Muscle glycogen stores are depleted during exercise and are rapidly repleted during the recovery period. To investigate the mechanism for this phenomenon, untrained male rats were run for 45 min on a motor-driven treadmill and the ability of their muscles to utilize glucose was then assessed during perfusion of their isolated hindquarters. Glucose utilization by the hindquarter was the same in exercised and control rats perfused in the absence of added insulin; however, when insulin (30-40,000 μU/ml) was added to the perfusate, glucose utilization was greater after exercise. Prior exercise lowered both, the concentration of insulin that half-maximally stimulated glucose utilization (exercise, 150 μU/ml; control, 480 μU/ml) and modestly increased its maximum effect. The increase in insulin sensitivity persisted for 4 h following exercise, but was not present after 24 h. The rate-limiting step in glucose utilization enhanced by prior exercise appeared to be glucose transport across the cell membrane, as in neither control nor exercised rats did free glucose accumulate in the muscle cell.


Erik A. Richter, Lawrence P. Garetto, Michael N. Goodman, Neil B. Ruderman


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