13C NMR studies of glycogen turnover in the perfused rat liver

Magnetic Resonance Imaging and Spectroscopy Methods to Study Hepatic Glucose Metabolism and Their Applications in the Healthy and Diabetic Liver

The liver plays an important role in whole-body glucose homeostasis by taking up glucose from and releasing glucose into the blood circulation. In the postprandial state, excess glucose in the blood circulation is stored in hepatocytes as glycogen. In the postabsorptive state, the liver produces glucose by breaking down glycogen and from noncarbohydrate precursors such as lactate. In metabolic diseases such as diabetes, these processes are dysregulated, resulting in abnormal blood glucose levels. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are noninvasive techniques that give unique insight into different aspects of glucose metabolism, such as glycogenesis, glycogenolysis, and gluconeogenesis, in the liver in vivo. Using these techniques, liver glucose metabolism has been studied in regard to a variety of interventions, such as fasting, meal intake, and exercise. Moreover, deviations from normal hepatic glucose metabolism have been investigated in both patients with type 1 and 2 diabetes, as well as the effects of antidiabetic medications. This review provides an overview of current MR techniques to measure hepatic glucose metabolism and the insights obtained by the application of these techniques in the healthy and diabetic liver.

Immunochemical Study of the Effect of F2Glc on Glycogen Synthase Translocation and Glycogen Synthesis in Isolated Rat Hepatocytes

The compound 2-deoxy-2-fluoro-α-D-glucopyranosyl fluoride (F₂Glc), which is a nonmetabolized superior glucose analogue, is a potent inhibitor of glycogen phosphorylase and pharmacological properties are reported. Glycogen phosphorylase (GP) and glycogen synthase (GS) are responsible of the degradation and synthesis, respectively, of glycogen which is a polymer of glucose units that provides a readily available source of energy in mammals. GP and GS are two key enzymes that modulate cellular glucose and glycogen levels; therefore, these proteins are suggested as potential targets for the treatment of diseases related to glycogen metabolism disorders. We studied by Western Blot technique that F₂Glc decreased GP activity, and we also showed that F₂Glc did not affect GS activity and its translocation from a uniform cytosolic distribution to the hepatocyte periphery, which is crucial for glycogen synthesis, using immunoblotting and immunofluorescence labeling techniques. F₂Glc specifically inhibits glycogenolysis pathway and permits a greater deposition of glycogen. These observations open up the possibility of further develop drugs that act specifically on GP. The ability to selectively inhibit GP, which is a key enzyme for the release of glucose from the hepatic glycogen reserve, may represent a new approach for the treatment of hyperglycemia in type 2 diabetes.

Effect of Repeated Glucagon Doses on Hepatic Glycogen in Type 1 Diabetes: Implications for a Bihormonal Closed-Loop System

OBJECTIVE

To evaluate subjects with type 1 diabetes for hepatic glycogen depletion after repeated doses of glucagon, simulating delivery in a bihormonal closed-loop system.

RESEARCH DESIGN AND METHODS

Eleven adult subjects with type 1 diabetes participated. Subjects underwent estimation of hepatic glycogen using (13)C MRS. MRS was performed at the following four time points: fasting and after a meal at baseline, and fasting and after a meal after eight doses of subcutaneously administered glucagon at a dose of 2 µg/kg, for a total mean dose of 1,126 µg over 16 h. The primary and secondary end points were, respectively, estimated hepatic glycogen by MRS and incremental area under the glucose curve for a 90-min interval after glucagon administration.

RESULTS

In the eight subjects with complete data sets, estimated glycogen stores were similar at baseline and after repeated glucagon doses. In the fasting state, glycogen averaged 21 ± 3 g/L before glucagon administration and 25 ± 4 g/L after glucagon administration (mean ± SEM) (P = NS). In the fed state, glycogen averaged 40 ± 2 g/L before glucagon administration and 34 ± 4 g/L after glucagon administration (P = NS). With the use of an insulin action model, the rise in glucose after the last dose of glucagon was comparable to the rise after the first dose, as measured by the 90-min incremental area under the glucose curve.

CONCLUSIONS

In adult subjects with well-controlled type 1 diabetes (mean A1C 7.2%), glycogen stores and the hyperglycemic response to glucagon administration are maintained even after receiving multiple doses of glucagon. This finding supports the safety of repeated glucagon delivery in the setting of a bihormonal closed-loop system.

NMR methodologies for studying mitochondrial bioenergetics

Nuclear magnetic resonance (NMR) spectroscopy is a technique with an increasing importance in the study of metabolic diseases. Its initial important role in the determination of chemical structures (1, 2) has been considerably overcome by its potential for the in vivo study of metabolism (3-5). The main characteristic that makes this technique so attractive is its noninvasiveness. Only nuclei capable of transitioning between energy states, in the presence of an intense and constant magnetic field, are studied. This includes abundant nuclei such as proton ((1)H) and phosphorous ((31)P), as well as stable isotopes such as deuterium ((2)H) and carbon 13 ((13)C). This allows a wide range of applications that vary from the determination of water distribution in tissues (as obtained in a magnetic resonance imaging scan) to the calculation of metabolic fluxes under ex vivo and in vivo conditions without the need to use radioactive tracers or tissue biopsies (as in a magnetic resonance spectroscopy (MRS) scan). In this chapter, some technical aspects of the methodology of an NMR/MRS experiment as well as how it can be used to study mitochondrial bioenergetics are overviewed. Advantages and disadvantages of in vivo MRS versus high-resolution NMR using proton high rotation magic angle spinning (HRMAS) of tissue biopsies and tissue extracts are also discussed.

A metabolic link between mitochondrial ATP synthesis and liver glycogen metabolism: NMR study in rats re-fed with butyrate and/or glucose

BACKGROUND

Butyrate, end-product of intestinal fermentation, is known to impair oxidative phosphorylation in rat liver and could disturb glycogen synthesis depending on the ATP supplied by mitochondrial oxidative phosphorylation and cytosolic glycolysis.

METHODS

In 48 hr-fasting rats, hepatic changes of glycogen and total ATP contents and unidirectional flux of mitochondrial ATP synthesis were evaluated by ex vivo 31P NMR immediately after perfusion and isolation of liver, from 0 to 10 hours after force-feeding with (butyrate 1.90 mg + glucose 14.0 mg.g-1 body weight) or isocaloric glucose (18.2 mg.g-1 bw); measurements reflected in vivo situation at each time of liver excision. The contribution of energetic metabolism to glycogen metabolism was estimated.

RESULTS

A net linear flux of glycogen synthesis (~11.10 ± 0.60 μmol glucosyl units.h-1.g-1 liver wet weight) occurred until the 6th hr post-feeding in both groups, whereas butyrate delayed it until the 8th hr. A linear correlation between total ATP and glycogen contents was obtained (r2 = 0.99) only during net glycogen synthesis. Mitochondrial ATP turnover, calculated after specific inhibition of glycolysis, was stable (~0.70 ± 0.25 μmol.min-1.g-1 liver ww) during the first two hr whatever the force-feeding, and increased transiently about two-fold at the 3rd hr in glucose. Butyrate delayed the transient increase (1.80 ± 0.33 μmol.min-1.g-1 liver ww) to the 6th hr post-feeding. Net glycogenolysis always appeared after the 8th hr, whereas flux of mitochondrial ATP synthesis returned to near basal level (0.91 ± 0.19 μmol.min-1.g-1 liver ww).

CONCLUSION

In liver from 48 hr-starved rats, the energy need for net glycogen synthesis from exogenous glucose corresponds to ~50% of basal mitochondrial ATP turnover. The evidence of a late and transient increase in mitochondrial ATP turnover reflects an energetic need, probably linked to a glycogen cycling. Butyrate, known to reduce oxidative phosphorylation yield and to induce a glucose-sparing effect, delayed the transient increase in mitochondrial ATP turnover and hence energy contribution to glycogen metabolism.

Fuel metabolism during exercise in euglycaemia and hyperglycaemia in patients with type 1 diabetes mellitus--a prospective single-blinded randomised crossover trial

AIMS/HYPOTHESIS

We assessed systemic and local muscle fuel metabolism during aerobic exercise in patients with type 1 diabetes at euglycaemia and hyperglycaemia with identical insulin levels.

METHODS

This was a single-blinded randomised crossover study at a university diabetes unit in Switzerland. We studied seven physically active men with type 1 diabetes (mean +/- SEM age 33.5 +/- 2.4 years, diabetes duration 20.1 +/- 3.6 years, HbA1c 6.7 +/- 0.2% and peak oxygen uptake [VO2peak] 50.3 +/- 4.5 ml min(-1) kg(-1)). Men were studied twice while cycling for 120 min at 55 to 60% of VO2peak, with a blood glucose level randomly set either at 5 or 11 mmol/l and identical insulinaemia. The participants were blinded to the glycaemic level; allocation concealment was by opaque, sealed envelopes. Magnetic resonance spectroscopy was used to quantify intramyocellular glycogen and lipids before and after exercise. Indirect calorimetry and measurement of stable isotopes and counter-regulatory hormones complemented the assessment of local and systemic fuel metabolism.

RESULTS

The contribution of lipid oxidation to overall energy metabolism was higher in euglycaemia than in hyperglycaemia (49.4 +/- 4.8 vs 30.6 +/- 4.2%; p < 0.05). Carbohydrate oxidation accounted for 48.2 +/- 4.7 and 66.6 +/- 4.2% of total energy expenditure in euglycaemia and hyperglycaemia, respectively (p < 0.05). The level of intramyocellular glycogen before exercise was higher in hyperglycaemia than in euglycaemia (3.4 +/- 0.3 vs 2.7 +/- 0.2 arbitrary units [AU]; p < 0.05). Absolute glycogen consumption tended to be higher in hyperglycaemia than in euglycaemia (1.3 +/- 0.3 vs 0.9 +/- 0.1 AU). Cortisol and growth hormone increased more strongly in euglycaemia than in hyperglycaemia (levels at the end of exercise 634 +/- 52 vs 501 +/- 32 nmol/l and 15.5 +/- 4.5 vs 7.4 +/- 2.0 ng/ml, respectively; p < 0.05).

CONCLUSIONS/INTERPRETATION

Substrate oxidation in type 1 diabetic patients performing aerobic exercise in euglycaemia is similar to that in healthy individuals revealing a shift towards lipid oxidation during exercise. In hyperglycaemia fuel metabolism in these patients is dominated by carbohydrate oxidation. Intramyocellular glycogen was not spared in hyperglycaemia.

Inhibition of the interaction between protein phosphatase 1 glycogen-targeting subunit and glycogen phosphorylase increases glycogen synthesis in primary rat hepatocytes

In Type 2 diabetes, increased glycogenolysis contributes to the hyperglycaemic state, therefore the inhibition of GP (glycogen phosphorylase), a key glycogenolytic enzyme, is one of the possibilities to lower plasma glucose levels. Following this strategy, a number of GPis (GP inhibitors) have been described. However, certain critical issues are associated with their mode of action, e.g. an impairment of muscle function. The interaction between GP and the liver glycogen targeting subunit (termed GL) of PP1 (protein phosphatase 1) has emerged as a new potential anti-diabetic target, as the disruption of this interaction should increase glycogen synthesis, potentially providing an alternative approach to counteract the enhanced glycogenolysis without inhibiting GP activity. We identified an inhibitor of the GL–GP interaction (termed GL–GPi) and characterized its mechanism of action in comparison with direct GPis. In primary rat hepatocytes, at elevated glucose levels, the GL–GPi increased glycogen synthesis similarly to direct GPis. Direct GPis significantly reduced the cellular GP activity, caused a dephosphorylation of the enzyme and decreased the amounts of GP in the glycogen-enriched fraction; the GL–GPi did not influence any of these parameters. Both mechanisms increased glycogen accumulation at elevated glucose levels. However, at low glucose levels, only direct GPis led to increased glycogen amounts, whereas the GL–GPi allowed the mobilization of glycogen because it did not block the activity of GP. Due to this characteristic, GL–GPi in comparison with GPis could offer an advantageous risk/benefit profile circumventing the potential downsides of a complete prevention of glycogen breakdown while retaining glucose- lowering efficacy, suggesting that inhibition of the GL–GP interaction may provide an attractive novel approach for rebalancing the disturbed glycogen metabolism in diabetic patients.

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Glycogen branches out: new perspectives on the role of glycogen metabolism in the integration of metabolic pathways

Glycogen is the storage form of carbohydrate for virtually every organism from yeast to primates. Most mammalian tissues store glucose as glycogen, with the major depots located in muscle and liver. The French physiologist Claude Bernard first identified a starch-like substance in liver and muscle and coined the term glycogen, or "sugar former," in the 1850s. During the 150 years since its identification, researchers in the field of glycogen metabolism have made numerous discoveries that are now recognized as significant milestones in biochemistry and cell signaling. Even so, more questions remain, and studies continue to demonstrate the complexity of the regulation of glycogen metabolism. Under classical definitions, the functions of glycogen seem clear: muscle glycogen is degraded to generate ATP during increased energy demand, whereas hepatic glycogen is broken down for release of glucose into the bloodstream to supply other tissues. However, recent findings demonstrate that the roles of glycogen metabolism in energy sensing, integration of metabolic pathways, and coordination of cellular responses to hormonal stimuli are far more complex.

Effects of insulin and cytosolic redox state on glucose production pathways in the isolated perfused mouse liver measured by integrated 2H and 13C NMR

A great deal is known about hepatic glucose production and its response to a variety of factors such as redox state, substrate supply and hormonal control, but the effects of these parameters on the flux through biochemical pathways which integrate to control glucose production are less clear. A combination of 13C and [2H]water tracers and NMR isotopomer analysis were used to investigate metabolic fluxes in response to altered cytosolic redox state and insulin. In livers isolated from fed mice and perfused with a mixture of substrates including lactate/pyruvate (10:1, w/w), hepatic glucose production had substantial contributions from glycogen, PEP (phosphoenolpyruvate) and glycerol. Inversion of the lactate/pyruvate ratio (1:10, w/w) resulted in a surprising decrease in the contribution from glycogen and an increase in that from PEP to glucose production. A change in the lactate/pyruvate ratio from 10:1 to 1:10 also stimulated flux through the tricarboxylic acid cycle (2-fold), while leaving oxygen consumption and overall glucose output unchanged. When lactate and pyruvate were eliminated from the perfusion medium, both gluconeogenesis and tricarboxylic-acid-cycle flux were dramatically lower. Insulin lowered glucose production by inhibiting glycogenolysis at both low and high doses, but only at high levels of insulin did gluconeogenesis or tricarboxylic-acid-cycle flux tend towards lower values (P<0.1). Our data demonstrate that, in the isolated mouse liver, substrate availability and cellular redox state have a dramatic impact on liver metabolism in both the tricarboxylic acid cycle and gluconeogenesis. The tight correlation of these two pathways under multiple conditions suggest that interventions which increase or decrease hepatic tricarboxylic-acid-cycle flux will have a concomitant effect on gluconeogenesis and vice versa.

Insulin induces a positive relationship between the rates of ATP and glycogen changes in isolated rat liver in presence of glucose; a 31P and 13C NMR study

Background

There is an emerging theory suggesting that insulin, which is known to be the predominant postprandial anabolic hormone, is also a major regulator of mitochondrial oxidative phosphorylation in human skeletal muscle. However, little is known about its effects in the liver. Since there is a theoretical relationship between glycogen metabolism and energy status, a simultaneous and continuous investigation of hepatic ATP and glycogen content was performed in intact and isolated perfused liver by ³¹P and ¹³C nuclear magnetic resonance (NMR) The hepatic rates of ATP and glycogen changes were evaluated with different concentrations of insulin and glucose during continuous and short-term supply.

Results

Liver from rats fed ad libitum were perfused with Krebs-Henseleit Buffer (KHB)(controls) or KHB containing 6 mM glucose, 30 mM glucose, insulin alone, insulin + 6 mM glucose, insulin + 30 mM glucose. In the control, glycogenolysis occurred at a rate of -0.53 ± 0.021 %·min^-1 and ATP content decreased at a rate of -0.28 ± 0.029 %·min^-1. In the absence of insulin, there was a close proportional relationship between the glycogen flux and the glucose concentration, whereas ATP rates never varied. With insulin + glucose, both glycogen and ATP rates were strongly related to the glucose concentration; the magnitude of net glycogen flux was linearly correlated to the magnitude of net ATP flux: flux_glycogen = 72.543(flux_ATP) + 172.08, R² = 0.98.

Conclusion

Only the co-infusion of 30 mM glucose and insulin led to (i) a net glycogen synthesis, (ii) the maintenance of the hepatic ATP content, and a strong positive correlation between their net fluxes. This has never previously been reported. The specific effect of insulin on ATP change is likely related to a rapid stimulation of the hepatic mitochondrial oxidative phosphorylation. We propose that variations in the correlation between rates of ATP and glycogen changes could be a probe for insulin resistance due to the action of substrates, drugs or pathologic situations. Consequently, any work evaluating insulin resistance on isolated organs or in vivo should determine both ATP and glycogen fluxes.

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.

Quantification of the glycogen 13C-1 NMR signal during glycogen synthesis in perfused rat liver

We studied glycogen synthesis from glucose in perfused livers of fed (n = 4) and 24 h starved (n = 7) rats. Glycogenolysis was inhibited by BAY R3401 (150 microM) and proglycosyn (100 microM). After 60 min, we replaced 99% (13)C-1 glucose by natural abundance glucose. This pulse-chase design allowed us to recognize residual ongoing futile glycogen turnover from the release of initially deposited (13)C-label, into the (13)C-free chase medium. Net residual turnover was less than 2 +/- 0.7% and 0.6 +/- 0.2% of 1-(13)C glycogen deposition rates of 0.31 +/- 0.04 and 0.99 +/- 0.04 micromol glucose g(-1) min(-1), in starved and fed livers, respectively. The 1-(13)C glycogen signal was monitored throughout the experiment with proton-decoupled (13)C NMR spectroscopy and analyzed in the time domain using AMARES. We noticed progressive line-broadening in any single experiment in the chase phase. One or a sum of two to three overlapping Lorentzians, with different exponential damping factors, were fitted to the signal. When the S/N was better than 40, the fit always delivered a small and a broad component. In the chase phase, the fit with a single Lorentzian resulted in a decline of glycogen signal by about 15 +/- 4 and 12 +/- 2% in starved and fed rats, respectively. This apparent decline in 1-(13)C glycogen signal could not be accounted for by the appearance of equivalent amounts of (13)C-labeled metabolites in the perfusate. The fit with a sum of two Lorentzians resulted in a decline of glycogen signal intensity of 7 +/- 5 and 5 +/- 3% in starved and fed rats, respectively, which reduced the apparent turnover to 8 +/- 9% and 6 +/- 4%, respectively. Quantification of the growing (13)C-1 glycogen signal requires a model function that accommodates changes in line shape throughout the period under study.

Carbohydrate cycling in micro-organisms: what can (13)C-NMR tell us?

The extension of (13)C-nuclear magnetic resonance (NMR) techniques to study cellular metabolism over recent years has provided valuable data supporting the occurrence, diversity and extent of carbon cycling in the carbohydrate metabolism of micro-organisms. The occurrence of such cycles, resulting from the simultaneous operation of different and sometimes opposite individual steps, is inherently related to the network organisation of cellular metabolism. These cycles are tentatively classified here as 'reversibility', 'metabolic' and 'substrate' cycles on the basis of their balance in carbon and cofactors. Current hypotheses concerning the physiological relevance of carbohydrate cycles are discussed in light of the (13)C-NMR data. They most likely represent system-level mechanisms for coherent and timely partitioning of carbon resources to fit with the various biosynthetic, energetic or redox needs of cells and/or additional strategies in the adaptive capacity of micro-organisms to face variation in environmental conditions.

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

NMR of glycogen in exercise

Natural-abundance 13C NMR spectroscopy is a non-invasive technique that enables in vivo assessments of muscle and/or liver glycogen concentrations. Over the last several years, 13C NMR has been developed and used to obtain information about human glycogen metabolism with diet and exercise. Since NMR is non-invasive, more data points are available over a specified time course, dramatically improving the time resolution. This improved time resolution has enabled the documentation of subtleties of muscle glycogen re-synthesis following severe glycogen depletion that were not previously observed. Muscle and liver glycogen concentrations have been tracked in several different human populations under conditions that include: (1) muscle glycogen recovery from intense localized exercise with normal insulin and with insulin suppressed; (2) muscle glycogen recovery in an insulin-resistant population; (3) muscle glycogen depletion during prolonged low-intensity exercise; (4) effect of a mixed meal on postprandial muscle and liver glycogen synthesis. The present review focuses on basic 13C NMR and gives results from selected studies.

Hepatic and peripheral glucose metabolism in intensive care patients receiving continuous high- or low-carbohydrate enteral nutrition

BACKGROUND

The suppression of endogenous glucose production during parenteral nutrition is impaired in critically ill patients. It is, however, unknown whether enteral administration of carbohydrates, which normally promote hepatic glucose uptake, improves hepatic glucose metabolism in such patients.

METHODS

We studied two groups of 7 patients during a 3-day continuous isocaloric enteral nutrition. A high-carbohydrate, low-lipid (EN-C) or a high-lipid, low-carbohydrate (EN-L) nutrient mixture was administered.

RESULTS

Endogenous glucose production assessed with [2H7]glucose was similarly increased in both groups, indicating absence of its suppression by carbohydrate feeding. Gluconeogenesis estimated from [13C]glucose synthesis during [13C]bicarbonate infusion also was not suppressed by EN-C compared with EN-L. Systemic appearance of exogenous glucose was monitored by enteral infusion of [6,6-2H]glucose and was not different from the rate of glucose equivalent administered enterally, indicating no significant hepatic uptake of glucose in both groups. Plasma glucose and insulin concentrations were slightly higher with EN-C, although not significantly, and plasma triglycerides were similar in both groups. Both nutrition formulas were well tolerated clinically.

CONCLUSIONS

These results indicate that enteral carbohydrate administration, whatever its quantity, fails to suppress endogenous glucose production and to promote net splanchnic glucose uptake in critically ill patients.

On the inhibition of hepatic glycogenolysis by fructose. A 31P-NMR study in perfused rat liver using the fructose analogue 2,5-anhydro-D-mannitol

Inhibition of hormone-stimulated hepatic glycogenolysis by fructose (Fru) has been attributed to accumulation of the competitive inhibitor Fru1P and/or to the associated depletion of the substrate phosphate (Pi). To evaluate the relative importance of either factor, we used the Fru analogue 2,5-anhydro-D-mannitol (aHMol). This analogue is avidly phosphorylated, traps Pi, and inhibits hormone-stimulated glycogenolysis, but it is not a gluconeogenic substrate, and hence does not confound glycogenolytic glucose production. Livers were continuously perfused with dibutyryl-cAMP (100 microM) to clamp phosphorylase in its fully activated a form. We administered aHMol (3.8 mM), and studied changes in glycogenolysis (glucose, lactate and pyruvate output) and in cytosolic Pi and phosphomonoester (PME), using in situ 31P-NMR spectroscopy (n = 4). Lobes of seven livers perfused outside the magnet were extracted for evaluation, by high-resolution 31P-NMR, of the evolution of aHMol1P and of aHMol(1,6)P2. After addition of aHMol, both glycogenolysis and the NMR Pi signal dropped precipitously, while the PME signal rose continuously and was almost entirely composed of aHMol1P. Inhibition of glycogenolysis in excess of the drop in Pi could be explained by continuing accumulation of aHMol1P. A subsequent block of mitochondrial ATP synthesis by KCN (1 mM) caused a rapid increase of Pi. Despite recovery of Pi to values exceeding control levels, glycogenolysis only recovered partially, attesting to the Pi-dependence of glycogenolysis, but also to inhibition by aHMol phosphorylation products. However, KCN resulted in conversion of the major part of aHMol1P into aHMol(1,6)P2. Residual inhibition of glycogenolysis was due to aHMol1P. Indeed, the subsequent withdrawal of aHMol caused a further gradual decrease in the proportion of aHMol1P (being converted into aHMol(1,6)P2, in the absence of de novo aHMol1P synthesis), and this resulted in a gradual de-inhibition of glycogenolysis, in the absence of marked changes in Pi. Glycogenolytic rates were consistently predicted by a model assuming non-saturated Pi kinetics and competition by aHMol1P exclusively: In conclusion, limited Pi availability and the presence of competitive inhibitors are decisive factors in the control of the in situ catalytic potential of phosphorylase a.

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.

Relationship between gluconeogenesis and phosphoenergetics in rat liver assessed by in vivo 13C and 31P NMR spectroscopy

The relationship between the phosphoenergetic state and gluconeogenesis in the liver after ischemic damage was investigated using living rats. The ATP level was determined with in vivo 31P nuclear magnetic resonance spectroscopy, and gluconeogenesis was evaluated with in vivo 31C NMR spectroscopy using L-[3-13C]alanine as a tracer. These two measurements were alternated repeatedly. The rats were divided into three groups: without ischemia (group A); with 10 min ischemia (group B); and with 30 min ischemia (group C). ATP was depleted to 20% of the preischemic state after 10 min ischemia and this level was maintained during 30 min ischemia. After reperfusion, the ATP level was partially restored, but the recovery was smaller in group C. Infusion of [3-13C]alanine was started immediately after the reperfusion. In vivo 13C NMR disclosed changes in the alanine C3, glutamine/glutamate C2 and C3, glucose C1-6, and glycogen C1 signals in the liver. After 60 min infusion of [3-13C]alanine, the ATP level correlated negatively with the signal intensity of alanine (r = -0.664, p = 0.008) and positively with those of glucose and glyogen (r = 0.586, p = 0.023, and r = 0.643, p = 0.011, respectively). These results suggest that the ATP level participates in gluconeogenesis and glycogenesis in the liver. Such multinuclear in vivo NMR observations might uncover new aspects of the metabolic function of the liver in the in vivo state.

13C-NMR relaxation in glycogen

This study is the first report on the multiexponential T2 relaxation of the 13C-1 carbon of glycogen. In contrast to T1 relaxation, which does not display observable multiexponential decay behavior, T2 relaxation is described by a continuous distribution of T2 times. Changes in molecular weight and sample viscosity, which affect the overall mobility of the glycogen particle have little influence on T1 and T2 relaxation times. This is in contradiction with earlier results that T2 is dominated by the overall motion of the glycogen particles [L.-H. Zang Biochemistry 29, 6815-6820 (1990)]. T1 depends strongly on the external field Bo and is almost temperature independent in the range 23-37 degrees C whereas T2 is field independent and varies appreciably with temperature. The experimental T1 and T2 relaxation data are shown to be consistent with existing theoretical models for relaxation, suitably modified to include a distribution of correlation times for the internal motions. The presence of fast decaying components (short T2) in the FID implies broad line components in the frequency spectrum and the corresponding need to appropriately set the integration limits for the quantification of the glycogen peak.

Effects of infused glucose on glycogen metabolism in healthy humans

In order to determine whether or not hepatic glycogen breakdown contributes to systemic glucose flux during glucose infusion, net carbohydrate oxidation (indirect calorimetry) and the total rate of glucose appearance (6,6(-2)H-glucose) were measured in six healthy women during infusion of U-13 C labelled glucose (22 mumol/kg/min). Glucose infusion completely suppressed endogeneous glucose production and increased net carbohydrate oxidation from 10.9 +/- 1.6 to 18.9 +/- 1.0 mumol/kg/min. To differentiate between the oxidation of endogenous (i.e. glycogen) and of exogenous carbohydrates, the 13CO2 production was measured and the oxidation of exogenous 13C labelled carbohydrate was calculated. For this purpose, the specific recovery factor in breath of 13CO2 issued from oxidation of uniformly labelled glucose was determined during infusions of equimolar amounts of 13C bicarbonate, 1-13C acetate and 2-13C acetate. The average recovery was 53.9 +/- 1.5%. The oxidation of exogenous carbohydrate was 20.9 +/- 0.7 mumol/kg/min. This value was slightly higher than net carbohydrate oxidation, indicating that no oxidation of endogenous, unlabelled carbohydrate, and, hence, no utilization of hepatic glycogen took place. These results indicate that (i) estimation of glucose oxidation from indirect calorimetry and tracer technology give concordant results when an appropriate factor of 13CO2 recovery in breath is used, and (ii) utilization of previously formed glycogen is inhibited during hyperglycaemia and hyperinsulinaemia.

Regulation of glycogen synthase activation in isolated hepatocytes

Glycogen synthase, the regulatory enzyme of glycogen synthesis undergoes multisite phosphorylation leading to its inactivation. The kinases responsible for this covalent modification (ex. cAMP-dependent protein kinase, protein kinase C and glycogen synthase kinase-3) are controlled by the second messengers generated by different hormones. The isolated hepatocytes has been used as one of the experimental models for studying this complex regulatory process. Inactivation of glycogen synthase by glucagon and vasopressin has been shown to be accompanied with incorporation of phosphate into the enzyme protein. Insulin has been shown to activate glycogen synthase by inhibition of kinases and activation of synthase phosphatase. Glycogen synthase is activated by several gluconeogenic substrates, in addition to glucose. Studies in hepatocytes with activators and inhibitors of protein kinase C show that this enzyme negatively controls glycogen synthase. The differential effects of the phosphatase inhibitors, calyculin A and okadaic acid in liver cells provide supporting evidence that protein phosphatase type-1 plays a major role in the regulation of glycogen synthase. Hepatocytes isolated from diabetic rats of both types (insulin-dependent and non-insulin-dependent) mimic the defective glycogen synthase activation seen in vivo.

Glycogen turnover in the isolated working rat heart

The isolated working rat heart was adapted for simultaneous determination of glycogen synthesis and degradation using a dual isotope technique. After prelabeling of glycogen with [U-14C]glucose, glycogenolysis was determined continuously from the washout of 14CO2 plus [14C]lactate. Glycogen synthesis was determined during the same period from incorporation of [5-3H]glucose. In the absence of added hormones, hearts were predominantly glycogenolytic (1.5 mumol/min/g, dry weight), and there was simultaneous synthesis (11% of the rate of glycogenolysis). The percentage of glucose taken up by the heart that could traverse the glycogen pool as a consequence of glycogen turnover was minor (5%). Insulin (10 milliunits/ml) predictably stimulated glycogen synthesis (3.6-fold) and nearly abolished glycogenolysis. Addition of glucagon (1 microgram/ml) increased contractile performance and initially stimulated glycogenolysis (3.8-fold) until glycogen was largely depleted. Net tritium incorporation was unaffected by glucagon. Both hormones stimulated glycolytic flux from exogenous glucose (3H2O from [5-3H]glucose) as well as total glycolytic flux (3H2O plus glycogenolysis). The initial stimulation in total glycolytic flux with glucagon was largely from glycogen, explaining the lag in stimulation from exogenous glucose. The relationship between the specific radioactivity and amount of glycogen remaining after different degrees of glycogenolysis suggests that the preference of glycogenolysis for newly synthesized glycogen is only partial.

Noninvasive assessment of in vivo glycogen kinetics in humans: effect of increased physical activity on glycogen breakdown and synthesis

In vivo glycogen kinetics was estimated with the simultaneous use of indirect calorimetry and tracer technology in healthy humans during 24-h periods with low or moderate physical activity (1 or 3 exercise sessions each day). Two 13C-carbohydrates meals were administered at 9.30 a.m. and 1.30 p.m., and one 12C-carbohydrates meal at 6.30 p.m. Net carbohydrate oxidation (net CHO ox) was measured over a 24 h period by indirect calorimetry and oxidation of 13C-labelled carbohydrates (13C CHO ox) was estimated from 13CO2 production. Glycogen breakdown, assessed for the period 8.15 a.m.-6.30 p.m. as the difference between net CHO ox and 13C CHO ox, was increased 1.6 times with three exercise sessions [123.3 (SEM 8.0) g] versus one session [77.9 (SEM 7.7) g, P < 0.0001]. Carbohydrate balances over 24 h were close to zero under both conditions, indicating that glycogen breakdown was matched by an equivalent glycogen synthesis. It was concluded that simultaneous use of indirect calorimetry and tracer technology may make possible the estimation of glycogen kinetics in humans. Moderate physical activity enhanced both glycogen breakdown and synthesis. This stimulation of glycogen metabolism may therefore play a role in the enhanced insulin sensitivity induced by physical exercise.

[13C]NMR studies of the effect of the somatostatin analogue octreotide on hepatic glycogenesis and glycogenolysis

NMR spectroscopy is a useful tool for monitoring multiple intermediate metabolic pathways in different organs in intact animals and humans. We report the effect of the somatostatin analogue octreotide on the fate of 13C-labeled glucose administered to fasted and well-fed rats as determined by NMR spectroscopy. The production of 13C-labeled glycogen and its subsequent breakdown after the end of infusion was identified with a time resolution of 7 min. Hepatic glycogen synthesis was not different between control and octreotide-treated animals but persisted for 15 min after the end of the infusion only in control animals. Glycogenolysis, however, was initiated immediately after the end of infusion in octreotide-treated animals where the half-life of glycogen was 40 min compared with 68 min in control animals. However, once initiated, the rate of glycogenolysis was not significantly altered by octreotide. Although octreotide had no effect on glucose signal intensities in fasted animals, 13C glucose signals were more intense in octreotide compared with control well-fed animals. In conclusion, octreotide alters rat hepatic metabolism by accelerating the onset of glycogenolysis and stimulating glucose accumulation without significantly interfering with glycogen synthesis.

Insulin and glucose suppress hepatic glycogenolysis by distinct enzymatic mechanisms

Both insulin and hyperglycemia can effectively suppress hepatic glucose output (HGO). We examined whether insulin and hyperglycemia specifically suppress liver net glycogen breakdown in a rat model in which glycogen is the major source of HGO. We further examined whether insulin and hyperglycemia act by similar or distinct enzymatic mechanisms. HGO, the rate of net glycogen loss, and glycogen phosphorylase and synthase activities were measured in fed, anesthetized rats infused with saline or insulin (7 mU/min/kg) while either maintaining plasma glucose at basal (7.8 +/- 0.2 mmol/L, euglycemic clamp [EC]) or at 10 mmol/L above basal (18 +/- 0.4 mmol/L, hyperglycemic clamp [HC]). During the basal period, the rate of HGO in each group was comparable to the rate of net glycogen breakdown, averaging 76 +/- 9 and 75 +/- 5 mumol/min/kg, respectively. Thus glycogen breakdown appeared to be a major source of ongoing HGO. Over the last 60 minutes of the experimental period, the rate of glycogenolysis averaged 69 +/- 8 mumol/min/kg in saline-treated rats; this could account for about 80% of the total HGO. During both EC and HC studies, HGO was suppressed (5.5 +/- 3 and -3.6 +/- 10 mumol/min/kg, respectively; P < .001 for each). Net glycogen breakdown decreased by 50% in EC rats (P < .05) and ceased in HC rats (P < .001). Glycogen synthase was predominantly in the active form in all three experimental groups (87% +/- 2%, 89% +/- 2%, and 95% +/- 3% in saline, EC, and HC rats, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Futile cycling of glycogen in Fibrobacter succinogenes as shown by in situ 1H-NMR and 13C-NMR investigation

Glycogen was synthesized during all the growth phases in the rumen anaerobic cellulolytic bacterium Fibrobacter succinogenes. Glycogen synthesis and degradation were monitored using in situ 13C and 1H-NMR spectroscopy in resting cells of F. succinogenes. The cells were incubated at 37 degrees C under anaerobic conditions with [1-13C]glucose and [2-13C]glucose. 1H-NMR spectra were used to quantify enrichment by 13C of metabolism products. Glucose was utilized for energy requirements of the bacterium, essentially via the Embden-Meyerhof pathway, leading to the synthesis of succinate and acetate, while glycogen was stored. From [1-13C]glucose, labeling occurred on C2 of succinate and acetate, and on both C1 and C6 of glycogen, the labeling on C1 being predominant. The C6-labeling of glycogen may be explained by scrambling and reversal of the glycolytic pathway at the triose-phosphate and fructose 1,6-bisphosphate level. When the bacteria were incubated first with [1-13C]glucose, then washed and incubated with [2-13C]glucose, the pattern of 13C labeling in the products of the metabolism, as shown by 13C and 1H-NMR spectra, indicated that glycogen was degraded at the same time as it was being stored, suggesting futile cycling of glycogen. The hydrolysis of previously stored glycogen can provide, in the presence of glucose, up to 30% of the carbon source for the bacteria.

Control of glycogen metabolism by gluconeogenic and ketogenic substrates in isolated hepatocytes from fed rats

1. This study was conducted to examine the effects of gluconeogenic and ketogenic substrates on the activities of the glycogen-metabolizing enzymes and on glycogenolysis in isolated hepatocytes from fed rats. 2. Gluconeogenic substrates like fructose, dihydroxyacetone or lactate turned out to stimulate the glucose-induced activation of glycogen synthase and this effect may be linked, to some extent, to the increase of the cellular glucose 6-phosphate concentration. 3. The effect of fructose was accompanied by the onset of glycogen synthesis. 4. Energetic substrates like fatty acids were also potent activators of glycogen synthase, especially in the presence of glucose. 5. When fatty acids were added alone or together with a physiological concentration of glucose, they induced or potentiated the inhibition of glycogen phosphorylase-a. 6. This inhibitory effect was mediated by a decrease of lactate release. 7. The stimulatory effect of amino acids on glycogen synthase seemed to be direct, non mediated by an inhibition of the phosphorylase-a activity although hepatic glycogenolysis markedly decreased. 8. Moreover, the amino acid action could be linked to their capacities to induce cell swelling and/or to limit proteolysis.

Assignment of the 1H chemical shifts of glycogen

Assignments of nearly all the 1H chemical shifts of glycogen are made by 2-D 1H-1H homonuclear and 13C-1H heteronuclear COSY. We demonstrated that it is possible to obtain well-resolved 2-D n.m.r. spectra for a large molecule like glycogen. The seven nonequivalent protons of the glucose residues in the alpha-(1----4)-linked chains, and of those at the nonreducing ends, were completely assigned. Distinct chemical shifts for H-1 and H-2 immediately adjacent to the alpha-(1----6) bonds at the branch points were also determined. Several modifications of previous 13C chemical shift assignments were made from the heteronuclear 2-D n.m.r. data.

Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR

The rate of net hepatic glycogenolysis was assessed in humans by serially measuring hepatic glycogen concentration at 3- to 12-hour intervals during a 68-hour fast with 13C nuclear magnetic resonance spectroscopy. The net rate of gluconeogenesis was calculated by subtracting the rate of net hepatic glycogenolysis from the rate of glucose production in the whole body measured with tritiated glucose. Gluconeogenesis accounted for 64 +/- 5% (mean +/- standard error of the mean) of total glucose production during the first 22 hours of fasting. In the subsequent 14-hour and 18-hour periods of the fast, gluconeogenesis accounted for 82 +/- 5% and 96 +/- 1% of total glucose production, respectively. These data show that gluconeogenesis accounts for a substantial fraction of total glucose production even during the first 22 hours of a fast in humans.

Quantitation of glycogen/glucose-1-P cycling in liver

A method is introduced for quantitating cycling between hepatic glycogen and glucose-1-P in humans. It depends on the administration of trace [2-3H,6-14C]galactose, a glucose load, and acetaminophen. The ratio of 3H to 14C in the glucuronide of the acetaminophen excreted in urine to that in the administered galactose provides the measure of the fraction of glycogen synthesized that is synthesized from glucose-1-P formed from glycogen. The quantity of glucose-1-P formed from glycogen that is not reconverted to glycogen is not measured. It is assumed that the glucuronide samples the UDP-glucose pool in liver from which glycogen is formed, the last glucosyl units formed from UDP-glucose in glycogen synthesis are the first broken down, and the equilibration of [2-3H]glucose-1-P with fructose-6-P is rapid relative to its conversion to UDP-glucose. During a 5-hour period, while three normal subjects and three non-insulin-dependent diabetics, who had fasted overnight, were infused with 4 mg/kg/min of glucose, the rate of glycogen breakdown, as measured using the method, was only a small percentage of the rate of glycogen synthesis.

Glucose metabolism and recycling by hepatocytes of OB/OB and ob/ob mice

Hepatocytes were prepared from livers of ob/ob (obese diabetic) mice and their lean (OB/OB) siblings that had been fasted for 24 h. The hepatocytes were incubated with [U-14C, 2-3H]-, [U-14C, 3-3H]-, and [U-14C, 6-3H]glucose at concentrations from 20 to 120 mM. 14C was recovered mainly in CO2, glycogen, and lactate. Tritium was recovered in water and glycogen. The yield in labeled products from [2-3H]glucose ranged from two to three times that from [U-14C]glucose. The yields from [3-3H]- and [6-3H]glucose were similar, and 1.3-1.7 times that from [U-14C]glucose. At 40 mM, total utilization of glucose by obese mice was about twice that for lean mice, but there was little difference at 120 mM. The rate of recycling between glucose and glucose 6-phosphate was calculated. An equation to calculate the rate of recycling of glucose from the 2-3H/U-14C ratio in glycogen is derived in the APPENDIX. Our results show that 1) the utilization of glucose by hepatocytes from obese diabetic mice exceeds that of their lean controls, 2) the rate of glucose phosphorylation in both groups greatly exceeds glucose uptake and the rate of glycogen synthesis, 3) glucose phosphorylation represents a difference between a high glucokinase rate and hydrolysis of glucose 6-phosphate, and 4) recycling of glucose carbon between glucose 6-phosphate and pyruvate occurs within mouse hepatocytes.

13C NMR relaxation times of hepatic glycogen in vitro and in vivo

The field dependence of relaxation times of the C-1 carbon of glycogen was studied in vitro by natural-abundance 13C NMR. T1 is strongly field dependent, while T2 does not change significantly with magnetic field. T1 and T2 were also measured for rat hepatic glycogen enriched with [1-13C]glucose in vivo at 4.7 T, and similar relaxation times were observed as those obtained in vitro at the same field. The in vitro values of T1 were 65 +/- 5 ms at 2.1 T, 142 +/- 10 ms at 4.7 T, and 300 +/- 10 ms at 8.4 T, while T2 values were 6.7 +/- 1 ms at 2.1 T, 9.4 +/- 1 ms at 4.7 T, and 9.5 +/- 1 ms at 8.4 T. Calculations based on the rigid-rotor nearest-neighbor model give qualitatively good agreement with the T1 field dependence with a best-fit correlation time of 6.4 X 10(-9) s, which is significantly smaller than tau M, the estimated overall correlation time for the glycogen molecule (ca. 10(-5) s). A more accurate fit of T1 data using a modified Lipari and Szabo approach indicates that internal fast motions dominate the T1 relaxation in glycogen. On the other hand, the T2 relaxation is dominated by the overall correlation time tau M while the internal motions are almost but not completely unrestricted.

No evidence for isotope discrimination of tritiated glucose tracers in measurements of glucose turnover rates in man

Under non-steady-state conditions, glucose turnover rates determined with tritiated glucose tracers are often underestimated. To examine whether isotope discrimination or a tracer contaminant can contribute to this, we compared the turnover rates of unlabelled and tritiated glucose under isotopic steady-state conditions. The turnover rates were measured in 20 healthy subjects at two insulin concentrations (79 +/- 3 mU.l-1 and 704 +/- 62 mU.l-1). Euglycaemia was maintained by infusing unlabelled glucose mixed with (3(3)H)- or (6(3)H)-glucose. In both studies, the isotopically determined glucose disposal rate was virtually identical to the exogenous glucose infusion rate (low insulin 7.66 +/- 0.48 vs 7.58 +/- 0.44 mg.kg-1.min-1, high insulin 13.36 +/- 0.74 vs 13.55 +/- 0.98 mg.kg-1.min-1). The individual values were correlated in both the low (r = 0.85, p less than 0.001) and high dose insulin (r = 0.81, p less than 0.001) studies. Tritiated glucose specific activities were also compared in arterialized and deep venous blood across forearm tissues during the high-dose insulin infusion. Glucose specific activities were similar in arterialized and deep venous blood when analysed with HPLC and conventional methods. In summary: (1) Under isotopic steady-state conditions the turnover rates of unlabelled and labelled glucoses are similar. (2) Unlabelled and labelled glucose are handled identically across forearm tissues. (3) We found no tracer impurity in our tritiated glucose preparations. We conclude that (3(3)H)- and (6(3)H)-glucose tracers can be used to reliably measure glucose turnover rates in man.

Measurement of myocardial glycogen synthesis in diabetic and fasted rats

Diabetes and fasting provoke an increase in heart glycogen content, despite a decline in the amount of active glycogen synthase present. To determine if the activity of glycogen synthase i is still rate limiting for glycogen synthesis, we used 13C-nuclear magnetic resonance to measure the in vivo rate of glycogen synthesis and compared this with the activity of glycogen synthase and phosphorylase measured in tissue extracts using physiological concentrations of substrates and activators. In the basal state the activity of glycogen synthase i was depressed in the diabetic and fasted hearts (P less than 0.01). The rate of heart glycogen synthesis was measured during a 50-min infusion of D-[1-13C]-glucose (10 mg/min) and insulin (1 U/min) and averaged 0.32 +/- 0.04 mumol.min-1.g wet wt-1 in controls and was diminished in both the diabetic (0.18 +/- 0.04 mumol.min-1.g wet wt-1) and fasted (0.16 +/- 0.03 mumol.min-1.g wet wt-1) and fasted (0.16 +/- 0.03 mumol.min-1.g wet wt-1) rats (P less than 0.05 for each). During the glucose and insulin infusion the average activity of glycogen synthase i was greater in control than diabetic or fasted hearts (P less than 0.01 for each) and approximated the rates of net glycogen synthesis in each group. In contrast, there were no significant differences in phosphorylase alpha activity, measured in tissue extracts, among the three groups. Furthermore, although this phosphorylase alpha activity greatly exceeded synthase activity, it did not appear to be expressed in vivo. We conclude that in normal, diabetic, and fasted rats, glycogen synthase is rate limiting for glycogen synthesis.

A minimal model of liver glycogen metabolism; feasibility for predicting flux rates

A minimal model of glycogen metabolism can allow the estimation of the flux rates in the glycogen pathway from the time course of the intermediates in the pathway, measured during substrate administration and hormonal stimulation. The comprehensive model of El-Refai & Bergman (Am. J. Physiol. 231, 1608, 1976) consisting of six compartments and 26 non-estimable parameters has successfully accounted for the responses of hepatic glycogenic intermediates in response to a glucose load in hepatocytes (Katz et al., J. biol. Chem. 253, 4530, 1978), in perfused liver (Nordlie et al., J. biol. Chem. 255, 1834, 1980) and during refeeding in vivo (Van DeWerve & Jeanrenaud, Am. J. Physiol. 247, E271, 1984). The comprehensive model is here reduced to a minimal model, consisting of five compartments representing extracellular and intracellular glucose, glucose-phosphate, uridine diphosphate glucose (UDPG), glycogen, and five parameters estimated from the hepatic response to a given stimulus. Estimation of these parameters requires the measurement of the net hepatic glucose balance, the net gluconeogenic flux, and the time course of glycogenic intermediates responding to a hormone or substrate stimulus. The hepatic glycogenolytic response predicted by the comprehensive model in response to an increase in glucagon is closely fitted by the minimal model. When Gaussian distributed random error was added, 0-5% SD in the glucose and glycogen compartments and 0-10% SD in the glucose-phosphate and UDPG compartments, the hepatic response predicted by the minimal model was virtually free of the added error, and the model parameters were found to be within 30% of their true values. When the minimal model was used to interpret the experimental response to an increase in glucose concentration it predicted that: (1) glucokinase can phosphorylate glucose at rates similar to maximal rates of net glycogen synthesis; (2) futile cycling at the glycogen/glucose-1-phosphate level can limit glycogen synthesis; and (3) glucose-6-phosphatase inhibition by glucose has a significant role in net glycogen synthesis.