EFFECT OF LOW GLYCOGEN ON CARBOHYDRATE AND ENERGY-METABOLISM IN HUMAN MUSCLE DURING EXERCISE

The effect of preexercise muscle glycogen content on the metabolic responses to exercise has been investigated. Seven men cycled at a work load calculated to elicit 75% of maximal oxygen uptake [211 +/- 17 (SE) W] on two occasions: 1) to fatigue (37.2 +/- 5.3 min) and 2) at the same work load and for the same duration as the first. Biopsies were obtained from the quadriceps femoris muscle before and after exercise. Before the first experiment, muscle glycogen was lowered by exercise and diet, and before the second experiment, muscle glycogen was elevated. In the low-glycogen condition (LG), muscle glycogen decreased from 182 +/- 15 at rest to 7 +/- 4 mmol glucosyl units/kg dry wt at fatigue, while in the high-glycogen condition (HG), glycogen decreased from 725 +/- 31 at rest to 353 +/- 53 mmol glucosyl units/kg dry wt at the end of exercise. Hexose monophosphates were not increased after LG exercise but increased approximately fivefold after HG exercise. Lactate increased more during HG exercise (LG = 16 +/- 5, HG = 61 +/- 7 mmol/kg dry wt; P less-than-or-equal-to 0.001), whereas IMP increased more during LG (LG = 2.8 +/- 0.6, HG = 0.9 +/- 0.2 mmol/kg dry wt; P less-than-or-equal-to 0.05). The increases in the sum of tricarboxylic acid cycle intermediates (TCAI; citrate + malate + fumarate) and acetylcarnitine (which is in equilibrium with acetyl CoA) were significantly greater during HG exercise (P less-than-or-equal-to 0.05). It is suggested that the increase in IMP during LG exercise reflects larger increases in free ADP and AMP at the enzymatic site (AMP deaminase) during the contraction. The larger increases in ADP and AMP during LG exercise are considered necessary to activate phosphofructokinase, the TCA cycle, and oxidative phosphorylation under conditions in which glycolysis does not provide adequate substrate for formation of acetyl CoA and TCAI.