Regulation of skeletal muscle glycogen phosphorylase and PDH at varying exercise power outputs
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This study investigated the transformational and posttransformational control of skeletal muscle glycogen phosphorylase and pyruvate dehydrogenase (PDH) at three exercise power outputs [35, 65, and 90% of maximal oxygen uptake (VO2 max)]. Seven untrained subjects cycled at one power output for 10 min on three separate occasions, with muscle biopsies at rest and 1 and 10 min of exercise. Glycogen phosphorylase in the more active (a) form was not significantly different at any time across power outputs (21. 4-29.6%), with the exception of 90%, where it fell significantly to 15.3% at 10 min. PDH transformation increased significantly from rest (average 0.53 mmol . kg wet muscle-1 . min-1) to 1 min of exercise as a function of power output (1.60 +/- 0.26, 2.77 +/- 0.29, and 3.33 +/- 0.31 mmol . kg wet muscle-1 . min-1 at 35, 65, and 90%, respectively) with a further significant increase at 10 min (4.45 +/- 0.35) at 90% VO2 max. Muscle lactate, acetyl-CoA, acetylcarnitine, and free ADP, AMP, and Pi were unchanged from rest at 35% VO2 max but rose significantly at 65 and 90%, with accumulations at 90% being significantly higher than 65%. The results of this study indicate that glycogen phosphorylase transformation is independent of increasing power outputs, despite increasing glycogenolytic flux, suggesting that flux through glycogen phosphorylase is matched to the demand for energy by posttransformational factors, such as free Pi and AMP. Conversely, PDH transformation is directly related to the increasing power output and the calculated flux through the enzyme. The rise in PDH transformation is likely due to increased Ca2+ concentration and/or increased pyruvate. These results demonstrate that metabolic signals related to contraction and the energy state of the cell are sensitive to the exercise intensity and coordinate the increase in carbohydrate use with increasing power output.
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