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1 Copenhagen Muscle Research Centre, Rigshospitalet, DK-2200 Copenhagen N, Denmark; and 2 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1
We examined the relationship between
tricarboxylic acid (TCA) cycle intermediate (TCAI) pool size, TCA cycle
flux (calculated from leg O2
uptake), and pyruvate dehydrogenase activity
(PDHa) in human skeletal muscle.
Six males performed moderate leg extensor exercise for 10 min, followed
immediately by intense exercise until exhaustion (3.8 ± 0.5 min).
The sum of seven measured TCAI (
TCAI) increased
(P 
0.05) from 1.39 ± 0.11 at
rest to 2.88 ± 0.31 after 10 min and to 5.38 ± 0.31 mmol/kg dry
wt at exhaustion. TCA cycle flux increased ~70-fold during submaximal
exercise and was ~100-fold higher than rest at exhaustion.
PDHa corresponded to 77 and 90%
of TCA cycle flux during submaximal and maximal exercise, respectively.
The present data demonstrate that a tremendous increase in TCA cycle
flux can occur in skeletal muscle despite a relatively small
change in TCAI pool size. It is suggested that the increase in TCAI
during exercise may primarily reflect an imbalance between the
rate of pyruvate production and its rate of oxidation in the TCA cycle.
pyruvate dehydrogenase complex; muscle oxygen uptake; amino acids; metabolism
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