COMPARISONS OF ATP TURNOVER IN HUMAN MUSCLE DURING ISCHEMIC AND AEROBIC EXERCISE USING P-31 MAGNETIC-RESONANCE SPECTROSCOPY

To investigate human muscle bioenergetics quantitatively in vivo, we u sed P-31 magnetic resonance spectroscopy to study the flexor digitorum superficialis of four adult males during dynamic ischemic and aerobic exercise at 0.50-1.00 W and during recovery from aerobic exercise. Du ring exercise, changes in pH and [PCr] were larger at higher power, bu t in aerobic exercise neither end-exercise [ADP] nor the initial poste xercise PCr resynthesis rate altered with power. In ischemic exercise we estimated total ATP synthesis from the rates of PCr depletion and g lycogenolysis (inferred using an analysis of proton buffering); this w as linear with power output. In aerobic exercise, again we estimated A TP synthesis rates due to phosphocreatine hydrolysis and glycogenolysi s (incorporating a correction for proton efflux) and also estimated ox idative ATP synthesis by difference, using the total ATP turnover rate established during ischemic exercise. We conclude that in early exerc ise oxidative ATP synthesis was small, increasing by the end of exerci se to a value close (as predicted) to the initial postexercise rate of PCr resynthesis. Furthermore, a plausible estimate of proton efflux d uring aerobic exercise can be inferred from the pH-dependence of proto n efflux in recovery.