Re-feeding after starvation involves a temporal shift in the control site of glycogen synthesis in rat muscle

Abstract

The starved-to-fed transition is accompanied by rapid glycogen deposition in skeletal muscles. On the basis of recent findings [Bra$ u, Ferreira, Nikolovski, Raja, Palmer and Fournier (1997) Biochem. J. 322, [303][304][305][306][307][308] that during recovery from exercise there is a shift from a glucose 6-phosphate\phosphorylation-based control of glycogen synthesis to a phosphorylation-based control alone, this paper seeks to establish whether a similar shift occurs in muscle during re-feeding after starvation in the rat. Chow re-feeding after 48 h of starvation resulted in glycogen deposition in all muscles examined (white, red and mixed quadriceps, soleus and diaphragm) to levels higher than those in the fed state. Although the early phase of re-feeding was associated with increases in glucose 6-phosphate levels in all muscles, there was no accompanying increase in the fractional

In late pregnancy insulin-dependent glucose transport/phosphorylation is selectively impaired and activation of glycogen synthase by insulin facilitated in skeletal muscles of 24-h starved rats

“…Statistically significant differences between unmated and 20-daypregnant rats in either the basal or hyperinsulinaemic states are indicated by ** p < 0.01; *** p < 0.001. Statistical analysis of the effects of hyperinsulinaemia is provided in the text pendent activity) was not observed in association with glycogen storage in slow-twitch skeletal muscle [14]. Showing impaired glucose transport and phosphorylation by skeletal muscle in the basal (24-h starved) state and after euglycaemic hyperinsulinaemia in late pregnancy afforded a further opportunity to examine the potential relation between glucose phosphorylation and changes in glycogen synthase phosphorylation state in vivo.…”

“…Activation of glycogen synthase through dephosphorylation results in a reduced dependence on glucose-6-phosphate (G6P) for activity [13]. During the early phase of refeeding after starvation, both glucose uptake/phosphorylation rates and glycogen synthase phosphorylation state contribute to the regulation of glycogen synthesis; however, later there is a shift to control based solely upon changes in glycogen synthase phosphorylation state [14]. Our study was designed to investigate the mechanism by which muscle glycogen storage is protected during refeeding after starvation in late pregnancy.…”

Lactate availability is not the major factor limiting muscle glycogen repletion during recovery from an intense sprint in previously active fasted rats

“…The elevated levels of glucose 6-phosphate at the onset of recovery in this study might contribute to the initial activation of glycogen synthesis, but the early return of this metabolite to basal pre-exercise levels, despite ongoing glycogen synthesis until late into recovery, suggests that glucose 6-phosphate does not play a major role in the control of glycogen synthesis during late recovery (Fig.·3). This constitutes one of several physiological conditions where changes in glucose 6-phosphate levels do not play a major role in the control of the rate of glycogen synthesis in skeletal muscles (James et al, 1998;Lawrence and Roach, 2000;Fournier et al, 2002).…”

“…The homogenates were centrifuged at 2000·g for 10·min, and the supernatants further diluted fivefold with glycerol-free buffer before assay. The fractional velocity was determined using an assay that consists of measuring the activity of the enzyme at a sub-saturating near-physiological level of UDP-glucose (0.03·mmol·l -1 ) in the presence of either low (0.1·mmol·l -1 ) or high (5.0·mmol·l -1 ) glucose 6-phosphate concentrations (Bräu et al, 1997;James et al, 1998;Ferreira et al, 2001). Under these conditions, the reaction rates of glycogen synthase in the presence of low or high glucose 6-phosphate levels were linear with respect to both the amount of extract used and incubation time.…”

“…Since the phosphorylation of glycogen synthase results in its inactivation when measured in the presence of low levels of glucose 6-phosphate, it follows that the higher the phosphorylation state of this enzyme, the lower its fractional velocity. The determination of the fractional velocity of glycogen synthase was performed as described in recent publications from this laboratory (James et al, 1998;Ferreira et al, 2001), using the filter paper method of Thomas et al (1968). Briefly, muscles previously weighed and ground were homogenised in the presence of 10 volumes of glycerol buffer [50·mmol·l -1 Tris-HCl (pH 7.8 at 25°C), 100·mmol·l -1 KF, 10·mmol·l -1 EDTA, 60% (v/v) glycerol at -20°C].…”