. While this may be an accurate representation of the control situation (where V trans Ϸ 0), under hyperammonemic conditions it is known that anaplerotic flux increases (32,33), and a more significant contribution from the rapidly labeling astrocytic glutamate pool will result. Therefore, it is likely that the hyperammonemic V gln determined here is an overestimate of this rate, as the model does not account for multiple precursor pools in its present form.…”

Section: Fig 2 Fits To the In Vivomentioning

confidence: 99%

In vivo ¹³ C NMR measurements of cerebral glutamine synthesis as evidence for glutamate–glutamine cycling

Sibson

Dhankhar²,

Mason³

et al. 1997

Proc. Natl. Acad. Sci. U.S.A.

316

346

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The cerebral tricarboxylic acid (TCA) cycle rate and the rate of glutamine synthesis were measured in rats in vivo under normal physiological and hyperammonemic conditions using 13 C NMR spectroscopy. In the hyperammonemic animals, blood ammonia levels were raised from control values of Ϸ0.05 mM to Ϸ0.35 mM by an intravenous ammonium acetate infusion. Once a steady-state of cerebral metabolites was established, a [1-13 C]glucose infusion was initiated, and 13 C NMR spectra acquired continuously on a 7-tesla spectrometer to monitor 13 C labeling of cerebral metabolites. The time courses of glutamate and glutamine C-4 labeling were fitted to a mathematical model to yield TCA cycle rate (V TCA ) and the f lux from glutamate to glutamine through the glutamine synthetase pathway (V gln ). Under hyperammonemia the value of V TCA was 0.57 ؎ 0.16 mol͞min per g (mean ؎ SD, n ‫؍‬ 6) and was not significantly different (unpaired t test; P > 0.10) from that measured in the control animals (0.46 ؎ 0.12 mol͞min per g, n ‫؍‬ 5). Therefore, the TCA cycle rate was not significantly altered by hyperammonemia. The measured rate of glutamine synthesis under hyperammonemia was 0.43 ؎ 0.14 mol͞min per g (mean ؎ SD, n ‫؍‬ 6), which was significantly higher (unpaired t test; P < 0.01) than that measured in the control group (0.21 ؎ 0.04 mol͞ min per g, n ‫؍‬ 5). We propose that the majority of the glutamine synthetase f lux under normal physiological conditions results from neurotransmitter substrate cycling between neurons and glia. Under hyperammonemia the observed increase in glutamine synthesis is comparable to the expected increase in ammonia transport into the brain and reported measurements of glutamine eff lux under such conditions. Thus, under conditions of elevated plasma ammonia an increase in the rate of glutamine synthesis occurs as a means of ammonia detoxification, and this is superimposed on the constant rate of neurotransmitter cycling through glutamine synthetase.Glutamine synthetase in the brain is predominantly an astrocytic enzyme that catalyzes the formation of glutamine from glutamate and ammonia. It has been suggested that this pathway forms part of a neurotransmitter cycle between neurons and glia (1). In this cycle, astrocytes take up neurotransmitter glutamate from the synaptic cleft, convert it to glutamine via glutamine synthetase, and release this glutamine into the extracellular space for uptake by neurons, where it is converted back to glutamate primarily by glutaminase. ␥-Aminobutyric acid (GABA) released from inhibitory neurons also may be taken up by astrocytes, converted to glutamate, and recycled to neruons in the same way. The function of such a cycle would be to remove the synaptically active metabolite from the synaptic cleft and to recycle the carbon skeletons back to the neuronal pool by a synaptically inactive form.While ammonia is a normal constituent of tissue, elevated concentrations of blood and brain ammonia have been found to interfere with cerebral energy metabolism and may redu...

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QUANTITATIVE ASPECTS OF CO₂ FIXATION IN MAMMALIAN BRAIN *IN VIVO**

Cited by 147 publications

References 0 publications

Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Role of Pyruvate Carboxylase in Facilitation of Synthesis of Glutamate and Glutamine in Cultured Astrocytes

In vivo ¹³ C NMR measurements of cerebral glutamine synthesis as evidence for glutamate–glutamine cycling

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QUANTITATIVE ASPECTS OF CO2 FIXATION IN MAMMALIAN BRAIN IN VIVO*

Cited by 147 publications

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Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Neuronal Pyruvate Carboxylation Supports Formation of Transmitter Glutamate

Role of Pyruvate Carboxylase in Facilitation of Synthesis of Glutamate and Glutamine in Cultured Astrocytes

In vivo 13 C NMR measurements of cerebral glutamine synthesis as evidence for glutamate–glutamine cycling

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QUANTITATIVE ASPECTS OF CO₂ FIXATION IN MAMMALIAN BRAIN *IN VIVO**

In vivo ¹³ C NMR measurements of cerebral glutamine synthesis as evidence for glutamate–glutamine cycling