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.