Hepatic Glycogen Synthase Activity Research Articles

A field study to optimize protein and lipid levels in diet for cage-cultured subadult rockfish Sebastes schlegelii

A feeding trial was conducted to evaluate growth, body composition, serum biochemistry, digestion and metabolism, and oxidation resistence of S. schlegelii fed 9 diets in a 3×3 factorial design (protein levels: 44 %, 48 %, 52 %; lipid levels: 6 %, 10 %, 14 %), to optimize protein and lipid levels for practical feed formula. Rockfish (114.25 g) were stocked into 27 cages in an offshore system and fed test diets to satiation twice daily for 8 weeks. Growth increased but feed conversion rate decreased with dietary protein increasing. Increasing dietary protein to >48 % led to reduced feeding intake but increased protein efficiency ratio. Lipid efficiency ratio increased with dietary protein increasing but decreased with dietary lipid increasing. Hepatosomatic index decreased coupling with the increases in condition factor and liver lipid content as dietary lipid increased to 14 %. Intraperitoneal fat ratio increased with dietary lipid increasing. Increasing dietary protein to > 48 % significantly elevated serum total protein and glucose concentrations, hepatic glycogen synthase activity. High-protein (52 %) diets elevated serum albumin and urea nitrogen concentrations, hepatic pyruvate kinase and alanine aminotransferase (ALT) activities but downregulated hepatic growth-hormone (GH) receptor. Activities of trypsin, lipase, serum superoxide dismutase and total antioxidant capacity decreased with the dietary lipid increasing. Increasing dietary lipid to > 10 % reduced serum GH concentration, and high-lipid (14 %) diets significantly enhanced serum triglyceride concentration and hepatic transaminases activities, and resulted in a visual increase of adipocytes in liver. Serum cholesterol and low-density lipoprotein decreased as dietary protein increased to 48–52 % or dietary lipid >10 %. Hepatic succinate dehydrogenase activity and serum IGF-1 significantly increased while hepatic fatty acid synthetase decreased as dietary protein increased to >48 % or dietary lipid >10 %. High-protein (52 %) diet or high-lipid (14 %) diet elevated serum malonaldehyde concentration. These findings suggested 48 % protein and 10 % lipid were the optimal in diet for subadult S. schlegelii.

A patient with glycogen storage disease type 0 and a novel sequence variant in GYS2: a case report and literature review

Glycogen storage disease type 0 (GSD0) is an autosomal recessive disorder caused by a sequence variant in the GYS2 gene, leading to decreased or absent activity of hepatic glycogen synthase. With a frequency of less than 1 in 1,000,000 individuals, GSD0 represents only around 1% of all glycogen storage disease cases but it might be underrecognized. A 13-month-old girl of reportedly unrelated parents presented with a decreased level of consciousness, twitching in her left cheek, and munching. During a fasting test, hyperketotic hypoglycemia was found. A novel homozygous GYS2 gene sequence variant p.Thr445Arg was later confirmed by next-generation gene sequencing. After establishing a cornstarch- and protein-rich diet, the hypoglycemic episodes subsided and the patient’s neurocognitive development was normal. To date, only 39 patients with 24 disease-causing gene variants have been identified in GSD0, and we review their characteristics. Because of the heterogeneous phenotypes, GSD0 is an underdiagnosed disorder. In patients with hyperketotic hypoglycemia and postprandial hyperglycemia, GYS2 gene analysis should be performed.

Role of leptin in the regulation of growth and carbohydrate metabolism in the ovine fetus during late gestation

Leptin is an important regulator of appetite and energy expenditure in adulthood, although its role as a nutritional signal in the control of growth and metabolism before birth is poorly understood. This study investigated the effects of leptin on growth, carbohydrate metabolism and insulin signalling in fetal sheep. Crown-rump length-measuring devices and vascular catheters were implanted in 12 sheep fetuses at 105-110 days of gestation (term 145 +/- 2 days). The fetuses were infused i.v. either with saline (0.9% NaCl; n = 6) or recombinant ovine leptin (0.5-1.0 mg kg(-1) day(-1); n = 6) for 5 days from 125 to 130 days when they were humanely killed and tissues collected. Leptin receptor mRNA and protein were expressed in fetal liver, skeletal muscle and perirenal adipose tissue. Throughout infusion, plasma leptin in the leptin-infused fetuses was 3- to 5-fold higher than in the saline-infused fetuses, although plasma concentrations of insulin, glucose, lactate, cortisol, catecholamines and thyroid hormones did not differ between the groups. Leptin infusion did not affect linear skeletal growth or body, placental and organ weights in utero. Hepatic glycogen content and activities of the gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in the leptin-infused fetuses were lower than in the saline-infused fetuses by 44, 48 and 36%, respectively; however, there were no differences in hepatic glycogen synthase activity or insulin signalling protein levels. Therefore, before birth, leptin may inhibit endogenous glucose production by the fetal liver when adipose energy stores and transplacental nutrient delivery are sufficient for the metabolic needs of the fetus. These actions of leptin in utero may contribute to the development of neonatal hypoglycaemia in macrosomic babies of diabetic mothers.

Role that phosphorylation of GSK3 plays in insulin and Wnt signalling defined by knockin analysis

The inactivation of glycogen synthase kinase (GSK)3 has been proposed to play important roles in insulin and Wnt signalling. To define the role that inactivation of GSK3 plays, we generated homozygous knockin mice in which the protein kinase B phosphorylation sites on GSK3alpha (Ser21) and GSK3beta (Ser9) were changed to Ala. The knockin mice were viable and were not diabetic. Using these mice we show that inactivation of GSK3beta rather than GSK3alpha is the major route by which insulin activates muscle glycogen synthase. In contrast, we demonstrate that the activation of muscle glycogen synthase by contraction, the stimulation of muscle glucose uptake by insulin, or the activation of hepatic glycogen synthase by glucose do not require GSK3 phosphorylation on Ser21/Ser9. GSK3 also becomes inhibited in the Wnt-signalling pathway, by a poorly defined mechanism. In GSK3alpha/GSK3beta homozygous knockin cells, Wnt3a induces normal inactivation of GSK3, as judged by the stabilisation of beta-catenin and stimulation of Wnt-dependent transcription. These results establish the function of Ser21/Ser9 phosphorylation in several processes in which GSK3 inactivation has previously been implicated.

A novel glycogen-targeting subunit of protein phosphatase 1 that is regulated by insulin and shows differential tissue distribution in humans and rodents.

Stimulation of glycogen-targeted protein phosphatase 1 (PP1) activity by insulin contributes to the dephosphorylation and activation of hepatic glycogen synthase (GS) leading to an increase in glycogen synthesis. The glycogen-targeting subunits of PP1, GL and R5/PTG, are downregulated in the livers of diabetic rodents and restored by insulin treatment. We show here that the mammalian gene PPP1R3E encodes a novel glycogen-targeting subunit of PP1 that is expressed in rodent liver. The phosphatase activity associated with R3E is slightly higher than that associated with R5/PTG and it is downregulated in streptozotocin-induced diabetes by 60-70% and restored by insulin treatment. Surprisingly, although mRNA for R3E is most highly expressed in rat liver and heart muscle, with only low levels in skeletal muscle, R3E mRNA is most abundant in human skeletal muscle and heart tissues with barely detectable levels in human liver. This species-specific difference in R3E mRNA expression has similarities to the high level of expression of GL mRNA in human but not rodent skeletal muscle. The observations imply that the mechanisms by which insulin regulates glycogen synthesis in liver and skeletal muscle are different in rodents and humans.

Glucose 6-Phosphate Produced by Gluconeogenesis and by Glucokinase Is Equally Effective in Activating Hepatic Glycogen Synthase

Glucose 6-phosphate (Glc-6-P) produced in cultured hepatocytes by direct phosphorylation of glucose or by gluconeogenesis from dihydroxyacetone (DHA) was equally effective in activating glycogen synthase (GS). However, glycogen accumulation was higher in hepatocytes incubated with glucose than in those treated with DHA. This difference was attributed to decreased futile cycling through GS and glycogen phosphorylase (GP) in the glucose-treated hepatocytes, owing to the partial inactivation of GP induced by glucose. Our results indicate that the gluconeogenic pathway and the glucokinase-mediated phosphorylation of glucose deliver their common product to the same Glc-6-P pool, which is accessible to liver GS. As observed in the treatment with glucose, incubation of cultured hepatocytes with DHA caused the translocation of GS from a uniform cytoplasmic distribution to the hepatocyte periphery and a similar pattern of glycogen deposition. We hypothesize that Glc-6-P has a major role in glycogen metabolism not only by determining the activation state of GS but also by controlling its subcellular distribution in the hepatocyte.

Loss of the hepatic glycogen-binding subunit (GL) of protein phosphatase 1 underlies deficient glycogen synthesis in insulin-dependent diabetic rats and in adrenalectomized starved rats.

Hepatic glycogen synthesis is impaired in insulin-dependent diabetic rats and in adrenalectomized starved rats, and although this is known to be due to defective activation of glycogen synthase by glycogen synthase phosphatase, the underlying molecular mechanism has not been delineated. Glycogen synthase phosphatase comprises the catalytic subunit of protein phosphatase 1 (PP1) complexed with the hepatic glycogen-binding subunit, termed GL. In liver extracts of insulin-dependent diabetic and adrenalectomized starved rats, the level of GL was shown by immunoblotting to be substantially reduced compared with that in control extracts, whereas the level of PP1 catalytic subunit was not affected by these treatments. Insulin administration to diabetic rats restored the level of GL and prolonged administration raised it above the control levels, whereas re-feeding partially restored the GL level in adrenalectomized starved rats. The regulation of GL protein levels by insulin and starvation/feeding was shown to correlate with changes in the level of the GL mRNA, indicating that the long-term regulation of the hepatic glycogen-associated form of PP1 by insulin, and hence the activity of hepatic glycogen synthase, is predominantly mediated through changes in the level of the GL mRNA.

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism.

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.

Effect of feeding, fasting, and diabetes on liver glycogen synthase activity, protein, and mRNA in rats.

Hepatic glycogen synthase activity is increased in diabetic animals. However, the relationship between enzymic activity, enzyme protein mass, and mRNA abundance has not been well characterized. In the present study, these relationships were determined in 3- and 8-day diabetic, fed and fasted rats. The results were compared to data obtained in normal fed and fasted animals. In normal rats, total synthase specific activity and protein mass were similar in the fed and fasted state. However, in fed animals, the synthase mRNA abundance was increased 1.7-fold. In 3-day diabetic rats, total synthase specific activity was increased approximately 29% compared to normal controls. It was unaffected by feeding and fasting and was associated with an approximate 15% increase in enzyme mass. Synthase mRNA was increased 1.8 and 2.6-fold in fasted and fed animals, respectively. In 8-day diabetic rats, total synthase specific activity was increased more than 2-fold compared to controls. However, the enzyme protein mass was decreased by approximately 20%. The mRNA abundance in 8-day diabetic fasted rats was only 30% of controls, while in fed rats it was increased by 40%. These data indicate that feeding and fasting have a major effect on synthase mRNA abundance which is independent of synthase activity, or protein mass, or both, in normal and diabetic animals. Total synthase specific activity increased with duration of diabetes. This was associated with only a modest change in protein mass. Thus, diabetes induces an increase in synthase catalytic efficacy. The specific activity of phosphorylase is decreased in diabetic rats.

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. (Hepatology 1996 Oct;24(4):902-7)

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.

Glucose 6-Phosphate Produced by Glucokinase, but Not Hexokinase I, Promotes the Activation of Hepatic Glycogen Synthase

In a previous study (O'Doherty, R. M., Lehman, D. L., Seoane, J., Gómez-Foix, A. M., Guinovart, J. J., and Newgard, C.B. (1996) J. Biol. Chem. 271, 20524-20530), we demonstrated that adenovirus-mediated overexpression of glucokinase but not hexokinase I has a potent enhancing effect on glycogen synthesis in primary hepatocytes. In an effort to understand the underlying mechanism of this differential effect of the two hexokinase isoforms, we have investigated changes in key intracellular metabolites and the activation state of glycogen synthase in cells treated with recombinant adenoviruses expressing the liver isoform of glucokinase (AdCMV-GKL) or hexokinase I (AdCMV-HKI). Glucose 6-phosphate (Glu-6-P) levels are elevated from approximately 1.5 nmol/mg protein to 8-10 nmol/mg protein in both AdCMV-GKL- and AdCMV-HKI-treated hepatocytes as glucose is raised from 1 to 5 mM, levels four times higher than those in untreated cells. In AdCMV-GKL-treated cells, Glu-6-P continues to accumulate at glucose levels greater than 5 mM, reaching a maximum of 120 nmol/mg protein in cells incubated at 25 mM glucose, a value 10 and 50 times greater than the maximal levels achieved in AdCMV-HKI-treated and untreated cells, respectively. In parallel with the changes observed in Glu-6-P levels, increases in UDP-Glc in AdCMV-HKI- and AdCMV-GKL-treated cells were most pronounced at low (1-5 mM) and high (25 mM) glucose levels, respectively. Despite the significant increases in Glu-6-P and UDP-Glc achieved in AdCMV-HKI-treated cells, only AdCMV-GKL-treated cells exhibited increases in glycogen synthase activity ratio and translocation of the enzyme from a soluble to a particulate form relative to untreated control cells. We conclude that Glu-6-P produced by overexpressed glucokinase is glycogenic because it effectively promotes activation of glycogen synthase. Glu-6-P produced by overexpressed hexokinase, in contrast, appears to be unable to exert the same regulatory effects, probably due to the different subcellular distribution of the two glucose-phosphorylating enzymes.

Role of norepinephrine in hepatic gluconeogenesis: evidence of aging and training effects.

This study examined the relationship among the sympathetic neurotransmitter norepinephrine (NE), hepatic gluconeogenesis, and glyconeogenesis in 63 (30 trained and 33 untrained) young (7 mo), middle-aged (15 mo), and old (25 mo) male Fischer 344 rats. Animals were trained 1 h/day, 5 days/wk for 10 wk at treadmill speeds of 75% of age-specific maximal capacity. Liver sections, removed at rest, were sliced and incubated in [14C]lactic acid and 0, 0.5, 1.0, 3.0, or 6.0 ng/ml NE. The rate of [14C]lactate incorporation into glucose was significantly greater in young compared with old animals in both training groups and at all NE concentrations. All trained animals had greater rates of glucose production from lactate than their untrained counterparts at 0.5, 1.0, 3.0, and 6.0 ng/ml NE. At each NE concentration, the old rats showed the lowest rates of glycogen synthesis from lactate. The untrained rats in all age groups were the least responsive to increases in NE concentration. Total hepatic glycogen synthase activity exhibited age-related declines as the young and middle-aged had significantly greater total activity compared with the old animals: 620.4 +/- 27.5, 590.0 +/- 37.9, and 436.3 +/- 44.5 disintegrations/min, respectively. No differences with training were found in total activity. The percent of glycogen synthase in the active form was significantly greater in young compared with old in both the trained (48.6 +/- 2.0 vs. 40.0 +/- 1.3% active) and untrained animals (44.7 +/- 2.2 vs. 35.4 +/- 1.5% active).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of activation of liver acetyl‐CoA carboxylase by cell swelling

The activation of hepatic glycogen synthase by the amino-acid-induced cell swelling has been attributed to the stimulation of [glycogen-synthase]-phosphatase resulting from an increase in the intracellular content in glutamate and aspartate, and a decrease in intracellular Cl-, which is a compensatory response to cell swelling [Meijer, A. J., Baquet, A., Gustafson, L., van Woerkom, G. M. & Hue, L. (1992) J. Biol. Chem. 267, 5823-5828]. Here we studied whether the activation of acetyl-CoA carboxylase by cell swelling could be explained by the same mechanism. The activation of endogenous or purified acetyl-CoA carboxylase was measured in gel-filtered liver extracts or cytosols. No activation could be observed under basal conditions but a fivefold stimulation was obtained with concentrations of glutamate (20-25 mM) found in hepatocytes incubated with glutamine. A similar stimulation was also observed with other dicarboxylic acids such as malonate and succinate, or with metal ions like Mg2+, Ca2+ and Mn2+ (10 mM). The addition of 50-100 mM Cl- was found to inhibit the activation of acetyl-CoA carboxylase by some 20-30%. Mg2+ was also found to stimulate the activation of the endogenous glycogen synthase. The glutamate-stimulated and Mg(2+)-stimulated activation of glycogen synthase and acetyl-CoA carboxylase was unaffected by 10 microM inhibitor-2, a specific inhibitory protein of protein phosphatase-1, but could be nearly completely blocked by the phosphatase inhibitor microcystin-LR. Our data suggest that the amino-acid-induced activation of acetyl-CoA carboxylase and glycogen synthase in the liver occurs by a common ionic mechanism.

Maternal malnutrition does not affect fetal hepatic glycogen synthase ontogeny.

Maternal malnutrition late in pregnancy results in the reduced storage of fetal hepatic glycogen in the final days of gestation and an accentuation of normal birth-related hypoglycemia. It was of interest to determine whether or not low glycogen levels resulted when maternal malnutrition disrupted the normal ontogeny of fetal hepatic glycogen synthase, an important glycogenic enzyme. A defect in this enzyme would be expected to seriously affect prenatal and postnatal glycogen synthesis. For this study, livers were removed from fetuses from malnourished (50% of normal dietary intake) mice, as well as from ad libitum-fed mice, and used for the determination of hepatic glycogen, glycogen synthase activity, and glycogen synthase protein levels. In this paper we report that maternal dietary restriction late in pregnancy produces growth-retarded fetuses with severely reduced hepatic glycogen levels, but the normal ontogenic changes in the quantity and activity of hepatic glycogen synthase were not affected. It is especially significant that the accumulation of glycogen synthase occurred despite the minimal level of natural substrate available for the enzyme. These results suggest that the accumulation and activity of hepatic glycogen synthase during late gestation is related to developmental events rather than levels of substrate or glycogen.

Localization of glycogen synthase activity in liver of fasted normal and adrenalectomized rats after injection of dexamethasone

Hepatic glycogen synthase activity was localized in normal and adrenalectomized (ADX) rats after fasting overnight and in fasted ADX rats after injection of dexamethasone (DEX) 2-8 h prior to sacrifice to stimulate glycogen synthesis. Cryostat sections were incubated in medium containing substrate to demonstrate glycogen synthase activity as indicated by glycogen synthesized during incubation. Sections from fasted normal rats showed limited dispersed glycogen synthase activity in both periportal and centrilobular regions. In contrast, activity for glycogen synthase in hepatocytes from fasted ADX rats appeared as large aggregates in random hepatocytes throughout the lobule. Two hours after injection of DEX the reaction product appeared as aggregates in some hepatocytes, but other cells revealed dispersed enzyme activity. Glycogen synthase activity was evident in more hepatocytes after 4 h treatment with DEX and after 8 h virtually all hepatocytes contained abundant reaction product. The results suggest that synthase activity becomes concentrated in limited regions of selected hepatocytes in fasted ADX rats. DEX stimulation of glycogen synthesis for 4-8 h results in increased enzyme activity.

Impaired glycogenic substrate activation of glycogen synthase is associated with depressed synthase phosphatase activity in diabetic rat liver.

Glucose and gluconeogenic substrates promote the activation of hepatic glycogen synthase in vivo and in vitro; activation occurs as inactive glycogen synthase D is dephosphorylated to active glycogen synthase I by glycogen synthase phosphatase. Impairments of glycogen accumulation and glycogen synthase activation in diabetes have been attributed to decreased glycogen synthase phosphatase activity. To determine the role of glycogen synthase phosphatases associated with cytosol and smooth endoplasmic reticulum in the impairment of glycogen synthase activation, livers of normal and streptozotocin-diabetic fed rats were sampled by freeze-clamping before and after perfusion with a mixture of 25 mM glucose, 10 mM glutamine, 4 mM lactate, and 1 mM pyruvate. Perfusion induced activation of glycogen synthase in normal rats, but activation was reduced in the diabetic rats in proportion to the severity of insulin deficiency (r = 0.72, P less than 0.0001). There was also a close correlation between insulin levels and glycogen synthase phosphatase activities of both cytosol (r = 0.76, P less than 0.0001) and SER (r = 0.71, P less than 0.0001) fractions. In contrast, glycogen phosphorylase phosphatase activity and inactivation of glycogen phosphorylase during perfusion were normal in the diabetic livers. This is the first demonstration that glycogen synthase phosphatase activities in both soluble and SER fractions of liver cells are closely related to circulating insulin levels, and that the impairment of glycogen synthesis in diabetes may result from deficient glycogen synthase phosphatase activity in both cell compartments.

Induction of hepatic glycogen synthesis by glucocorticoids is not mediated by insulin

Administration of 0.1 or 1 mg of prednisolone to fed mice caused a 5-fold activation of glycogen synthase in the liver after 3 h, without significant changes in the circulating levels of glucose or insulin, or in the hepatic concentration of cyclic AMP. Adrenalectomized fasted rats responded to Cortisol (10 mg) with an increased glycaemia and a progressive activation of hepatic glycogen synthase after 2–4 h, but without an increase in the very low insulinaemia. These results are incompatible with the prevailing hypothesis that glucocorticoids provoke hepatic glycogen synthesis through an extra secretion of insulin. It is discussed that the acute effect of glucocorticoids is to inhibit rather than stimulate the release of insulin.

Effects of gold thioglucose treatment on enzymes of glycogen metabolism in liver and skeletal muscle in mice

Portions of liver and skeletal muscles from GTG-obese and lean mice were analyzed for glycogen synthase and glycogen phosphorylase activities, and for tissue levels of glycogen, free fatty acids, and triglycerides. Hepatic glycogen synthase activity in GTG-treated mice was comparable to that in control animals, but glycogen phosphorylase activity was increased and hepatic glycogen content was decreased in GTG mice. The activities of both enzymes in pectoralis muscle were comparable in both GTG-treated and control mice, and muscle glycogen levels were similar, as well. Triglyceride levels were markedly elevated in both liver and diaphragm from GTG-treated mice. The results suggest that glycogen synthesis in both liver and skeletal muscle is insulin resistant in GTG-obese mice, and that concomitant changes in lipid metabolism occur in these tissues.

The effect of enteric galactose on neonatal canine carbohydrate metabolism

Newborn pups were assigned to a fasting group or to a group receiving intravenous glucose alimentation. Glucose turnover was determined during steady state equilibration of simultaneously infused [6- 3H] glucose. Thereafter, pups from each group received 0.625 g/Kg of either oral [U- 14C] galactose or [U- 14C] glucose. In fasted or intravenously alimented pups enteric glucose resulted in a rapid and sustained elevation of blood glucose concentrations. Systemic appearance of carbon-14 label from enteric glucose increased rapidly as did the enrichment of blood [ 14C] glucose specific activity. In those pups given enteric galactose, blood glucose values were equivalent to that in the glucose fed groups, however carbon-14 appearing in blood glucose and blood glucose specific activity were significantly lower. The peak values for the rates of appearance and disappearance of systemic glucose were significantly lower in pups fed galactose than among pups fed glucose. Glucose clearance was also significantly lower in these pups despite equivalent plasma insulin responses. Among fasting pups hepatic glycogen content was significantly higher in those given either oral glucose or galactose when compared to a completely starved control group. In contrast, among alimented pups galactose administration significantly enhanced hepatic glycogen content compared to those fed glucose. Similarly, enteric substrate label incorporation into hepatic glycogen was enhanced in both groups given oral labeled galactose. In addition, hepatic glycogen synthase (glucose-6-phosphate independent) activity was increased only among alimented pups fed galactose when compared to completely fasted pups. In conclusion these data suggest that following gastrointestinal galactose administration, hepatic carbohydrate uptake is augmented while glycogen synthesis may be enhanced. Augmented glycogen synthesis following galactose administration may reflect alterations in hepatic glycogen synthase activity or enhanced hepatic carbohydrate uptake.