Cardiac glycogen repletion after exercise: influence of synthase and glucose 6-phosphate

Previous studies have shown that cardiac glycogen content is replenished to supranormal levels after exercise. To better understand this phenomenon glycogen synthase (GS) %I activity as well as glucose 6-phosphate (G-6-P) and glycogen (G) concentrations were measured in hearts from normal and alloxan diabetic rats killed at various time intervals before or after a treadmill run to exhaustion. In normal animals at exhaustion G was reduced but GS %I and G-6-P were elevated. During 2-8 h of recovery G was elevated, %I depressed, and G-6-P unchanged when compared to their preexercise values. By contrast, diabetic hearts that had elevated G and low %I were characterized by abnormally high G-6-P concentrations. At exhaustion in these hearts %I increased significantly in response to a decrease in G. These findings suggest that glycogen repletion occurs in normal heart as a result of the combined increases in GS %I and G-6-P levels present at the cessation of work. In both normal and diabetic hearts there exists an inverse relationship between G content and GS %I activity.