Hexosephosphate formation and the regulation of glycolysis in muscle

The steady state concentrations of the hexosephosphates, and tracer data with ³²P_i suggest that the formation of glucose-6-phosphate in resting muscle takes place predominantly from the glucose and P_i present in the extracellular phase, and that the phosphorylase system is essentially inactive under these conditions. The rate of glycogenolysis in tetanic contraction is such that it is adequate to support the very high rate of lactic acid formation under these extreme conditions. The apparent primary rate-limiting step in glycolysis is the phosphohexose isomerase reaction, both in resting muscle and in contraction. The activity of phosphofructokinase in resting muscle is sufficient so that this reaction is not rate-limiting; however, the activation of this enzyme in tetanic contraction is not necessarily adequate to the situation, and this reaction may then become rate-limiting. The tracer data indicate that the formation of fructose-1,6-diphosphate in contraction may take place by some mechanism other than the phosphofructokinase reaction.