Exercise and exhaustion effects on glycogen synthesis pathways

Female Sprague-Dawley rats were infused with [1-¹³C]glucose to measure the effect of endurance training and the effect of various metabolic conditions on pathways of hepatic glycogen synthesis. Four metabolic states [sedentary (S), trained (T), sedentary exhausted (SE), and trained exhausted (TE)] were studied. T and TE rats were trained on a motor-driven treadmill (30 m/min, 15% grade, 1.0 h/day, 5 days/wk) for 8-10 wk. After a 24-h fast, SE and TE rats were run to exhaustion (sedentary average = 78 min, trained average = 155 min) at a training pace and immediately infused with labeled glucose for 2 h. S and T rats were infused after a 24-h fast. After infusion, tissues were removed and glycogen was isolated and hydrolyzed to glucose. The glucose was measured for distribution of ¹³C by using nuclear magnetic resonance. Glycogen was synthesized predominantly by the indirect pathway for all metabolic states, indicating that infused glucose was first metabolized primarily in the peripheral tissue. The direct-pathway utilization was greater in rested S than in rested T animals (30 vs. 14%); however, for exhausted animals, the trained use of the direct pathway was greater (22 vs. 9%). Both TE and rested T animals utilize the indirect pathway a comparable amount. Sedentary animals, on the other hand, dramatically decreased utilization of the direct pathway, with exhaustive exercise changing from 30 to 9%. The results indicate that endurance training modifies glucose utilization during glycogen synthesis after fasting and exhaustive exercise.