UDPglucose: alpha-I,4-glucan alpha-4-glucosyltransferase in heart regulation of the activity of the transferase in vivo and in vitro in rat. A dissociation in the action of insulin on transport and on transferase conversion

D-chiro-inositol glycans in insulin signaling and insulin resistance

Classical actions of insulin involve increased glucose uptake from the bloodstream and its metabolism in peripheral tissues, the most important and relevant effects for human health. However, nonoxidative and oxidative glucose disposal by activation of glycogen synthase (GS) and mitochondrial pyruvate dehydrogenase (PDH) remain incompletely explained by current models for insulin action. Since the discovery of insulin receptor Tyr kinase activity about 25 years ago, the dominant paradigm for intracellular signaling by insulin invokes protein phosphorylation downstream of the receptor and its primary Tyr phosphorylated substrates-the insulin receptor substrate family of proteins. This scheme accounts for most, but not all, intracellular actions of insulin. Essentially forgotten is the previous literature and continuing work on second messengers generated in cells in response to insulin. Treatment and even prevention of diabetes and metabolic syndrome will benefit from a more complete elucidation of cellular-signaling events activated by insulin, to include the actions of second messengers such as glycan molecules that contain D-chiro-inositol (DCI). The metabolism of DCI is associated with insulin sensitivity and resistance, supporting the concept that second messengers have a role in responses to and resistance to insulin.

Exercise training attenuated the PKB and GSK-3 dephosphorylation in the myocardium of ZDF rats

Cardiac dysfunction is a severe secondary effect of Type 2 diabetes. Recruitment of the protein kinase B/glycogen synthase kinase-3 pathway represents an integral event in glucose homeostasis, albeit its regulation in the diabetic heart remains undefined. Thus the following study tested the hypothesis that the regulation of protein kinase B/glycogen synthase kinase-3 was altered in the myocardium of the Zucker diabetic fatty rat. Second, exercise has been shown to improve glucose homeostasis, and, in this regard, the effect of swimming training on the regulation of protein kinase B/glycogen synthase kinase-3 in the diabetic rat heart was examined. In the sedentary Zucker diabetic fatty rats, glucose levels were elevated, and cardiac glycogen content increased, compared with wild type. A 13-wk swimming regimen significantly reduced plasma glucose levels and cardiac glycogen content and partially normalized protein kinase B-serine473, protein kinase B-threonine308, and glycogen synthase kinase-3alpha phosphorylation in Zucker diabetic fatty rats. In conclusion, hyperglycemia and increased cardiac glycogen content in the Zucker diabetic fatty rats were associated with dysregulation of protein kinase B/glycogen synthase kinase-3 phosphorylation. These anomalies in the Zucker diabetic fatty rat were partially normalized with swimming. These data support the premise that exercise training may protect the heart against the deleterious consequences of diabetes.

Prostaglandylinositol cyclic phosphate (cPIP): a novel second messenger of insulin action. Comparative analysis of two kinds of "insulin mediators"

Insulin induces a broad spectrum of effects over a wide time interval. It also stimulates the phosphorylation of some cellular proteins, while decreasing the state of phosphorylation of others. These observations indicate the presence of different, but not necessarily mutually exclusive, pathways of insulin action. One well-known pathway represents a phosphorylation cascade initiated by the tyrosine kinase activity of the insulin receptor followed by involvement of different MAP-kinases. Another pathway suggests the existence of low molecular weight insulin mediators whose synthesis and/or release is initiated by insulin. Comparable analysis of two kinds of insulin mediators, namely inositolphosphoglycans and prostaglandylinositol cyclic phosphate (cPIP), has been carried out. It has been shown that the expression of a number of enzymes, such as phospholipase A(2), phospholipase C, cyclo-oxygenase and IRS-1-like enzyme, could regulate the biosynthesis of cPIP in both normal and diabetes-related conditions. Data on the activity of a key enzyme of cPIP biosynthesis termed cPIP synthase (IRS-1-like enzyme) in various monkey tissues before and twice during an euglycemic hyperinsulinemic clamp have been presented. It has been concluded that in vivo insulin increases cPIP synthase activity in both liver and subcutaneous adipose tissue of lean normal monkeys. It has been also suggested that abnormal production of cPIP could be related to several pathologies including glucocorticoid-induced insulin resistance and diabetic embryopathy. Further studies on cPIP and other types of insulin mediators are necessary to aid our understanding of insulin action.

The effect of oral casein on hepatic glycogen metabolism in fasted rats

Casein hydrolysate administration to fasted rats resulted in a biphasic response of glycogen synthase. Fifteen minutes after the protein meal, synthase R (active form) was increased. This was associated with a transient increase in hepatic glucose and glucose-6-phosphate (G6P) concentrations. Both glucose and G6P are known to stimulate synthase phosphatase activity, which would result in activation of synthase. Portal plasma insulin concentration was directly related to the amount of synthase R present. By 1 hour after the meal, synthase R activity was decreased compared with the control activity. Hepatic glycogen concentration was variable during the first 30 minutes after the meal, and then decreased progressively. Portal plasma glucagon concentration and phosphorylase a activity were elevated at all time points. The data suggest that the increased portal plasma glucagon concentration is the major hormonal signal for glycogen metabolism during the second hour following a pure protein meal. However, during the first 30 minutes glycogenolysis is attenuated, perhaps due to the transient increase in insulin and an increased intracellular G6P concentration.

Comparative characterization of human and rat liver glycogen synthase

Glycogen synthase from human liver was studied, and its properties were compared with those of rat liver glycogen synthase. The rat and human liver glycogen synthases were similar in their pH profile, in their kinetic constants for the substrate UDP-glucose and the activator glucose 6-phosphate, and in their elution profiles from Q-Sepharose. The apparent molecular weight of the human synthase subunit was 80,000-85,000 by gel electrophoresis, which is similar to that of the rat enzyme. In addition, antibodies to rat liver glycogen synthase recognized human liver glycogen synthase, indicating that the enzymes of these two species share antigenic determinants. However, there were significant differences between the two enzymes. In particular, the total activity of the human enzyme was higher than that of the rat. The human enzyme, purified to near homogeneity, had a specific activity of 40 U/mg protein compared with 20 U/mg protein for the rat enzyme. The active forms of the rat enzyme had greater thermal stability than those of the human enzyme, but the human enzyme was more stable on storage in various buffers. Last, amino acid analysis indicated differences between the enzymes of the two species. Since glycogen synthase is an important enzyme in liver glycogen synthesis, the characterization of this enzyme in the human will help provide insight regarding human liver glycogen synthesis.

Substrate specific activation by glucose 6-phosphate of the dephosphorylation of muscle glycogen synthase

The activation of glycogen synthase by insulin is in many instances stimulated by the presence of extracellular glucose. Previous observations in cell extracts, glycogen pellets and other crude systems suggest that this stimulation may be due to an increase in glucose 6-phosphate, which activates the dephosphorylation of glycogen synthase by protein phosphatases. Using purified rabbit muscle glycogen synthase D and protein phosphatases 1 and 2A, the types responsible for the activation of muscle synthase, it was found that glucose 6-phosphate, at low, physiological concentrations, stimulated the dephosphorylation of glycogen synthase. Both types of phosphatase were stimulated to the same extent when acting on glycogen synthase. The dephosphorylation of other protein substrates of the phosphatases was either not affected or inhibited by glucose 6-phosphate. It appears that the stimulatory effect of glucose 6-phosphate at physiological concentrations is apparently specific for glycogen synthase, and most likely due to an allosteric configuration change of this enzyme which facilitates its dephosphorylation. In addition, the effects of other reported modulators of glycogen synthase dephosphorylation, AMP, ATP and Mg2+, were studied in this 'in vitro' system.

An improved assay for hepatic glycogen synthase in liver extracts with emphasis on synthase R

An assay for measurement of optimal amounts of glycogen synthase R, the physiologically active form of the enzyme, in liver tissue extracts is described. Tissue extracts enriched in synthase R had a pH profile different from those reported for synthase D and synthase I. In tissue extracts, synthase I had a broad pH optimum but maximal activity was present at pH 7.0-9.0. Synthase D had a sharp pH optimum at pH 8.5 and had little activity at pH 7.0, either in the presence or in the absence of glucose 6-phosphate (G6P). In extracts enriched in synthase R, the pH optimum was 7.0-8.0 without G6P, but 8.0 with G6P. The synthase R activity without G6P rapidly decreased at a higher pH. The proportion of synthase in the physiologically active form traditionally has been reported as an activity ratio based upon the activity in the presence and absence of G6P. The assay has been performed at a single pH. Because of the differences in pH profile, we recommend that the enzyme be measured at pH 7.0 in the absence of G6P and pH 8.5-8.8 in the presence of G6P. In previous assays the substrate UDP-Glc concentration used often has been less than saturating, and the G6P concentration generally has been excessive. A substrate concentration of 11 mM UDP-Glc was found to be necessary for maximal activity. A G6P concentration of 2 mM is adequate for measurement of the D form of the enzyme.

[Enzyme activity of cardiac glycogen metabolism: study of an in situ hypoxia protocol in the rat]

Myocardial hypoxia, induced by arrest of the artificial ventilation of anaesthetized open-chest rats, was utilized in order to study some aspects of the regulation of myocardial glycogen metabolism. Atenolol, a cardioselective beta-adrenergic receptor antagonist, and verapamil, an inhibitor of sarcolemmal calcium transfer, were used to determine the respective role of adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium in the activation of the enzymes of glycogen phosphorolysis and synthesis. Glycogen degradation is reduced by atenolol treatment, as a consequence of a reduced activation of glycogen phosphorylase. Verapamil treatment has no significant effect, neither on the enzyme activation nor on the glycogen utilization. The activation of glycogen synthase, expressed by the conversion of the enzyme from the D to the I form, which results from the decrease in glycogen stores during hypoxia, is lowered under the effect of both drugs. However, in the beta-blocker treatment case, this effect results from a lower glycogen depletion while this effect is more specific in hearts from rats treated with verapamil. Under the effect of verapamil, the reduction of synthase activation, for a similar depletion of glycogen stores, was confirmed by experiments using isolated rat hearts submitted to ischaemia. These results show that: 1. the glycogenolysis in the hypoxic myocardium in situ is mainly controlled by a cAMP-dependent enzyme conversion or by metabolic allosteric effectors; 2. the activation of myocardial glycogen synthase, which is essentially correlated to the reduction of glycogen stores, is also calcium-dependent and most probably totally cAMP-independent.

Acute effects of ingestion of carbohydrate, protein, or fat on cardiac glycogen metabolism in rats

Administration of large doses of insulin to intact rats has been shown to stimulate cardiac glycogen synthase phosphatase activity. This results in an activation of glycogen synthase. However, whether a more physiologic stimulus for insulin secretion also resulted in activation of synthase had not been studied. In the present study, rats were fed glucose, casein hydrolysate, or a mixed meal after a 24-hour fast as a means of physiologically stimulating insulin secretion. Lard was fed as a noninsulinotropic nutrient for comparative purposes. Plasma insulin was significantly increased by 15 minutes in rats given a mixed meal or glucose, but surprisingly, no change was observed in rats fed casein hydrolysate. As expected, no change in plasma insulin was observed in rats fed lard. Synthase phosphatase activity was stimulated in rats fed a mixed meal, glucose, or casein hydrolysate, but not in rats fed lard. Likewise, the proportion of synthase in the active (I) form was significantly increased in rats fed a mixed meal, glucose, or casein hydrolysate, but not in rats fed lard. The increase in phosphatase activity and the increased proportion of synthase in the active form following ingestion of casein hydrolysate was unexpected since this occurred in the absence of an increase in insulin. The proportion of phosphorylase in the active (a) form decreased in rats fed glucose, but remained unchanged in rats fed a mixed meal, casein hydrolysate, or lard. The cardiac glycogen concentration decreased dramatically in rats fed casein hydrolysate. Following the other meals, there was either no change or the decrease was modest.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in cardiac glycogen synthase and phosphorylase activities following stimulation of beta-adrenergic receptors in rats

Following a subcutaneous injection of isoprenaline into rats (5 mg X kg-1 b.w.) the cardiac glycogen stores were depleted by about 90% in less than 15 min. Complete restoration of myocardial glycogen was slow (more than 7-8 hours) despite an elevated glycogen synthase activity. A cardioselective beta-adrenergic receptor blockade (using atenolol) resulted in a complete restoration of glycogen stores in 30 min. The results indicate that the potential of myocardial tissue for glycogenogenesis is great but this capability is obscured by continuous glycogenolysis induced by a long-term activation of phosphorylase. The relative importance of beta-receptor stimulation and actual glycogen level in the control of cardiac synthase is discussed.

Dephosphorylation of glycogen synthase in rat heart extracts by E. coli alkaline phosphatase. Use of an exogenous phosphatase to study substrate-mediated regulation of dephosphorylation

Regulation of the dephosphorylation of glycogen synthase in extracts from rat heart has been studied by adding exogenous phosphatase to the extract. These experiments were possible only because the endogenous protein phosphatase activity of the extract could be inhibited by KF under conditions where alkaline phosphatase activity was not. The concentration of substrate (glycogen synthase from the heart extract) and catalyst (purified E. coli alkaline phosphatase) could be varied independently, by adding known amounts of alkaline phosphatase to the KF-containing heart extracts. Alkaline phosphatase could completely dephosphorylate glycogen synthase while phosphorylase was unchanged. The rate of dephosphorylation was proportional to both the concentration of alkaline phosphatase added to the tissue extract and the amount of glycogen synthase in the extract. The Km for glycogen synthase was close to the concentration found in heart tissue. The Km and the maximum rate of dephosphorylation were both dependent on the phosphorylation state of the glycogen synthase. Less phosphorylated enzyme forms were dephosphorylated faster. These results indicate the necessity for precise control of many variables in studying the rate of glycogen synthase dephosphorylation. Alkaline phosphatase-catalyzed dephosphorylation could be inhibited by physiological concentrations of glycogen. Glycogen synthase dephosphorylation in extracts from fasted-refed rats was less sensitive to glycogen inhibition than in extracts from normal animals. The phosphorylation state of the glycogen synthase in these animals was assessed by kinetic studies to show that differences in phosphorylation state probably could not account for the observations. Fasting led to a decreased rate of dephosphorylation of glycogen synthase due to both an apparent change in kinetic properties of glycogen synthase as a substrate for alkaline phosphatase, and an increased inhibitory effect of glycogen. Stable modifications of glycogen synthase caused by altered nutritional states in the animals are thought to produce these effects.

The influence of free fatty acids on glycogen recovery in rat heart after exercise

Glycogen supercompensation is the term used to denote the abnormally high levels of glycogen found in the heart shortly after an exercise-induced reduction of the substrate. Using rats, we tested whether this condition was linked to the use of plasma free fatty acids (FFA), which normally rise with exercise. Before a 1-h swim, animals received an injection of either saline (S) or nicotinic acid (NA). The nicotinic acid treatment dramatically suppressed the rise in plasma FFA observed in the S-group. Exercise caused a significant but similar reduction (35-38%) of the myocardial glycogen content in both groups. After 1 h of recovery in the S-group, myocardial glycogen reached a value of 30.3 +/- 1.7 mumol x g-1 or 113% of that measured before the exercise began. In contrast, the value for hearts from the NA-group with reduced FFA levels was 24.0 +/- 1.9 mumol x g-1 or only 91% of that measured before exercise. After 2 h the values were 33.8 +/- 1.4 and 29.0 +/- 1.9 mumol x g-1 respectively. These data indicate that glycogen repletion in rat heart after exercise is related to the amount of FFA present in the plasma. We suggest that carbohydrate metabolism is diverted towards synthesis and storage as a result of the glycolytic inhibition exerted by the increased use of fat as an energy source as previously observed in hearts from fasted or diabetic animals.

Resynthesis of muscle glycogen stores during recovery from prolonged exercise in non-diabetic and diabetic subjects

Six male juvenile diabetics were compared with six non-diabetic male subjects regarding the rate of muscle glycogen resynthesis during recovery after prolonged exercise. The glycogen synthesis were similar in the two groups, which indicates that diabetics can participate in strenuous daily physical activity just as non-diabetics.

Enzyme activity during the metabolism of glycogen. II. Cytochemical study of glycogen synthetase in the sensory cells of the tuberous organ of Gnathonemus petersii (Mormyridae)

Glycogen synthetase (2.4.1.11) forms I (independent or active) and D (dependent or passive) as well as the enzymes active in the transformation of the pathways, protein kinase and phosphatase transferase, were studied in the sensory cells and glycogen rich epidermal cells of the weakly electric fish Gnathonemus petersii (Mormyridae). For light microscopy an indirect cytochemical method which differentiated between glycogen originally present and that produced during incubation in the presence of UDPG was used. This differentiation was obtained by iodine, PAS and alpha and beta amylases. Glycogen synthetase is present in the sensory cells in the I and D forms. The epidermal cells only contain the D form. Protein kinase (active I yields D) has only been found in the sensory cells but phosphatase transferase (active D yields I) has been found in both the epidermal cells and the sensory cells, but only within certain organs. Electron microscopy studies of glycogen synthetase I and D and protein kinase were restricted to the sensory cells only. As with the light microscope it was possible to differentiate between native glycogen and newly formed glycogen. This was done using ultrathin sections and staining with uranyl acetate, lead citrate or by the PATAg reaction. It was possible from these observations to locate precisely the positions of these enzymes. In fact, glycogen synthetase I and D are found both in the sensory cytoplasm and in the sensory cavity with the polysaccharide filaments. Protein kinase is also abundant in the sensory cytoplasm especially in the periphery of the cell near the microvillary border.

Synthesis of muscle glycogen during recovery after prolonged severe exercise in diabetic and non-diabetic subjects

Glycogen synthesis rate in skeletal muscle studied in six juvenile diabetic and six non-diabetic males ingesting a carbohydrate rich diet during 12 h of resting recovery after exhaustive bicycle exercise. The diabetic subjects took their regular insulin. Blood samples and muscle biopsies were obtained at rest prior to exercise, immediately after cessation of exercise and after 2,4,6.9 and 12 h of recovery. A marked decrease in muscle glycogn content was observed in response to exercise in both groups of subjects. Mean glycogen utilization rate was the same in the two groups. Glycogen synthesis rate during the first 4 h or recovery was 6.4 +/- 0.6 mmol glucosyl units/kg w.w./h in the diabetic subjects and 7.2 +/- 0.7 mmol glycosyl units/kg w.w./h in the non-diabetic subjects. During the next 8 h glycogen synthesis rate was approximately 1/3 of that being 2.0 +/- 0.3 and 2.4 +/- 0.5 mmol glucosyl units/kg w.w./h in the two groups respectively. Glycogen synthetase I-activity increased markedly in response to exercise in both groups of subjects. However, no differences were observed between the groups. No significant differences in muscle glucose 6-phosphate concentrations were observed between the two groups. Plasma glucose levels were significantly higher in the diabetic than in the non-diabetic subjects. It is concluded that glycogen synthesis during recovery following prolonged severe exercise can proceed at the same rate in diabetic subjects taking their regular insulin as in non-diabetic subjects.

Anti-insulin actions of a bovine pituitary diabetogenic peptide on glycogen synthesis

The site(s) of action of a bovine pituitary diabetogenic peptide that produces hyperglycemia and hyperinsulinemia in vivo (dogs or humans) was investigated in vitro. When rat diaphragms were incubated with the peptide in the presence of insulin, the peptide depressed insulin-mediated (a) glucose uptake, (b) glycogen synthesis, and (c) glycogen synthase activation (conversion of D to I form). Incubation with the peptide alone resulted in small increases in (a), (b), and (c). Insulin-mediated glycogen synthase kinase inactivation was inhibited when both insulin and peptide were present (d), whereas glycogen synthase kinase activity was lowered by the peptide alone. High doses of insulin completely reversed inhibitory effects of the peptide on glycogen synthesis. Therefore, the hyperglycemic and anti-insulin properties of this peptide in vivo can possibly be explained by the partial blocking action of the peptide on insulin-mediated glucose uptake and glycogenesis.