Recovery of muscle glycogen concentrations in sled dogs during prolonged exercise

Purpose: To determine the depletion of muscle glycogen during five consecutive days of endurance exercise in Alaskan sled dogs consuming a high-fat, low-carbohydrate diet. Methods: Forty-two fit Alaskan sled dogs were used in the study, of which six dogs served as nonexercising control animals. The remaining 36 dogs ran 160 km·d^-1 for up to 5 d while consuming a diet providing approximately 50% of calories as fat and 15% as carbohydrate. Muscle biopsies were performed on six randomly selected dogs before feeding and within 4 h after each 160-km run was completed. Muscle samples were prepared for analysis of glycogen content and myosin ATPase staining. Serum creatine kinase (CK) activity was measured once before exercise and after each 160-km run. Results: Thirty-three of 36 dogs completed the runs. Muscle glycogen concentration was highest in sedentary dogs (340 ± 102 mmol·kg^-1 dry weight), declined to 73 ± 16 after 160 km and subsequently increased to similar levels between 320 and 800 km (320 km: 177 ± 34; 800 km: 213 ± 44). Postexercise serum CK activity was significantly elevated throughout the study. Conclusion: Skeletal muscle in Alaskan sled dogs has remarkable glyconeogenic ability as demonstrated by repletion to greater than 50% of resting muscle glycogen concentrations after the second of five consecutive 160-km runs even when fed a low-carbohydrate, high-fat diet. Whether this finding is attributable to rapid repletion of muscle glycogen during brief recovery periods versus progressive utilization of alternative substrates remains to be investigated.