GLYCOGEN RESYNTHESIS IN HUMAN MUSCLE-FIBER TYPES FOLLOWING EXERCISE-INDUCED GLYCOGEN DEPLETION

1. Studies investigating muscle glycogen resynthesis in man have usually examined mixed-fibred biopsies or have used histochemical methods to estimate single fibre resynthesis. Since the accuracy of the latter is open to debate, this study investigated glycogen resynthesis in type I and II fibres using biochemical methods of analysis. 2. Seven subjects performed one-legged cycling exercise to exhaustion. During the initial 2 h (kg body mass (BM))(-1), and a high carbohydrate diet thereafter. Muscle biopsy samples were obtained from both legs at exhaustion, and from the exercised leg after 3, 10 and 24 h of recovery. 3. In the initial 3 h of recovery, there was a 25 +/- 8% higher rate of resynthesis in type I compared with type II fibres (41 +/- 3 and 31 +/- 4 mmol glucosyl units (kg dry mass (DM))(-1) h(-1), respectively; P < 0.05). Between 3 and 10 h of recovery, resynthesis in type I fibres declined by 60 +/- 13% to 15 +/- 4 mmol glucosyl units (kg DM)(-1) h(-1) (P < 0.01), whilst the rate in type II fibres was maintained. Good agreement was found when relating the mixed-fibred muscle glycogen concentration to the mean concentration found in type I and type II fibres (r = 0.96). 4. A discrepancy was found to exist with histochemically derived data reported in the literature. The higher initial glycogen resynthesis rate in type I fibres may be attributable to fibre-type differences in glucose uptake and disposal. Thereafter the fibre glycogen concentration appears to exert a negative feedback on further resynthesis.