title: Metabolic and nonmetabolic components of fatigue monitored with 31P-NMR.
abstract: The goal of this study was to determine the roles of metabolic and nonmetabolic factors in muscle fatigue. Rat gastrocnemius muscles were fatigued by stimulation of the nerve (n = 6) or muscle (n = 4, after 2 days of denervation). 31Phosphorus nuclear magnetic resonance spectroscopy was used to measure levels of intracellular inorganic phosphate (Pi) and hydrogen ions (H+) (which are thought to inhibit contraction) and the high-energy phosphates, phosphocreatine (PCr), and ATP. For both indirect and direct stimulation, with fatigue to approximately 60% initial tetanic force, [Pi] increased from approximately 3.5 mmol/L to approximately 20 mmol/L and [PCr] decreased from approximately 27 mmol/L to approximately 9 mmol/L. However, with continued fatigue to 25-35% initial tetanic force, neither [Pi] or [PCr] changed further. [ATP] and pH changed only slightly during fatigue. The results are consistent with early fatigue arising from metabolic inhibition of contraction; but later fatigue arising independent of metabolites, due to impaired activation beyond the neuromuscular junction.
authors.affiliation: Department of Medicine, University of California, San Francisco 94121.
authors.forename: A J
authors.initials: AJ
authors.lastname: Baker
authors.forename: P J
authors.initials: PJ
authors.lastname: Carson
authors.forename: R G
authors.initials: RG
authors.lastname: Miller
authors.forename: M W
authors.initials: MW
authors.lastname: Weiner
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D004558:Electric Stimulation; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009132:Muscles / Q000294:innervation / Q000378:metabolism / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism; D010725:Phosphocreatine / Q000378:metabolism; D010758:Phosphorus; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D010710:Phosphates; D010725:Phosphocreatine; D010758:Phosphorus; D000255:Adenosine Triphosphate