title: Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized ¹³C magnetic resonance.
abstract: BACKGROUND
  The heart relies on continuous energy production and imbalances herein impair cardiac function directly. The tricarboxylic acid (TCA) cycle is the primary means of energy generation in the healthy myocardium, but direct noninvasive quantification of metabolic fluxes is challenging due to the low concentration of most metabolites. Hyperpolarized (13)C magnetic resonance spectroscopy (MRS) provides the opportunity to measure cellular metabolism in real time in vivo. The aim of this work was to noninvasively measure myocardial TCA cycle flux (VTCA) in vivo within a single minute.
  
  
  RESULTS
  Hyperpolarized [1-(13)C]acetate was administered at different concentrations in healthy rats. (13)C incorporation into [1-(13)C]acetylcarnitine and the TCA cycle intermediate [5-(13)C]citrate was dynamically detected in vivo with a time resolution of 3s. Different kinetic models were established and evaluated to determine the metabolic fluxes by simultaneously fitting the evolution of the (13)C labeling in acetate, acetylcarnitine, and citrate. VTCA was estimated to be 6.7±1.7 μmol·g(-1)·min(-1) (dry weight), and was best estimated with a model using only the labeling in citrate and acetylcarnitine, independent of the precursor. The TCA cycle rate was not linear with the citrate-to-acetate metabolite ratio, and could thus not be quantified using a ratiometric approach. The (13)C signal evolution of citrate, i.e. citrate formation was independent of the amount of injected acetate, while the (13)C signal evolution of acetylcarnitine revealed a dose dependency with the injected acetate. The (13)C labeling of citrate did not correlate to that of acetylcarnitine, leading to the hypothesis that acetylcarnitine formation is not an indication of mitochondrial TCA cycle activity in the heart.
  
  
  CONCLUSIONS
  Hyperpolarized [1-(13)C]acetate is a metabolic probe independent of pyruvate dehydrogenase (PDH) activity. It allows the direct estimation of VTCA in vivo, which was shown to be neither dependent on the administered acetate dose nor on the (13)C labeling of acetylcarnitine. Dynamic (13)C MRS coupled to the injection of hyperpolarized [1-(13)C]acetate can enable the measurement of metabolic changes during impaired heart function.
authors: Laboratory of Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; Institute of Physics of Biological Systems, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. Electronic address: jbastiaansen.mri@gmail.com.
authors: Jessica A M
authors: JA
authors: Bastiaansen
authors: Institute of Physics of Biological Systems, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
authors: Tian
authors: T
authors: Cheng
authors: Department of Radiology, University of Geneva, Geneva, Switzerland.
authors: Hongxia
authors: H
authors: Lei
authors: Laboratory of Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; Department of Radiology, University of Geneva, Geneva, Switzerland; Department of Radiology, University of Lausanne, Lausanne, Switzerland.
authors: Rolf
authors: R
authors: Gruetter
authors: Institute of Physics of Biological Systems, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
authors: Arnaud
authors: A
authors: Comment
mesh_terms: D000108:Acetylcarnitine; D000818:Animals; D002247:Carbon Isotopes / Q000008:administration & dosage; D002952:Citric Acid Cycle; D006801:Humans; D008279:Magnetic Resonance Imaging; D008929:Mitochondria, Heart / Q000378:metabolism / Q000473:pathology; D009206:Myocardium / Q000378:metabolism* / Q000473:pathology; D051381:Rats; D014233:Tricarboxylic Acids / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D014233:Tricarboxylic Acids; D000108:Acetylcarnitine
keywords: Acetate; Acetylcarnitine; Citrate; Dynamic nuclear polarization; Metabolism