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 O-2 consumption ((V) over dot O-2max)]. 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% (V) over dot O-2max, 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 mu mol/kg dry muscle). During exercise at 35% (V) over dot O-2max, M-CoA decreased fi om rest at 1 min (1.85 +/- 0.29 to 1.20 +/- 0.12 mu mol/kg dry muscle) but returned to rest level by 10 min (1.86 +/- 0.25 mu mol/kg dry muscle). M-CoA content did not decrease during cycling at 65% (V) over dot O-2max. At 90% (V) over dot O-2max, 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% (V) overdot O-2max. Furthermore, M-CoA content does not increase during exercise at 90% (V) over dot O-2max and does not contribute to the lower rate of fat oxidation at this power output.