ATP synthesis and proton handling in muscle during short periods of exercise and subsequent recovery

We used (31P)-magnetic resonance spectroscopy to study proton buffering in finger flexor muscles of eight healthy men ( 25 - 45 yr), during brief (18-s) voluntary finger flexion exercise (0.67-Hz contraction at 10% maximum voluntary contraction; 50/50 duty cycle) and 180-s recovery. Phosphocreatine (PCr) concentration fell 19 +/- 2% during exercise and then recovered with half time = 0.24 +/- 0.01 min. Cell pH rose by 0.058 +/- 0.003 units during exercise as a result of H+ consumption by PCr splitting, which ( assuming no lactate production or H+ efflux) implies a plausible non-P-i buffer capacity of 20 +/- 3 mmol . l intracellular water(-1) . pH unit(-1). There was thus no evidence of significant glycogenolysis to lactate during exercise. Analysis of PCr kinetics as a classic linear response suggests that oxidative ATP synthesis reached 48 +/- 2% of ATP demand by the end of exercise; the rest was met by PCr splitting. Postexercise pH recovery was faster than predicted, suggesting "excess proton" production, with a peak value of 0.6 +/- 0.2 mmol/l intracellular water at 0.45 min of recovery, which might be due to, e. g., proton influx driven by cellular alkalinization, or a small glycolytic contribution to PCr resynthesis in recovery.