Skeletal-muscle glycogen synthesis during the starved-to-fed transition in the rat

Glycogen content regulates insulin- but not contraction-mediated glycogen synthase activation in the rat slow-twitch soleus muscles

AIM

The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined.

METHODS

Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin.

RESULTS

Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser(641) (R = -0.93, P < 0.001) and Ser(645,649,653,657) (R = -0.85, P < 0.001) phosphorylation, but not with Ser(7) phosphorylation.

CONCLUSION

Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser(641) and Ser(645,649,653,657) seems to regulate GS activity in soleus.

Effect of acetic acid feeding on the circadian changes in glycogen and metabolites of glucose and lipid in liver and skeletal muscle of rats

The aim of the present study is to investigate the effect of acetic acid feeding on the circadian changes in glycogen concentration in liver and skeletal muscle. Rats were provided meal once daily (09.00–13.00 hours) for 10d. On the 11th day, they were either killed immediately or given 9g diet containing either 0 (control) or 0·7g/kg-diet acetic acid beginning at 09.00 hours for 4h, as in the previous regimen. Rats in the fed group were killed at 4, 8 or 24h after the start of feeding. At 4h after the start of feeding, the acetic acid group had significantly greater liver and gastrocnemius muscle glycogen concentrations (P<0·05). Also, at this same point, liver xylulose-5-phosphate, a key stimulator of glycolysis, the ratio of fructose-1,6-bisphosphate to fructose-6-phosphate in skeletal muscle, which reflects phosphofructokinase-1 activity, and liver malonyl-CoA, an allosteric inhibitor of carnitine palmitoyl-transferase, were significantly lower in the acetic acid group than in the control group (P<0·05). In addition, the acetic acid group had a significantly lower serum lactate concentration and lower ratio of insulin to glucagon than the control group at the same point (P<0·05). We conclude that a diet containing acetic acid may enhance glycogen repletion but not induce supercompensation, a large increase in the glycogen level that is beneficial in improving performance, in liver and skeletal muscle by transitory inhibition of glycolysis. Further, we indicate the possibility of a transient enhancement of fatty acid oxidation in liver by acetic acid feeding.

Effect of fasting on the intracellular metabolic partition of intravenously infused glucose in humans

The effects of fasting on the pathways of insulin-stimulated glucose disposal were explored in three groups of seven normal subjects. Group 1 was submitted to a euglycemic hyperinsulinemic clamp ( approximately 100 microU/ml) after both a 12-h and a 4-day fast. The combined use of [3-(3)H]- and [U-(14)C]glucose allowed us to demonstrate that fasting inhibits, by approximately 50%, glucose disposal, glycolysis, glucose oxidation, and glycogen synthesis via the direct pathway. In group 2, in which the clamp glucose disposal during fasting was restored by hyperglycemia (155 +/- 15 mg/dl), fasting stimulated glycogen synthesis (+29 +/- 2%) and inhibited glycolysis (-32 +/- 3%) but only in its oxidative component (-40 +/- 3%). Results were similar in group 3 in which the clamp glucose disposal was restored by a pharmacological elevation of insulin ( approximately 2,800 microU/ml), but in this case, both glycogen synthesis and nonoxidative glycolysis participated in the rise in nonoxidative glucose disposal. In all groups, the reduction in total carbohydrate oxidation (indirect calorimetry) induced by fasting markedly exceeded the reduction in circulating glucose oxidation, suggesting that fasting also inhibits intracellular glycogen oxidation. Thus prior fasting favors glycogen retention by three mechanisms: 1) stimulation of glycogen synthesis via the direct pathway; 2) preferential inhibition of oxidative rather than nonoxidative glycolysis, thus allowing carbon conservation for glycogen synthesis via the indirect pathway; and 3) suppression of intracellular glycogen oxidation.

Relative insensitivity of avian skeletal muscle glycogen to nutritive status

Previous studies in avian species have reported time-dependent losses in muscle glycogen with prolonged feed withdrawal (FW). However, cervical dislocation was used to collect tissues, a method that results in significant involuntary muscle convulsions. In this study, cervical dislocation alone was found to reduce muscle glycogen by 23%, therefore, barbiturate overdose was used to collect tissue samples before and after FW, at the end of refeeding, and from continuously fed controls at each interval. Additionally, plasma samples from 6-wk-old male chickens were taken at the initiation and end of a 24-hr feed withdrawal, and at various times during refeeding. After 24 hr of FW, liver glycogen decreased markedly (77%; P < 0.05), whereas muscle glycogen decreased slightly and transiently, such that it returned to and remained at control levels, even after prolonged (72 hr) FW. Plasma glucose was decreased, whereas glucagon was elevated after a 24-hr feed withdrawal (P < 0.05), when compared with control concentrations. Muscle glycogen levels were not significantly increased over control levels after refeeding, but liver glycogen was increased by 380% (P < 0.05). Feed deprivation followed by refeeding resulted in increased circulating insulin and glucose levels when compared with control levels. Therefore, by using methods of tissue collection that ensure that muscle glycogen determinations are not confounded by artifactual degradation, these results verify that regulation of avian muscle glycogen stores is similar to that in mammals.

Mechanisms of whole-body glycogen deposition after oral glucose in normal subjects. Influence of the nutritional status

It is known that prior fasting enhances whole-body glycogen retention after glucose ingestion. To identify the involved mechanisms, 33 normal volunteers underwent a total fast, varying between 14 h and 4 days, and ingested thereafter 75 g glucose labeled with [14C]glucose. Measurements of oral glucose oxidation (expired 14CO2, corrected for incomplete recovery) and total carbohydrate (CHO) oxidation (indirect calorimetry) were performed over the following 5 h. These data allowed us to calculate oral glucose storage (uptake oxidation), glycogen oxidation (CHO oxidation - oral glucose oxidation), and net CHO balance (oral glucose uptake - CHO oxidation). As compared with an overnight fast, prolonged fasting (4 days) inhibited the uptake (64.8 vs. 70.3 g/5 h; P < 0.01) and the oxidation (10.9 vs. 20.0 g/5 h; P < 0.001) of oral glucose and stimulated slightly its conversion to glycogen (53.9 vs. 50.3 g/5 h; P < 0.05). The latter effect played only a minor role in the marked increase in net CHO balance (52.3 vs. 25.2 g/5 h; P < 0.001), which was almost entirely related to a decrease in glycogen oxidation (1.6 vs. 25.1 g/5 h; P < 0.001). Considering the whole series of data, including intermediate durations of fast, it was observed that the modifications in postprandial CHO metabolism, induced by fasting, correlated strongly with basal CHO oxidation, suggesting that the degree of initial glycogen depletion is a major determinant of glycogen oxidation and net CHO storage. Thus, prior fasting stimulates postprandial glycogen retention, mainly through an inhibition of the glycogen turnover that exists in overnight-fasted subjects, during the absorptive period.

Re-feeding 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 deposition in skeletal muscles. On the basis of recent findings [Bräu, Ferreira, Nikolovski, Raja, Palmer and Fournier (1997) Biochem. J. 322, 303-308] 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 velocity of glycogen synthase except in the white quadriceps muscle. This finding, together with the observation that the fractional velocity of glycogen synthase in most muscles was already high in the starved state, suggests that in the initial phase of glycogen deposition the phosphorylation state of the enzyme may be adequate to support net glycogen synthesis. In the later phase of re-feeding, the progressive decrease in the fractional velocity of glycogen synthase in association with a decrease in the rate of glycogen deposition suggests that glycogen synthesis is controlled primarily by changes in the phosphorylation state of glycogen synthase. In conclusion, this study suggests that there is a temporal shift in the site of control of glycogen synthesis as glycogen deposition progresses during re-feeding after starvation.

pH is regulated differently by glucose in skeletal muscle from fed and starved rats: a study using 31P-NMR spectroscopy

The aim of this study was to determine whether exogenous glucose metabolism influences the pH in superfused EDL muscle from growing rats fed or starved for 48 h (body weight 55 and 45 g, respectively). Energy state and intracellular pH of muscle were repeatedly monitored by 31P-nuclear magnetic resonance spectroscopy (31P-NMRS); glycogen and other energy metabolites were assayed enzymatically in muscle extracts at the end of the experiment. In EDL muscles from starved rats superfused with glucose for 4 h, intracellular pH was elevated (7-7.3), lactate concentration low, glycogen repletion very intense and citrate synthase activity high. We conclude that glucose was routed mainly toward both oxidative phosphorylation and glycogen synthesis in EDL muscles after food deprivation of rats. In contrast, the major pathway in muscles from fed rats may be glycolysis because the glycogen pool remained constant throughout the experiment. The additional and minor pH component (in the range of 6.5 to 6.8) seen in muscles from fed rats, even in the presence of exogenous glucose, might be due to impaired glucose utilization because this component appears also in muscles from starved rats superfused without glucose or with a nonmetabolizable analog of glucose. Consequently, direct pH measurement by 31P-NMR may be considered to be a precise criterion for evaluation of differences in metabolic potentialities of muscle studied ex vivo in relation to the nutritional state of rats.

Differential regulation of glycogen synthase by insulin and glucose in vivo in skeletal muscles of the rat

Glucose 6-phosphate (G-6-P)-independent glycogen synthase (GSa) and glycogen synthase (GS) total activities were measured in muscles from 24-h-starved rats. Intravenous glucose tolerance tests (0.5 g/kg body wt) were used to produce physiological, transient increases in insulin and glucose concentrations. GS activation occurred at approximately 10 min after glucose administration with peak activation at approximately 15 min. GS activation was reversed approximately 15 min after insulin and glucose concentrations had returned to basal. No differences existed between fast- and slow-twitch muscles. Hyperinsulinemia (approximately 160 mU/ml) in the absence of hyperglycemia elicited 1.5-fold activation of GS (P < 0.001) in two of three fast-twitch muscles but did not activate GS in slow-twitch muscles. Glucose infusion (glycemia approximately 8 mM; insulin approximately 40 mU/ml) significantly (P < 0.01) increased the percentage of total GS in the GSa form in four of the five muscles. Hyperglycemia with modest hyperinsulinemia evoked greater enhancement of GSa activity in fast-twitch muscle than insulin alone at a higher concentration (P < 0.01). In summary, hyperinsulinemia without hyperglycemia does not result in maximal activation of GS in fast-twitch muscle, and a rise in glycemia is obligatory for GS activation by insulin in slow-twitch muscle. The data support an important role for glycemia in modulating the response of skeletal muscle GS to insulin and provide further evidence of heterogeneity among skeletal muscle types.

Mechanisms involved in the coordinate regulation of strategic enzymes of glucose metabolism

In this review, we evaluate the relative regulatory importance of specific strategic enzymes (in particular glycogen synthase, acetyl-CoA carboxylase [ACC] and the pyruvate dehydrogenase complex [PDH]) for carbohydrate utilization as an anabolic precursor and as an energy substrate during the nutritional transitions between the fed and fasted states. The involvement of the specific protein kinases contributing to the inactivation of these enzymes by phosphorylation [cyclic AMP-dependent protein kinase, AMP-activated protein kinase and PDH kinase] in achieving each regulatory response is also assessed. We demonstrate a striking temporal correlation between hepatic glycogen mobilization and PDH and ACC inactivation by phosphorylation during the immediate postabsorptive period; in contrast, rates of hepatic glycogen synthesis and PDH and ACC expressed activities do not change in parallel during refeeding. The results are consistent with shifting of the primary sites of control for overall hepatic carbon flux during the fed-to-starved and starved-to-fed nutritional transitions achieved, at least in part, by a complex pattern of regulation by protein phosphorylation and metabolites which is critically dependent on the precise nutritional status. Data are also presented that demonstrate asynchronous suppression of glucose uptake/phosphorylation and pyruvate oxidation in cardiac and skeletal muscle during progressive starvation. Analogous asynchrony is observed in the reactivation of these processes in cardiac and skeletal muscle during refeeding after starvation. We provide evidence in support of the concept that selective suppression of pyruvate oxidation in oxidative muscles during early starvation and during the initial phase of refeeding is achieved because of differential sensitivity of glucose uptake/phosphorylation and pyruvate oxidation to lipid-fuel utilization. We discuss the relative importance of regulatory events governing local fatty acid production and utilization (via lipoprotein lipase and carnitine palmitoyltransferase 1, respectively) or overall fatty acid supply (dictated by events at the adipocyte) for fuel utilization by muscle during nutritional transitions. Finally, we assess the regulatory importance of glycogen synthesis in determining overall rates of glucose clearance by skeletal muscle during alimentary hyperglycemia and hyperinsulinemia.

Ethanol and glycogen synthesis in cardiothoracic and skeletal muscles following glucose re-feeding after starvation in the rat

The pattern of glycogen deposition in individual cardiothoracic and skeletal muscles in response to oral and intraperitoneal glucose administration was examined in 40 h-starved rats. Rates of glycogen synthesis were consistently higher in oxidative muscles than in non-oxidative muscles. Intragastric ethanol administration was associated with an impaired glycaemic response and the almost total abolition of glycogen deposition in oxidative muscles in response to oral or intraperitoneal glucose re-feeding. This effect was dose-dependent and differential, in that ethanol produced no equivalent impairment in glycogen deposition in non-oxidative muscles. Ethanol treatment also selectively promoted glycogenolysis in oxidative muscles in the starved state. There was positive correlation (P < 0.001) between the decrease in glycogen levels in soleus and diaphragm muscles in response to increasing ethanol doses and blood glucose and lactate concentrations after intraperitoneal glucose administration, implying that the basis for the impairment in glycogen synthesis may be diminished glucose availability. The mechanism whereby ethanol may differentially compromise carbohydrate metabolism in oxidative muscles is discussed.

Glucose utilization and disposition by skeletal muscle during unrestricted feeding

We measured glucose utilization index (GUI) values in individual skeletal muscles of conscious rats during the light (quiescent) and dark (feeding/activity) phases. There was a 2-3-fold variation in muscle GUI values, with peak values observed at the end of the dark phase and minimum values observed at 6-9 h into the light phase. GUI values in working muscles (soleus and adductor longus) were consistently higher than in non-working muscles (tibialis anterior and extensor digitorum longus), indicating that working muscles make the major contribution of the total skeletal muscle mass to glucose disposal during unrestricted feeding. There was a clear overall increase in muscle glycogen deposition during the first 9 h of the dark phase; this was concomitant with an increase in food consumption. Peak glycogen concentrations were reached after 9 h of darkness, but subsequently declined. The pattern of changes in muscle GUI values during the light and dark phases is discussed in relation to the role of insulin in facilitating glucose clearance.

Heterogeneity of glycogen synthesis upon refeeding following starvation

1. Starvation of rats for 40 hr decreased the body weight, liver weight and blood glucose concentration. The hepatic and skeletal muscle glycogen concentrations were decreased by 95% (from 410 mumol/g tissue to 16 mumol/g tissue) and 55% (from 40 mumol/g tissue to 18.5 mumol/g tissue), respectively. 2. Fine structural analysis of glycogen purified from the liver and skeletal muscle of starved rats suggested that the glycogenolysis included a lysosomal component, in addition to the conventional phosphorolytic pathway. In support of this the hepatic acid alpha-glucosidase activity increased 1.8-fold following starvation. 3. Refeeding resulted in liver glycogen synthesis at a linear rate of 40 mumol/g tissue per hr over the first 13 hr of refeeding. The hepatic glycogen store were replenished by 8 hr of refeeding, but synthesis continued and the hepatic glycogen content peaked at 24 hr (approximately 670 mumol/g tissue). 4. Refeeding resulted in skeletal muscle glycogen synthesis at an initial rate of 40 mumol/g tissue per hr. The muscle glycogen store was replenished by 30 min of refeeding, but synthesis continued and the glycogen content peaked at 13 hr (approximately 50 mumol/g tissue). 5. Both liver and skeletal muscle glycogen synthesis were inhomogeneous with respect to molecular size; high molecular weight glycogen was initially synthesised at a faster rate than low molecular weight glycogen. These observations support suggestions that there is more than a single site of glycogen synthesis.

Glucose disposal by skeletal muscle in response to re-feeding after progressive starvation

We investigated the extent to which increases in glucose utilization indices (GUIs) in individual skeletal muscles during chow re-feeding after 6 h, 24 h or 48 h starvation are related to the antecedent duration of starvation. Chow re-feeding after either acute or prolonged starvation led to an increase in glucose disposal by the muscle mass. Glucose intolerance after prolonged starvation was not associated with lower values of GUI in skeletal muscle. In both working and non-working muscles, the increment in GUI during the first 2 h of re-feeding was less after acute than after prolonged starvation. In non-working muscles the differential responses to re-feeding were due to higher GUI values after re-feeding rather than lower pre-prandial GUI values. Therefore the contribution of non-working muscles to glucose clearance is higher as the antecedent period of starvation is extended. Rates of glycogen deposition in non-working muscles after refeeding were similar to absolute values of GUI, and a strong relationship existed between measured GUI values and rates of glycogen deposition.

Regulation of glucose uptake in fast and slow skeletal muscles with fasting and refeeding

1. The decrease in wet weight and noncollagen protein (NCP) was faster and greater in extensor digitorum longus (EDL) during fasting than in soleus (Sol) muscles in rats. 2. During refeeding, recovery was completed faster in Sol than in EDL. 3. Glucose uptake in skeletal muscle increased significantly during fasting on both a per wet weight and NCP basis. 4. This increase was faster and greater in EDL than Sol. 5. The initial increase in glucose uptake was greater during refeeding than fasting only in EDL.

Glucose utilization and disposal in cardiothoracic and skeletal muscles during the starved-to-fed transition in the rat

Glucose utilization indices (GUI) increased to fed values in diaphragm and oxidative skeletal muscles and exceeded fed values in non-oxidative muscles within 2 h of re-feeding chow to 48 h-starved rats. Cardiac GUI reached fed values only after 7 h. Glycogen deposition accounted for most of the glucose phosphorylated in skeletal muscle over the first 2 h in oxidative muscles and over the first 4 h in non-oxidative muscles. In oxidative muscles, the contribution of glycogen deposition to total glucose 6-phosphate disposal diminished as re-feeding was extended from 2 to 6 h.

Formation of gluconeogenic precursors in rat skeletal muscle during fasted-refed transition

During the fasted-refed transition, hepatic glycogen repletion from glucose can occur by the direct and indirect pathway. In the indirect pathway, glucose is first metabolized to 3-carbon intermediates that then are converted in the liver to glucose 6-phosphate via the gluconeogenic pathway before conversion to glycogen. The present study evaluated whether skeletal muscle is a major source of 3-carbon intermediates (i.e., lactate, pyruvate, and alanine) during refeeding of 1-day fasted rats. Arteriovenous differences for lactate, pyruvate, and alanine across the anesthetized rat hindlimbs were used to evaluate muscle metabolism in the fed, fasted, and refed state. In the fasted state, liver glycogen was depleted, and muscle released 3-carbon intermediates. One hour after refeeding, hepatic glycogen was 30% repleted, and blood lactate, pyruvate, and alanine increased. Despite this, the release of alanine by muscle diminished at this time and lactate was removed. At 4 h after refeeding, 3-carbon intermediates were all released by hindlimb tissue but in an amount not greater than in the fasted state. Overall, these results suggest that skeletal muscle in the rat is not a major source of 3-carbon precursors for early postprandial hepatic glycogen repletion via the indirect pathway, nor is the rise in 3-carbon intermediates in blood during refeeding caused by their increased output by muscle.

Initial glucose kinetics and hormonal response to a gastric glucose load in unrestrained post-absorptive and starved rats

A gastric [U-14C]glucose load (4.8 mg/g body wt.) was delivered to unrestrained post-absorptive or 30 h-starved rats bearing peripheral and portal vein catheters and continuously perfused with [3-3H]glucose, in order to compare their metabolic and hormonal responses. In the basal state, portal and peripheral glycaemia were less in starved rats than in rats in the post-absorptive period (P less than 0.01), whereas blood lactate was similar. Portal insulinaemia (P less than 0.05) and protal glucagonaemia (P less than 0.005) were lower in starved rats, but insulin/glucagon ratio was higher in post-absorptive rats (P less than 0.005). The glucose turnover rate was decreased by starvation (P less than 0.005). After glucose ingestion, blood glucose was similar in post-absorptive and starved rats. A large portoperipheral gradient of lactate appeared in starved rats. Portal insulinaemia reached a peak at 9 min, and was respectively 454 +/- 68 and 740 +/- 65 mu-units/ml in starved and post-absorptive rats. Portal glucagonaemia remained stable, but was higher in post-absorptive rats (P less than 0.05). At 60 min after the gastric glucose load, 30% of the glucose was delivered at the periphery in both groups. The total glucose appearance rate was higher in starved rats (P less than 0.05), as was the glucose utilization rate (P less than 0.05), whereas the rate of appearance of exogenous glucose was similar. This was due to a non-suppressed hepatic glucose production in the starved rats, whereas it was totally suppressed in post-absorptive rats. At 1 h after the glucose load, the increase in both liver and muscle glycogen concentration was greater in starved rats. Thus short-term fasting induces an increased portal lactate concentration after a glucose load, and produces a state of liver insulin unresponsiveness for glucose production, whereas the sensitivity of peripheral tissues for glucose utilization is unchanged or even increased. This might allow preferential replenishment of the peripheral stores of glycogen.

Fuel selection and carbon flux during the starved-to-fed transition

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Effects of re-feeding after prolonged starvation on pyruvate dehydrogenase activities in heart, diaphragm and selected skeletal muscles of the rat

We investigated the capacity for pyruvate oxidation in skeletal muscle, diaphragm and heart after starvation and re-feeding. Starvation for 48 h decreased pyruvate dehydrogenase (PDH) activity in soleus (by 47%), extensor digitorum longus (64%), gastrocnemius (86%), diaphragm (87%), adductor longus (90%), tibialis anterior (92%) and heart (99%). Chow re-feeding increased PDH activity in all muscles to 43-78% of the fed value within 2 h. However, complete re-activation was not observed for at least 4-6 h, during which time hepatic glycogen was replenished. We discuss the importance of muscle PDH activity in relation to sparing carbohydrate for hepatic glycogen synthesis.