In vitro and at low glucose 6-phosphate concentrations, the dependent form of rat muscle glycogen synthetase is inhibited more markedly by several metabolites (adenosine triphosphate, adenosine diphosphate, inorganic phosphate, etc.) than the independent form (Piras, R., Rothman, L. B., and Cabib, E. (1968), Biochemistry 7, 56). To test the significance of these findings the levels of the metabolites involved in glycogen metabolism and the forms of glycogen synthetase and phosphorylase have been simultaneously determined under different physiological situations. In situ electrical stimulation of the posterior muscles of the rat thigh produced a tetanic contraction with the resulting increased level of phosphorylase a, glucose 6-phosphate, pyruvate, lactate, and inorganic phosphate, and a decrease of glycogen and creatine phosphate. Within 10 min of the subsequent recovery period all the metabolites returned to the resting values. The rate of glycogen resynthesis in vivo was also studied during this period. Rates increased during the first 4-min recovery, reaching a maximum of 3.2 mM/min. The largest fraction of glycogen synthetase in the independent form was found 4-5 min after the end of a 10-sec stimulation. The levels of adenosine triphosphate, adenosine diphosphate, adenosine monophosphate, and uridine diphosphate glucose did not vary significantly throughout the experiments. A comparison was carried out between the rates of glycogen resynthesis in vivo and those obtained in vitro under conditions which intended to simulate the concentrations of metabolites and the forms of glycogen synthetase found at rest, during contraction, and recovery. The relative rates of glycogen synthesis obtained when only glycogen synthetase was present in the assays, or the enzyme and glucose 6-phosphate, were different from those found in vivo. On the other hand, when the inhibitory metabolites were included in the assay, the relative rates were similar to those found in vivo, suggesting that these effectors play a role in the regulation of the enzyme. It is concluded that the regulation of glycogen synthetase activity under the conditions of muscular contraction and recovery can be best explained by the interconversion between the dependent and independent forms of the enzyme, and the differential inhibition and reversion by effectors of the two forms of glycogen synthetase. Both mechanisms act concomitantly, and have the effect of switching on and off gylcogen synthetase activity according to the physiological situation. © 1969, American Chemical Society. All rights reserved.
