RESYNTHESIS OF GLYCOGEN IN SKELETAL-MUSCLE FROM STANDARD-BRED TROTTERS AFTER REPEATED BOUTS OF EXERCISE

Objective-To determine glycogen resynthesis rate and changes in plasma metabolite concentrations in horses before and after repeated exercis e. Animals-6 clinically normal Standardbred trotters. Procedure-Horses trotted distances of 3,000, 3,000, and 2,000 m (trial A) and 3 days l ater, trotted 2,100, 2,100, and 1,600 m (trial B). Horses had 1 hour r est periods between bouts of exercise. Trotting speed was increased wi th each exercise bout, up to a near maximal. Muscle biopsy specimens a nd venous blood samples were obtained before each trial and 0, 4, 24, 48, and 72 hours after the third bout. Blood samples were also taken b etween exercise bouts. Muscle glycogen content and plasma glucose, gly cerol, nonesterified fatty acid, and triglyceride concentrations were determined. Results-Muscle glycogen content was significantly decrease d immediately after exercise from 473 +/- 45 to 329 +/- 79 mmol/kg of dry weight in trial A, and from 472 +/- 128 to 347 +/- 59 mmol/kg in t rial B. Further decreases were measured 4 hours after exercise. Glycog en resynthesis was negligible 24 hours after exercise. Basal muscle co ncentrations of glycogen were obtained 72 hours after exercise in tria l A (472 +/- 128 mmol/kg), but not in trial B (279 +/- 52 mmol/kg). Pl asma concentrations of glucose were greater than or equal to before-ex ercise values. Plasma concentrations of lipid metabolites, glycerol, t riglycerides, and nonesterified fatty acids, were less than before-exe rcise values 2 to 72 hours after exercising. Conclusions-Repeated bout s of exercise decrease glycogen repletion rate, which is not attributa ble to hypoglycemia, but may be influenced by limited availability of lipids for energy production.