In situ regulation of glycolysis in tetanized cat skeletal muscle

Abstract The concentrations of metabolites in tetanized cat gastrocnemius muscle, in situ , have been measured to determine the enzymic sites of regulation of glycolysis in mammalian skeletal muscle. Increases of approximately 10-fold in the levels of glucose-6-P and fructose-6-P were found. The concentrations of pyruvate and laetate also increased, but none of the other glycolytic intermediates or citrate accumulated appreciably. The concentration of creatine-P in the tetanized muscle decreased, but those of ATP, ADP, and AMP were maintained at control levels. These findings show that the phosphohexose isomerase reaction is at, or near, equilibrium even during the greatly increased glycolytic flux accompanying contraction, thus precluding it as the rate-limiting step in glycolysis, as has been suggested recently. The marked accumulation of hexosemonophosphate in tetanus and the maintenance of levels of known activators of phosphofructokinase, i.e., ADP, AMP, and fructose-1,6-diP, near those of the unstimulated muscle, suggest that, even during tetanus, phosphofructokinase is functioning at a rate less than optimal and is limiting the rate of glycolysis. Also, the importance of the phosphorylase reaction is demonstrated by the finding that 90% or more of the hexose entering the glycolytic pathway during a 30-second tetanic contraction stems from glycogen rather than from blood sugar. Thus these results are in accord with the concept of glycogenolytic control in skeletal muscle by phosphorylase and phosphofructokinase.