title: Evaluation of cerebral acetate transport and metabolic rates in the rat brain in vivo using 1H-[13C]-NMR.
abstract: Acetate is a well-known astrocyte-specific substrate that has been used extensively to probe astrocytic function in vitro and in vivo. Analysis of amino acid turnover curves from (13)C-acetate has been limited mainly for estimation of first-order rate constants from exponential fitting or calculation of relative rates from steady-state (13)C enrichments. In this study, we used (1)H-[(13)C]-Nuclear Magnetic Resonance spectroscopy with intravenous infusion of [2-(13)C]acetate-Na(+) in vivo to measure the cerebral kinetics of acetate transport and utilization in anesthetized rats. Kinetics were assessed using a two-compartment (neuron/astrocyte) analysis of the (13)C turnover curves of glutamate-C4 and glutamine-C4 from [2-(13)C]acetate-Na(+), brain acetate levels, and the dependence of steady-state glutamine-C4 enrichment on blood acetate levels. The steady-state enrichment of glutamine-C4 increased with blood acetate concentration until 90% of plateau for plasma acetate of 4 to 5 mmol/L. Analysis assuming reversible, symmetric Michaelis-Menten kinetics for transport yielded 27+/-2 mmol/L and 1.3+/-0.3 micromol/g/min for K(t) and T(max), respectively, and for utilization, 0.17+/-0.24 mmol/L and 0.14+/-0.02 micromol/g/min for K(M_util) and V(max_util), respectively. The distribution space for acetate was only 0.32+/-0.12 mL/g, indicative of a large excluded volume. The astrocytic and neuronal tricarboxylic acid cycle fluxes were 0.37+/-0.03 micromol/g/min and 1.41+/-0.11 micromol/g/min, respectively; astrocytes thus comprised approximately 21%+/-3% of total oxidative metabolism.
authors.affiliation: Department of Diagnostic Radiology, Magnetic Resonance Research Center, Yale University School of Medicine, New Haven, Connecticut, USA. abpatel@ccmb.res.in
authors.forename: Anant B
authors.initials: AB
authors.lastname: Patel
authors.forename: Robin A
authors.initials: RA
authors.lastname: de Graaf
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Rothman
authors.forename: Kevin L
authors.initials: KL
authors.lastname: Behar
authors.forename: Graeme F
authors.initials: GF
authors.lastname: Mason
mesh_terms: D000085:Acetates / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D001692:Biological Transport / Q000502:physiology; D001921:Brain / Q000378:metabolism*; D001923:Brain Chemistry / Q000502:physiology; D002247:Carbon Isotopes; D005973:Glutamine / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009474:Neurons / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D000085:Acetates; D002247:Carbon Isotopes; D005973:Glutamine