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31P magnetization transfer measurements of Pi→ATP flux in exercising human muscle

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Sleigh, Alison 
Savage, David B 
Williams, Guy B 
Porter, David 
Carpenter, T Adrian 


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.



31P magnetization transfer, Pi↔ATP exchange, exercising muscle, saturation transfer, Adenosine Triphosphate, Adult, Exercise, Female, Glyceric Acids, Glycolysis, Humans, Kinetics, Magnetic Resonance Spectroscopy, Male, Muscle, Skeletal, Phosphates, Phosphorus Isotopes, Rest

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Journal of Applied Physiology

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American Physiological Society
Medical Research Council (G0701532)
Wellcome Trust (093875/Z/10/Z)
Wellcome Trust (091551/Z/10/A)
Wellcome Trust (107064/Z/15/Z)
This work was funded by the Clinical Research Infrastructure Grant and the Siemens MAGNETOM 3T Verio scanner is funded by the NIHR via an award to the Cambridge NIHR/Wellcome Trust Clinical Research Facility. D.B. Savage is supported by the Wellcome Trust (091551).