title: In vivo quantification of neuro-glial metabolism and glial glutamate concentration using 1H-[13C] MRS at 14.1T.
abstract: Astrocytes have recently become a major center of interest in neurochemistry with the discoveries on their major role in brain energy metabolism. An interesting way to probe this glial contribution is given by in vivo (13) C NMR spectroscopy coupled with the infusion labeled glial-specific substrate, such as acetate. In this study, we infused alpha-chloralose anesthetized rats with [2-(13) C]acetate and followed the dynamics of the fractional enrichment (FE) in the positions C4 and C3 of glutamate and glutamine with high sensitivity, using (1) H-[(13) C] magnetic resonance spectroscopy (MRS) at 14.1T. Applying a two-compartment mathematical model to the measured time courses yielded a glial tricarboxylic acid (TCA) cycle rate (Vg ) of 0.27 ± 0.02 μmol/g/min and a glutamatergic neurotransmission rate (VNT ) of 0.15 ± 0.01 μmol/g/min. Glial oxidative ATP metabolism thus accounts for 38% of total oxidative metabolism measured by NMR. Pyruvate carboxylase (VPC ) was 0.09 ± 0.01 μmol/g/min, corresponding to 37% of the glial glutamine synthesis rate. The glial and neuronal transmitochondrial fluxes (Vx (g) and Vx (n) ) were of the same order of magnitude as the respective TCA cycle fluxes. In addition, we estimated a glial glutamate pool size of 0.6 ± 0.1 μmol/g. The effect of spectral data quality on the fluxes estimates was analyzed by Monte Carlo simulations. In this (13) C-acetate labeling study, we propose a refined two-compartment analysis of brain energy metabolism based on (13) C turnover curves of acetate, glutamate and glutamine measured with state of the art in vivo dynamic MRS at high magnetic field in rats, enabling a deeper understanding of the specific role of glial cells in brain oxidative metabolism. In addition, the robustness of the metabolic fluxes determination relative to MRS data quality was carefully studied.
authors.affiliation: Laboratory for Functional and Metabolic Imaging (LIFMET), Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
authors.forename: Bernard
authors.initials: B
authors.lastname: Lanz
authors.forename: Lijing
authors.initials: L
authors.lastname: Xin
authors.forename: Philippe
authors.initials: P
authors.lastname: Millet
authors.forename: Rolf
authors.initials: R
authors.lastname: Gruetter
mesh_terms: D000818:Animals; D002247:Carbon Isotopes; D002540:Cerebral Cortex / Q000737:chemistry / Q000378:metabolism; D004734:Energy Metabolism / Q000502:physiology*; D018698:Glutamic Acid / Q000378:metabolism*; D006859:Hydrogen; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D056004:Molecular Dynamics Simulation; D009010:Monte Carlo Method; D009457:Neuroglia / Q000737:chemistry / Q000378:metabolism*; D009474:Neurons / Q000737:chemistry / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D002247:Carbon Isotopes; D018698:Glutamic Acid; D006859:Hydrogen
keywords: 13C MRS; acetate metabolism; glial TCA cycle; glutamate/glutamine cycle; metabolic modeling; neurotransmission