TRAINING DECREASES MUSCLE GLYCOGEN TURNOVER DURING EXERCISE

The present study was undertaken to determine the effects of endurance training on glycogen kinetics during exercise. A new model describing glycogen kinetics was applied to quantitate the rates of synthesis an d degradation of glycogen. Trained and untrained rats were infused wit h a 25% glucose solution with 6-H-3-glucose and U-C-14-lactate at 1.5 and 0.5 mu Ci . min(-1) (where 1 Ci = 3.7 x 10(10) Bq), respectively, during rest (30 min) and exercise (60 min). Blood samples were taken a t IO-min intervals starting just prior to isotopic infusion, until the cessation of exercise. Tissues harvested after the cessation of exerc ise were muscle (soleus, deep, and superficial vastus lateralis, gastr ocnemius), liver, and heart. Tissue glycogen was quantitated and analy zed for incorporation of H-3 and C-14 via liquid scintillation countin g. There were no net decreases in muscle glycogen concentration from t rained rats, whereas muscle glycogen concentration decreased to as muc h as 64% (P < 0.05) in soleus in muscles from untrained rats after exe rcise. Liver glycogen decreased in both trained (30%) and untrained (4 0%) rats. Glycogen specific activity increased in all tissues after ex ercise indicating isotope incorporation and, thus, glycogen synthesis during exercise. There were no differences in muscle glycogen synthesi s rates between trained and untrained rats after exercise. However, tr aining decreased muscle glycogen degradation rates in total muscle (i. e., the sum of the degradation rates of all of the muscles sampled) te nfold(P < 0.05). We have applied a model to describe glycogen kinetics in relation to glucose and lactate metabolism during exercise in trai ned and untrained rats. Training significantly decreases muscle glycog en degradation rates during exercise.