Bioenergetics and ATP Synthesis during Exercise: Role of Group III/IV Muscle Afferents.

Purpose: To investigate the role of the group III/IV muscle afferents in the bioenergetics of exercising skeletal muscle beyond constraining the magnitude of metabolic perturbation. Methods: Eight healthy men, performed intermittent isometric knee-extensor exercise to task failure at ~58% maximal voluntary contraction (MVC) under control conditions (CTRL) and with lumbar intrathecal fentanyl to attenuate group III/IV leg muscle afferents (FENT). Intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4[spacing macron]), adenosine triphosphate (ATP), and pH were determined using phosphorous magnetic resonance spectroscopy (31P-MRS). Results: The magnitude of metabolic perturbation was significantly greater in FENT compared to CTRL for [Pi] (37.8 +/- 16.8 vs. 28.6 +/- 8.6 mM), [H2PO4[spacing macron]] (24.3 +/- 12.2 vs. 17.9 +/- 7.1 mM), and [ATP] (75.8 +/- 17.5 vs. 81.9 +/- 15.8 % of baseline), while there was no significant difference in [PCr] (4.5 +/- 2.4 vs. 4.4 +/- 2.3 mM) or pH (6.51 +/- 0.10 vs. 6.54 +/- 0.14). The rate of perturbation in [PCr], [Pi], [H2PO4[spacing macron]], and pH was significantly faster in FENT compared to CTRL. Oxidative ATP synthesis was not significantly different between conditions. However, anaerobic ATP synthesis, through augmented creatine kinase and glycolysis reactions, was significantly greater in FENT than CTRL, resulting in a significantly greater ATP cost of contraction (0.049 +/- 0.016 vs. 0.038 +/- 0.010 mM[middle dot]min-1[middle dot]N-1). Conclusion: Not only do group III/IV muscle afferents constrain magnitude of perturbation in intramuscular Pi, H2PO4[spacing macron], and ATP during small muscle mass exercise, but also appear to play a role in maintaining efficient skeletal muscle contractile function in men. (C) 2017 American College of Sports Medicine

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