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PubMed1991

Regulation of glycogen resynthesis following exercise. Dietary considerations.

With the cessation of exercise, glycogen repletion begins to take place rapidly in skeletal muscle and can result in glycogen levels higher than those present before exercise. Understanding the rate-limiting steps that regulate glycogen synthesis will provide us with strategies to increase the resynthesis of glycogen during recovery from exercise, and thus improve performance. Given the importance of muscle glycogen to endurance performance, various factors which may optimise glycogen resynthesis rate and insure complete restoration have been of interest to both the scientist and athlete. The time required for complete muscle glycogen resynthesis after prolonged moderate intensity exercise is generally considered to be 24 hours provided approximately 500 to 700g of carbohydrate is ingested. Muscle glycogen synthesis rate is highest during the first 2 hours after exercise. Ingestion of 0.70g glucose/kg bodyweight every 2 hours appears to maximise glycogen resynthesis rate at approximately 5 to 6 mumol/g wet weight/h during the first 4 to 6 hours after exhaustive exercise. Further enhancement of glycogen resynthesis rate with ingestion of greater than 0.70g glucose/kg bodyweight appears to be limited by the constraints imposed by gastric emptying. Ingestion of glucose or sucrose results in similar muscle glycogen resynthesis rates while glycogen synthesis in liver is better served with the ingestion of fructose. Also, increases in muscle glycogen content during the first 4 to 6 hours after exercise are greater with ingestion of simple as compared with complex carbohydrate. Glycogen synthase activity is a key component in the regulation of glycogen resynthesis. Glycogen synthase enzyme exists in 2 states: the less active, more phosphorylated (D) form which is under allosteric control of glucose-6-phosphate, and the more active, less phosphorylated (I) form which is independent of glucose-6-phosphate. There is generally an inverse relationship between glycogen content in muscle and the percentage synthase in the activated (I) form. Exercise and insulin by themselves activate glycogen synthase by conversion to glycogen synthase I. Although small changes in the activity ratio (% I form) can lead to large changes in the rate of glycogen synthesis, glycogen synthase I appears to increase very little (approximately 25%) in response to glycogen depletion and returns to pre-exercise levels as glycogen levels return to normal. Thus glycogen resynthesis, which may increase 3- to 5-fold, may also be influenced by glucose-6-phosphate, which can activate glycogen synthase in the D form.(ABSTRACT TRUNCATED AT 400 WORDS)

Biological Transport↗
PubMed1979

Glycogen synthesis by rat hepatocytes.

1. Hepatocytes from starved rats or fed rats whose glycogen content was previously depleted by phlorrhizin or by glucagon injections, form glycogen at rapid rates when incubated with 10mM-glucose, gluconeogenic precursors (lactate, glycerol, fructose etc.) and glutamine. There is a net synthesis of glucose and glycogen. 14C from all three types of substrate is incorporated into glycogen, but the incorporation from glucose represents exchange of carbon atoms, rather than net incorporation. 14C incorporation does not serve to measure net glycogen synthesis from any one substrate. 2. With glucose as sole substrate net glucose uptake and glycogen deposition commences at concentrations of about 12--15mM. Glycogen synthesis increases with glucose concentrations attaining maximal values at 50--60mM, when it is similar to that obtained in the presence of 10mM glucose and lactate plus glutamine. 3. The activities of the active (a) and total (a+b) forms of glycogen synthase and phosphorylase were monitored concomitant with glycogen synthesis. Total synthase was not constant during a 1 h incubation period. Total and active synthase activity increased in parallel with glycogen synthesis. 4. Glycogen phosphorylase was assayed in two directions, by conversion of glycose 1-phosphate into glycogen and by the phosphorylation of glycogen. Total phosphorylase was assyed in the presence of AMP or after conversion into the phosphorylated form by phosphorylase kinase. Results obtained by the various methods were compared. Although the rates measured by the procedures differ, the pattern of change during incubation was much the same. Total phosphorylase was not constant. 5. The amounts of active and total phosphorylase were highest in the washed cell pellet. Incubation in an oxygenated medium, with or without substrates, caused a prompt and pronounced decline in the assayed amounts of active and total enzyme. There was no correlation between phosphorylase activity and glycogen synthesis from gluconeogenic substrates. With fructose, active and total phosphorylase activities increased during glycogen syntheses. 6. In glycogen synthesis from glucose as sole substrate there was a decline in phosphorylase activities with increased glucose concentration and increased rates of glycogen deposition. The decrease was marked in cells from fed rats. 7. To determine whether phosphorolysis and glycogen synthesis occur concurrently, glycogen was prelabelled with [2-3H,1-14C]-galactose. During subsequent glycogen deposition there was no loss of activity from glycogen in spite of high amounts of assayable active phosphorylase.

Animals↗
PubMed1995

Superiority of acid extractable glycogen for detection of metabolic changes during myocardial ischaemia.

Various methods for extraction and isolation of myocardial glycogen show different yields and identify different glycogen subsets. The aim of the present study was to identify a glycogen fraction exposed to changes during myocardial ischaemia. Endomyocardial biopsies from 10 pigs were sampled before cardioplegia, after cardioplegic arrest, and after reperfusion. Glycogen yields were compared following five extraction procedures: (1) hot alkaline tissue digestion, (2) homogenization in perchloric acid and subsequent determination in homogenate, (3) homogenization in perchloric acid and subsequent determination in supernatant, (4) homogenization in perchloric acid and subsequent determination in the precipitate redissolved in hot alkaline and (5) homogenization in homogenisation buffer with lysating capacity. Glycogen was isolated on filter-paper and determined enzymatically. Hot alkaline tissue digestion yielded the highest glycogen amounts (63.5 +/- 18.3 nmol/mg wet weight). Glycogen yields in perchloric homogenate and supernatant were 51%, perchloric precipitate 47%, and buffer 30% of these obtained with hot alkaline. Glycogen yields in hot alkaline were comparable to the sum of those obtained in perchloric supernatant ("acid extractable glycogen") and redissolved precipitate ("heavily extracted glycogen") confirming that glycogen yields obtained with hot alkaline digestion represent "total glycogen". Acid extractable glycogen showed superior analytical characteristics compared with the other methods. Acid extractable glycogen demonstrated a consistent decrease during ischaemia whereas total glycogen and glycogen extracted in homogenization buffer tended to decrease. Glycogen in perchloric precipitate remained unchanged during ischaemia. These findings support a revival of the concept that tissue contains two forms of glycogen. Decreases in myocardial glycogen content during myocardial ischaemia are best observed with acid extractable glycogen.

Animals↗
PubMed2004

Hormonal regulation of glycogen metabolism in white muscle slices from rainbow trout (Oncorhynchus mykiss Walbaum).

To test the hypothesis that cortisol and epinephrine have direct regulatory roles in muscle glycogen metabolism and to determine what those roles might be, we developed an in vitro white muscle slice preparation from rainbow trout (Oncorhynchus mykiss Walbaum). In the absence of hormones, glycogen-depleted muscle slices obtained from exercised trout were capable of significant glycogen synthesis, and the amount of glycogen synthesized was inversely correlated with the initial postexercise glycogen content. When postexercise glycogen levels were <5 micromol/g, about 4.3 micromol/g of glycogen were synthesized, but when postexercise glycogen levels were >5 micromol/g, only about 1.7 micromol/g of glycogen was synthesized. This difference in the amount of glycogen synthesized was reflected in the degree of activation of glycogen synthase. Postexercise glycogen content also influenced the response of the muscle to 10(-8) M epinephrine and 10(-8) M dexamethasone (a glucocorticoid analog). At high glycogen levels (>5 micromol/g), epinephrine and dexamethasone stimulated glycogen phosphorylase activity and net glycogenolysis, whereas at low (<5 micromol/g) glycogen levels, glycogenesis and activation of glycogen synthase activity prevailed. These data clearly indicate not only is trout muscle capable of in situ glycogenesis, but the amount of glycogen synthesized is a function of initial glycogen content. Furthermore, whereas dexamethasone and epinephrine directly stimulate muscle glycogen metabolism, the net effect is dependent on initial glycogen content.

Animals↗
PubMed2004

Spatial reorganization of glycogen synthase upon activation in 3T3-L1 adipocytes.

The dephosphorylation of glycogen synthase is a key step in the stimulation of glycogen synthesis by insulin. To further investigate the hormonal regulation of glycogen synthase activity, enzymatic localization in 3T3-L1 adipocytes was determined by immunocytochemistry and confocal microscopy. In basal cells, glycogen synthase and the protein phosphatase-1-glycogen-targeting subunit, protein targeting to glycogen (PTG), were diffusely distributed throughout the cell. Insulin treatment had no effect on PTG distribution but resulted in a reorganization of glycogen synthase into punctate clusters. Glycogen synthase aggregation was restricted to discrete cellular sites, presumably where glycogen synthesis occurred. Omission of extracellular glucose or substitution with 2-deoxy-glucose blocked the insulin-induced redistribution of glycogen synthase. Addition of the glycogenolytic agent forskolin after insulin stimulation disrupted the clusters of glycogen synthase protein, restoring the immunostaining pattern to the basal state. Conversely, adenoviral-mediated overexpression of PTG resulted in the insulin-independent dephosphorylation of glycogen synthase and a redistribution of the enzyme from the cytosolic- to glycogen-containing fractions. The effects of PTG on glycogen synthase activity were mediated by multisite dephosphorylation, which was enhanced by insulin and 2-deoxy-glucose, and required a functional glycogen synthase-binding domain on PTG. However, PTG overexpression did not induce distinct glycogen synthase clustering in fixed cells, presumably because cellular glycogen levels were increased more than 7-fold under these conditions, resulting in a diffusion of sites where glycogen elongation occurred. Cumulatively, these data indicate that the hormonal regulation of glycogen synthesis rates in 3T3-L1 adipocytes is mediated in part through changes in the subcellular localization of glycogen synthase.

3T3 Cells↗
PubMed2003

Determinants of post-exercise glycogen synthesis during short-term recovery.

The pattern of muscle glycogen synthesis following glycogen-depleting exercise occurs in two phases. Initially, there is a period of rapid synthesis of muscle glycogen that does not require the presence of insulin and lasts about 30-60 minutes. This rapid phase of muscle glycogen synthesis is characterised by an exercise-induced translocation of glucose transporter carrier protein-4 to the cell surface, leading to an increased permeability of the muscle membrane to glucose. Following this rapid phase of glycogen synthesis, muscle glycogen synthesis occurs at a much slower rate and this phase can last for several hours. Both muscle contraction and insulin have been shown to increase the activity of glycogen synthase, the rate-limiting enzyme in glycogen synthesis. Furthermore, it has been shown that muscle glycogen concentration is a potent regulator of glycogen synthase. Low muscle glycogen concentrations following exercise are associated with an increased rate of glucose transport and an increased capacity to convert glucose into glycogen. The highest muscle glycogen synthesis rates have been reported when large amounts of carbohydrate (1.0-1.85 g/kg/h) are consumed immediately post-exercise and at 15-60 minute intervals thereafter, for up to 5 hours post-exercise. When carbohydrate ingestion is delayed by several hours, this may lead to ~50% lower rates of muscle glycogen synthesis. The addition of certain amino acids and/or proteins to a carbohydrate supplement can increase muscle glycogen synthesis rates, most probably because of an enhanced insulin response. However, when carbohydrate intake is high (> or =1.2 g/kg/h) and provided at regular intervals, a further increase in insulin concentrations by additional supplementation of protein and/or amino acids does not further increase the rate of muscle glycogen synthesis. Thus, when carbohydrate intake is insufficient (<1.2 g/kg/h), the addition of certain amino acids and/or proteins may be beneficial for muscle glycogen synthesis. Furthermore, ingestion of insulinotropic protein and/or amino acid mixtures might stimulate post-exercise net muscle protein anabolism. Suggestions have been made that carbohydrate availability is the main limiting factor for glycogen synthesis. A large part of the ingested glucose that enters the bloodstream appears to be extracted by tissues other than the exercise muscle (i.e. liver, other muscle groups or fat tissue) and may therefore limit the amount of glucose available to maximise muscle glycogen synthesis rates. Furthermore, intestinal glucose absorption may also be a rate-limiting factor for muscle glycogen synthesis when large quantities (>1 g/min) of glucose are ingested following exercise.

Dietary Carbohydrates↗
PubMed1996

Progressive decrease in tissue glycogen content in rats with long-term cholestasis.

Liver and skeletal muscle glycogen metabolism were investigated in rats 1 and 4 weeks after bile duct ligation (BDL) and in pair-fed, sham-operated control rats. Livers were subjected to morphometric analysis to express glycogen content and enzyme activities per mL hepatocytes. One week after BDL, the hepatic glycogen content was 28.8 +/- 13.8 versus 38.6 +/- 16.4 mg/mL hepatocyte in BDL and control rats, respectively. Total activity of glycogen synthase (50.2 +/- 7.0 vs. 63.5 +/- 9.4 mU/mL hepatocytes) and glycogen phosphorylase (59.4 +/- 12.9 vs. 90.8 +/- 18.9 U/mL) were significantly reduced in BDL whereas the active fraction of glycogen synthase (27 +/- 6 vs. 38 +/- 5%) but not of glycogen phosphorylase was reduced. The skeletal muscle glycogen content was not different between BDL and control rats. Four weeks after BDL, hepatic glycogen content was further reduced (20.5 +/- 14.2 vs. 52.9 +/- 6.4 mg/mL). Total activity of glycogen synthase (38.8 +/- 12.1 vs. 60.1 +/- 4.6 mU/mL hepatocytes) and glycogen phosphorylase (127 +/- 19 vs. 178 +/- 33 U/mL hepatocytes) were both reduced in BDL rats as were their corresponding active fractions (30 +/- 18 vs. 66 +/- 8% and 58 +/- 10 vs. 76 +/- 10). At this time point, the glycogen content in soleus muscle was decreased by 64% in BDL. The glucagon plasma concentration was increased in BDL rats at both time points. There were positive correlations between the volume fraction and both hepatic glycogen content and total activity of hepatic glycogen synthase. Plasma glucagon and the active fraction of hepatic glycogen synthase were negatively correlated. The current studies show a progressive decrease in the hepatic and skeletal muscle glycogen content in BDL rats. The observed decrease in the activities of glycogen synthase and phosphorylase suggest that reduced glycogen synthesis is the major mechanism leading to the reduction in the hepatic glycogen content in BDL rats.

Animals↗