title: 31P magnetization transfer measurements of Pi→ATP flux in exercising human muscle.
abstract: Fundamental criticisms have been made over the use of (31)P magnetic resonance spectroscopy (MRS) magnetization transfer estimates of inorganic phosphate (Pi)→ATP flux (VPi-ATP) in human resting skeletal muscle for assessing mitochondrial function. Although the discrepancy in the magnitude of VPi-ATP is now acknowledged, little is known about its metabolic determinants. Here we use a novel protocol to measure VPi-ATP in human exercising muscle for the first time. Steady-state VPi-ATP was measured at rest and over a range of exercise intensities and compared with suprabasal oxidative ATP synthesis rates estimated from the initial rates of postexercise phosphocreatine resynthesis (VATP). We define a surplus Pi→ATP flux as the difference between VPi-ATP and VATP. The coupled reactions catalyzed by the glycolytic enzymes GAPDH and phosphoglycerate kinase (PGK) have been shown to catalyze measurable exchange between ATP and Pi in some systems and have been suggested to be responsible for this surplus flux. Surplus VPi-ATP did not change between rest and exercise, even though the concentrations of Pi and ADP, which are substrates for GAPDH and PGK, respectively, increased as expected. However, involvement of these enzymes is suggested by correlations between absolute and surplus Pi→ATP flux, both at rest and during exercise, and the intensity of the phosphomonoester peak in the (31)P NMR spectrum. This peak includes contributions from sugar phosphates in the glycolytic pathway, and changes in its intensity may indicate changes in downstream glycolytic intermediates, including 3-phosphoglycerate, which has been shown to influence the exchange between ATP and Pi catalyzed by GAPDH and PGK.
authors.affiliation: Wolfson Brain Imaging Centre, University of Cambridge School of Clinical Medicine, Cambridge Biomedical Campus, United Kingdom; National Institute for Health Research/Wellcome Trust Clinical Research Facility at Cambridge University Hospitals NHS Foundation Trust, Cambridge Biomedical Campus, United Kingdom; as626@wbic.cam.ac.uk.
authors.forename: Alison
authors.initials: A
authors.lastname: Sleigh
authors.affiliation: University of Cambridge Metabolic Research Laboratories, Wellcome Trust-Medical Research Council Institute of Metabolic Science, Cambridge Biomedical Campus, United Kingdom;
authors.forename: David B
authors.initials: DB
authors.lastname: Savage
authors.affiliation: Wolfson Brain Imaging Centre, University of Cambridge School of Clinical Medicine, Cambridge Biomedical Campus, United Kingdom;
authors.forename: Guy B
authors.initials: GB
authors.lastname: Williams
authors.affiliation: Fraunhofer MEVIS, Institute for Medical Image Computing, Bremen, Germany;
authors.forename: David
authors.initials: D
authors.lastname: Porter
authors.affiliation: Wolfson Brain Imaging Centre, University of Cambridge School of Clinical Medicine, Cambridge Biomedical Campus, United Kingdom;
authors.forename: T Adrian
authors.initials: TA
authors.lastname: Carpenter
authors.affiliation: Department of Biochemistry, University of Cambridge, United Kingdom; Cancer Research UK Cambridge Institute, Li Ka Shing Centre, University of Cambridge, Cambridge Biomedical Campus, United Kingdom;
authors.forename: Kevin M
authors.initials: KM
authors.lastname: Brindle
authors.affiliation: Magnetic Resonance and Image Analysis Research Centre, University of Liverpool, United Kingdom; and Department of Musculoskeletal Biology and MRC - Arthritis Research UK Centre for Integrated research into Musculoskeletal Ageing, Institute of Ageing and Chronic Disease, University of Liverpool, United Kingdom.
authors.forename: Graham J
authors.initials: GJ
authors.lastname: Kemp
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000328:Adult; D015444:Exercise / Q000502:physiology*; D005260:Female; D005988:Glyceric Acids / Q000378:metabolism; D006019:Glycolysis / Q000502:physiology; D006801:Humans; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism*; D010759:Phosphorus Isotopes / Q000378:metabolism*; D012146:Rest / Q000502:physiology
chemical_list: D005988:Glyceric Acids; D010710:Phosphates; D010759:Phosphorus Isotopes; C005156:3-phosphoglycerate; D000255:Adenosine Triphosphate
keywords: 31P magnetization transfer; Pi↔ATP exchange; exercising muscle; saturation transfer