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Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5; and Department of Clinical Chemistry, Huddinge University Hospital, S 141 86 Huddinge, Sweden
To investigate the regulation of intramuscular
fuel selection, we measured the malonyl-CoA (M-CoA) content in human
skeletal muscle at three exercise power outputs [35, 65, and 90%
maximal rate of O2 consumption
(
O2 max)]. Four
males and four females cycled for 10 min at one power output on three
separate occasions with muscle biopsies sampled at rest and at 1 and 10 min. The respiratory exchange ratio was 0.84 ± 0.03, 0.92 ± 0.02, and >1.0 at 35, 65 and 90%
O2 max, respectively.
Muscle lactate content increased and phosphocreatine content decreased
as a function of power output. Pyruvate dehydrogenase
a activity increased from 0.40-0.64 mmol · kg wet
muscle
1 · min
1
at rest to 1.57 ± 0.28, 2.80 ± 0.41, and 3.28 ± 0.27 mmol · kg wet
muscle
1 · min
1
after 1 min of cycling at the three power outputs, respectively. Mean
resting M-CoA contents were similar at all power outputs (1.85-1.98 µmol/kg dry muscle). During exercise at 35%
O2 max, M-CoA decreased from rest at 1 min (1.85 ± 0.29 to 1.20 ± 0.12 µmol/kg dry muscle) but returned to rest level by 10 min (1.86 ± 0.25 µmol/kg dry muscle). M-CoA content did not decrease
during cycling at 65%
O2 max. At 90%
O2 max, M-CoA did not
increase despite significant acetyl-CoA accumulation (the substrate for M-CoA synthesis). The data suggest that a decrease in M-CoA content is
not required for the increase in free fatty acid uptake and oxidation
that occurs during exercise at 35 and 65%
O2 max. Furthermore,
M-CoA content does not increase during exercise at 90%
O2 max and does not
contribute to the lower rate of fat oxidation at this power
output.
fatty acid oxidation; acetyl-coenzyme A; acetyl-coenzyme A carboxylase; carnitine palmitoyltransferase I; high-performance liquid chromatography; malonyl-coenzyme A
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