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

Abstract

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.

Keywords: AHA Scientific Statements; metabolic pathways; metabolism, cardiac; molecular biology; radionuclide imaging; systems biology.

Figure 1

The main metabolic pathways for fatty acids, glucose, lactate, ketone body, and amino acids as represented in the KEGG (Kyoto Encyclopedia of Genes and Genomes) Atlas global metabolism map viewer. Reprinted from Okuda et al. Copyright © 2008, The Authors (see: http://creativecommons.org/licenses/by-nc/2.0/uk/).

Figure 2

Hypothetical sequence from metabolic gene expression (central dogma) to energy transfer in the cardiomyocyte (physiology). The major methodologies are denoted in italics. See the Metabolic Pathways and Networks and Metabolomics sections of the text for further detail. PTM indicates posttranslational modification.

Figure 3

Hypothetical example of the impact of an activator (A) or repressor (B) on a metabolic parameter that exhibits a time-of-day-dependent oscillation. A large proportion of metabolic parameters exhibit time-of-day–dependent oscillations in the heart. As such, the time of day at which cardiac metabolism is assessed can markedly impact the results yielded and the conclusions drawn. In this hypothetical example, assessment of the metabolic parameter around 12:00 noon (light phase) would yield results consistent with the impact of the activator, whereas assessment at 12:00 midnight would not (A). Conversely, assessment of the metabolic parameter around 12:00 midnight (dark phase) would yield results consistent with the impact of the repressor, whereas assessment at 12:00 noon would not (B). Minimum and maximum indicate the physiological range for the metabolic parameter.

Figure 4

Top, End-diastolic transverse positron emission tomography images for sham, transverse aortic constriction (TAC), and TAC mice treated with propranolol at baseline and on day 1 and day 7 after surgery. The images show an increase in uptake of the glucose tracer analog [¹⁸F]2-deoxy-D-glucose (FDG) in TAC mice starting at day 1, indicative of the metabolic adaptation in pressure-overload left ventricular hypertrophy. Bottom, A, Measured rates of myocardial FDG uptake in vivo, showing K_i (in mL·g⁻¹·min⁻¹). Bottom, B, Measured left ventricular ejection fraction from dynamic gated positron emission tomography images in vivo. All values are mean ± SEM. *P<0.05 vs baseline, TAC-propranolol, and sham groups. Reprinted from Zhong et al with permission. Copyright © 2013, the Society of Nuclear Medicine and Molecular Imaging, Inc.

Figure 5

Serial ³¹P magnetic resonance spectra from a heart treated with verapamil during control and an untreated perfused rat heart, as well as at the end of 30 minutes of hypoxia and at the end of 30 minutes of reperfusion. During hypoxia, phosphocreatine (PCr) and ATP decrease, whereas inorganic phosphate (Pi) increases and an additional resonance (monophosphate esters [MPE], mostly glycolytic intermediates and AMP) appears. During reoxygenation, PCr and Pi recover, but ATP does not. Verapamil pretreatment attenuates these changes. NAD indicates nicotine adenine dinucleotide. Reprinted from Neubauer et al with permission from Elsevier. Copyright © 1989, Elsevier Ltd.

Figure 6

Abbreviated schematic of carbon-13 (¹³C) isotope labeling for nuclear magnetic resonance detection of enriched glutamate. Catabolism of ¹³C-enriched metabolic precursors contributes to [¹³C]acetyl-CoA, which enters the Krebs cycle via the citrate synthase reaction. Oxidation of ¹³C citrate forms ¹³C-enriched α-ketoglutarate (αKG). Either ¹³C αKG is then oxidized to form ¹³C succinate, which introduces the ¹³C into the second span of the Krebs cycle, or ¹³C αKG is exported across the mitochondrial membrane to the cytosol via a rate-determining process through the reversible αKG (or oxoglutarate) malate (MAL) carrier (OMC). Depending on the experimental protocol, various carbon positions with the metabolic intermediates can be isotopically labeled from the delivery of differential ¹³C enrichment schemes within an individual precursor or a mixed supply of fuels for acetyl-CoA production and oxidative metabolism (fats, carbohydrates, and ketones). The carrier-mediated exchange of αKG for MAL has been shown to be detectable and to introduce a separable rate to the formation of ¹³C glutamate that is coupled to but distinct from flux through the Krebs cycle. This exchange can occur in tandem with exchange through the unidirectional, glutamate-aspartate carrier and associated transamination reactions (not shown) to form the MAL-aspartate shuttle, the mechanism for transferring reducing equivalents from the cytosol to the mitochondria by coupling to NAD⁺/NADH redox reactions. CIT indicates citrate; FUM, fumarate; ISO, isocitrate; NAD, nicotinamide adenine dinucleotide; OAA, oxaloacetate; and SUC, succinate. Reprinted from Lewandowski et al with permission of Springer. Copyright © 2013, Springer Science + Business Media, LLC.

Figure 7

Compartmental model of myocardial protein turnover. A, The theoretical model for the precursor/product relationship described in the equation under Measuring Metabolism. F and P refer to free amino acids in the precursor and bound amino acids in the protein compartments, respectively. K_s and K_d are first-order rate constants for the rates of transfer of amino acids in and out of the protein compartment. B, The same compartments, including known pathways of precursor and product uptake and release from myocardial proteins. tRNA indicates transfer ribonucleic acid. Modified with permission of FASEB from Samarel; permission conveyed through Copyright Clearance Center, Inc. Copyright © 1991, FASEB.