Transiently increased glutamate cycling in rat PFC is associated with rapid onset of antidepressant-like effects.

Several drugs have recently been reported to induce rapid antidepressant effects in clinical trials and rodent models. Although the cellular mechanisms involved remain unclear, reports suggest that increased glutamate transmission contributes to these effects. Here, we demonstrate that the antidepressant-like efficacy of three unique drugs, with reported rapid onset antidepressant properties, is coupled with a rapid transient rise in glutamate cycling in the medial prefronal cortex (mPFC) of awake rats as measured by ex vivo 1H-[13C]-nuclear magnetic resonance spectroscopy. Rats were acutely pretreated by intraperitoneal injection with a single dose of ketamine (1, 3, 10, 30 and 80 mg kg-1), Ro 25-6981 (1, 3 and 10 mg kg-1), scopolamine (5, 25 and 100 µg kg-1) or vehicle (controls). At fixed times after drug injection, animals received an intravenous infusion of [1,6-13C2]glucose for 8 min to enrich the amino-acid pools of the brain with 13C, followed by rapid euthanasia. The mPFC was dissected, extracted with ethanol and metabolite 13C enrichments were measured. We found a clear dose-dependent effect of ketamine and Ro 25-6981 on behavior and the percentage of 13C enrichment of glutamate, glutamine and GABA (γ-aminobutyric acid). Further, we also found an effect of scopolamine on both cycling and behavior. These studies demonstrate that three pharmacologically distinct classes of drugs, clinically related through their reported rapid antidepressant actions, share the common ability to rapidly stimulate glutamate cycling at doses pertinent for their antidepressant-like efficacy. We conclude that increased cycling precedes the antidepressant action at behaviorally effective doses and suggest that the rapid change in cycling could be used to predict efficacy of novel agents or identify doses with antidepressant activity.

Spectroscopic assessment of alterations in macromolecule and small-molecule metabolites in human brain after stroke.

BACKGROUND AND PURPOSE: We sought to measure the temporal evolution and spatial distribution of lesion macromolecules and small molecules (lactate, N-acetyl compounds, creatine, and choline) in stroke patients by using short echo time in vivo proton MR spectroscopy. METHODS: Single-voxel spectra with TE=22 ms were obtained with and without inversion recovery suppression of small-molecule resonances from 30 examinations of 24 patients 3 to 214 days after stroke. Subtraction of the suppressed from the unsuppressed spectra yielded metabolite spectra without overlap from macromolecules. Two-dimensional spectroscopic images were acquired with macromolecule and small-molecule suppression from 5 additional patients. RESULTS: Macromolecule signals were elevated in lesions relative to normal brain and tended to increase in the subacute period, even as lactate peaks declined. Regions of increased lactate, increased macromolecule signal at 1.3 ppm, and decreased N-acetyl compounds were closely correlated in the 2D spectroscopic images. CONCLUSIONS: Short echo time spectra can be acquired in vivo in a manner that improves signal-to-noise ratio over long echo experiments and resolves overlapping macromolecule and small-molecule signals. The prominent macromolecule signals seen in the subacute period in association with persistently elevated lactate may represent mobile lipids in macrophages or other cells.

Quantitative functional imaging of the brain: towards mapping neuronal activity by BOLD fMRI.

Quantitative magnetic resonance imaging (MRI) and spectroscopy (MRS) measurements of energy metabolism (i.e. cerebral metabolic rate of oxygen consumption, CMR(O2)), blood circulation (i.e. cerebral blood flow, CBF, and volume, CBV), and functional MRI (fMRI) signal over a wide range of neuronal activity and pharmacological treatments are used to interpret the neurophysiologic basis of blood oxygenation level dependent (BOLD) image-contrast at 7 T in glutamatergic neurons of rat cerebral cortex. Multi-modal MRI and MRS measurements of CMR(O2), CBF, CBV and BOLD signal (both gradient-echo and spin-echo) are used to interpret the neuroenergetic basis of BOLD image-contrast. Since each parameter that can influence the BOLD image-contrast is measured quantitatively and separately, multi-modal measurements of changes in CMR(O2), CBF, CBV, BOLD fMRI signal allow calibration and validation of the BOLD image-contrast. Good agreement between changes in CMR(O2) calculated from BOLD theory and measured by (13)C MRS, reveals that BOLD fMRI signal-changes at 7 T are closely linked with alterations in neuronal glucose oxidation, both for activation and deactivation paradigms. To determine the neurochemical basis of BOLD, pharmacological treatment with lamotrigine, which is a neuronal voltage-dependent Na(+) channel blocker and neurotransmitter glutamate release inhibitor, is used in a rat forepaw stimulation model. Attenuation of the functional changes in CBF and BOLD with lamotrigine reveals that the fMRI signal is associated with release of glutamate from neurons, which is consistent with a link between neurotransmitter cycling and energy metabolism. Comparisons of CMR(O2) and CBF over a wide dynamic range of neuronal activity provide insight into the regulation of energy metabolism and oxygen delivery in the cerebral cortex. The current results reveal the energetic and physiologic components of the BOLD fMRI signal and indicate the required steps towards mapping neuronal activity quantitatively by fMRI at steady-state. Consequences of these results from rat brain for similar calibrated BOLD fMRI studies in the human brain are discussed.

Lactate efflux and the neuroenergetic basis of brain function.

In the unstimulated brain energy is primarily supplied by the oxidation of glucose. However the oxygen-to-glucose index (OGI), which is the ratio of metabolic rates of oxygen to glucose, CMR(O2)/CMR(glc), diverges from the theoretical value of 6 as activity is increased. In vivo measurements of brain lactate show its concentration to increase with stimulation. The decreasing OGI with stimulation had led to the suggestion that activation, unlike resting activity, is supported by anaerobic glycolysis. To date a unifying concept that accommodates glucose oxidation at rest with lactate generation and OGI decrease during stimulation of brain is lacking. Furthermore, energetics that change with increasing activity are not consistent with a neuroenergetic model that has been proposed from 1-(13)C-glucose MRS experiments. That model, based upon in vivo MRS measurements and cellular studies by Pellerin and Magistretti, showed that glutamate neurotransmitter cycling was coupled to glucose oxidation over a wide range of brain activities from rest down to deep anesthesia. Here we reconcile these paradoxical observations by suggesting that anaerobic glucose consumption (which can provide energy rapidly) increases with activation to meet the power requirements of millisecond neuronal firing. It is proposed, in accord with our neuroenergetic model, that the extra glucose mobilized rapidly for glial clearance of glutamate, is not needed for the oxidative processes that are responsible for neuronal firing and glutamate release, and consequently it is effluxed as lactate. A stoichiometric relation between OGI and lactate concentration is derived from the neuroenergetic model, showing that the enhanced glucose uptake during activation is consistent with neuronal activity being energetically supported by glucose oxidation.

Measurement of beta-hydroxybutyrate in acute hyperketonemia in human brain.

We report the measurement of D-beta-hydroxybutyrate (BHB) in the brains of six normal adult subjects during acute infusions of BHB. We used high field in vivo (1)H magnetic resonance (MR) spectroscopy in the occipital lobe in conjunction with an acute infusion protocol to elevate plasma BHB levels from overnight fasted levels (0.20 +/- 0.10 mM) to a steady state value of 2.12 +/- 0.30 mM. At this level of hyperketonemia, we determined a tissue BHB level of 0.24 +/- 0.04 mM. No increases in brain lactate levels were seen in these data. The concentrations of BHB and lactate were both considerably lower in comparison with previous data acquired in fasted adult subjects. This suggests that up-regulation of the monocarboxylic acid transporter occurs with fasting.

Glutamine is the major precursor for GABA synthesis in rat neocortex in vivo following acute GABA-transaminase inhibition.

The objective of the present study was to assess the degree to which astrocytic glutamine provides carbon for net synthesis of GABA in the rat neocortex in vivo. Isotopic labeling of GABA and glutamate from astrocytic glutamine was followed in halothane anesthetized and ventilated rats during an intravenous infusion of [2-(13)C]glucose. A net increase in GABA was achieved by administration of the GABA-transaminase inhibitor, gabaculine to suppress catabolism of GABA and recycling of (13)C label. (13)C Percentage enrichments of GABA, glutamate and glutamine were assessed in tissue extracts using (13)C-edited (1)H nuclear magnetic resonance at 8.4 T. GABA levels increased 2.6 micromol/g at 2 h and 6.1 micromol/g at 5 h after gabaculine, whereas glutamate and glutamine decreased in toto by 5.6 micromol/g at 2 h and 3.1 micromol/g at 5 h. Selective enrichment of glutamine, glutamate, and GABA C3's over other carbon positions was observed consistent with a precursor role for astrocytic glutamine. Between 1 h (control) and 3 h (gabaculine-treated) of [2-(13)C]glucose infusion, (13)C percentage enrichment increased in glutamine C3 (from 3.2+/-0.5 to 7.0+/-0.9%), glutamate C3 (from 1.8+/-0.5 to 3.4+/-0.9%), and GABA C3 (from 2.7+/-1.6 to 4.8+/-0.4%). The measured incremental [3-(13)C]GABA concentration (0.15 micromol/g) was close to the predicted value (0.13 micromol/g) that would be expected if the increase in GABA were produced entirely from glutamine compared to glutamate (0.07 micromol/g) based on the average precursor enrichments between 1 and 3 h. We conclude that glutamine is the major source of GABA carbon in the rat neocortex produced acutely following GABA-T inhibition by gabaculine in vivo.

Decrease in GABA synthesis rate in rat cortex following GABA-transaminase inhibition correlates with the decrease in GAD(67) protein.

gamma-Aminobutyric acid (GABA) synthesis in the brain is mediated by two major isoforms of glutamic acid decarboxylase, GAD(65) and GAD(67). The contribution of these isoforms to GABA synthesis flux (V(GAD)) is not known quantitatively. In the present study we compared V(GAD) in cortex of control and vigabatrin-treated rats under alpha-chloralose/70% nitrous oxide anesthesia, with total GAD activity and GAD isoform composition (GAD(65) and GAD(67)) measured by enzymatic assay and quantitative immunoblotting. V(GAD) was determined by re-analysis of 13C NMR data obtained ex vivo and in vivo during infusions of [1-13C]glucose using an extension of a model of glutamate-glutamine cycling that included a discrete GABAergic neuronal compartment with relevant interconnecting fluxes. V(GAD) was significantly lower in vigabatrin-treated rats (0.030-0.05 micromol/min per g, P<0.003) compared to the non-treated control group (0.10-0.15 micromol/min per g). The 67-70% decrease in V(GAD) was associated with a 13% decrease in total GAD activity (P=0.01) and a selective 44+/-15% decrease in GAD(67) protein (from 0.63+/-0.10 to 0.35+/-0.08 microg protein/mg tissue, P<0.05); GAD(65) protein was unchanged. The reduction in GAD(67) protein could account for a maximum of approximately 65% of the decrease in V(GAD) in vigabatrin-treated animals suggesting that inhibition of GAD(65) must have also occurred in these experiments, although product inhibition of GAD(67) by increased GABA could play a role. GAD(67) could account for 56-85% of cortical GABA synthesis flux under basal conditions and the entire flux after vigabatrin treatment.

Detection of gamma-aminobutyric acid (GABA) by longitudinal scalar order difference editing.

Two novel spectral editing techniques for the in vivo detection of gamma-aminobutyric acid (GABA) are presented. The techniques rely on the generation of longitudinal scalar order (LSO) coherences, which in combination with J-difference editing results in the selective detection of GABA. The utilization of LSO coherences makes the editing sequences insensitive to phase and frequency instabilities. Furthermore, the spectral editing selectivity can be increased independent of the echo time, thereby opening the echo time for state-of-the-art water suppression and/or spatial localization techniques. The performance of the LSO editing techniques is theoretically demonstrated with product operator calculations and density matrix simulations and experimentally evaluated on phantoms in vitro and on human brain in vivo.

In vivo nuclear magnetic resonance studies of glutamate-gamma-aminobutyric acid-glutamine cycling in rodent and human cortex: the central role of glutamine.

It has been recognized for many years that the metabolism of brain glutamate and gamma-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters, is linked to a substrate cycle between neurons and astrocytes involving glutamine. However, the quantitative significance of these fluxes in vivo was not known. Recent in vivo 13C and 15N NMR studies in rodents and 13C NMR in humans indicate that glutamine synthesis is substantial and that the total glutamate-GABA-glutamine cycling flux, necessary to replenish neurotransmitter glutamate and GABA, accounts for >80% of net glutamine synthesis. In studies of the rodent cortex, a linear relationship exists between the rate of glucose oxidation and total glutamate-GABA-glutamine cycling flux over a large range of cortical electrical activity. The molar stoichiometric relationship (approximately 1:1) found between these fluxes suggests that they share a common mechanism and that the glutamate-GABA-glutamine cycle is coupled to a major fraction of cortical glucose utilization. Thus, glutamine appears to play a central role in the normal functional energetics of the cerebral cortex.

Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle.

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

Topiramate rapidly raises brain GABA in epilepsy patients.

PURPOSE: The short- and long-term pharmacodynamic effects of topiramate (TPM) on brain gammay-aminobutyric acid (GABA) metabolism were studied in patients with complex partial seizures. METHODS: In vivo measurements of GABA, homocarnosine, and pyrrolidinone were made of a 14-cc volume in the occipital cortex using 1H spectroscopy with a 2.1-Tesla magnetic resonance spectrometer and an 8-cm surface coil. Fifteen patients (four men) were studied serially after the first, oral dose (100 mg) of TPM. RESULTS: The first dose of TPM increased brain GABA within 1 h. Within 4 h, GABA was increased by 0.9 mM (95% CI, 0.7-1.1). Brain GABA remained elevated for > or =24 h. Pyrrolidinone and homocarnosine increased slowly during the first day. Daily TPM therapy (median, 300 mg; range, 200-500) increased GABA (0.3 mM; 95% CI, 0.1-0.5), homocarnosine (0.4 mM; 95% CI, 0.3-0.5), and pyrrolidinone (0.15 mM; 95% CI, 0.10-0.19), compared with levels before TPM. There was no dose response evident with daily TPM doses of 200-500 mg. CONCLUSIONS: TPM promptly elevates brain GABA and presumably offers partial protection against further seizures within hours of the first oral dose. Patients may expect to experience the effects of increased homocarnosine and pyrrolidinone within 24 h.

Reductions in occipital cortex GABA levels in panic disorder detected with 1h-magnetic resonance spectroscopy.

BACKGROUND: There is preclinical evidence and indirect clinical evidence implicating gamma-aminobutyric acid (GABA) in the pathophysiology and treatment of human panic disorder. Specifically, deficits in GABA neuronal function have been associated with anxiogenesis, whereas enhancement of GABA function tends to be anxiolytic. Although reported peripheral GABA levels (eg, in cerebrospinal fluid and plasma) have been within reference limits in panic disorder, thus far there has been no direct assessment of brain GABA levels in this disorder. The purpose of the present work was to determine whether cortical GABA levels are abnormally low in patients with panic disorder. METHODS: Total occipital cortical GABA levels (GABA plus homocarnosine) were assessed in 14 unmedicated patients with panic disorder who did not have major depression and 14 retrospectively age- and sex-matched control subjects using spatially localized (1)H-magnetic resonance spectroscopy. All patients met DSM-IV criteria for a principal current diagnosis of panic disorder with or without agoraphobia. RESULTS: Patients with panic disorder had a 22% reduction in total occipital cortex GABA concentration (GABA plus homocarnosine) compared with controls. This finding was present in 12 of 14 patient-control pairs and was not solely accounted for by medication history. There were no significant correlations between occipital cortex GABA levels and measures of illness or state anxiety. CONCLUSIONS: Panic disorder is associated with reductions in total occipital cortex GABA levels. This abnormality might contribute to the pathophysiology of panic disorder.

Differentiation of glucose transport in human brain gray and white matter.

Localized 1H nuclear magnetic resonance spectroscopy has been applied to determine human brain gray matter and white matter glucose transport kinetics by measuring the steady-state glucose concentration under normoglycemia and two levels of hyperglycemia. Nuclear magnetic resonance spectroscopic measurements were simultaneously performed on three 12-mL volumes, containing predominantly gray or white matter. The exact volume compositions were determined from quantitative T1 relaxation magnetic resonance images. The absolute brain glucose concentration as a function of the plasma glucose level was fitted with two kinetic transport models, based on standard (irreversible) or reversible Michaelis-Menten kinetics. The steady-state brain glucose levels were similar for cerebral gray and white matter, although the white matter levels were consistently 15% to 20% higher. The ratio of the maximum glucose transport rate, V(max), to the cerebral metabolic utilization rate of glucose, CMR(Glc), was 3.2 +/- 0.10 and 3.9 +/- 0.15 for gray matter and white matter using the standard transport model and 1.8 +/- 0.10 and 2.2 +/- 0.12 for gray matter and white matter using the reversible transport model. The Michaelis-Menten constant K(m) was 6.2 +/- 0.85 and 7.3 +/- 1.1 mmol/L for gray matter and white matter in the standard model and 1.1 +/- 0.66 and 1.7 +/- 0.88 mmol/L in the reversible model. Taking into account the threefold lower rate of CMR(Glc) in white matter, this finding suggests that blood--brain barrier glucose transport activity is lower by a similar amount in white matter. The regulation of glucose transport activity at the blood--brain barrier may be an important mechanism for maintaining glucose homeostasis throughout the cerebral cortex.

Cerebral energetics and the glycogen shunt: neurochemical basis of functional imaging.

Positron-emission tomography and functional MRS imaging signals can be analyzed to derive neurophysiological values of cerebral blood flow or volume and cerebral metabolic consumption rates of glucose (CMR(Glc)) or oxygen (CMR(O(2))). Under basal physiological conditions in the adult mammalian brain, glucose oxidation is nearly complete so that the oxygen-to-glucose index (OGI), given by the ratio of CMR(O(2))/CMR(Glc), is close to the stoichiometric value of 6. However, a survey of functional imaging data suggests that the OGI is activity dependent, moving further below the oxidative value of 6 as activity is increased. Brain lactate concentrations also increase with stimulation. These results had led to the concept that brain activation is supported by anaerobic glucose metabolism, which was inconsistent with basal glucose oxidation. These differences are resolved here by a proposed model of glucose energetics, in which a fraction of glucose is cycled through the cerebral glycogen pool, a fraction that increases with degree of brain activation. The "glycogen shunt," although energetically less efficient than glycolysis, is followed because of its ability to supply glial energy in milliseconds for rapid neurotransmitter clearance, as a consequence of which OGI is lowered and lactate is increased. The value of OGI observed is consistent with passive lactate efflux, driven by the observed lactate concentration, for the few experiments with complete data. Although the OGI changes during activation, the energies required per neurotransmitter release (neuronal) and clearance (glial) are constant over a wide range of brain activity.

Contribution of net hepatic glycogen synthesis to disposal of an oral glucose load in humans.

The contribution of hepatic glycogen synthesis to whole body glucose disposal after an oral glucose load was examined using (13)C nuclear magnetic resonance (NMR) spectroscopy to measure liver glycogen content in healthy, volunteers after an overnight fast. In group 1 (n = 14), hepatic glycogen synthesis was measured using (13)C-NMR spectroscopy for 240 minutes after ingestion of 98 +/- 1 g glucose. Liver volumes were measured using magnetic resonance imaging (MRI). To assess the direct (glucose --> glucose-6-P --> glucose-1-P --> uridine diphosphate (UDP)-glucose --> glycogen) and indirect (3-carbon units --> --> glycogen) pathways of liver glycogen synthesis, group 2 (n = 6) was studied with an identical glucose load enriched with [1-(13)C]glucose along with acetaminophen to noninvasively assess the (13)C enrichment in hepatic UDP-glucose. The fasting hepatic glycogen content was 305 +/- 17 mmol/L liver, and the liver volume was 1.46 +/- 0.07 L. For the initial 180 minutes after ingestion of glucose, hepatic glycogen concentrations increased linearly (r =.94, P =.0006) achieving a maximum concentration of 390 +/- 7 mmol/L liver and then remained constant until the end of the study. The mean maximum rate of net hepatic glycogen synthesis was 0.48 +/- 0.07 mmol/L liver-minute. Total liver glycogen synthesis could account for 16.7 +/- 3.8 g (17% +/- 4%) of the glucose ingested, and of this, 10.5 +/- 2.4 g (63% +/- 7%) was synthesized by the direct pathway. In conclusion, after ingestion of 98 g of glucose: (1) 16.7 +/- 3.8 g (17% +/- 4%) glucose was stored in the liver as glycogen, and (2) 63% +/- 7% (10.5 +/- 2.4 g) of this glycogen was formed via the direct pathway.

Studies of metabolic compartmentation and glucose transport using in vivo MRS.

Organs consist of several types of cells with specialized functions. This cellular localization of function is often referred to as compartmentation. Due to the intrinsic low sensitivity of MR methods it is generally not possible in vivo to obtain images or spectra of single cells. Instead the MRS signal is the sum of the signal from millions of cells and multiple cell types. A major challenge in using MRS to study biological processes such as metabolism and transport is to devise measurements that provide cell-specific information from this mix. Fortunately nature has helped the MR scientist by in several cases nearly completely localizing metabolic pathways and their associated metabolites in specific cell types. The chemical specificity of MRS allows the concentrations and synthesis rates of these metabolites to be measured, providing information about the compartmentation of metabolism and function. In this review examples are presented from MRS studies of metabolic trafficking between neurons and astrocytes in the brain, brain glucose transport, and the role of muscle glucose transport in insulin resistance and diabetes. The concepts and approaches used in these studies are generally applicable for studying cellular metabolic compartmentation in a wide range of systems.

Homocarnosine and seizure control in juvenile myoclonic epilepsy and complex partial seizures.

OBJECTIVE: To assess the relationship between seizure control and gamma-aminobutyric acid (GABA), homocarnosine, and pyrrolidinone levels in the visual cortex of patients with epilepsy taking valproate or lamotrigine. Previous studies suggested that poor seizure control was associated with low GABA and homocarnosine levels. METHODS: In vivo measurements of GABA, homocarnosine, and pyrrolidinone were made in a 14-cm(3) volume of the occipital cortex using (1)H spectroscopy with a 2.1-Tesla MR spectrometer and an 8-cm surface coil. Twenty-six adults (eight men) taking valproate or lamotrigine were recruited; 12 had complex partial seizures (CPS) and 14 had juvenile myoclonic epilepsy (JME). RESULTS: Median homocarnosine levels were normal for patients with JME and below normal for patients with CPS. Better seizure control was associated with higher homocarnosine levels for both groups. Median GABA was below normal for patients with JME, lower than for patients with CPS. Brain GABA was lowest in patients with JME even when seizure control was excellent. Pyrrolidinone levels were above normal in almost all patients with JME. CONCLUSIONS: Low GABA levels are associated with poor seizure control in patients with CPS, but not in JME. Higher homocarnosine levels are associated with better seizure control in both types of epilepsy.

Flux control in the rat gastrocnemius glycogen synthesis pathway by in vivo 13C/31P NMR spectroscopy.

To determine the relative contributions of glucose transport/hexokinase, glycogen synthase (GSase), and glycolysis to the control of insulin-stimulated muscle glycogen synthesis, we combined 13C and 31P NMR to quantitate the glycogen synthesis rate and glucose 6-phosphate (G-6-P) levels in rat (Sprague-Dawley) gastrocnemius muscle during hyperinsulinemia at euglycemic (E) and hyperglycemic (H) glucose concentrations under thiopental anesthesia. Flux control was calculated using metabolic control analysis. The combined control coefficient of glucose transport/hexokinase (GT/Hk) for glycogen synthesis was 1.1 +/- 0.03 (direct measure) and 1.14-1.16 (calculated for a range of glycolytic fluxes), whereas the control coefficient for GSase was much lower (0.011-0.448). We also observed that the increase in in vivo [G-6-P] from E to H (0.22 +/- 0.03 to 0.40 +/- 0.03 mM) effects a supralinear increase in the in vitro velocity of GSase, from 14.6 to 26.1 mU. kg(-1). min(-1) (1.8-fold). All measurements suggest that the majority of the flux control of muscle glycogen synthesis is at the GT/Hk step.

The "glycogen shunt" in exercising muscle: A role for glycogen in muscle energetics and fatigue.

Stimulated by recent (13)C and (31)P NMR studies of exercising muscle, we propose a model of the energetics of contraction. Previous studies of energetics have followed energy consumption. However, the rapidity of contraction, in 10-40 msec, requires that energy be delivered rapidly, so that the muscle has power requirements of rapid energy expenditure that are ultimately met by the slower averaged consumption of carbon and oxygen from blood. We propose that energy is supplied in milliseconds by glycogenolysis and that between contractions, glycogenesis refills the pools. The energy for glycogenesis is supplied by oxidative phosphorylation. This mechanism utilizes the rapid conversion of glycogen phosphorylase, the "fight-or-flight" enzyme, to its active form. Lactate is necessarily generated by this pathway to serve as a time buffer between fast and slow energy needs, which resolves the paradoxical generation of lactate in well oxygenated tissue. Consequences of the glycogen shunt are compatible with numerous biochemical and physiological experiments. The model provides a possible mechanism for muscle fatigue, suggesting that at low but nonzero glycogen concentrations, there is not enough glycogen to supply millisecond energy needs.

In vivo (13)C NMR measurement of neurotransmitter glutamate cycling, anaplerosis and TCA cycle flux in rat brain during.

The aims of this study were twofold: (i) to determine quantitatively the contribution of glutamate/glutamine cycling to total astrocyte/neuron substrate trafficking for the replenishment of neurotransmitter glutamate; and (ii) to determine the relative contributions of anaplerotic flux and glutamate/glutamine cycling to total glutamine synthesis. In this work in vivo and in vitro (13)C NMR spectroscopy were used, with a [2-(13)C]glucose or [5-(13)C]glucose infusion, to determine the rates of glutamate/glutamine cycling, de novo glutamine synthesis via anaplerosis, and the neuronal and astrocytic tricarboxylic acid cycles in the rat cerebral cortex. The rate of glutamate/glutamine cycling measured in this study is compared with that determined from re-analysis of (13)C NMR data acquired during a [1-(13)C]glucose infusion. The excellent agreement between these rates supports the hypothesis that glutamate/glutamine cycling is a major metabolic flux ( approximately 0.20 micromol/min/g) in the cerebral cortex of anesthetized rats and the predominant pathway of astrocyte/neuron trafficking of neurotransmitter glutamate precursors. Under normoammonemic conditions anaplerosis was found to comprise 19-26% of the total glutamine synthesis, whilst this fraction increased significantly during hyperammonemia ( approximately 32%). These findings indicate that anaplerotic glutamine synthesis is coupled to nitrogen removal from the brain (ammonia detoxification) under hyperammonemic conditions.