Glycogen synthase activation in human skeletal muscle: Effects of diet and exercise

Abstract

We investigated the role of glycogen synthase in supranormal resynthesis (super-compensation) of skeletal muscle glycogen after exhaustive exercise. Six healthy men exercised 60 min by cycling with one leg at 75% VO2(max) recovered 3 days on a low-carbohydrate diet, exercised again, and recovered 4 days on high-carbohydrate diet. Glycogen and glycogen synthase activities at several glucose-6-phosphate (G6P) concentrations were measured in biopsy samples of m. vastus lateralis. Dietary alterations alone did not affect glycogen, whereas exercise depleted glycogen stores. After the second exercise bout, glycogen returned to normal within 24 hr and reached supercompensated levels by 48 hr of recovery. Glycogen synthase activation state strikingly increased after exercise in exercised muscle and remained somewhat elevated for the first 48 hr of recovery in both muscles. We suggest that forms of glycogen synthase intermediate to I (G6P-independent) and D (G6P-dependent) forms are present in vivo, and glycogen supercompensation can in part be explained by the formation of intermediate forms of glycogen synthase that exhibit relatively low activity ratios, but an increased sensitivity to activation by G6P.