THE REGULATION OF MUSCLE GLYCOGEN RESYNTHESIS IN FED RATS DURING RECOVERY FROM HIGH-INTENSITY EXERCISE

Abstract

The aim of this study was to determine the role of phosphorylation state of glycogen synthase and glycogen phosphorylase in the regulation of muscle glycogen repletion in fed rats recovering from high-intensity exercise. Groups of rats in the absorptive state were swum to exhaustion and allowed to recover for up to 60 minutes without access to food. Swimming to exhaustion resulted in substantial glycogen breakdown and lactate accumulation in the tibialis anterior, red, white and mixed gastrocnemius muscles, whereas the glycogen content in the soleus muscle remained stable. During 30 minutes of recovery, significant and rapid repletion of glycogen occurred in all muscles examined except the soleus muscle. Lactate returned to basal rapidly within 15 minutes after exercise in all muscles examined. A significant decrease in liver glycogen concentration was observed following exercise but returned to basal within 15 minutes of recovery. Exercise resulted in significant elevation of plasma glucose level. However, plasma glucose returned to pre-exercise level within 15 minutes of recovery. These findings indicate that liver glycogen and plasma glucose, other than lactate, are likely to be major carbon precursors for the repletion of muscle glycogen in fed rats following high-intensity exercise. At the onset of recovery, the fractional velocities of glycogen synthase in the red, white and mixed gastrocnemius muscles were higher than basal and remained elevated for 15 minutes before returning to pre-exercise levels within 30 minutes after exercise. In contrast, the activity ratios of glycogen phosphorylase in the same muscles were lower than basal and remained low for 15 minutes before increasing to pre-exercise levels within 30 minutes of recovery. This pattern of changes in glycogen synthase and phosphorylase activities, never observed before, following high-intensity exercise suggests that the integrated regulation of the phosphorylation state of both glycogen synthase and phosphorylase in the control of glycogen resynthesis in the skeletal muscle is sensitive to the nutritional status of the animal.