Discrepancy in stoichiometry of steady-state muscle [Pi] and [PCr] induced by exercise

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Abstract
Phosphorus nuclear magnetic resonance was used to quantify the relations between metabolic phosphates, intracellular pH, and work rate in forearm muscle of six adult men over a range of work rates from 1.0 to 3.5 W. Three work rates were studied in each of four sessions (either 1.0, 2.0, and 3.0 or 1.5, 2.5, and 3.5 W), with measurements made before and during each bout, thereby permitting the partition of the variance attributable to rest, work-dependent, and time-dependent metabolic functions by regression analysis. There were no time-dependent changes in either [ATP] or intracellular [H+] as assessed during the rest intervals between bouts of exercise. In contrast, the total nuclear magnetic resonance (NMR)-visible phosphorus pool (TVPP) decreased with time, with both phosphocreatine (PCr) and inorganic phosphate (Pi) contributing significantly to TVPP reduction. Muscle [ATP] was unchanged by work at all intensities. Intracellular [H+] increased moderately and proportionately to work rate. [PCr] decreased and [Pi] increased in proportion to work rate, with the work-dependent coefficient for PCr consumption approximately 1.5 times that of Pi production. Neither Pi line width nor motion artifact accounted for the decrease in TVPP, so the reduced Pi accumulation in exercise may represent its sequestering in some NMR-invisible muscle pool and/or loss to the blood. Whatever the process involved, it is proportional to work rate and persists for at least 10–15 min after exercise.