Pyruvate dehydrogenase and glycogen synthase activity at transition from fasted to fed state

The aim of the present study was to evaluate whether the PDC and GS activities at the transition from fasted into fed state are consistent with indirect pathway for glycogen synthesis, as suggested previously. Refeeding of glucose given to rats after 72 hr of starvation did not reactivate PDC in the liver; however, the PDC activity in the muscle was increased. In comparison to PDC, glucose refeeding leads to an opposite effect on GS in both liver and muscle as evidenced by the immediate increase in the active form of GS. The low activity of liver PDC restricts 3-carbon flux through the Krebs cycle and enables their transfer to the gluconeogenic pathway for glycogen synthesis. In contrast, an immediate activation of muscle PDC following refeeding indicates that 3-carbon flux will be oxidized in the citric acid cycle, which thereby eliminates the indirect pathway for glycogen synthesis in this tissue. Glucose infusion increased plasma lactate, insulin, and glycogen content in the liver and muscle to the same extent as observed in the fed rats. The results are in agreement with the suggestion that at the transition from fasted to fed state, liver glycogen synthesis occurs mainly from 3-carbon precursors.