Metabolic studies of pyruvate- and lactate-perfused guinea pig hearts by 13C NMR. Determination of substrate preference by glutamate isotopomer distribution

Guinea pig hearts perfused in vitro with 99% enriched [3-13C]pyruvate and [3-13C]lactate have been examined by 13C NMR spectroscopy at 75.45 MHz. Resonances from the intracellular metabolites, glutamate, aspartate, alanine, citrate, malate, lactate, and acetylcarnitine are detected in the [3-13C]pyruvate-perfused hearts while glutamate is the only metabolite observed in the [3-13C]lactate-perfused hearts. Extracts obtained from individual freeze-clamped hearts run under high resolution conditions show similar distributions of metabolites indicating that the intracellular concentrations of these metabolites are indeed quite dependent upon the substrate being utilized. The excellent signal to noise in the spectra of lactate-perfused hearts allows observation of spin-spin coupling between 13C-enriched nuclei in the various glutamate isotopomers within the intact heart. It is shown that the steady-state distribution of glutamate isotopomers is determined by the amount of 12C- versus 13C-enriched acetyl-CoA entering the citric acid cycle and this provides the basis for a direct determination of substrate utilization by the heart in the presence of competing substrates. Similar information may be derived from the 13C spectrum of an extract prepared from a single [3-13C]pyruvate-perfused heart. Our results indicate that lactate is preferred over glucose by the guinea pig heart even in the presence of insulin.     

Effect of Echinococcus multilocularis on the origin of acetyl-coA entering the tricarboxylic acid cycle in host liver

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy was employed to investigate alterations in hepatic carbohydrate metabolism in Meriones unguiculatus infected with Echinococcus multilocularis. Following portal vein injections of an equimolar mixture of [1,2-13C2]acetate and [3-13C]lactate, perchloric acid extracts of the livers were prepared and NMR spectra obtained. Isotopomer analysis using glutamate resonances in these spectra showed that the relative contributions of endogenous and exogenous substrates to the acetyl-CoA entering the tricarboxylic acid cycle differed significantly between infected and control groups. The mole fraction of acetyl-CoA that was derived from endogenous, unlabelled sources (F(U)) was 0.50 +/- 0.10 in controls compared to 0.34 +/- 0.04 in infected animals. However, the fraction of acetyl-CoA derived from [3-13C]lactate (FLL) was larger in livers of infected animals than those from controls with values of 0.27 +/- 0.04 and 0.18 +/- 0.04, respectively. Similarly, the fraction of acetyl-CoA derived from [1,2-13C2]acetate (FLA) was larger in livers of infected animals compared to those in controls; the fractions were 0.38 +/- 0.01 and 0.32 +/- 0.07, respectively. The ratio of FLA:FLL was significantly smaller in the infected group with a value of 1.42 +/- 0.18 compared to 1.74 +/- 0.09 for the controls. These results indicate that alveolar hydatid disease has a pronounced effect on the partitioning of substrates within the pathways of carbohydrate metabolism in the host liver.     

Evaluation of carbon flux and substrate selection through alternate pathways involving the citric acid cycle of the heart by 13C NMR spectroscopy

A previous 13C NMR technique (Malloy, C. R., Sherry, A.D., and Jeffrey, F.M.H. (1987) FEBS Lett. 212, 58-62) for measuring the relative flux of molecules through the oxidative versus anaplerotic pathways involving the citric acid cycle of the rat heart has been extended to include a complete analysis of the entire glutamate 13C spectrum. Although still simple in practice, this more sophisticated model allows an evaluation of 13C fractional enrichment of molecules entering both the oxidative and anaplerotic pathways under steady-state conditions. The method was used to analyze 13C NMR spectra of intact hearts or their acid extracts during utilization of 13C-enriched pyruvate, propionate, acetate, or various combinations thereof. [2-13C]Pyruvate was used to prove that steady-state flux of pyruvate through pyruvate carboxylase is significant during co-perfusion of pyruvate and acetate, and we demonstrate for the first time that a nine-line 13C multiplet may be detected in an intact, beating heart. Acetate or pyruvate alone provided about 86% of the acetyl-CoA; in combination, about 65% of the acetyl-CoA was derived from acetate, about 30% was derived from pyruvate, and the remainder from endogenous sources. Propionate reduced the contribution of exogenous acetate to acetyl-CoA to 77% and also reduced the oxidation of endogenous substrates. Equations are presented which allow this same analysis on multiply labeled substrates, making this technique extremely powerful for the evaluation of substrate selection and relative metabolic flux through anaplerotic and oxidative pathways in the intact heart.     

Metabolism of alpha-D-[1,2-13C]glucose pentaacetate and alpha-D-glucose penta[2-13C]acetate in rat hepatocytes

Hepatocytes from fed rats were incubated for 120 min in the presence of alpha-D-[1,2-13C]glucose pentaacetate (1.7 mM), both D-[1,2-13C]glucose (1.7 mM) and acetate (8.5 mM), alpha-D-glucose penta[2-13C]acetate (1.7 mM), or D-[1,2-13C]glucose (8.3 mM). The amounts of 13C-enriched L-lactate and D-glucose and those of acetate and beta-hydroxybutyrate recovered in the incubation medium were comparable under the first two experimental conditions. The vast majority of D-glucose isotopomers consisted of alpha- and beta-D[1,2-13C]glucose. The less abundant single-labeled isotopomers of D-glucose were equally labeled on each C atom. The output of 13C-labeled L-lactate, mainly L-[2-13C]lactate and L-[3-13C]lactate, was 1 order of magnitude lower than that found in hepatocytes exposed to 8.3 mM D-[1,2-13C]glucose, in which case the total production of the single-labeled species of D-glucose was also increased and that of the C3- or C4-labeled hexose was lower than that of the other 13C-labeled isotopomers. In cells exposed to alpha-D-glucose penta[2-13C]acetate, the large majority of 13C atoms was recovered as [2-13C]acetate and, to a much lesser extent, beta-hydroxybutyrate labeled in position 2 and/or 4. Nevertheless, L-[2-13C]lactate, L-[3-13C]lactate, and single-labeled D-glucose isotopomers were also produced in amounts higher or comparable to those found in cells exposed to alpha-D-[1,2-13C]glucose pentaacetate. However, a modest preferential labelling of the C6-C5-C4 moiety of D-glucose, relative to its C1-C2-C3 moiety, and a lesser isotopic enrichment of the C3 (or C4), relative to that of C1 (or C6) and C2 (or C5), were now observed. These findings indicate that, despite extensive hydrolysis of alpha-D-glucose pentaacetate (1.7 mM) in the hepatocytes, the catabolism of its D-glucose moiety is not more efficient than that of unesterified D-glucose, tested at the same molar concentration (1.7 mM) in the presence of the same molar concentration of unesterified acetate (8.5 mM), and much lower than that found at a physiological concentration of the hexose (8.3 mM). The present results also argue against any significant back-and-forth interconversion of D-glucose 6-phosphate and triose phosphates, under conditions in which sizeable amounts of D-glucose are formed de novo from 13C-enriched Krebs cycle intermediates generated from either D-[1,2-13C]glucose or [2-13C]acetate.     

Neuronal-glial interactions in rats fed a ketogenic diet

Glucose is the preferred energy substrate for the adult brain. However, during periods of fasting and consumption of a high fat, low carbohydrate (ketogenic) diet, ketone bodies become major brain fuels. The present study was conducted to investigate how the ketogenic diet influences neuronal-glial interactions in amino acid neurotransmitter metabolism. Rats were kept on a standard or ketogenic diet. After 21 days all animals received an injection of [1-(13)C]glucose plus [1,2-(13)C]acetate, the preferential substrates of neurons and astrocytes, respectively. Extracts from cerebral cortex and plasma were analyzed by (13)C and (1)H nuclear magnetic resonance spectroscopy and HPLC. Increased amounts of valine, leucine and isoleucine and a decreased amount of glutamate were found in the brains of rats receiving the ketogenic diet. Glycolysis was decreased in ketotic rats compared with controls, evidenced by the reduced amounts of [3-(13)C]alanine and [3-(13)C]lactate. Additionally, neuronal oxidative metabolism of [1-(13)C]glucose was decreased in ketotic rats compared with controls, since amounts of [4-(13)C]glutamate and [4-(13)C]glutamine were lower than those of controls. Although the amount of glutamate from [1-(13)C]glucose was decreased, this was not the case for GABA, indicating that relatively more [4-(13)C]glutamate is converted to GABA. Astrocytic metabolism was increased in response to ketosis, shown by increased amounts of [4,5-(13)C]glutamine, [4,5-(13)C]glutamate, [1,2-(13)C]GABA and [3,4-(13)C]-/[1,2-(13)C]aspartate derived from [1,2-(13)C]acetate. The pyruvate carboxylation over dehydrogenation ratio for glutamine was increased in the ketotic animals compared to controls, giving further indication of increased astrocytic metabolism. Interestingly, pyruvate recycling was higher in glutamine than in glutamate in both groups of animals. An increase in this pathway was detected in glutamate in response to ketosis. The decreased glycolysis and oxidative metabolism of glucose as well as the increased astrocytic metabolism, may reflect adaptation of the brain to ketone bodies as major source of fuel.     

Contribution of exogenous substrates to acetyl coenzyme A: measurement by 13C NMR under non-steady-state conditions

A method is presented for the rapid determination of substrate selection in a manner that is not restricted to conditions of metabolic and isotopic steady state. Competition between several substrates can be assessed directly and continuously in a single experiment, allowing the effect of interventions to be studied. It is shown that a single proton-decoupled 13C NMR spectrum of glutamate provides a direct measure of the contribution of exogenous 13C-labeled substrates to acetyl-CoA without measurement of oxygen consumption and that steady-state conditions need not apply. Two sets of experiments were performed: one in which a metabolic steady state but a non-steady-state 13C distribution was achieved and another in which both metabolism and labeling were not at steady state. In the first group, isolated rat hearts were supplied with [1,2-13C]acetate, [3-13C]lactate, and unlabeled glucose. 13C NMR spectra of extracts from hearts perfused under identical conditions for 5 or 30 min were compared. In spite of significant differences in the spectra, the measured contributions of acetate, lactate, and unlabeled sources to acetyl-CoA were the same. In the second set of experiments, the same group of labeled substrates was used in a regional ischemia model in isolated rabbit hearts to show regional differences in substrate utilization under both metabolic and isotopic non steady state. This sensitive probe of substrate selection was also demonstrated in intact hearts where excellent time resolution (3 min) of substrate selection was feasible.(ABSTRACT TRUNCATED AT 250 WORDS)     

Astrocyte metabolism is disturbed in the early development of experimental hydrocephalus

The proper diagnosis of the arrested or the progressive form of hydrocephalus has a critical impact on treatment, but remains difficult. The assessment of early changes in cerebral metabolism might help in the development of adequate non-invasive diagnostic tools. This study examined the alterations in label incorporation in neurotransmitter amino acids and other compounds in kaolin-induced progressive hydrocephalus in rats by means of magnetic resonance spectroscopy (MRS) combined with the administration of [1-13C]glucose and [1,2-13C]acetate. Some 2, 4 and 6 weeks after kaolin injection into the cisterna magna, cerebrum, brainstem and cerebellum were dissected. Interestingly, labelling of most amino acids derived from [1-13C]glucose showed no alterations, whereas labelling from [1,2-13C]acetate was affected. Two weeks after induction of hydrocephalus the taurine concentration was decreased, whereas the concentration of [1,2-13C]lactate was increased in the cerebrum and that of [1,2-13C]GABA in the brainstem. Furthermore, labelling from [1,2-13C]acetate was significantly decreased in [4,5-13C]glutamate, [1,2-13C]glutamate and [1,2-13C]GABA in cerebrum from 4 weeks after hydrocephalus induction. The concentration of N-acetylaspartate, a neuronal marker, was unchanged. However, labelling of the acetyl group from [1-13C]glucose was decreased in cerebellum and brainstem at 6 weeks after the induction of hydrocephalus. As glucose is metabolized predominately by neurones, whereas acetate is exclusively taken up by astrocytes, these results indicate that mostly astrocytic, and only later neuronal, metabolism is disturbed in the kaolin model of hydrocephalus. If verified in patients using in vivo MRS, impaired astrocyte metabolism might serve as an early indication for operative treatment.     

Complex Glutamate Labeling from [U-13C]glucose or [U-13C]lactate in Co-cultures of Cerebellar Neurons and Astrocytes

Glutamate metabolism was studied in co-cultures of mouse cerebellar neurons (predominantly glutamatergic) and astrocytes. One set of cultures was superfused (90 min) in the presence of either [U-¹³C]glucose (2.5 mM) and lactate (1 mM) or [U-¹³C]lactate (1 mM) and glucose (2.5 mM). Other sets of cultures were incubated in medium containing [U-¹³C]lactate (1 mM) and glucose (2.5 mM) for 4 h. Regardless of the experimental conditions cell extracts were analyzed using mass spectrometry and nuclear magnetic resonance spectroscopy. ¹³C labeling of glutamate was much higher than that of glutamine under all experimental conditions indicating that acetyl-CoA from both lactate and glucose was preferentially metabolized in the neurons. Aspartate labeling was similar to that of glutamate, especially when [U-¹³C]glucose was the substrate. Labeling of glutamate, aspartate and glutamine was lower in the cells incubated with [U-¹³C]lactate. The first part of the pyruvate recycling pathway, pyruvate formation, was detected in singlet and doublet labeling of alanine under all experimental conditions. However, full recycling, detectable in singlet labeling of glutamate in the C-4 position was only quantifiable in the superfused cells both from [U-¹³C]glucose and [U-¹³C]lactate. Lactate and alanine were mostly uniformly labeled and labeling of alanine was the same regardless of the labeled substrate present and higher than that of lactate when superfused in the presence of [U-¹³C]glucose. These results show that metabolism of pyruvate, the precursor for lactate, alanine and acetyl-CoA is highly compartmentalized. Special issue dedicated to John P. Blass.     

13C NMR studies of glycogen turnover in the perfused rat liver

To assess whether hepatic glycogen is actively turning over under conditions which promote net glycogen synthesis we perfused livers from 24-h fasted rats with 20 mM D-[1-13C]glucose, 10 mM L-[3-13C]alanine, 10 mM L-[3-13C]lactate, and 1 microM insulin for 90 min followed by a 75-min "chase" period with perfusate of the same composition containing either 13C-enriched or unlabeled substrates. The peak height of the C-1 resonance of the glucosyl subunits in glycogen was monitored, in real time, using 13C NMR techniques. During the initial 90 min the peak height of the C-1 resonance of glycogen increased at almost a constant rate reflecting a near linear increase in net glycogen synthesis, which persisted for a further 75 min if 13C-enriched substrates were present during the "chase" period. However, when the perfusate was switched to the unenriched substrates, the peak height of the C-1 resonance of glycogen declined in a nearly linear manner reflecting active glycogenolysis during a time of net glycogen synthesis. By comparing the slopes of the curve describing the time course of the net [1-13C] glucose incorporation into glycogen with the rate of net loss of 13C label from the C-1 resonance of glycogen during the "chase" period we estimated the relative rate of glycogen breakdown to be 60% of the net glycogen synthetic rate. Whether this same phenomenon occurs to such an appreciable extent in vivo remains to be determined.     

Cerebral metabolism of [1,2-13C2]acetate as detected by in vivo and in vitro 13C NMR

The metabolism of [1,2-13C2]acetate in rat brain was studied by in vivo and in vitro 13C NMR spectroscopy, in particular by taking advantage of the homonuclear 13C-13C spin coupling patterns. Well nourished rats were infused with [1,2-13C2]acetate or [1-13C]acetate in the jugular vein, and the in situ kinetics of 13C labeling during the infusion period was followed by 13C NMR techniques. The in vivo 13C NMR spectra showed signals from (i) the C-1 carbon of [1,2-13C2] acetate or [1-13C]acetate, (ii) 13CO3H-, and (iii) the natural abundance 13C carbons of sufficiently mobile fatty acids. Methanol/HCl/perchloric acid extracts of the brains were prepared and were further analyzed by high resolution 13C NMR. The homonuclear 13C-13C spin coupling patterns after infusion of [1,2-13C2]acetate showed very different isotopomer populations in glutamate, glutamine, and gamma-aminobutyric acid. Analyzing the relative proportions of these isotopomers revealed (i) two different glutamate compartments in the rat brain characterized by the presence and absence, respectively, of glutamine synthase activity, (ii) two different tricarboxylic acid cycles, one preferentially metabolizing [(1,2-13C2]acetate, the other mainly using unlabeled acetyl-coenzyme A, (iii) a hitherto unknown cerebral pyruvate recycling system associated with the tricarboxylic acid cycle, metabolizing primarily unlabeled acetyl-coenzyme A, and (iv) a predominant production of gamma-aminobutyric acid in the glutamate compartment lacking glutamine synthase.     

Use of 13C in biosynthetic studies. The labelling pattern in tenellin enriched from isotope-labelled acetate, methionine, and phenylalanine

The biogenetic origin of the carbon atoms in tenellin has been established by adding 13C-enriched compounds to cultures of Beauveria bassiana, and determining the isotopic distribution in the metabolite by 13C nuclear magnetic resonance spectrometry. Tenellin is formed by condensation of an acetate-derived polyketide chain with a phenylpropanoid unit that may be phenylalanine. Alternate carbon atoms of the polyketide chain were labelled with sodium [1(-13C)]- and [2-(13C]-acetate; sodium [1,2-(13C)]acetate was incorporated as intact two-carbon units, the presence of which in tenellin was apparent from coupling between adjacent 13C-enriched carbons. Substituent methyl groups of the polyketide-derived alkenyl chain were labelled with L-[Me-13C]methionine. The labelling patterns from DL-[carboxy-13C]phenylalanine and DL-[alpha-13C]phenylalanine indicated a rearrangement of the propanoid component at some stage in the synthesis. The mass spectrum of tenellin from cultures administered L-[15N]phenylalanine showed isotopic enrichment similar to that obtained with 13C- or 14C-labelled phenylalanine. During incorporation of L-[carboxy-14C, beta-3H]phenylalanine 96% of the tritium label was lost, discounting the possibility of a 1,2-hydride shift during biosynthesis of the metabolite.     

In vivo effects of adenosine A(2) receptor agonist and antagonist on neuronal and astrocytic intermediary metabolism studied with ex vivo (13)C MR spectroscopy.

The effect of adenosine A(2) receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-ethylcarboxamidoadenosine (CGS 21680) and antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) on [1-(13)C]glucose and [1,2-(13)C]acetate metabolism was studied in rats by (13)C magnetic resonance (MR) spectroscopy and HPLC. In the cortex a significant reduction was observed in the amounts of [2-(13)C]GABA and [3-(13)C]aspartate from [1-(13)C]glucose in CGS 21680. In the subcortex the concentration of labelled [4-(13)C]glutamate was increased in both treatment groups. The amounts of [2 + 3-(13)C]succinate and [3-(13)C]lactate were increased in the CGS 21680 group compared to control, and the DMPX group showed an increase in the total amount of [6-(13)C]N-acetyl aspartate compared to control in the subcortex. Astrocyte metabolism was only affected in the cortex as shown by a decrease in the pyruvate carboxylase/pyruvate dehydrogenase ratio in glutamate and glutamine in the treatment groups. Labelling from [1,2-(13)C]acetate was not much affected by CGS 21680 or DMPX. However, the amount of [1,2-(13)C]acetate in cortex and subcortex was reduced in the DMPX group. In the cortex a reduction in the labelling of [3-(13)C]GABA in the DMPX group compared to control and an increase in the total amount of taurine in both treatment groups was detected. The present study shows that A(2) receptor agonist and antagonist have similar effects; however, in cortex GABAergic neurones and astrocytes were affected in contrast to subcortex, where glutamatergic neurones showed the greatest changes.     

A multinuclear magnetic resonance study of a cls11 mutant showing the Pet- phenotype of Saccharomyces cerevisiae

Energetic and intermediary metabolism was studied in a Pet- mutant of Saccharomyces cerevisiae with a calcium-sensitive phenotype that shows an inability to grow when cultured in a medium containing non-fermentable substrates. The perchloric acid extracts were prepared from suspensions of cls11 mutant and wild-type cells incubated with [1,3-13C]glycerol or [2-13]acetate, and analyzed by 31P, 13C and 1H NMR. 31P- and 1H-NMR spectra showed significant differences between cls11 and wild-type cells at the level of amino acids, the storage carbohydrate trehalose (higher in mutant cells), and sugar phosphates (higher in wild-type cells). 13C-NMR spectra revealed major differences in the steady-state labelling of glutamate carbons. For incubations with [1,3-13C]glycerol, we estimated from the relative 13C enrichment of glutamate carbons that acetyl-CoA C2 is 43% C13 labelled in wild-type and 10% 13C labelled in mutant cells, respectively. For incubations with [2-13C]acetate, we calculated that the ratio of the relative flux through the glyoxylate shunt versus oxidative reactions is 58% in wild-type cells and 44% in the cls11 mutant cells. Again, a dilution of the relative enrichment of C2 of acetyl-CoA was observed in the mutant cells (89%) compared to the wild-type cells (97%). These results are discussed in terms of pleiotropic defects in non-fermentable carbon metabolism in mutant cells.     

Determination of flux through different metabolite pathways in Saccharomyces cerevisiae by 1H-NMR and 13C-NMR spectroscopy.

We propose an experimental approach combining 1H-NMR and 13C-NMR spectroscopy to investigate metabolite flux in cells under physiological conditions and present a mathematical model giving the relationships between the following different parameters. 13C fractional enrichment, fluxes in competing pathways, metabolite concentration and experimental time. This model has been used for determining the absolute and/or relative values of five fluxes involving pyruvate, ethanol, acetyl-CoA and glutamate via the Krebs cycle in glucose-grown repressed Saccharomyces cerevisiae cells fed with [1-13C]glucose and/or unlabeled ethanol. The glucose consumption and the production of various compounds such as ethanol, glycerol, trehalose etc. were studied qualitatively and/or quantitatively as a function of time. The 13C fractional enrichment of [2-13C]ethanol was determined by observing the proton resonance of the methyl group. Addition of 25 mM unlabeled ethanol shows no significant effect on the glucose consumption or the production of any metabolites. However unlabeled ethanol exerts a strong influence on the enrichment of glutamate C4, but only induces an insignificant change on glutamate C2 and C3. Apart from the fact that ethanol is a potential precursor of acetyl-CoA as expected, these results indicate that (a) the probability for citrate and 2-oxoglutarate to make one turn or more in the Krebs cycle is negligible and (b) the scrambling between C4 and C3 via the glyoxylate shunt is virtually absent. The flux of ethanol formation from pyruvate is about three-times and nine-times greater than that of ethanol consumption and acetyl-CoA formation, respectively, from pyruvate via pyruvate dehydrogenase. Without addition of unlabeled ethanol, the ratio of the integrated resonance of glutamate (C2 + C3)/C4 reflecting the activity of pyruvate carboxylase relative to that of citrate synthase, is about 1.1. By comparing the absolute values of the different fluxes, it was found that 88% of the glucose was used to synthetize ethanol but the observed concentration of ethanol in the supernatant represents only 58% of the glucose consumption. The validity of the present model was supported by the data obtained from similar experiments using unlabeled ethanol and non-NMR techniques.     

Carbon flux through citric acid cycle pathways in perfused heart by 13C NMR spectroscopy

Mathematical models of the TCA cycle derived previously for 14C tracer studies have been extended to 13C NMR to measure the 13C fractional enrichment of [2-13C]acetyl-CoA entering the cycle and the relative activities of the oxidative versus anaplerotic pathways. The analysis is based upon the steady-state enrichment of 13C into the glutamate carbons. Hearts perfused with [2-13C]acetate show low but significant activity of the anaplerotic pathways. Activation of two different anaplerotic pathways is demonstrated by addition of unlabeled propionate or pyruvate to hearts perfused with [2-13C]acetate. In each case, the amount of [2-13C]acetate being oxidized and the relative carbon flux through anaplerotic versus oxidative pathways are evaluated.     

Acetate Utilization in Lactococcus lactis Deficient in Lactate Dehydrogenase: a Rescue Pathway for Maintaining Redox Balance

Acetate was shown to improve glucose fermentation in Lactococcus lactis deficient in lactate dehydrogenase. 13C and 1H nuclear magnetic resonance studies using [2-13C]glucose and [2-(13)C]acetate as substrates demonstrated that acetate was exclusively converted to ethanol. This novel pathway provides an alternative route for NAD+ regeneration in the absence of lactate dehydrogenase.     

Metabolism of [1-(13)C)glucose and [2-(13)C]acetate in the hypoxic rat brain.

The effects of hypoxia on the metabolism of the central nervous system were investigated in rats submitted to a low oxygen atmosphere (8% O(2); 92% N(2)). [1-(13)C]glucose and [2-(13)C]acetate were used as substrates, this latter being preferentially metabolized by glial cells. After 1-h substrate infusion, the incorporation of 13C in brain metabolites was determined by NMR spectroscopy. Under hypoxia, an important hyperglycemia was noted. As a consequence, when using labeled glucose, the specific enrichment of brain glucose C1 was lower (48.2+/-5.1%) than under normoxia (66.9+/-2.5%). However, relative to this specific enrichment, the (13)C incorporation in amino acids was increased under hypoxia. This suggested primarily a decreased exchange between blood and brain lactate. The glutamate C2/C4 enrichment ratio was higher under hypoxia (0.62+/-0.01) than normoxia (0.51+/-0.06), indicating a lower glutamate turnover relative to the neuronal TCA cycle activity. The glutamine C2/C4 enrichment ratio was also higher under hypoxia (0.87+/-0.07 instead of 0.65+/-0.11), indicating a new balance in the contributions of different carbon sources at the acetyl-CoA level. When using [2-(13)C]acetate as substrate, no difference in glutamine enrichment appeared under hypoxia, whereas a significant decrease in glutamate, aspartate, alanine and lactate enrichments was noted. This indicated a lower trafficking between astrocytes and neurons and a reduced tricarboxylic acid cycle intermediate recycling of pyruvate.     

Metabolism of (1-(13)C) glucose and (2-(13)C, 2-(2)H(3)) acetate in the neuronal and glial compartments of the adult rat brain as detected by [(13)C, (2)H] NMR spectroscopy

Ex vivo ?(13)C, (2)H? NMR spectroscopy allowed to estimate the relative sizes of neuronal and glial glutamate pools and the relative contributions of (1-(13)C) glucose and (2-(13)C, 2-(2)H(3)) acetate to the neuronal and glial tricarboxylic acid cycles of the adult rat brain. Rats were infused during 60 min in the right jugular vein with solutions containing (2-(13)C, 2-(2)H(3)) acetate and (1-(13)C) glucose or (2-(13)C, 2-(2)H(3)) acetate only. At the end of the infusion the brains were frozen in situ and perchloric acid extracts were prepared and analyzed by high resolution (13)C NMR spectroscopy (90.5 MHz). The relative sizes of the neuronal and glial glutamate pools and the contributions of acetyl-CoA molecules derived from (2-(13)C, (2)H(3)) acetate or (1-(13)C) glucose entering the tricarboxylic acid cycles of both compartments, could be determined by the analysis of (2)H-(13)C multiplets and (2)H induced isotopic shifts observed in the C4 carbon resonances of glutamate and glutamine. During the infusions with (2-(13)C, 2-(2)H(3)) acetate and (1-(13)C) glucose, the glial glutamate pool contributed 9% of total cerebral glutamate being derived from (2-(13)C, 2-(2)H(3)) acetyl-CoA (4%), (2-(13)C) acetyl-CoA (3%) and recycled (2-(13)C, 2-(2)H) acetyl-CoA (2%). The neuronal glutamate pool accounted for 91% of the total cerebral glutamate being mainly originated from (2-(13)C) acetyl-CoA (86%) and (2-(13)C, 2-(2)H) acetyl-CoA (5%). During the infusions of (2-(13)C, 2-(2)H(3)) acetate only, the glial glutamate pool contributed 73% of the cerebral glutamate, being derived from (2-(13)C, 2-(2)H(3)) acetyl-CoA (36%), (2-(13)C, 2-(2)H) acetyl-CoA (27%) and (2-(13)C) acetyl-CoA (10%). The neuronal pool contributed 27% of cerebral glutamate being formed from (2-(13)C) acetyl-CoA (11%) and recycled (2-(13)C, 2-(2)H) acetyl-CoA (16%). These results illustrate the potential of ?(13)C, (2)H? NMR spectroscopy as a novel approach to investigate substrate selection and metabolic compartmentation in the adult mammalian brain.     

Incorporation and utilization of [3-13C]lactate and [1,2-13C]acetate by rat skeletal muscle.

Skeletal muscle can utilize many different substrates, and traditional methodologies allow only indirect discrimination between oxidative and nonoxidative uptake of substrate, possibly with contamination by metabolism of other internal organs. Our goal was to apply 1H- and 13C-nuclear magnetic resonance spectroscopy to monitor the patterns of [3-13C]lactate and [1,2-13C]acetate (model of simple carbohydrates and fats, respectively) utilization in resting vs. contracting muscle extracts of the isolated perfused rat hindquarter. Total metabolite concentrations were measured by using NADH-linked fluorometric assays. Fractional oxidation of [3-13C]lactate was unchanged by contraction despite vascular endogenous lactate accumulation. Although label accumulated in several citric acid cycle (CAC) intermediates, contraction did not increase the concentration of CAC intermediates in any muscle extracts. We conclude that 1) the isolated rat hindquarter is a viable, well-controlled model for measuring skeletal muscle 13C-labeled substrate utilization; 2) lactate is readily oxidized even during contractile activity; 3) entry and exit from the CAC, via oxidative and nonoxidative pathways, is a component of normal muscle metabolism and function; and 4) there are possible differences between gastrocnemius and soleus muscles in utilization of nonoxidative pathways.     

13C-NMR study of glycerol metabolism in rabbit renal cells of proximal convoluted tubules

Perchloric acid extracts of rabbit renal proximal convoluted tubular cells (PCT) incubated with [2-13C]glycerol and [1,3-13C]glycerol were investigated by 13C-NMR spectroscopy. These 13C-NMR spectra enabled us to determine cell metabolic pathways of glycerol in PCT cells. The main percentage of 13C-label, arising from 13C-enriched glycerol, was found in glucose, lactate, glutamine and glutamate. So far it can be concluded that glycerol is a suitable substrate for PCT cells and is involved in gluconeogenesis and glycolysis as well in the Krebs cycle intermediates. Label exchange and label enrichment in 13C-labelled glucose, arising from [2-13C]glycerol and [1,3-13C]glycerol, is explained by label scrambling through the pentose shunt and a label exchange in the triose phosphate pool. From relative enrichments it is estimated that the ratio of the pyruvate kinase flux to the gluconeogenetic flux is 0.97:1 and that the ratio of pyruvate carboxylase activity relative to pyruvate dehydrogenase activity is 2.0:1. Our results show that 13C-NMR spectroscopy, using 13C-labelled substrates, is a powerful tool for the examination of renal metabolism.