REFEEDING AFTER STARVATION INVOLVES A TEMPORAL SHIFT IN THE CONTROL SITE OF GLYCOGEN-SYNTHESIS IN RAT MUSCLE

The starved-to-fed transition is accompanied by rapid glycogen deposit ion in skeletal muscles. On the basis of recent findings [Brau, Ferrei ra, Nikolovski, Raja, Palmer and Fournier (1997) Biochem. J. 322, 303- 308] that during recovery from exercise there is a shift from a glucos e 6-phosphate/phosphorylation-based control of glycogen synthesis to a phosphorylation-based control alone, this paper seeks to establish wh ether a similar shift occurs in muscle during re-feeding after starvat ion in the rat. Chow re-feeding after 48 h of starvation resulted in g lycogen deposition in all muscles examined (white, red and mixed quadr iceps, soleus and diaphragm) to levels higher than those in the fed st ate. Although the early phase of re-feeding was associated with increa ses in glucose 6-phosphate levels in all muscles, there was no accompa nying increase in the fractional velocity of glycogen synthase except in the white quadriceps muscle. This finding, together with the observ ation that the fractional velocity of glycogen synthase in most muscle s was already high in the starved state, suggests that in the initial phase of glycogen deposition the phosphorylation state of the enzyme m ay be adequate to support net glycogen synthesis. In the later phase o f re-feeding, the progressive decrease in the fractional velocity of g lycogen synthase in association with a decrease in the rate of glycoge n deposition suggests that glycogen synthesis is controlled primarily by changes in the phosphorylation state of glycogen synthase. In concl usion, this study suggests that there is a temporal shift in the site of control of glycogen synthesis as glycogen deposition progresses dur ing re-feeding after starvation.