title: Metabolic determinants of the onset of acidosis in exercising human muscle: a 31P-MRS study.
abstract: Onset of intracellular acidosis during muscular exercise has been generally attributed to activation or hyperactivation of nonoxidative ATP production but has not been analyzed quantitatively in terms of H(+) balance, i.e., production and removal mechanisms. To address this issue, we have analyzed the relation of intracellular acidosis to H(+) balance during exercise bouts in seven healthy subjects. Each subject performed a 6-min ramp rhythmic exercise (finger flexions) at low frequency (LF, 0.47 Hz), leading to slight acidosis, and at high frequency (HF, 0.85 Hz), inducing a larger acidosis. Metabolic changes were recorded using (31)P-magnetic resonance spectroscopy. Onset of intracellular acidosis was statistically identified after 3 and 4 min of exercise for HF and LF protocols, respectively. A detailed investigation of H(+) balance indicated that, for both protocols, nonoxidative ATP production preceded a change in pH. For HF and LF protocols, H(+) consumption through the creatine kinase equilibrium was constant in the face of increasing H(+) generation and efflux. For both protocols, changes in pH were not recorded as long as sources and sinks for H(+) approximately balanced. In contrast, a significant acidosis occurred after 4 min of LF exercise and 3 min of HF exercise, whereas the rise in H(+) generation exceeded the rise in H(+) efflux at a nearly constant H(+) uptake associated with phosphocreatine breakdown. We have clearly demonstrated that intracellular acidosis in exercising muscle does not occur exclusively as a result of nonoxidative ATP production but, rather, reflects changes in overall H(+) balance.
authors.affiliation: Centre de Résonance Magnétique Biologique et Médicale, Unité Mixte de Recherche Centre National de la Recherche Scientifique 6612, and Faculté de Médecine de Marseille, France.
authors.forename: M
authors.initials: M
authors.lastname: Roussel
authors.forename: J P
authors.initials: JP
authors.lastname: Mattei
authors.forename: Y
authors.initials: Y
authors.lastname: Le Fur
authors.forename: B
authors.initials: B
authors.lastname: Ghattas
authors.forename: P J
authors.initials: PJ
authors.lastname: Cozzone
authors.forename: D
authors.initials: D
authors.lastname: Bendahan
mesh_terms: D000138:Acidosis / Q000378:metabolism*; D000328:Adult; D000693:Anaerobiosis; D001639:Bicarbonates / Q000378:metabolism; D003402:Creatine Kinase / Q000378:metabolism; D015444:Exercise / Q000502:physiology*; D006003:Glycogen / Q000096:biosynthesis; D006801:Humans; D006859:Hydrogen / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D018482:Muscle, Skeletal / Q000201:enzymology / Q000378:metabolism / Q000502:physiology*; D010725:Phosphocreatine / Q000378:metabolism; D012146:Rest / Q000502:physiology
chemical_list: D001639:Bicarbonates; D010725:Phosphocreatine; D006859:Hydrogen; D006003:Glycogen; D003402:Creatine Kinase