Time course of insulin sensitivity and skeletal muscle glycogen synthase activity after a single bout of exercise in horses.

The research article explores how a single intense exercise bout impacts insulin sensitivity, muscle glycogen and GLUT4 content, and glycogen synthase activity in horses. The study showed that the exercise resulted in no increase in glucose infusion rate, a significant reduction in muscle glycogen content, and higher glycogen synthase activity, all of which suggest a slow rate of muscle glycogen resynthesis in horses post-exercise.

Research Design and Procedure

• The study involved eight horses that underwent intense exercise; after which their insulin sensitivity, skeletal muscle glycogen and GLUT4 content, and glycogen synthase activity were tracked at 0.5, 4, and 24 hours post-exercise. As a control, these metrics were also examined after 48 hours of rest.
• To measure insulin sensitivity, a euglycemic-hyperinsulinemic clamp (EHC) was used. This is a common method for gauging how well the body uses insulin.

Key Findings

• The glucose infusion rate was not increased with exercise, and it was significantly decreased at 0.5 hours after exercise. The glucose infusion rate is crucial because it indicates how effectively glucose can be transported into cells, a process that insulin regulates.
• Before each EHC, the muscle glycogen content was lower in exercised horses than in rested ones, indicating that exercise depletes muscle glycogen stores. However, GLUT4 content was unchanged. GLUT4 is a protein that facilitates glucose transportation in cells.
• The active-to-total glycogen synthase activity ratio was higher in horses after exercise than in the control. This finding suggests that exercise increased glycogen synthase activity, which is integral for glycogen synthesis.

Conclusions and Interpretations

• An inverse correlation was found between glycogen content and glycogen synthase activity ratio. But, no relationship was concluded between glycogen synthase activity and the glucose infusion rate.
• Overall, the findings suggest that intense exercise results in a substantial reduction in muscle glycogen content and an increase in glycogen synthase activity. However, it does not enhance insulin sensitivity as determined by EHC. This outcome aligns with previous observations of the slow rate of muscle glycogen resynthesis in equines after exercise.