SEPARATE MEASURES OF ATP UTILIZATION AND RECOVERY IN HUMAN SKELETAL-MUSCLE

1. The chemical changes during contractile activity were separated fro m recovery metabolism in the forearm flexor musculature in normal huma n subjects using P-31 nuclear magnetic resonance (NMR) spectroscopy. P ercutaneous, supramaximal twitch stimulation of the median and ulnar n erves was used in combination with temporary ischaemia of the forearm to characterize the summed ATPase activity. The recovery following res toration of blood flow provided a measure of oxidative ATP synthesis a ctivity. These processes were measured based on the dynamics of creati ne phosphate (PCr) content. 2. Muscle oxygen stores were depleted usin g ischaemia without stimulation as indicated by PCr breakdown after 25 0 +/- 33 s (mean +/- S.D.; n = 5), which provided a measure of the bas al metabolic rate (0.008 +/- 0.002 mM s-1, n = 5). 3. The PCr breakdow n rate during twitch stimulation of the oxygen-depleted muscle was con stant at 1 Hz at 0.15 +/- 0.03 mm PCr per second or per twitch (n = 8) . A constant cost per twitch was found from 0.5 to 2 Hz stimulation (d epletion of PCr per twitch = 0.15 mm per twitch). 4. No net anaerobic recovery of PCr was found during a 2 min post-stimulation ischaemia. 5 . Upon restoration of blood flow, PCr recovery followed an exponential time course with a time constant of 63 +/- 14 s (n = 8). From these r ecovery rates, the capacity for oxidative phosphorylation was estimate d to be 0.4 mm s-1. 6. This experimental approach defines a non-invasi ve and quantitative measure of human muscle ATPase rate and ATP synthe tase rate.