EXERCISE AND EXHAUSTION EFFECTS ON GLYCOGEN-SYNTHESIS PATHWAYS

Female Sprague-Dawley rats were infused with [1-C-13]glucose to measur e 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 trai ned exhausted (TE)] were studied. T and TE rats were trained on a moto r-driven treadmill (30 m/min, 15% grade, 1.0 h/day: 5 days/wk) for 8-1 0 wk. After a 24-h fast, SE and TE rats were run to exhaustion (sedent ary average = 78 min, trained average = 155 min) at a training pace an d 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 gl ycogen was isolated and hydrolyzed to glucose. The glucose was measure d for distribution of C-13 by using nuclear magnetic resonance. Glycog en was synthesized predominantly by the indirect pathway for all metab olic states, indicating that infused glucose was first metabolized pri marily in the peripheral tissue. The direct-pathway utilization was gr eater 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, dramaticall y decreased utilization of the direct pathway, with exhaustive exercis e changing from 30 to 9%. The results indicate that endurance training modifies glucose utilization during glycogen synthesis after fasting and exhaustive exercise.