REGULATION OF MUSCLE GLYCOGEN-PHOSPHORYLASE ACTIVITY FOLLOWING SHORT-TERM ENDURANCE TRAINING

The purpose of this study was to examine the regulation (hormonal, sub strate, and allosteric) of muscle glycogen phosphorylase (Phos) activi ty and glycogenolysis after short-term endurance training. Eight untra ined males completed 6 days of cycle exercise (2 h/day) at 65% of maxi mal O-2 uptake (Vo(2max)). Before and after training subjects cycled f or 15 min at 80% of VO2max, and muscle biopsies and blood samples were obtained at 0 and 30 s, 7.5 and 15 min, and 0, 5, 10, and 15 min of e xercise. VO2max was unchanged with training but citrate synthase (CS) activity increased by 20%. Muscle glycogenolysis was reduced by 42% du ring the 15-min exercise challenge following training (198.8 +/- 36.9 vs. 115.4 +/- 25.1 mmol/kg dry muscle), and plasma epinephrine was blu nted at 15 min of exercise. The Phos a mole fraction was unaffected by training. Muscle phosphocreatine utilization and free P-i and ANLP ac cumulations were reduced with training at 7.5 and 15 min of exercise. It is concluded that posttransformational control of Phos, exerted by reductions in substrate (free P-i) and allosteric modulator (free AMP) contents, is responsible for a blunted muscle glycogenolysis after 6 days of endurance training. The increase in CS activity suggests that the reduction of muscle glycogenolysis was due in part to an enhanced mitochondrial potential.