Insulin-independent glycogen supercompensation in isolated mouse skeletal muscle: role of phosphorylase inactivation

Glycogen supercompensation (increase in muscle glycogen content above basal) is an established phenomenon induced by unknown mechanisms. It consists of both insulin-dependent and -independent components. Here, we investigate insulin-independent glycogen supercompensation in isolated, intact extensor digitorum longus muscles from mice. Muscles were stimulated electrically, incubated in vitro with 5.5 mM glucose for up to 16 h and then analysed for glycogen, glucose uptake and enzyme activities. Basal glycogen was 84±6 µmol glucosyl units/g dry muscle and was depleted by 80% after 10 min contraction. Glycogen increased after contraction, reaching a peak value of 113±9 µmol glucosyl units/g dry muscle (P<0.05 vs. basal) by 6 h, and returned to basal values by 16 h (84±8). Maximal activities of glycogen synthase, phosphorylase and α-glucosidase were not significantly altered by contraction or during the 6-h recovery period. Glycogen synthase fractional activity (0.17/7.2 mM glucose-6-P; inversely related to phosphorylation state of the enzyme) was increased about twofold early after contraction but then decreased and was slightly lower than baseline during the period of supercompensation (4–6 h). Phosphorylase fractional activity (±adenosine monophosphate; directly related to phosphorylation state of the enzyme) decreased to 60% of basal after contraction and decreased further during the initial 4 h of recovery to 40% of basal (P<0.01 vs. basal). After 4 h recovery, glucose uptake was slightly (50%) higher in the stimulated than in the non-stimulated muscle (P<0.01). Thus, insulin-independent glycogen supercompensation involves inactivation of phosphorylase and hence an inhibition of glycogen breakdown.