INFLUENCE OF MUSCLE GLYCOGEN ON GLYCOGENOLYSIS AND GLUCOSE-UPTAKE DURING EXERCISE IN HUMANS

To examine the effects of alterations in preexercise muscle glycogen a vailability on glycogenolysis and glucose uptake during exercise, 12 a ctive but untrained men [22.8 +/- 1.6 (SE) yr, 71.7 +/- 2.0 kg, peak p ulmonary oxygen uptake 3.85 +/- 0.16 l/min] were studied during 40 min of cycle ergometer exercise at 65-70% peak pulmonary oxygen uptake on two separate occasions, at least 1 wk apart. Preexercise muscle glyco gen concentrations were manipulated by having the subjects perform gly cogen-lowering exercise either 24 or 48 h before a trial, in combinati on with either high or low dietary carbohydrate intake. In series I (n = 7), increasing muscle glycogen from 90.3 +/- 6.0 to 124.7 +/- 10.8 mmol/kg wet wt increased muscle glycogenolysis during exercise (62.7 /- 7.9 vs. 49.1 +/- 6.6 mmol/kg; P < 0.05). Similarly, in series 2 (n = 5) when muscle glycogen was reduced from 96.2 +/- 6.6 to 53.7 +/- 6. 0 mmol/kg, glycogen utilization during exercise was reduced from 51.8 +/- 4.6 to 28.3 +/- 3.8 mmol/kg (P < 0.05). The altered muscle glycoge n utilization was associated with alterations in carbohydrate oxidatio n during exercise, without effect on tracer ([H-3]glucose)-determined glucose uptake. These results indicate that preexercise muscle glycoge n availability influences muscle glycogenolysis, but not glucose uptak e, during exercise.