Substrates for muscle glycogen synthesis in recovery from intense exercise in man

1. Intramuscular glyconeogenesis from lactate after intense exercise was examined by using the one-legged knee extension model which enables evaluation of metabolism in a well-defined muscle group. 2. In seven subjects measurements of leg blood flow and arterial-venous differences of various substrates were performed in individuals after intense, exhaustive knee extensor exercise lasting 3.0 min. Muscle glycogen and lactate concentrations were determined in the quadriceps muscle immediately after exercise and three times during 1 h of recovery. 3. Muscle glycogen increased from 93.7 +/- 6.7 (+/- S.E.M.) to 108.8 +/- 8.1 mmol (kg wet wt)-1 during the recovery period. Muscle lactate was 27.1 +/- 2.1 mmol (kg wet wt)-1 at the end of exercise and decreased to 14.5 +/- 2.1, 6.7 +/- 1.1, and 3.0 +/- 0.5 mmol (kg wet wt)-1 after 3, 10 and 60 min of recovery, respectively. 4. More than two-thirds of the lactate that accumulated in the muscle during the intense exercise was released into the blood. It was estimated that between 13 and 27% of the lactate could have been converted to glycogen. This corresponded to a glycogen resynthesis rate from lactate of 0.17-0.34 and 0.002 mmol glucosyl units min-1 (kg wet wt)-1 for the first 10 and last 50 min of recovery, respectively. 5. The O2 debt of the leg was 1.5 l of which the resynthesis of ATP, creatine phosphate (CP) and glycogen and reloading of haemoglobin (Hb) and myoglobin (Mb) only could account for one-third. It is proposed that the elevated oxygen uptake during recovery is linked to the metabolic use of intramuscular triacylglycerol.