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

Assessing Cardiac Metabolism: A Scientific Statement From the American Heart Association

In a complex system of interrelated reactions, the heart converts chemical energy to mechanical energy. Energy transfer is achieved through coordinated activation of enzymes, ion channels, and contractile elements, as well as structural and membrane proteins. The heart's needs for energy are difficult to overestimate. At a time when the cardiovascular research community is discovering a plethora of new molecular methods to assess cardiac metabolism, the methods remain scattered in the literature. The present statement on "Assessing Cardiac Metabolism" seeks to provide a collective and curated resource on methods and models used to investigate established and emerging aspects of cardiac metabolism. Some of those methods are refinements of classic biochemical tools, whereas most others are recent additions from the powerful tools of molecular biology. The aim of this statement is to be useful to many and to do justice to a dynamic field of great complexity.

The use of magnetic resonance methods in translational cardiovascular research

Magnetic resonance methods are widely applicable to research questions posed in translational cardiovascular studies. The main intent of this review was to offer the cardiovascular translational research scientist a "menu" of magnetic resonance (MR) approaches that can be applied to answering research questions posed in a variety of experimental situations including those involving the use of human subjects. Obviously, this menu is not comprehensive and many other topics could have been selected for emphasis. However, we hope that the material presented encompasses a broad enough slice of the field to stimulate thinking about the possible applications of MR methods to specific research questions.

Metabolic shifts and myocyte hypertrophy in deflazacort treatment of mdx mouse cardiomyopathy

We tested the hypothesis that treatment of mdx mouse muscular dystrophy with the glucocorticoid deflazacort prevents cardiomyopathic lesions and is accompanied by changes in metabolism and gene expression that reflect the improved tissue integrity. Cardiac muscle pathology, expression of alpha-cardiac myosin heavy chain, DNA synthesis, laminin, and basic fibroblast growth factor (bFGF) were examined to characterize dystrophy and changes with treatment. The potential of proton magnetic resonance spectroscopy (H-NMRS) to track cardiac dystrophy and deflazacort effects was also studied. Deflazacort (but not equipotent prednisone) reproducibly decreased lesion prevalence and severity. Treatment also produced cardiomyocyte hypertrophy and a 5.4-fold increase in alpha-cardiac myosin content. Expression of bFGF messenger RNA (mRNA), notable around lesions, rose 3.3-fold, and laminin expression rose 2.1-fold after deflazacort. Studies using H-NMRS showed a cardiac "signature" with less glycine and taurine than limb muscle or diaphragm and shifts with progression of dystrophy (distinct from normal aging) in many metabolites. Increased taurine, acetate, and succinate were present after 2 weeks of deflazacort treatment but were not present after 4 weeks. Although paired kinetic and functional studies of myocardium will be needed to determine the origin of such changes, these results demonstrate the potential application of H-NMRS to monitor clinical heart disease and treatment. In addition, the metabolic effects of deflazacort were substantial in preventing the progression of cardiomyopathy in mdx mice and included increased expression of protectant and stabilizing factors and hypertrophy of cardiac myocytes.

In-vivo myocardial substrate alteration during perfused ventricular fibrillation

OBJECTIVES

Earlier work suggests the in-vivo heart alters its substrate utilization as a function of cardiac work. Previous work has also demonstrated the high oxygen requirements of the heart during ventricular fibrillation (VF). The authors hypothesized that myocardial substrate utilization during VF with perfusion is similar to the normal beating heart under conditions of increased workload.

METHODS

Myocardial substrate selection was studied in the in-vivo porcine myocardium using 13carbon nuclear magnetic resonance (13C NMR) under conditions of increased cardiac work (dobutamine group) and VF with extracorporeal perfusion (VF group). Once the animal preparation was completed, metabolic steady state was achieved with the infusion of unlabeled acetate into the left anterior descending (LAD) coronary artery. The infused substrate was then changed to [2-13C] acetate and glutamate pool labeling was monitored by 13C NMR. The glutamate C4 resonance areas at baseline and after intervention of either increased workload (dobutamine group) or perfused VF (VF group) were compared within groups using paired t-tests.

RESULTS

Baseline aortic and great cardiac vein lactates, glucose levels, blood gases, hemoglobin levels, and temperatures were similar between groups. In both groups, there was a significant decrease from baseline in the labeling of C4 glutamate peaks (dobutamine group: 20.2+/-14.9 vs 84.7+/-32.7, p = 0.002; and VF group: 49.8+/-24.4 vs 83.9+/-24.4, p = 0.02), indicating selection against acetate oxidation in favor of other endogenous substrates.

CONCLUSIONS

In the in-vivo heart, despite the absence of functional contractions, changes in substrate utilization during perfused VF are similar to changes that occur with increased workload in the normal beating heart.

13C-NMR spectroscopic evaluation of the citric acid cycle flux in conditions of high aspartate transaminase activity in glucose-perfused rat hearts

A new mathematical model, based on the observation of 13C-NMR spectra of two principal metabolites (glutamate and aspartate), was constructed to determine the citric acid cycle flux in the case of high aspartate transaminase activity leading to the formation of large amounts of labeled aspartate and glutamate. In this model, the labeling of glutamate and aspartate carbons by chemical and isotopic exchange with the citric acid cycle are considered to be interdependent. With [U-13C]Glc or [1,2-(13)C]acetate as a substrate, all glutamate and aspartate carbons can be labeled. The isotopic transformations of 32 glutamate isotopomers into 16 aspartate isotopomers or vice versa were studied using matrix operations; the results were compiled in two matrices. We showed how the flux constants of the citric acid cycle and the 13C-enrichment of acetyl-CoA can be deduced from 13C-NMR spectra of glutamate and/or aspartate. The citric acid cycle flux in beating Wistar rat hearts, aerobically perfused with [U-13C]glucose in the absence of insulin, was investigated by 13C-NMR spectroscopy. Surprisingly, aspartate instead of glutamate was found to be the most abundantly-labeled metabolite, indicating that aspartate transaminase (which catalyses the reversible reaction: (glutamate + oxaloacetate <--> 2-oxoglutarate + aspartate) is highly active in the absence of insulin. The amount of aspartate was about two times larger than glutamate. The quantities of glutamate (G0) or aspartate (A0) were approximately the same for all hearts and remained constant during perfusion: G0 = (0.74 +/- 0.03) micromol/g; A0 = (1.49 +/- 0.05) micromol/g. The flux constants, i.e., the fraction of glutamate and aspartate in exchange with the citric acid cycle, were about 1.45 min(-1) and 0.72 min(-1), respectively; the flux of this cycle is about (1.07 +/- 0.02) micromol min(-1) g(-1). Excellent agreement between the computed and experimental data was obtained, showing that: i) in the absence of insulin, only 41% of acetyl-CoA is formed from glucose while the rest is derived from endogenous substrates; and ii) the exchange between aspartate and oxaloacetate or between glutamate and 2-oxoglutarate is fast in comparison with the biological transformation of intermediate compounds by the citric acid cycle.