Effects of exercise and conditioning on rat heart glycogen and glycogen synthase

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Abstract
Glycogen content and glycogen synthase activity were studied in hearts of nonconditioned and treadmill-conditioned male Long-Evans rats. There was no significant difference in glycogen content, synthase I (G-6-P-independent activity), synthase D (G-6-P-dependent activity), I + D activity or synthase activation state (I/(I + D)) between nonconditioned rats and rested, conditioned rats. Glycogen and synthase were also measured in rat hearts from 0 to 24 h after a single mild or strenuous run. In nonconditioned rats, the mild run produced slight glycogen supercompensation 22-150 min after the run. Synthase activation preceded supercompensation, appearing activated at 9 min postrun, but returning to control by 22 min. In nonconditioned rats, the strenuous run produced significant glycogen supercompensation 1-16 h postrun. Synthase activation preceded supercompensation, being activated at 9 min after the run and declining over the next 2 h. In conditioned rats, the mild run did not result in significant supercompensation or synthase activation. The strenuous run produced slight glycogen supercompensation, similar to that in nonconditioned rats after the mild run. Synthase was activated immediately after this exercise. Exercise conditioning does not alter the resting level of glycogen or glycogen synthase activity but does alter the pattern of glycogen supercompensation following a given bout of exercise. The patterns of glycogen synthase activation correlate with the glycogen supercompensation patterns observed.