To determine the relationship between cerebral Glc metabolism and glutamatergic neuronal function, we used 13 C NMR spectroscopy to measure, simultaneously, the rates of the tricarboxylic acid cycle and Gln synthesis in the rat cortex in vivo. From these measurements, we calculated the rates of oxidative Glc metabolism and glutamate-neurotransmitter cycling between neurons and astrocytes (a quantitative measure of glutamatergic neuronal activity). By measuring the rates of the tricarboxylic acid cycle and Gln synthesis over a range of synaptic activity, we have determined the stoichiometry between oxidative Glc metabolism and glutamateneurotransmitter cycling in the cortex to be close to 1:1. This finding indicates that the majority of cortical energy production supports functional (synaptic) glutamatergic neuronal activity. Another implication of this result is that brain activation studies, which map cortical oxidative Glc metabolism, provide a quantitative measure of synaptic glutamate release.Glc metabolism is the major pathway of energy production in the mature brain (1). During brain activation, increases in Glc metabolism directly form the basis of brain functional mapping by using both 2-deoxyglucose autoradiography (2, 3) and positron-emission tomography (4) and indirectly influence signal changes observed with functional MRI (5). Despite the extensive use of these methods for mapping brain function, the mechanism linking Glc metabolism and functional neuronal activity and the fraction of cerebral energy production that supports neuronal function are still unknown.Glutamate is the major excitatory neurotransmitter in the brain (6), and a high percentage of cortical neurons are glutamatergic (7). It has been proposed that a neuronalastrocytic neurotransmitter cycle exists in the brain in which glutamate from the neuronal pool is released into the synaptic cleft as a neurotransmitter, taken up by astrocytes, converted to Gln, and returned to the neuron in this synaptically inactive form where it is converted back to glutamate (6). The development of in vivo 13 C NMR spectroscopy has enabled the direct investigation of cerebral glutamate metabolism (8, 9). We recently have shown that the rate of glutamateneurotransmitter cycling between neurons and astrocytes can be calculated by using the flux of the 13 C label from glutamate to Gln in the rat brain in vivo during a [1-13 C]Glc infusion (10). Thus, we can obtain an in vivo measure of glutamatergic neuronal activity. In the same experiment, the flux of the 13 C label from [1-13 C]Glc into glutamate yields a simultaneous in vivo measurement of the cerebral tricarboxylic acid (TCA) cycle rate, from which oxidative Glc consumption can be derived (11,12). Therefore, by using the combined measurement of these two fluxes, we can determine quantitatively the stoichiometry between cerebral Glc metabolism and glutamatergic-synaptic activity in vivo.In the present study, we have used direct 13 C NMR spectroscopy to determine the cerebral (primarily cortical)...

To examine the extent to which the defect in insulin action in subjects with non-insulin-dependent diabetes mellitus (NIDDM) can be accounted for by impairment of muscle glycogen synthesis, we performed combined hyperglycemic-hyperinsulinemic clamp studies with [13C]glucose in five subjects with NIDDM and in six age- and weight-matched healthy subjects. The rate of incorporation of intravenously infused [1-13C]glucose into muscle glycogen was measured directly in the gastrocnemius muscle by means of a nuclear magnetic resonance (NMR) spectrometer with a 15.5-minute time resolution and a 13C surface coil. The steady-state plasma concentrations of insulin (approximately 400 pmol per liter) and glucose (approximately 10 mmol per liter) were similar in both study groups. The mean (+/- SE) rate of glycogen synthesis, as determined by 13C NMR, was 78 +/- 28 and 183 +/- 39 mumol-glucosyl units per kilogram of muscle tissue (wet weight) per minute in the diabetic and normal subjects, respectively (P less than 0.05). The mean glucose uptake was markedly reduced in the diabetic (30 +/- 4 mumol per kilogram per minute) as compared with the normal subjects (51 +/- 3 mumol per kilogram per minute; P less than 0.005). The mean rate of nonoxidative glucose metabolism was 22 +/- 4 mumol per kilogram per minute in the diabetic subjects and 42 +/- 4 mumol per kilogram per minute in the normal subjects (P less than 0.005). When these rates are extrapolated to apply to the whole body, the synthesis of muscle glycogen would account for most of the total-body glucose uptake and all of the nonoxidative glucose metabolism in both normal and diabetic subjects. We conclude that muscle glycogen synthesis is the principal pathway of glucose disposal in both normal and diabetic subjects and that defects in muscle glycogen synthesis have a dominant role in the insulin resistance that occurs in persons with NIDDM.

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Recent 13 C NMR studies in rat models have shown that the glutamate͞glutamine cycle is highly active in the cerebral cortex and is coupled to incremental glucose oxidation in an Ϸ1:1 stoichiometry. To determine whether a high level of glutamatergic activity is present in human cortex, the rates of the tricarboxylic acid cycle, glutamine synthesis, and the glutamate͞glutamine cycle were determined in the human occipital͞parietal lobe at rest. During an infusion of [1-13 C]-glucose, in vivo 13 C NMR spectra were obtained of the time courses of label incorporation into [4-13 C]-glutamate and [4-13 C]-glutamine. Using a metabolic model we have validated in the rat, we calculated a total tricarboxylic acid cycle rate of 0.77 ؎ 0.07 mol͞min͞g (mean ؎ SD, n ‫؍‬ 6), a glucose oxidation rate of 0.39 ؎ 0.04 mol͞min͞g, and a glutamate͞ glutamine cycle rate of 0.32 ؎ 0.05 mol͞min͞g (mean ؎ SD, n ‫؍‬ 6). In agreement with studies in rat cerebral cortex, the glutamate͞glutamine cycle is a major metabolic f lux in the resting human brain with a rate Ϸ80% of glucose oxidation.The regulation of the release and re-uptake of the excitatory neurotransmitter glutamate is critical for mammalian brain function (1, 2). Glutamate released from the neuron may be cleared from the synaptic cleft through uptake by neuronal or glial glutamate transporters (3, 4). Recent studies have supported the glia as the major pathway of glutamate clearance (3). Neurons lack the enzymes necessary to perform net glutamate synthesis and depend on the glia to supply precursors. One of the pathways proposed for neuronal glutamate repletion is the glutamate͞glutamine cycle (5-8). In this pathway, glutamate taken up by the glia is converted to glutamine by glutamine synthetase (9-11). Glutamine then is released to the extracellular fluid, where it is taken up by neurons and is converted back to glutamate by the action of phosphate-activated glutaminase (12).The rate of the glutamate͞glutamine cycle has been controversial because of difficulties in performing measurements in the living brain. The prevailing belief has been that the glutamate͞glutamine cycle is a minor metabolic flux relative to total cellular glutamate metabolism. This view is largely based on the small size of the vesicular glutamate pool compared with other cellular glutamate pools (13,14). Additional evidence comes from the low flux of isotope from [1-13 C] glucose into glutamine in studies of brain slices (15).We have demonstrated that in vivo 13 C NMR may be used to measure the rate of glutamine labeling (16, 17) from [1-13 C] glucose in human occipital͞parietal cortex. These and subsequent studies (18) demonstrated that, in contrast with results from nonactivated brain slices (15), glutamine labeling is rapid. However, the rate of the glutamate͞glutamine cycle was not uniquely determined from these first experiments because of the inability to distinguish the glutamate͞glutamine cycle from other sources of glutamine labeling. The major alternate pathway of brain glutamine metaboli...

To quantitate hepatic glycogenolysis, liver glycogen concentration was measured with 13C nuclear magnetic resonance spectroscopy in seven type II diabetic and five control subjects during 23 h of fasting. Net hepatic glycogenolysis was calculated by multiplying the rate of glycogen breakdown by the liver volume, determined from magnetic resonance images. Gluconeogenesis was calculated by subtracting the rate of hepatic glycogenolysis from the whole body glucose production rate, measured using 16-3HIglucose. Liver glycogen concentration 4 h after a meal was lower in the diabetics than in the controls; 131±20 versus 282±60 mmol/liter liver (P < 0.05). Net hepatic glycogenolysis was decreased in the diabetics, 1.3±0.2 as compared to 2.8±0.7 umol/(kg body wt X min) in the controls (P < 0.05). Whole body glucose production was increased in the diabetics as compared to the controls, 11.1±0.6 versus 8.9±0.5 gmol/(kg body wt X min) (P < 0.05). Gluconeogenesis was consequently increased in the diabetics, 9.8±0.7 as compared to 6.1±0.5,gmol/(kg body wt X min) in the controls (P < 0.01), and accounted for 88±2% of total glucose production as compared with 70±6% in the controls (P < 0.05). In conclusion: increased gluconeogenesis is responsible for the increased whole body glucose production in type II diabetes mellitus after an overnight fast. (J. Clin. Invest. 1992. 90:1323-1327

Prior 13C magnetic resonance spectroscopy (MRS) experiments, which simultaneously measured in vivo rates of total glutamate-glutamine cycling (V(cyc(tot))) and neuronal glucose oxidation (CMR(glc(ox), N)), revealed a linear relationship between these fluxes above isoelectricity, with a slope of approximately 1. In vitro glial culture studies examining glutamate uptake indicated that glutamate, which is cotransported with Na+, stimulated glial uptake of glucose and release of lactate. These in vivo and in vitro results were consolidated into a model: recycling of one molecule of neurotransmitter between glia and neurons was associated with oxidation of one glucose molecule in neurons; however, the glucose was taken up only by glia and all the lactate (pyruvate) generated by glial glycolysis was transferred to neurons for oxidation. The model was consistent with the 1:1 relationship between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) measured by 13C MRS. However, the model could not specify the energetics of glia and gamma-amino butyric acid (GABA) neurons because quantitative values for these pathways were not available. Here, we review recent 13C and 14C tracer studies that enable us to include these fluxes in a more comprehensive model. The revised model shows that glia produce at least 8% of total oxidative ATP and GABAergic neurons generate approximately 18% of total oxidative ATP in neurons. Neurons produce at least 88% of total oxidative ATP, and take up approximately 26% of the total glucose oxidized. Glial lactate (pyruvate) still makes the major contribution to neuronal oxidation, but approximately 30% less than predicted by the prior model. The relationship observed between DeltaCMR(glc(ox), N) and DeltaV(cyc(tot)) is determined by glial glycolytic ATP as before. Quantitative aspects of the model, which can be tested by experimentation, are discussed.

The rate of net hepatic glycogenolysis was assessed in humans by serially measuring hepatic glycogen concentration at 3- to 12-hour intervals during a 68-hour fast with 13C nuclear magnetic resonance spectroscopy. The net rate of gluconeogenesis was calculated by subtracting the rate of net hepatic glycogenolysis from the rate of glucose production in the whole body measured with tritiated glucose. Gluconeogenesis accounted for 64 +/- 5% (mean +/- standard error of the mean) of total glucose production during the first 22 hours of fasting. In the subsequent 14-hour and 18-hour periods of the fast, gluconeogenesis accounted for 82 +/- 5% and 96 +/- 1% of total glucose production, respectively. These data show that gluconeogenesis accounts for a substantial fraction of total glucose production even during the first 22 hours of a fast in humans.

Brain lactate concentration is usually assumed to be stable except when pathologic conditions cause a mismatch between glycolysis and respiration. Using newly developed 1H NMR spectroscopic techniques that allow measurement of lactate in vivo, we detected lactate elevations of 0.3-0.9 mM in human visual cortex during physiologic photic stimulation. The maximum rise appeared in the first few minutes; thereafter lactate concentration declined while stimulation continued. The results are consistent with a transient excess of glycolysis over respiration in the visual cortex, occurring as a normal response to stimulation in the physiologic range.Glucose and oxygen-the principal substrates ofbrain energy metabolism-are consumed by that organ at matched rates that ordinarily maintain stable lactate concentrations. Brain lactate elevations due to lack of oxygen or increased energy demand to the degree of status epilepticus are well-known phenomena, and extensive research on them has created a general impression that brain lactate elevation always reflects pathologic conditions. However, several recent reports suggest that brain activity within the physiologic range may cause brain lactate to rise. In an earlier study using nuclear magnetic resonance spectroscopy (MRS) in vivo, we found that lactate rose in posterior cerebral cortex of rabbits when electric shocks were delivered to the optic nerves (1). Ueki et al. (2) demonstrated lactate elevation in rat somatosensory cortex due to forepaw stimulation. In humans studied by positron emission tomography (PET), Fox et al. (3) showed that visual stimulation caused 30-50% increases in blood flow and glucose uptake of visual cortex, whereas oxygen extraction rose no more than 5%. Newly developed MRS techniques permit repeated noninvasive detection of lactate in a few cc of human brain (4-6). We have used such techniques to show that photic stimulation does indeed cause a clear, although transient, elevation of lactate in human visual cortex; a preliminary report has appeared (7).