Effects of a rapid change in muscle glycogen availability on metabolic and hormonal responses during exercise

To evaluate the metabolic and hormonal adaptations following a rapid change in muscle glycogen availability, 14 subjects had their muscle glycogen content increased in one leg (IG) and decreased in the other (DG). In group A (n = 7), subjects exercised on a bicycle ergometer at 70% maximal oxygen uptake for 20 min using the DG leg. Without resting these same subjects exercised another 20 min using the IG leg. Subjects in group B (n = 7) followed the same single-leg exercise protocol but in the reverse order. In order to get some information on the time sequence of these possible adaptations, blood samples were collected at rest and at the beginning and the end of each exercise period (min 5, 20, 25, and 40). Results indicated that 5 min after the switch from the DG leg to the IG leg, transient increases in plasma free fatty acids (1.20 to 1.39 meq X 1(-1)) and serum insulin (10.1 to 12 mu X 1(-1)) concentrations occurred. Between minute 25 and 40 of exercise, the DG to IG switch was accompanied by a decrease in free fatty acids and glycerol concentrations as well as an increase in lactate levels. An opposite response was observed in the IG to DG condition during the same time span. Plasma norepinephrine, epinephrine, glucagon, and serum cortisol concentrations were not significantly affected by the leg change. These results suggest a rapid preferential use of muscle glycogen when available and a time lag in the response of the extramuscular substrate mobilization factors.