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Exercise Physiology and Metabolism Laboratory, Department of Physiology, The University of Melbourne, Parkville, Victoria 3052, Australia
A depletion of phosphocreatine (PCr), fall in
the total adenine nucleotide pool (TAN = ATP + ADP + AMP), and increase
in TAN degradation products inosine 5'-monophosphate (IMP) and
hypoxanthine are observed at fatigue during prolonged exercise at 70%
maximal O2 uptake in untrained
subjects [J. Baldwin, R. J. Snow, M. F. Carey, and M. A. Febbraio. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol.
46): R295-R300, 1999]. The present study aimed to examine
whether these metabolic changes are also prevalent when exercise is
performed below the blood lactate threshold (LT). Six healthy,
untrained humans exercised on a cycle ergometer to voluntary exhaustion
at an intensity equivalent to 93 ± 3% of LT (~65% peak
O2 uptake). Muscle biopsy samples
were obtained at rest, at 10 min of exercise, ~40 min before fatigue
(F
40 =143 ± 13 min), and at fatigue (F = 186 ± 31 min).
Glycogen concentration progressively declined
(P < 0.01) to very low levels at
fatigue (28 ± 6 mmol glucosyl U/kg dry wt). Despite this, PCr
content was not different when F40 was compared with F and was
only reduced by 40% when F was compared with rest (52.8 ± 3.7 vs.
87.8 ± 2.0 mmol/kg dry wt; P < 0.01). In addition, TAN concentration was not reduced, IMP did not
increase significantly throughout exercise, and hypoxanthine was not
detected in any muscle samples. A significant correlation
(r = 0.95;
P < 0.05) was observed between
exercise time and glycogen use, indicating that glycogen availability
is a limiting factor during prolonged exercise below LT. However, because TAN was not reduced, PCr was not depleted, and no correlation was observed between glycogen content and IMP when glycogen stores were
compromised, fatigue may be related to processes other than those
involved in muscle high-energy phosphagen metabolism.
total adenine nucleotides; phosphocreatine; lactate threshold; glycogen
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