Skeletal muscle malonyl-CoA content at the onset of exercise at varying power outputs in humans

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 (VO2 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% VO2 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 micromol/kg dry muscle). During exercise at 35% VO2 max, M-CoA decreased from rest at 1 min (1.85 +/- 0.29 to 1.20 +/- 0.12 micromol/kg dry muscle) but returned to rest level by 10 min (1.86 +/- 0.25 micromol/kg dry muscle). M-CoA content did not decrease during cycling at 65% VO2 max. At 90% VO2 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% VO2 max. Furthermore, M-CoA content does not increase during exercise at 90% VO2 max and does not contribute to the lower rate of fat oxidation at this power output.