Dynamic changes in 13C NMR spectra of intact hearts under conditions of varied metabolite enrichment

Metabolic flux in the driver's seat during cardiac health and disease

Cardiac function is a dynamic process that must adjust efficiently to the immediate demands of physical state and activity. So too, the metabolic support of cardiac function is a dynamic process that must respond, in time, to the demands of cardiac function and viability. Flux through metabolic pathways provides chemical energy and generates signaling molecules that regulate activity among intracellular compartments to meet these demands. Thus, flux through metabolic pathways provides a dynamic mode of support of cardiomyocytes during physiological and pathophysiological challenges. Any inability of metabolic flux to keep pace with the demands of the cardiomyocyte results in progressive dysfunction that contributes to cardiac disease. Thus, the priority in maintaining and regulating flux through metabolic pathways in the cardiomyocyte cannot be understated. Great potential exists in current efforts to elucidate metabolic mechanisms as therapeutic targets for the diseased heart. As a consequence, detecting metabolic flux in the functioning myocardium of the heart, under normal and diseased conditions, is essential in elucidating the metabolic basis of contractile dysfunction. As a companion to the 2022 ISHR Research Achievement Award lecture, this review examines the use and applications of stable isotope kinetics to quantify metabolic flux through intermediary pathways and the exchange and transport of intermediates across the mitochondrial membrane and sarcolemma of intact functioning hearts in determining how these intracellular events are coordinated to support cardiac function and health. Finally, this work reviews recently demonstrated metabolic defects in diseased hearts and the potential for metabolic alleviation of heart disease.

Multinuclear Magnetic Resonance Spectroscopy at Ultra-High-Field: Assessing Human Cerebral Metabolism in Healthy and Diseased States

The brain is a highly energetic organ. Although the brain can consume metabolic substrates, such as lactate, glycogen, and ketone bodies, the energy metabolism in a healthy adult brain mainly relies on glucose provided via blood. The cerebral metabolism of glucose produces energy and a wide variety of intermediate metabolites. Since cerebral metabolic alterations have been repeatedly implicated in several brain disorders, understanding changes in metabolite levels and corresponding cell-specific neurotransmitter fluxes through different substrate utilization may highlight the underlying mechanisms that can be exploited to diagnose or treat various brain disorders. Magnetic resonance spectroscopy (MRS) is a noninvasive tool to measure tissue metabolism in vivo. ¹H-MRS is widely applied in research at clinical field strengths (≤3T) to measure mostly high abundant metabolites. In addition, X-nuclei MRS including, ¹³C, ²H, ¹⁷O, and ³¹P, are also very promising. Exploiting the higher sensitivity at ultra-high-field (>4T; UHF) strengths enables obtaining unique insights into different aspects of the substrate metabolism towards measuring cell-specific metabolic fluxes in vivo. This review provides an overview about the potential role of multinuclear MRS (¹H, ¹³C, ²H, ¹⁷O, and ³¹P) at UHF to assess the cerebral metabolism and the metabolic insights obtained by applying these techniques in both healthy and diseased states.

A model for determining cardiac mitochondrial substrate utilisation using stable 13C-labelled metabolites

INTRODUCTION

Relative oxidation of different metabolic substrates in the heart varies both physiologically and pathologically, in order to meet metabolic demands under different circumstances. ¹³C labelled substrates have become a key tool for studying substrate use-yet an accurate model is required to analyse the complex data produced as these substrates become incorporated into the Krebs cycle.

OBJECTIVES

We aimed to generate a network model for the quantitative analysis of Krebs cycle intermediate isotopologue distributions measured by mass spectrometry, to determine the ¹³C labelled proportion of acetyl-CoA entering the Krebs cycle.

METHODS

A model was generated, and validated ex vivo using isotopic distributions measured from isolated hearts perfused with buffer containing 11 mM glucose in total, with varying fractions of universally labelled with ¹³C. The model was then employed to determine the relative oxidation of glucose and triacylglycerol by hearts perfused with 11 mM glucose and 0.4 mM equivalent Intralipid (a triacylglycerol mixture).

RESULTS

The contribution of glucose to Krebs cycle oxidation was measured to be 79.1 ± 0.9%, independent of the fraction of buffer glucose which was U-¹³C labelled, or of which Krebs cycle intermediate was assessed. In the presence of Intralipid, glucose and triglyceride were determined to contribute 58 ± 3.6% and 35.6 ± 0.8% of acetyl-CoA entering the Krebs cycle, respectively.

CONCLUSION

These results demonstrate the accuracy of a functional model of Krebs cycle metabolism, which can allow quantitative determination of the effects of therapeutics and pathology on cardiac substrate metabolism.

Examination of primary metabolic pathways in a murine hybridoma with carbon-13 nuclear magnetic resonance spectroscopy

Primary metabolism of a murine hybridoma was probed with (13)C nuclear magnetic resonance (NMR) spectroscopy. Cells cultured in a hollow fiber bioreactor were serially infused with [1-(13)C] glucose, [2-(13)C] glucose, and [3-(13)C] glutamine. In vivo spectroscopy of the culture was used in conjunction with off-line spectroscopy of the medium to determine the intracellular concentration of several metabolic intermediates and to determine fluxes for primary metabolic pathways. Intracellular concentrations of pyruvate and alanine were very high relative to levels observed in normal quiescent mammalian cells. Estimates made from labeling patterns in lactate indicate that 76% of pyruvate is derived directly from glycolysis; some is also derived from the malate shunt, the pyruvate/melate shuttle associated with lipid synthesis and the pentose phosphate pathway. The rate of formation of pyruvate from the pentose phosphate pathway was estimated to be 4% of that from glycolysis; This value is a lower limit and the actual value may be higher. Incorporation of pyruvate into the tricarboxylic acid (TCA) cycle appears to occur through only pyruvate dehydrogenase; no pyruvate carboxylase activity was detected. The malate shunt rate was approximately equal to the rate of glutamine uptake. The rate of incorporation of glucosederived acetyl-CoA into lipids was 4% of the glucose uptake rate. The TCA cycle rate between isocitrate and alpha-ketoglutarate was 110% of the glutamine uptake rate.

[1-13C]glucose MRS in chronic hepatic encephalopathy in man

[1-13C]-labeled glucose was infused intravenously in a single dose of 0.2 g/kg body weight over 15 min in six patients with chronic hepatic encephalopathy, and three controls. Serial 13C MR spectra of the brain were acquired. Patients exhibited the following characteristics relative to normal controls: 1) Cerebral glutamine concentration was increased (12.6 +/- 3.8 vs. 6.5 +/- 1.9 mmol/kg, P < 0.006) and glutamate was reduced (8.2 +/- 1.0 vs. 9.9 +/- 0.6 mmol/kg, P < 0.02). 2) 13C incorporation into glutamate C4 and C2 positions was reduced in patients (80 min after start of infusion C4: 0.43 +/- 0.09 vs. 0.84 +/- 0.15 mmol/kg, P < 0.001; C2: 0.20 +/- 0.03 vs. 0.45 +/- 0.07 mmol/kg, P < 0.0001). 3) 13C incorporation into bicarbonate was delayed (90 +/- 21 vs. 40 +/- 10 min, P < 0.003), and the time interval between detection of glutamate C4 and C2 labeling was longer in patients (22 +/- 8 vs. 12 +/- 3 min, P < 0.03). 4) Glutamate C2 turnover time was reduced in chronic hepatic encephalopathy (17.1 +/- 6.8 vs. 49.6 +/- 8.7 min, P < 0.0002). 5) 13C accumulation into glutamine C2 relative to its substrate glutamate C2 increased progressively with the severity of clinical symptoms (r = 0.96, P < 0.01). These data indicate disturbed neurotransmitter glutamate/glutamine cycling and reduced glucose oxidation in chronic hepatic encephalopathy. [1-13C] glucose MRS provides novel insights into disease progression and the pathophysiology of chronic hepatic encephalopathy.

Use of a single (13)C NMR resonance of glutamate for measuring oxygen consumption in tissue

A kinetic model of the citric acid cycle for calculating oxygen consumption from (13)C nuclear magnetic resonance (NMR) multiplet data has been developed. Measured oxygen consumption (MVO(2)) was compared with MVO(2) predicted by the model with (13)C NMR data obtained from rat hearts perfused with glucose and either [2-(13)C]acetate or [3-(13)C]pyruvate. The accuracy of MVO(2) measured from three subsets of NMR data was compared: glutamate C-4 and C-3 resonance areas; the doublet C4D34 (expressed as a fraction of C-4 area); and C-4 and C-3 areas plus several multiplets of C-2, C-3, and C-4. MVO(2) determined by set 2 (C4D34 only) gave the same degree of accuracy as set 3 (complete data); both were superior to set 1 (C-4 and C-3 areas). Analysis of the latter suffers from the correlation between citric acid cycle flux and exchange between alpha-ketoglutarate and glutamate, resulting in greater error in estimating MVO(2). Analysis of C4D34 is less influenced by correlation between parameters, and this single measurement provides the best opportunity for a noninvasive measurement of oxygen consumption.

Cytosolic redox state mediates postischemic response to pyruvate dehydrogenase stimulation

Augmented pyruvate oxidation via pharmacological stimulation of pyruvate dehydrogenase (PDH) during reperfusion has been related to improved recovery of postischemic hearts independent of glycolytic activity. This study examined recovery of postischemic rabbit hearts during activation of PDH with dichloroacetate (DCA) in the presence of lactate, as a source of pyruvate, to determine the response to substrate-dependent changes in cytosolic redox state. After 10 min of ischemia, isolated hearts were reperfused with either 2.5 mM or 0. 5 mM pyruvate (Pyr) or 2.5 mM lactate (Lac), with or without 5 mM DCA. (13)C-enriched substrates allowed NMR assessment of metabolic perturbations. During normal perfusion, Pyr and Lac supported similar mechanical work. Increasing Pyr oxidation restored postischemic rate-pressure product to 82 +/- 4 and 88 +/- 6% of preischemic values during reperfusion with 2.5 and 0.5 mM Pyr, respectively, vs. 61 +/- 6 and 45 +/- 14% for untreated 2.5 and 0.5 mM Pyr, respectively (P < 0.05). In contrast, increasing Lac oxidation did not benefit recovery of RPP in untreated (44 +/- 7%) vs. DCA-treated 36 +/- 4% hearts. Thus the benefit of PDH activation for contractile recovery of postischemic hearts is mediated by the source of pyruvate, which also influences cytosolic redox state.

Effects of aminooxyacetate on glutamate compartmentation and TCA cycle kinetics in rat hearts

The nonspecific transaminase inhibitor aminooxyacetate (AOA) has multiple influences on the dynamics of 13C appearance in glutamate in rat hearts as measured by 13C nuclear magnetic resonance (NMR) without altering O2 consumption or tricarboxylic acid (TCA) cycle flux. These include the following: 1) a reduced rate of 13C enrichment at glutamate C3 and C4; 2) a near coalescence of the C3 and C4 fractional enrichment curves; 3) a dramatic alteration in the time-dependent evolution of the glutamate C4 multiplets, C4S and C4D34; and 4) a decrease in the NMR visibility of glutamate. A fit of the 13C fractional enrichment curves of glutamate C4 and C3 in the absence of inhibitor to a kinetic model of the TCA cycle gave values for transaminase flux of 7.5 mumol.min-1.g dry wt-1 and TCA cycle flux of 7.5 mumol.min-1.g dry wt-1, thereby confirming reports by others that the kinetics of 13C enrichment of glutamate C3 and C4 in heart tissue is significantly affected by flux through reactions other than TCA cycle. The 13C fractional enrichment data collected in the presence of 0.5 mM AOA could not be fitted using this same kinetic model. However, kinetic simulations demonstrated that the time-dependent changes in C4S and C4D34 are only consistent with a 10-fold reduction in the size of intermediate pools undergoing rapid turnover in the TCA cycle. We conclude that inhibition of glutamic-oxalacetic transaminase by AOA effectively reduces the size of the alpha-ketoglutarate pool in rapid exchange with the TCA cycle. Our data indicate that changes in glutamate multiplet areas in the 13C NMR spectra of heart (as demonstrated by glutamate C4S and C4D34) are more sensitive to alterations in metabolic pool sizes in exchange with the TCA cycle than are measurements of 13C fractional enrichment at glutamate C3 and C4.

Dehydrogenase regulation of metabolite oxidation and efflux from mitochondria in intact hearts

To test how alpha-ketoglutarate dehydrogenase (alpha-KGDH) activity influences the balance between oxidative flux and transmitochondrial metabolite exchange, we monitored these rates in isolated mitochondria and in perfused rabbit hearts at an altered kinetics (Km) of alpha-KGDH for alpha-ketoglutarate (alpha-KG). In isolated mitochondria, relative Km dropped from 0.23 mM at pH = 7.2 to 0.10 mM at pH 6.8 (P < 0.05), and alpha-KG efflux decreased from 126 to 95 nmol.min-1.mg-1. In intact hearts, Km was reduced with low intracellular pH, while matching control workload and respiratory rate with increased Ca2+ (pHi = 7.20, perfusate CaCl2 = 1.5 mM; pHi = 6.89, perfusate CaCl2 = 3 +/- 1 mM). Sequential 13C nuclear magnetic resonance spectra from hearts oxidizing [2-13C]acetate provided tricarboxylic acid cycle flux and the exchange rate between alpha-KG and cytosolic glutamate (F1). Tricarboxylic acid cycle flux was 10 mumol.min-1.g-1 in both groups, but F1 fell from a control of 9.3 +/- 0.6 to 2.8 +/- 0.4 mumol.min-1.g-1 at low Km. The results indicate that increased activity of alpha-KGDH occurs at the expense of alpha-KG efflux during support of normal workloads.

Dynamic 13C NMR analysis of pyruvate and lactate oxidation in the in vivo canine myocardium: evidence of reduced utilization with increased work

In this work, substrate selection was monitored in the left ventricle of the canine myocardium by following pyruvate and lactate oxidation under in vivo conditions at basal and elevated workloads. These studies were conducted in the open chest model using dynamic 13C NMR techniques in the presence and absence of dichloroacetic acid (DCA), a well-known activator of pyruvate dehydrogenase (PDH). Following the infusion of (3-(13)C) pyruvate or (3-(13)C) lactate into the left anterior descending artery, highly variable 13C enrichments of glutamate, alanine, aspartate, and citrate were noted under low (RPP < 14,500 mmHg/min), intermediate (RPP = 15,000-25,000 mmHg/min), and high (RPP > 25,500 mmHg/min) rate pressure products (RPP). At low workloads, the myocardium typically oxidized the infused (3-(13)C) pyruvate or (3-(13)C) lactate and incorporated the labeled carbon into the glutamate pool as expected. However, in a few notable instances (n = 3), 13C-enriched pyruvate and lactate were unable to label the glutamate pool under in vivo conditions even at the lowest RPPs, indicating a lack of selection for these substrates by the tricarboxylic acid (TCA) cycle. Nonetheless, the levels of glutamate C4 enrichment observed at low workloads could usually be enhanced by infusion of DCA. Importantly, 13C NMR extract analysis revealed that (3-(13)C) pyruvate or (3-(13)C) lactate labeling of the glutamate pool was reduced (< 20%) at high workloads in spite of increased DCA concentrations.

Altered metabolite exchange between subcellular compartments in intact postischemic rabbit hearts

To examine metabolic regulation in postischemic hearts, we examined oxidative recycling of 13C within the glutamate pool (GLU) of intact rabbit hearts. Isolated hearts oxidized 2.5 mmol/L [2-13C]acetate during normal conditions (n = 6) or during reperfusion after 10 minutes of ischemia (n = 5). 13C-Nuclear magnetic resonance spectra were acquired every 1 minute. Kinetic analysis of 13C incorporation into GLU provided both tricarboxylic acid (TCA) cycle flux and the interconversion rate (F1) between the TCA cycle intermediate, alpha-ketoglutarate (alpha-KG), and the largely cytosolic GLU. The rate-pressure product in postischemic hearts was 46% of normal (P < .05). No difference in substrate utilization occurred between groups, with acetate accounting for 92% of the carbon units entering the TCA cycle at the citrate synthase step. TCA cycle flux in postischemic hearts was normal (normal hearts, 10.7 mumol.min-1.g-1; postischemic hearts, 9.4 mumol.min-1.g-1), whereas F1 was 72% lower at 2.9 +/- 0.4 versus 10.2 +/- 2.5 mumol.min-1.g-1 (mean +/- SE) in normal hearts (P < .05). From additional hearts perfused with 2.5 mmol/L [2-13C]acetate plus supplemental 5 mmol/L glucose, any potential differences in endogenous carbohydrate availability were proved not to account for the reduced rate alpha-KG and GLU exchange, which remained depressed in postischemic hearts. However, specific activities of the transaminase enzyme, catalyzing chemical exchange of alpha-KG and GLU, were the same, and transaminase flux was 100 mumol.min-1.g-1 in postischemic hearts versus 68 mumol.min-1.g-1 in normal hearts. Normal transaminase activity and the increased flux in postischemic hearts are contrary to the reduced F1. The findings indicate reduced metabolite transport rates across the mitochondrial membranes of stunned myocardium, particularly through the reversible alpha-KG-malate carrier.

Detection of 13C-labeled metabolites in the in vivo canine heart by B1 insensitive heteronuclear coherent polarization transfer and comparison of signal enhancement with NOE

A recently developed adiabatic coherent polarization transfer enhancement technique [H. Merkle, H. Wei, M. Garwood, K. Uğurbil. J. Magn. Reson, 99, 480-494 (1992)] was employed to perform 13C spectroscopy in the intact canine heart in vivo during [2-13C]-acetate infusion into the left descending coronary artery, the results were compared with 13C spectra obtained with conventionally employed nuclear Overhauser enhancement. The results demonstrate that both methods can be performed by using surface coils to obtain in vivo 13C spectra and that coherent polarization transfer provides better enhancement than NOE for [2-13C]-acetate but not for short T2 compounds.

Oxidation of acetate in rabbit skeletal muscle: detection by 13C NMR spectroscopy in vivo

The results of a proton-decoupled and Overhauser-enhanced 13C NMR study of acetate metabolism in skeletal muscle are reported. [2-13C]Acetate was infused intravenously over 2 h into anesthetized rabbits, and skeletal muscle in the lateral thigh was monitored by 13C NMR spectroscopy at 4.7 T. Stable 13C enrichment in carbons 2, 3, and 4 of glutamate was observed at the end of the infusion, and the half-time for enrichment was 17 min for glutamate C4 and 50 min for glutamate C2 and C3. The contribution of exogenous acetate to acetylcoenzyme A was nearly equal in skeletal muscle and heart in vivo (83-87%, measured in tissue extracts), comparable with earlier perfused heart studies in which acetate was the sole available substrate. Although relative flux through the combined anaplerotic pathways (relative to citric acid cycle flux) was higher in quiescent skeletal muscle (28%) compared with hearts (3%) from the same animals, actual anaplerotic flux was estimated to be substantially higher in heart than in skeletal muscle after correcting for differences in citric acid cycle flux in the two tissues.

Multiplet structure of 13C NMR signal from glutamate and direct detection of tricarboxylic acid (TCA) cycle intermediates

For the first time, 13C NMR signals are shown from 13C-enriched, low-level tricarboxylic acid (TCA) cycle intermediates from extracts of normal cardiac tissue. As the low tissue content of the key intermediates alpha-ketoglutarate (alpha-KG) and succinate (SUC) in normal, well perfused tissues has until now precluded direct NMR detection from intact tissues and tissue extracts, 13C NMR signal from glutamate has generally been used to infer the isotopomer patterns of intermediates that are in chemical exchange with glutamate. However, the required assumptions regarding intracellular compartmentation for such indirect analysis have not been previously tested, as glutamate is largely cytosolic while the TCA cycle enzymes are located in the mitochondria. Chromatographic isolation of alpha-KG and SUC from heart tissue extracts allowed isotopomer analysis to be performed for comparison with that of glutamate. At steady state, a direct relationship between glutamate and alpha-ketoglutarate isotopomers was found, but succinate isotopomers matched those of glutamate only in hearts that displayed negligible contributions from the oxidation of unlabeled endogenous carbon sources.

Kinetic analysis of dynamic 13C NMR spectra: metabolic flux, regulation, and compartmentation in hearts

Control of oxidative metabolism was studied using 13C NMR spectroscopy to detect rate-limiting steps in 13C labeling of glutamate. 13C NMR spectra were acquired every 1 or 2 min from isolated rabbit hearts perfused with either 2.5 mM [2-13C]acetate or 2.5 mM [2-13C]butyrate with or without KCl arrest. Tricarboxylic acid cycle flux (VTCA) and the exchange rate between alpha-ketoglutarate and glutamate (F1) were determined by least-square fitting of a kinetic model to NMR data. Rates were compared to measured kinetics of the cardiac glutamate-oxaloacetate transaminase (GOT). Despite similar oxygen use, hearts oxidizing butyrate instead of acetate showed delayed incorporation of 13C label into glutamate and lower VTCA, because of the influence of beta-oxidation: butyrate = 7.1 +/- 0.2 mumol/min/g dry wt; acetate = 10.1 +/- 0.2; butyrate + KCl = 1.8 +/- 0.1; acetate + KCl = 3.1 +/- 0.1 (mean +/- SD). F1 ranged from a low of 4.4 +/- 1.0 mumol/min/g (butyrate + KCl) to 9.3 +/- 0.6 (acetate), at least 20-fold slower than GOT flux, and proved to be rate limiting for isotope turnover in the glutamate pool. Therefore, dynamic 13C NMR observations were sensitive not only to TCA cycle flux but also to the interconversion between TCA cycle intermediates and glutamate.

Pyruvate dehydrogenase influences postischemic heart function

BACKGROUND

The pyruvate dehydrogenase (PDH) enzyme complex determines the extent of carbohydrate oxidation in the myocardium. PDH is in a largely inactive state during early reperfusion of postischemic myocardium. The resultant decrease in pyruvate oxidation in postischemic hearts has been documented with 13C nuclear magnetic resonance (NMR) spectroscopy. This study demonstrates that counteracting depressed pyruvate oxidation can enhance contractile recovery in the absence of increases in either glycolytic activity or glucose oxidation. The findings indicate that increased incorporation of carbon units from pyruvate into the intermediates of the oxidative pathways by PDH influences the metabolic efficiency and mechanical work of postischemic hearts.

METHODS AND RESULTS

Isolated rabbit hearts were situated in an NMR magnet and perfused or reperfused (10 minutes of ischemia) with 2.5 mmol/L [3-13C]pyruvate as sole substrate to target PDH directly and bypass the glycolytic pathway. Hearts were observed with or without activation of PDH with dichloroacetate. Mechanical function and oxygen consumption (MVO2) were monitored. 13C and 31P NMR spectroscopy allowed observations of pyruvate oxidation and bioenergetic state in intact, functioning hearts. Metabolite content and 13C enrichment levels were then determined with in vitro NMR spectroscopy and biochemical assay. PDH activation did not affect performance of normal hearts. Postischemic hearts with augmented pyruvate oxidation (dichloroacetate-treated) sustained improved mechanical function throughout 40 minutes of reperfusion. Rate-pressure-product (RPP) increased from 8300 +/- 1800 (mean +/- SEM) in untreated postischemic hearts to 21,300 +/- 2400 in hearts treated with dichloroacetate (P < .05). Oxygen use per unit work [MVO2 multiplied by 10(4) divided by RPP] was improved from 1.50 +/- 0.13 to 1.14 +/- 0.11 (P < .05) without differences in high-energy phosphate content between treated and untreated hearts. Values of dP/dt were also consistently higher, by as much as 185%, during reperfusion with dichloroacetate. Postischemic hearts displayed reduced pyruvate oxidation from the incorporation of 13C into the tissue glutamate pool. With the tissue alanine level as a marker of 13C-enriched pyruvate availability in the cell, the ratio of labeled glutamate to alanine was only 58% of the control value during early reperfusion. With dichloroacetate, that ratio was 167% greater than that of untreated hearts (P < .05). By the end of the reperfusion period, the 13C enrichment of the tissue glutamate pool by pyruvate oxidation was elevated from dichloroacetate treatment (untreated, 62 +/- 7%; DCA-treated, 81 +/- 6%; P < .05), but glycogen content was similar in both groups and 13C enrichment of tissue alanine remained unchanged, near 60%, indicating no increases in glycolytic end-product formation.

CONCLUSIONS

Metabolic reversal of contractile dysfunction was achieved in isolated hearts by counteracting depressed PDH activity in the postischemic myocardium. Improved cardiac performance did not result from, nor require, increased glycolysis secondary to the activation of PDH. Rather, restoring carbon flux through PDH alone was sufficient to improve mechanical work by postischemic hearts.

Construction of a 28-mm 1H/13C probe and actively shielded Z-gradient set for operation in a 9.4 T/89 mm magnet

In this work, we present a new RF and gradient assembly for operation in a 9.4 Tesla/89 mm magnet. This assembly was designed in order to enable 1H-NMR perfusion studies that are based on proton-observed carbon-edited approaches or gradient selected double quantum coherence. The RF portion of this probe assembly is comprised of a modified Alderman-Grant coil and a saddle coil operating at 400 and 100 MHz, respectively. These coils are surrounded by an actively shielded Z gradient, which also allows for the use of gradient-based water suppression without the need for carbon selection. We demonstrate that this probe can be used to implement gradient selected double quantum coherence experiments resulting in a high degree of water suppression.

Transaminase reaction rates, transport activities and TCA cycle analysis by post-steady state 13C NMR

In this work, we present the post-steady state analysis of the TCA cycle and a closed form solution to the rate of label washout from the C4 carbon of glutamic acid through the transaminases and the malate-aspartate shuttle and then through alpha-ketoglutarate dehydrogenase. We demonstrate using a model of this problem that the rate of label washout depends not only on the flux through alpha-ketoglutarate dehydrogenase, but most importantly on the activity of the malate-aspartate shuttle as determined by the forward and reverse fluxes through the transaminases and by the rate of transport of glutamate and alpha-ketoglutarate across the mitochondrial membrane.

Nuclear magnetic resonance evaluation of metabolic and respiratory support of work load in intact rabbit hearts

Pre-steady-state 13C nuclear magnetic resonance (NMR) spectra can provide a nondestructive probe of metabolic events associated with the physiology of intact organs. Therefore, the relation between phosphorylation state and intermediary metabolism in rabbit hearts, oxidizing [2-13C]acetate, was examined with a combination of 31P and 13C NMR. Multiple enrichment of the tissue glutamate pool with 13C as an index of metabolic turnover within the tricarboxylic acid cycle was readily observed as a function of work load. Dynamic changes in pre-steady-state 13C spectra evolved according to work load and correlated closely to respiratory rate in rabbit hearts perfused 1) under normal conditions (n = 7), 2) at basal metabolic rates (20 mM KCl arrest, n = 5), 3) and at heightened contractile state (10(-7) M isoproterenol, n = 7). The ratio of signal intensity arising from the secondary labeling sites within glutamate (C-2 and C-3) to that of the initial labeling site (C-4) reached steady state within 8.5 minutes in isoproterenol-treated hearts versus 18.5 minutes in control hearts. Work load did not affect glutamate concentration or fractional enrichment at the C-4 position, although an unlabeled fraction of glutamate persisted. Arrested hearts displayed slowed evolution of steady-state 13C enrichment with increased contributions from anaplerotic sources for tricarboxylic acid intermediate formation (32%) as compared with control (9%). Thus, the response of mitochondrial dehydrogenase activity to the demands of cardiac performance is likely to influence the recruitment of anabolic sources supplying the tricarboxylic acid cycle.(ABSTRACT TRUNCATED AT 250 WORDS)