Regulation of glycolysis in intermittent exercise in man.

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Abstract
Healthy male volunteers (7) performed intermittent exercise (15 s work, 15 s rest) at a high work load for 60 min and 6 subjects performed continuous exercise at an equally high load to exhaustion, which occurred after 4-6 min. Muscle biopsies were obtained from the lateral portion of the quadriceps muscle before intermittent exercise and after the end of a work period and the end of the subsequent rest period at 5, 15, 30 and 60 min of exercise, as well as before, immediately after and about 15, 30, 60 and 180 s after continuous exercise. The reduction in glycogen content was smaller and glucose-6-phosphate, glycerol-1-phosphate, lactate and malate contents were lower after both work and rest periods in intermittent compared with continuous exercise, indicating a lower rate of glycolysis. ATP and CP (creatine phosphate) levels had decreased at the end of work periods in intermittent exercise but increased to slightly below basal in the subsequent rest periods. A still larger decrease in ATP and CP levels was found after continuous exercise to exhaustion and a progressive increase occurred over the 3 min of recovery. In each rest period during intermittent exercise, citrate levels increased to above basal, they increased in the recovery phase after continuous exercise, although more slowly. ATP, CP and citrate probably act as regulatory factors of glycolysis in human muscle by retarding certain rate limiting steps. The increase in G-6-P[glucose-b-phosphate]/F-1-6-P2 [fructose-1,6, diphosphate] ratio in rest periods of intermittent intense exercise and in the recovery phase of continuous intense exercise suggests that glycolysis is retarded at the phosphofructokinase reaction. These factors may contribute to the relative increase in lipid utilization during intense intermittent compared to continuous exercise.