PCB126 induced toxic actions on liver energy metabolism is mediated by AhR in rats

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in the regulation of biological responses to more planar aromatic hydrocarbons, like TCDD. We previously described the sequence of events following exposure of male rats to a dioxin-like polychlorinated biphenyl (PCB) congener, 3,3',4,4',5-pentachlorobiphenyl (PCB126), that binds avidly to the AhR and causes various types of toxicity including metabolic syndrome, fatty liver, and disruption of energy homeostasis. The purpose of this study was, to investigate the role of AhR to mediate those toxic manifestations following sub-acute exposure to PCB126 and to examine possible sex differences in effects. For this goal, we created an AhR knockout (AhR-KO) model using CRISPR/Cas9. Comparison was made to the wild type (WT) male and female Holtzman Sprague Dawley rats. Rats were injected with a single IP dose of corn oil vehicle or 5 μmol/kg PCB126 in corn oil and necropsied after 28 days. PCB126 caused significant weight loss, reduced relative thymus weights, and increased relative liver weights in WT male and female rats, but not in AhR-KO rats. Similarly, significant pathologic changes were visible which included necrosis and regeneration in female rats, micro- and macro-vesicular hepatocellular vacuolation in males, and a paucity of glycogen in livers of both sexes in WT rats only. Hypoglycemia and lower IGF1, and reduced serum non-esterified fatty acids (NEFAs) were found in serum of both sexes of WT rats, low serum cholesterol levels only in the females, and no changes in AhR-KO rats. The expression of genes encoding enzymes related to xenobiotic metabolism (e.g. CYP1A1), gluconeogenesis, glycogenolysis, and fatty acid oxidation were unaffected in the AhR-KO rats following PCB126 exposure as opposed to WT rats where expression was significantly upregulated (PPARα, females only) or downregulated suggesting a disrupted energy homeostasis. Interestingly, Acox2, Hmgcs, G6Pase and Pc were affected in both sexes, the gluconeogenesis and glucose transporter genes Pck1, Glut2, Sds, and Crem only in male WT-PCB rats. These results show the essential role of the AhR in glycogenolysis, gluconeogenesis, and fatty acid oxidation, i.e. in the regulation of energy production and homeostasis, but also demonstrate a significant difference in the effects of PCB126 in males verses females, suggesting higher vulnerability of glucose homeostasis in males and more changes in fatty acid/lipid homeostasis in females. These differences in effects, which may apply to more/all AhR agonists, should be further analyzed to identify health risks to specific groups of highly exposed human populations.

Recombinant Bovine Growth Hormone-Induced Metabolic Remodelling Enhances Growth of Gilthead Sea-Bream (Sparus aurata): Insights from Stable Isotopes Composition and Proteomics

Growth hormone and insulin-like growth factors (GH/IGF axis) regulate somatic growth in mammals and fish, although their action on metabolism is not fully understood in the latter. An intraperitoneal injection of extended-release recombinant bovine growth hormone (rbGH, Posilac^®) was used in gilthead sea bream fingerlings and juveniles to analyse the metabolic response of liver and red and white muscles by enzymatic, isotopic and proteomic analyses. GH-induced lipolysis and glycogenolysis were reflected in liver composition, and metabolic and redox enzymes reported higher lipid use and lower protein oxidation. In white and red muscle reserves, rBGH increased glycogen while reducing lipid. The isotopic analysis of muscles showed a decrease in the recycling of proteins and a greater recycling of lipids and glycogen in the rBGH groups, which favoured a protein sparing effect. The protein synthesis capacity (RNA/protein) of white muscle increased, while cytochrome-c-oxidase (COX) protein expression decreased in rBGH group. Proteomic analysis of white muscle revealed only downregulation of 8 proteins, related to carbohydrate metabolic processes. The global results corroborated that GH acted by saving dietary proteins for muscle growth mainly by promoting the use of lipids as energy in the muscles of the gilthead sea bream. There was a fuel switch from carbohydrates to lipids with compensatory changes in antioxidant pathways that overall resulted in enhanced somatic growth.

Role of nitration in control of phosphorylase and glycogenolysis in mouse skeletal muscle

Phosphorylase is one of the most carefully studied proteins in history, but knowledge of its regulation during intense muscle contraction is incomplete. Tyrosine nitration of purified preparations of skeletal muscle phosphorylase results in inactivation of the enzyme and this is prevented by antioxidants. Whether an altered redox state affects phosphorylase activity and glycogenolysis in contracting muscle is not known. Here, we investigate the role of the redox state in control of phosphorylase and glycogenolysis in isolated mouse fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) muscle preparations during repeated contractions. Exposure of crude muscle extracts to H₂O₂ had little effect on phosphorylase activity. However, exposure of extracts to peroxynitrite (ONOO^-), a nitrating/oxidizing agent, resulted in complete inactivation of phosphorylase (half-maximal inhibition at ∼200 µM ONOO^-), which was fully reversed by the presence of an ONOO^- scavanger, dithiothreitol (DTT). Incubation of isolated muscles with ONOO^- resulted in nitration of phosphorylase and marked inhibition of glycogenolysis during repeated contractions. ONOO^- also resulted in large decreases in high-energy phosphates (ATP and phosphocreatine) in the rested state and following repeated contractions. These metabolic changes were associated with decreased force production during repeated contractions (to ∼60% of control). In contrast, repeated contractions did not result in nitration of phosphorylase, nor did DTT or the general antioxidant N-acetylcysteine alter glycogenolysis during repeated contractions. These findings demonstrate that ONOO^- inhibits phosphorylase and glycogenolysis in living muscle under extreme conditions. However, nitration does not play a significant role in control of phosphorylase and glycogenolysis during repeated contractions.NEW & NOTEWORTHY Here we show that exogenous peroxynitrite results in nitration of phosphorylase as well as inhibition of glycogenolysis in isolated intact mouse skeletal muscle during short-term repeated contractions. However, repeated contractions in the absence of exogenous peroxynitrite do not result in nitration of phosphorylase or affect glycogenolysis, nor does the addition of antioxidants alter glycogenolysis during repeated contractions. Thus phosphorylase is not subject to redox control during repeated contractions.

Mild Physiologic Hyperglycemia Induces Hepatic Insulin Resistance in Healthy Normal Glucose-Tolerant Participants

CONTEXT

Chronic hyperglycemia worsens skeletal muscle insulin resistance and β-cell function. However, the effect of sustained physiologic hyperglycemia on hepatic insulin sensitivity is not clear.

OBJECTIVE

To examine the effect of sustained physiologic hyperglycemia (similar to that observed in patients with type 2 diabetes) on endogenous (primarily reflecting hepatic) glucose production (EGP) in healthy individuals.

DESIGN

Volunteers participated in a three-step hyperinsulinemic (10, 20, 40 mU/m2 per minute) euglycemic clamp before and after a 48-hour glucose infusion to increase plasma glucose concentration by ∼40 mg/dL above baseline. EGP was measured with 3-3H-glucose before and after chronic glucose infusion.

PARTICIPANTS

Sixteen persons with normal glucose tolerance [eight with and eight without a family history (FH) of diabetes] participated in the study.

MAIN OUTCOME MEASURE

EGP.

RESULTS

Basal EGP increased following 48 hours of glucose infusion (from a mean ± SEM of 2.04 ± 0.08 to 3.06 ± 0.29 mg/kgffm⋅ min; P < 0.005). The hepatic insulin resistance index (basal EGP × fasting plasma insulin) markedly increased following glucose infusion (20.1 ± 1.8 to 51.7 ± 6.6; P < 0.005) in both FH+ and FH- subjects.

CONCLUSION

Sustained physiologic hyperglycemia for as little as 48 hours increased the rate of basal hepatic glucose production and induced hepatic insulin resistance in health persons with normal glucose tolerance, providing evidence for the role of glucotoxicity in the increase in hepatic glucose production in type 2 diabetes.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

Although efficient compared to computers, the human brain utilizes energy at 10-fold the rate of other organs by mass. How the brain is supplied with sufficient on-demand energy to support its activity in the absence of neuronal storage capacity remains unknown. Neurons are not capable of meeting their own energy requirements, instead energy supply in the brain is managed by an oligocellular cartel composed of neurons, glia and the local vasculature (NGV), wherein glia can provide the ergogenic metabolite lactate to the neuron in a process called the astrocyte-to-neuron shuttle (ANLS). The only means of energy storage in the brain is glycogen, a polymerized form of glucose that is localized largely to astrocytes, but its exact role and conditions of use are not clear. In this computational model we show that neuromodulatory stimulation by norepinephrine induces astrocytes to recover glucosyl subunits from glycogen for use in a glycolytic process that favors the production of lactate. The ATP and NADH produced support metabolism in the astrocyte while the lactate is exported to feed the neuron. Thus, rapid energy demands by both neurons and glia in a stimulated brain can be met by glycogen mobilization.

Gintonin, a Ginseng-Derived Exogenous Lysophosphatidic Acid Receptor Ligand, Protects Astrocytes from Hypoxic and Re-oxygenation Stresses Through Stimulation of Astrocytic Glycogenolysis

Astrocytes are a unique brain cell-storing glycogen and express lysophosphatidic acid (LPA) receptors. Gintonin is a ginseng-derived exogenous G protein-coupled LPA receptor ligand. Accumulating evidence shows that astrocytes serve as an energy supplier to neurons through astrocytic glycogenolysis under physiological and pathophysiological conditions. However, little is known about the relationships between LPA receptors and astrocytic glycogenolysis or about the roles of LPA receptors in hypoxia and re-oxygenation stresses. In the present study, we examined the functions of gintonin-mediated astrocytic glycogenolysis in adenosine triphosphate (ATP) production, glutamate uptake, and cell viability under normoxic, hypoxic, and re-oxygenation conditions. The application of gintonin or LPA to astrocytes induced glycogenolysis in concentration- and time-dependent manners. The stimulation of gintonin-mediated astrocytic glycogenolysis was achieved through the LPA receptor-Gα_q/11 protein-phospholipase C-inositol 1,4,5-trisphosphate receptor-intracellular calcium ([Ca²⁺]_i) transient pathway. Gintonin treatment to astrocytes increased the phosphorylation of brain phosphorylase kinase, with sensitive manner to K252a, an inhibitor of phosphorylase kinase. Gintonin-mediated astrocytic glycogenolysis was blocked by isofagomine, a glycogen phosphorylase inhibitor. Gintonin additionally increased astrocytic glycogenolysis under hypoxic and re-oxygenation conditions. Moreover, gintonin increased ATP production, glutamate uptake, and cell viability under the hypoxic and re-oxygenation conditions. Collectively, we found that the gintonin-mediated [Ca²⁺]_i transients regulated by LPA receptors were coupled to astrocytic glycogenolysis and that stimulation of gintonin-mediated astrocytic glycogenolysis was coupled to ATP production and glutamate uptake under hypoxic and re-oxygenation conditions, ultimately protecting astrocytes. Hence, the gintonin-mediated astrocytic energy that is modulated via LPA receptors helps to protect astrocytes under hypoxia and re-oxygenation stresses.

Insulin Regulates Glycogen Synthesis in Human Endometrial Glands Through Increased GYS2

CONTEXT

Glycogen synthesis is a critical metabolic function of the endometrium to prepare for successful implantation and sustain embryo development. Yet, regulation of endometrial carbohydrate metabolism is poorly characterized. Whereas glycogen synthesis is attributed to progesterone, we previously found that the metabolic B isoform of the insulin receptor is maximally expressed in secretory-phase endometrium, indicating a potential role of insulin in glucose metabolism.

OBJECTIVE

We sought to determine whether insulin or progesterone regulates glycogen synthesis in human endometrium.

DESIGN, PARTICIPANTS, OUTCOME MEASUREMENTS

Endometrial epithelial cells were isolated from 28 healthy women and treated with insulin, medroxyprogesterone (MPA), or vehicle. Intracellular glycogen and the activation of key enzymes were quantified.

RESULTS

In epithelia, insulin induced a 4.4-fold increase in glycogen, whereas MPA did not alter glycogen content. Insulin inactivated glycogen synthase (GS) kinase 3α/β (GSK3α/β), relieving inhibition of GS. In a regulatory mechanism, distinct from liver and muscle, insulin also increased GS by 3.7-fold through increased GS 2 (GYS2) gene expression.

CONCLUSIONS

We demonstrate that insulin, not progesterone, directly regulates glycogen synthesis through canonical acute inactivation of GSK3α/β and noncanonical stimulation of GYS2 transcription. Persistently elevated GS enables endometrium to synthesize glycogen constitutively, independent of short-term nutrient flux, during implantation and early pregnancy. This suggests that insulin plays a key, physiological role in endometrial glucose metabolism and underlines the need to delineate the effect of maternal obesity and hyperinsulinemia on fertility and fetal development.

Refeeding after caloric restriction reverses altered liver glucose release

CONTEXT

Caloric restriction increases liver glucose release (LGR), but it is not known if this is a permanent condition.

OBJECTIVE

To investigate if refeeding after caloric restriction reverses the high LGR.

MATERIALS AND METHODS

Rats were organised in six-pups litters (GC); 12-pups litters with either 50% caloric restriction from 21 to 80 days of age (GR) or fed at will from 50 to 80 days of age (GRL). Liver perfusion was made at the age of 80 days.

RESULTS

LGR was higher in the GR both during basal and adrenaline-stimulated conditions. Refeeding after caloric restriction decreased it to values close to those of GC rats.

DISCUSSION

The altered LGR of GR rats was reversed by refeeding (group GRL). The influence of hypothalamic neuropetides on these hepatic changes is suggested.

CONCLUSIONS

Enhanced LGR under caloric restriction is not programmed by early feeding; instead, it is determined by the current nutritional conditions.

CCL2 Induces the Production of β2 Adrenergic Receptors and Modifies Astrocytic Responses to Noradrenaline

The decline in brain noradrenaline levels is associated with the progression of certain neurodegenerative diseases. This seems to be due, at least in part, to the ability of noradrenaline to limit glial activation and to reduce the damage associated with it. Our previous studies of the mechanisms involved in this process indicate that noradrenaline induces the production of the chemokine CCL2 in astrocytes. While CCL2 can protect neurons against certain injuries, its overproduction has also proven to be harmful and to prevent noradrenaline neuroprotective effects. Therefore, in this study, we analyze if the modifications caused to astrocytes by an excessive production of CCL2 may alter their response to noradrenaline. Using primary cultures of rat cortical astrocytes, we observed that CCL2 enhances the production of beta 2 adrenergic receptors in these cells. While this potentiates noradrenaline signaling through cAMP, the activation of the transcription factor CREB is inhibited by CCL2. Furthermore, although CCL2 potentiates noradrenaline induction of glycogenolysis, this does not translate into an augmented release of lactate, one of the processes through which astrocytes help support neurons. Additionally, other neuroprotective actions of noradrenaline, such as the production of brain derived neurotrophic factor and the inhibition of the inducible nitric oxide synthase in astrocytes were modified by CCL2. These data suggest that some of the central nervous system alterations related to CCL2 could be due to its effects on adrenergic receptors and its interference with noradrenaline signaling.

Alteration of hepatic cells glucose metabolism as a non-cholinergic detoxication mechanism in counteracting diazinon-induced oxidative stress

The aim of this study was to evaluate effects of acute exposure to various doses of diazinon, a widely used synthetic organophosphorus (OP) insecticide on plasma glucose, hepatic cells key enzymes of glycogenolysis and gluconeogenesis, and oxidative stress in rats. Diazinon was administered by gavage at doses of 15, 30 and 60 mg/kg. The liver was perfused and removed under anaesthesia. The activities of glycogen phosphorylase (GP), phosphoenolpyruvate carboxykinase (PEPCK), thiobarbituric acid reactive substances (TBARS) and total antioxidant capacity (TAC) were analysed in liver homogenate. Administration of diazinon (15, 30 and 60 mg/kg) increased plasma glucose concentrations by 101.43% (P=0.001), 103.68% (P=0.000) and 160.65% (P=0.000) of control, respectively. Diazinon (15, 30 and 60 mg/kg) increased hepatic GP activity by 43.5% (P=0.05), 70.3% (P=0.00) and 117.2% (P=0.02) of control, respectively. In addition, diazinon (30 and 60 mg/kg) increased hepatic PEPCK by 77.3% (P=0.000) and 93.5% (P=0.000) of control, respectively. Diazinon (30 and 60 mg/kg) decreased liver TAC by 38% (P=0.046) and 48% (P=0.000) of control, respectively. Also diazinon (30 and 60 mg/kg) increased hepatic cell liver lipid peroxidation by 77% (P=0.05) and 280% (P=0.000) of control. The correlations between plasma glucose and hepatic cells TBARS (r2=0.537, P=0.02), between plasma glucose and ChE activity (r2=0.81, P=0.049) and between plasma glucose and hepatic cells GP activity (r2=0.833, P=0.04) were significant. It is concluded that the liver cells are a site of toxic action of diazinon. Diazinon increases glucose release from liver into blood through activation of glycogenolysis and gluconeogenesis as a detoxication non-cholinergic mechanism to overwhelm diazinon-induced toxic stress. The results are in accordance with the hypothesis that OPs are a predisposing factor of diabetes.

Glucocorticoid antagonism limits adiposity rebound and glucose intolerance in young male rats following the cessation of daily exercise and caloric restriction

Severe caloric restriction (CR), in a setting of regular physical exercise, may be a stress that sets the stage for adiposity rebound and insulin resistance when the food restriction and exercise stop. In this study, we examined the effect of mifepristone, a glucocorticoid (GC) receptor antagonist, on limiting adipose tissue mass gain and preserving whole body insulin sensitivity following the cessation of daily running and CR. We calorically restricted male Sprague-Dawley rats and provided access to voluntary running wheels for 3 wk followed by locking of the wheels and reintroduction to ad libitum feeding with or without mifepristone (80 mg·kg(-1)·day(-1)) for 1 wk. Cessation of daily running and CR increased HOMA-IR and visceral adipose mass as well as glucose and insulin area under the curve during an oral glucose tolerance test vs. pre-wheel lock exercised rats and sedentary rats (all P < 0.05). Insulin sensitivity and glucose tolerance were preserved and adipose tissue mass gain was attenuated by daily mifepristone treatment during the post-wheel lock period. These findings suggest that following regular exercise and CR there are GC-induced mechanisms that promote adipose tissue mass gain and impaired metabolic control in healthy organisms and that this phenomenon can be inhibited by the GC receptor antagonist mifepristone.

Glucagon sensitivity and clearance in type 1 diabetes: insights from in vivo and in silico experiments

Glucagon use in artificial pancreas for type 1 diabetes (T1D) is being explored for prevention and rescue from hypoglycemia. However, the relationship between glucagon stimulation of endogenous glucose production (EGP) viz., hepatic glucagon sensitivity, and prevailing glucose concentrations has not been examined. To test the hypothesis that glucagon sensitivity is increased at hypoglycemia vs. euglycemia, we studied 29 subjects with T1D randomized to a hypoglycemia or euglycemia clamp. Each subject was studied at three glucagon doses at euglycemia or hypoglycemia, with EGP measured by isotope dilution technique. The peak EGP increments and the integrated EGP response increased with increasing glucagon dose during euglycemia and hypoglycemia. However, the difference in dose response based on glycemia was not significant despite higher catecholamine concentrations in the hypoglycemia group. Knowledge of glucagon's effects on EGP was used to develop an in silico glucagon action model. The model-derived output fitted the obtained data at both euglycemia and hypoglycemia for all glucagon doses tested. Glucagon clearance did not differ between glucagon doses studied in both groups. Therefore, the glucagon controller of a dual hormone control system may not need to adjust glucagon sensitivity, and hence glucagon dosing, based on glucose concentrations during euglycemia and hypoglycemia.

A low-protein diet combined with low-dose endotoxin leads to changes in glucose homeostasis in weanling rats

Severe malnutrition is a leading cause of global childhood mortality, and infection and hypoglycemia or hyperglycemia are commonly present. The etiology behind the changes in glucose homeostasis is poorly understood. Here, we generated an animal model of severe malnutrition with and without low-grade inflammation to investigate the effects on glucose homeostasis. Immediately after weaning, rats were fed diets containing 5 [low-protein diet (LP)] or 20% protein [control diet (CTRL)], with or without repeated low-dose intraperitoneal lipopolysaccharide (LPS; 2 mg/kg), to mimic inflammation resulting from infections. After 4 wk on the diets, hyperglycemic clamps or euglycemic hyperinsulinemic clamps were performed with infusion of [U-(13)C6]glucose and [2-(13)C]glycerol to assess insulin secretion, action, and hepatic glucose metabolism. In separate studies, pancreatic islets were isolated for further analyses of insulin secretion and islet morphometry. Glucose clearance was reduced significantly by LP feeding alone (16%) and by LP feeding with LPS administration (43.8%) compared with control during the hyperglycemic clamps. This was associated with a strongly reduced insulin secretion in LP-fed rats in vivo as well as ex vivo in islets but signficantly enhanced whole body insulin sensitivity. Gluconeogenesis rates were unaffected by LP feeding, but glycogenolysis was higher after LP feeding. A protein-deficient diet in young rats leads to a susceptibility to low-dose endotoxin-induced impairment in glucose clearance with a decrease in the islet insulin secretory pathway. A protein-deficient diet is associated with enhanced peripheral insulin sensitivity but impaired insulin-mediated suppression of hepatic glycogenolysis.

Silver nanoparticles stimulate glycogenolysis in rainbow trout (Oncorhynchus mykiss) hepatocytes

Silver nanoparticles (AgNPs) are found in many consumer products yet their biological effects on non-target aquatic organisms are yet to be fully understood. This research aimed to investigate the effects of AgNPs on cell signaling in rainbow trout (Oncorhynchus mykiss) hepatocytes. We focused on the β-adrenoreceptor (AR), which mediates glycogenolysis, and the glucocorticoid receptor (GCR), which mediates gluconeogenesis. These two receptors have been extensively studied in trout hepatocytes due to their key roles during the stress response to increase glucose availability (among other things), allowing the organisms to cope with the stressor. We show for the first time that AgNPs at a concentration of 1 μg/mL did not interfere with the function of either the β-AR or the GCR systems in rainbow trout hepatocytes, but at the concentration of 10 μg/mL AgNPs stimulated glycogenolysis which was apparently receptor-independent. This study suggests that AgNPs could affect hormone-regulated cell signaling pathways at a concentration of 10 μg/mL.

The action of p-synephrine on hepatic carbohydrate metabolism and respiration occurs via both Ca(2+)-mobilization and cAMP production

Citrus aurantium extracts, which contain large amounts of p-synephrine, are widely used for weight loss purposes and as appetite suppressants. In the liver, C. aurantium (bitter orange) extracts affect hemodynamics, carbohydrate metabolism, and oxygen uptake. The purpose of the present work was to quantify the action of p-synephrine and also to obtain indications about its mechanism of action, a task that would be difficult to accomplish with C. aurantium extracts due to their rather complex composition. The experimental system was the isolated perfused rat liver. p-Synephrine significantly stimulated glycogenolysis, glycolysis, gluconeogenesis, and oxygen uptake. The compound also increased the portal perfusion pressure and the redox state of the cytosolic NAD(+)/NADH couple. A Ca(2+)-dependency for both the hemodynamic and the metabolic effects of p-synephrine was found. p-Synephrine stimulated both cAMP overflow and the initial Ca(2+) release from the cellular stores previously labeled with (45)Ca(2+). The metabolic and hemodynamic actions of p-synephrine were strongly inhibited by α-adrenergic antagonists and moderately affected by β-adrenergic antagonists. The results allow to conclude that p-synephrine presents important metabolic and hemodynamic effects in the liver. These effects can be considered as both catabolic (glycogenolysis) and anabolic (gluconeogenesis), they are mediated by both α- and β-adrenergic signaling, require the simultaneous participation of both Ca(2+) and cAMP, and could be contributing to the overall stimulation of metabolism that usually occurs during weight loss periods.

A novel experimental strategy to assess the metabolic effects of selective activation of a G(q)-coupled receptor in hepatocytes in vivo

Increased hepatic glucose production is a key pathophysiological feature of type 2 diabetes. Like all other cell types, hepatocytes express many G protein-coupled receptors (GPCRs) that are linked to different functional classes of heterotrimeric G proteins. The important physiological functions mediated by G(s)-coupled hepatic glucagon receptors are well-documented. In contrast, little is known about the in vivo physiological roles of hepatocyte GPCRs that are linked to G proteins of the G(q) family. To address this issue, we established a transgenic mouse line (Hep-Rq mice) that expressed a G(q)-linked designer receptor (Rq) in a hepatocyte-selective fashion. Importantly, Rq could no longer bind endogenous ligands but could be selectively activated by a synthetic drug, clozapine-N-oxide. Clozapine-N-oxide treatment of Hep-Rq mice enabled us to determine the metabolic consequences caused by selective activation of a G(q)-coupled GPCR in hepatocytes in vivo. We found that acute Rq activation in vivo led to pronounced increases in blood glucose levels, resulting from increased rates of glycogen breakdown and gluconeogenesis. We also demonstrated that the expression of the V(1b) vasopressin receptor, a G(q)-coupled receptor expressed by hepatocytes, was drastically increased in livers of ob/ob mice, a mouse model of diabetes. Strikingly, treatment of ob/ob mice with a selective V(1b) receptor antagonist led to reduced glucose excursions in a pyruvate challenge test. Taken together, these findings underscore the importance of G(q)-coupled receptors in regulating hepatic glucose fluxes and suggest novel receptor targets for the treatment of type 2 diabetes.

Colesevelam suppresses hepatic glycogenolysis by TGR5-mediated induction of GLP-1 action in DIO mice

Bile acid sequestrants are nonabsorbable resins designed to treat hypercholesterolemia by preventing ileal uptake of bile acids, thus increasing catabolism of cholesterol into bile acids. However, sequestrants also improve hyperglycemia and hyperinsulinemia through less characterized metabolic and molecular mechanisms. Here, we demonstrate that the bile acid sequestrant, colesevelam, significantly reduced hepatic glucose production by suppressing hepatic glycogenolysis in diet-induced obese mice and that this was partially mediated by activation of the G protein-coupled bile acid receptor TGR5 and glucagon-like peptide-1 (GLP-1) release. A GLP-1 receptor antagonist blocked suppression of hepatic glycogenolysis and blunted but did not eliminate the effect of colesevelam on glycemia. The ability of colesevelam to induce GLP-1, lower glycemia, and spare hepatic glycogen content was compromised in mice lacking TGR5. In vitro assays revealed that bile acid activation of TGR5 initiates a prolonged cAMP signaling cascade and that this signaling was maintained even when the bile acid was complexed to colesevelam. Intestinal TGR5 was most abundantly expressed in the colon, and rectal administration of a colesevelam/bile acid complex was sufficient to induce portal GLP-1 concentration but did not activate the nuclear bile acid receptor farnesoid X receptor (FXR). The beneficial effects of colesevelam on cholesterol metabolism were mediated by FXR and were independent of TGR5/GLP-1. We conclude that colesevelam administration functions through a dual mechanism, which includes TGR5/GLP-1-dependent suppression of hepatic glycogenolysis and FXR-dependent cholesterol reduction.

The glucagon receptor is involved in mediating the body weight-lowering effects of oxyntomodulin

Oxyntomodulin (OXM) is a peptide secreted postprandially from the L-cells of the gut that has a weak affinity for both the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR). Peripheral administration of OXM in humans and rodents causes weight loss reducing food intake and increasing energy expenditure. It has been suggested that OXM modulates energy intake solely through GLP1R agonism. Because glucagon decreases food intake in rodents and humans, we examined whether activation of the GCGR is involved in the body weight-lowering effects of OXM. We identified an equipotent GLP1R-selective peptide agonist that differs from OXM by only one residue (Q3→E, OXMQ3E), but has no significant GCGR agonist activity in vitro and ~100-fold reduced ability to stimulate liver glycogenolysis. Chronic treatment of obese mice with OXM and OXMQ3E demonstrated that OXM exhibits superior weight loss and lipid-lowering efficacy, and antihyperglycemic activity that is comparable to the corresponding GLP1R-selective agonist. Studies in Glp1r(-/-) mice and coadministration of OXM and a GCGR antagonist revealed that the antiobesity effect of OXM requires activation of both GLP1R and GCGR. Our data provide new insight into the mechanism of action of OXM and suggest that activation of GCGR is involved in the body weight-lowering action of OXM.

Nickel induces hyperglycemia and glycogenolysis and affects the antioxidant system in liver and white muscle of goldfish Carassius auratus L

The toxicity of nickel to mammals is well studied, whereas information on nickel effects on fish is scant. Goldfish exposure to 10-50 mg L(-1) of waterborne Ni(2+) for 96 h showed reduced glycogen levels by 27-33% and 37-40% in liver and white muscle, respectively, accompanied by substantial increases in blood glucose levels (by 15-99%). However, indices of oxidative damage to proteins (carbonyl proteins) and lipids (lipid peroxides) were largely unaffected by nickel exposure. In liver, the activities of antioxidant enzymes, superoxide dismutase (SOD) and glutathione peroxidase (GPx), were not affected by Ni(2+) treatment, while catalase activity was elevated by 26%. In white muscle, however, substantial increases in SOD (by 38-147%) and GPx (by 2.5-5.5-fold) activities appeared to compensate for decreased catalase activity (by 59-69%) in order to resist Ni-induced oxidative perturbations. Both hepatic and muscular glutathione reductase activities were suppressed by 10-30% and 12-21%, respectively, after goldfish exposure to all Ni(2+) concentrations used. However, the activity of glucose-6-phosphate dehydrogenase was remarkably enhanced (by 1.6-5.4-fold) in white muscle of Ni-exposed fish, indicating a strong potential increase in NADPH production under Ni exposure. Thus, the exposure of goldfish to 10-50 mg L(-1) of Ni(2+) for 96 h induces glycogenolysis and hyperglycemia, showing some similarities with a hypoxia response, and leads to a substantial activation of defense systems against reactive oxygen species in liver and white muscle in tissue-specific and concentration-dependent manner.

Acute activation of central GLP-1 receptors enhances hepatic insulin action and insulin secretion in high-fat-fed, insulin resistant mice

Glucagon-like peptide-1 (GLP-1) receptor knockout (Glp1r(-/-)) mice exhibit impaired hepatic insulin action. High fat (HF)-fed Glp1r(-/-) mice exhibit improved, rather than the expected impaired, hepatic insulin action. This is due to decreased lipogenic gene expression and triglyceride accumulation. The present studies overcome these secondary adaptations by acutely modulating GLP-1R action in HF-fed wild-type mice. The central GLP-1R was targeted given its role as a regulator of hepatic insulin action. We hypothesized that acute inhibition of the central GLP-1R impairs hepatic insulin action beyond the effects of HF feeding. We further hypothesized that activation of the central GLP-1R improves hepatic insulin action in HF-fed mice. Insulin action was assessed in conscious, unrestrained mice using the hyperinsulinemic euglycemic clamp. Mice received intracerebroventricular (icv) infusions of artificial cerebrospinal fluid, GLP-1, or the GLP-1R antagonist exendin-9 (Ex-9) during the clamp. Intracerebroventricular Ex-9 impaired the suppression of hepatic glucose production by insulin, whereas icv GLP-1 improved it. Neither treatment affected tissue glucose uptake. Intracerebroventricular GLP-1 enhanced activation of hepatic Akt and suppressed hypothalamic AMP-activated protein kinase. Central GLP-1R activation resulted in lower hepatic triglyceride levels but did not affect muscle, white adipose tissue, or plasma triglyceride levels during hyperinsulinemia. In response to oral but not intravenous glucose challenges, activation of the central GLP-1R improved glucose tolerance. This was associated with higher insulin levels. Inhibition of the central GLP-1R had no effect on oral or intravenous glucose tolerance. These results show that inhibition of the central GLP-1R deteriorates hepatic insulin action in HF-fed mice but does not affect whole body glucose homeostasis. Contrasting this, activation of the central GLP-1R improves glucose homeostasis in HF-fed mice by increasing insulin levels and enhancing hepatic insulin action.

Lactate produced by glycogenolysis in astrocytes regulates memory processing

When administered either systemically or centrally, glucose is a potent enhancer of memory processes. Measures of glucose levels in extracellular fluid in the rat hippocampus during memory tests reveal that these levels are dynamic, decreasing in response to memory tasks and loads; exogenous glucose blocks these decreases and enhances memory. The present experiments test the hypothesis that glucose enhancement of memory is mediated by glycogen storage and then metabolism to lactate in astrocytes, which provide lactate to neurons as an energy substrate. Sensitive bioprobes were used to measure brain glucose and lactate levels in 1-sec samples. Extracellular glucose decreased and lactate increased while rats performed a spatial working memory task. Intrahippocampal infusions of lactate enhanced memory in this task. In addition, pharmacological inhibition of astrocytic glycogenolysis impaired memory and this impairment was reversed by administration of lactate or glucose, both of which can provide lactate to neurons in the absence of glycogenolysis. Pharmacological block of the monocarboxylate transporter responsible for lactate uptake into neurons also impaired memory and this impairment was not reversed by either glucose or lactate. These findings support the view that astrocytes regulate memory formation by controlling the provision of lactate to support neuronal functions.

Changes in calcium fluxes in mitochondria, microsomes, and plasma membrane vesicles of livers from monosodium L-glutamate-obese rats

The purpose of this work was to evaluate if the fat liver accumulation interferes with intracellular calcium fluxes and the liver glycogenolytic response to a calcium-mobilizing α(1)-adrenergic agonist, phenylephrine. The animal model of monosodium L-glutamate (MSG)-induced obesity was used. The adult rats develop obesity and steatosis. Calcium fluxes were evaluated through measuring the (45)Ca(2+) uptake by liver microsomes, inside-out plasma membrane, and mitochondria. In the liver, assessments were performed on the calcium-dependent glycogenolytic response to phenylephrine and the glycogen contents. The Ca(2+) uptake by microsomes and plasma membrane vesicles was reduced in livers from obese rats as a result of reduction in the Ca(2+)-ATPase activities. In addition, the plasma membrane Na(+)/K(+)-ATPase was reduced. All these matched effects could contribute to elevated resting intracellular calcium levels in the hepatocytes. Livers from obese rats, albeit smaller and with similar glycogen contents to those of control rats, released higher amounts of glucose in response to phenylephrine infusion, which corroborates these observations. Mitochondria from obese rats exhibited a higher capacity of retaining calcium, a phenomenon that could be attributed to a minor susceptibility of the mitochondrial permeability transition pore opening.

Aroclor 1254 disrupts liver glycogen metabolism and enhances acute stressor-mediated glycogenolysis in rainbow trout

The objective of this study was to investigate the impact of short-term exposure to polychlorinated biphenyls on the acute stress response in rainbow trout. Fish were exposed to dietary Aroclor1254 (10mg kg(-1) body mass/day) for 3 days and then subjected to a 3-min handling disturbance and sampled over a 24h recovery after the stressor exposure. In the pre-stress fish, PCB exposure significantly elevated aryl hydrocarbon receptor (AhR) and cytochrome P4501A1 (Cyp1A1) mRNA abundance and Cyp1A protein expression confirming AhR activation. There was no significant effect of PCB on plasma cortisol and glucose levels, while plasma lactate levels were significantly elevated compared to the sham group. PCB exposure significantly elevated liver glycogen content and hexokinase activity, whereas lactate dehydrogenase activity was depressed. Short-term PCB exposure did not modify the acute stressor-induced plasma cortisol, glucose and lactate responses. Liver glycogen content dropped significantly after stressor exposure in the PCB group but not in the sham group. This was matched by a significantly higher liver LDH activity and a lower HK activity during recovery in the PCB group suggesting enhanced glycolytic capacity to fuel hepatic metabolism. Liver AhR, but not Cyp1A1, transcript levels were significantly reduced during recovery from handling stressor in the Aroclor fed fish. Collectively, this study demonstrates that short-term PCB exposure may impair the liver metabolic performance that is critical to cope with the enhanced energy demand associated with additional stressor exposure in rainbow trout.

Study of muscle glycogen content in both sexes of an Indian teleost Clarias batrachus (Linn.) exposed to different concentrations of arsenic

The present investigation has been conducted to study the effect of different sublethal concentrations of arsenic (5, 10 and 15 mg/L) on the muscle glycogen content in an Indian teleost, Clarias batrachus, during their post-spawning period (October to January).The species were exposed to various arsenic concentrations for six consecutive days i.e.,144 h. Significant differences in average muscle glycogen content were found in the treated male and female C. batrachus specimens. However, significantly higher (P < 0.01) muscle glycogen content was observed in male fish when compared to females in the untreated specimens. Arsenic caused muscle glycogenolysis in both sexes of C. batrachus. The depletions were not always dose (arsenic concentration) and time dependent, although higher concentrations were more glycogenolytic than the lower concentrations. After 96 h of treatment with arsenic, less depletion of muscle glycogen content was recorded in both sexes of fish.

Role of serotonin and/or norepinephrine in the MDMA-induced increase in extracellular glucose and glycogenolysis in the rat brain

The acute administration of MDMA has been shown to promote glycogenolysis and increase the extracellular concentration of glucose in the striatum. In the present study the role of serotonergic and/or noradrenergic mechanisms in the MDMA-induced increase in extracellular glucose and glycogenolysis was assessed. The relationship of these responses to the hyperthermia produced by MDMA also was examined. The administration of MDMA (10mg/kg, i.p.) resulted in a significant and sustained increase of 65-100% in the extracellular concentration of glucose in the striatum, as well as in the prefrontal cortex and hippocampus, and a 35% decrease in brain glycogen content. Peripheral blood glucose was modestly increased by 32% after MDMA treatment. Treatment of rats with fluoxetine (10mg/kg, i.p.) significantly attenuated the MDMA-induced increase in extracellular glucose in the striatum but had no effect on MDMA-induced glycogenolysis or hyperthermia. Treatment with prazosin (1mg/kg, i.p.) did not alter the glucose or glycogen responses to MDMA but completely suppressed MDMA-induced hyperthermia. Finally, propranolol (3mg/kg, i.p.) significantly attenuated the MDMA-induced increase in extracellular glucose and glycogenolysis but did not alter MDMA-induced hyperthermia. The present results suggest that MDMA increases extracellular glucose in multiple brain regions, and that this response involves both serotonergic and noradrenergic mechanisms. Furthermore, beta-adrenergic and alpha-adrenergic receptors appear to contribute to MDMA-induced glycogenolysis and hyperthermia, respectively. Finally, hyperthermia, glycogenolysis and elevated extracellular glucose appear to be independent, unrelated responses to acute MDMA administration.

A case of McArdle disease: efficacy of vitamin B6 on fatigability and impaired glycogenolysis

McArdle disease is a glycogenetic myopathy caused by a deficit of myophosphorylase inherited in an autosomal recessive pattern. Here, we report a case of McArdle disease in which fatigability was the only subjective complaint. Objective neurological findings were normal except for very mild muscle weakness in limbs and an elevated serum creatine kinase level. Ischemic forearm exercise test showed deficient glycogenolysis. In the muscle biopsy specimen, periodic acid Schiff (PAS) stained subsarcolemmal glycogen was increased and the muscle phosphorylase A activity was decreased. After administration of vitamin B6, fatigability was diminished and ischemic forearm exercise test showed improved glycogenolysis. Vitamin B6 may be beneficial for McArdle disease, especially for its easy fatigability.

Regulation of net hepatic glycogenolysis and gluconeogenesis by epinephrine in humans

The relative contributions of net hepatic glycogenolysis (NHG) and gluconeogenesis to rates of glucose production during a physiological increment in plasma epinephrine concentrations, independent of changes in plasma insulin concentrations, were determined in seven fasting, healthy young subjects. Plasma insulin concentrations were kept constant by infusing somatostatin (0.1 microg.kg(-1).min(-1)) and replacing basal insulin (24 pmol.m(-2).min(-1)). Epinephrine (1.2 microg.m(-2).min(-1)) was infused for 90 min while NHG was assessed directly by (13)C magnetic resonance spectroscopy. The rate of glucose production was assessed using [6,6-(2)H(2)]glucose, and gluconeogenesis was calculated as the difference between the rate of glucose production and NHG. Plasma epinephrine concentrations increased rapidly from approximately 100 to approximately 2,000 pmol/l (P < 0.00001) accompanied by an increase in plasma glucose concentrations from 4.3 +/- 0.2 to 13.3 +/- 0.3 mmol/l at 90 min (P = 0.00001). This increase in plasma epinephrine concentration resulted in a 2.5-fold increase in glucose production (from 14.4 +/- 1.0 micromol.kg(-1).min(-1) to 35.7 +/- 2.0 micromol.kg(-1).min(-1), P < 0.0001), which lasted for approximately 60 min (phase 1), after which glucose production decreased to 31.2 +/- 1.9 micromol.kg(-1).min(-1) (P < 0.0001 vs. basal) during the last 30 min of the epinephrine infusion (phase 2). Hepatic glycogen concentrations decreased almost linearly during phase 1, and rates of NHG were 19.9 +/- 3.0 micromol.kg(-1).min(-1) (P = 0.005 vs. basal), which could account for approximately 60% of glucose production. During phase 2, NHG decreased to 7.3 +/- 2.8 micromol.kg(-1).min(-1) (P = 0.02 vs. peak), accounting for only approximately 20% of glucose production. In conclusion, in the presence of basal plasma insulin and glucagon concentrations, a physiological increase in plasma epinephrine concentrations stimulates glucose production with an initial, 60-min transient phase caused by stimulation of NHG and a second phase that can mostly be attributed to a twofold increase in rates of gluconeogenesis.

Structure-activity relationships of flavonoids as potential inhibitors of glycogen phosphorylase

Flavonoids are ubiquitous components in vegetables, fruits, tea, and wine. Therefore, they are often consumed in large quantities in our daily diet. Several flavonoids have been shown to have potential as antidiabetic agents. In the present study, we focused on inhibition of glycogen phosphorylase (GP) by flavonoids. 6-Hydroxyluteolin, hypolaetin, and quercetagetin were identified as good inhibitors of dephosphorylated GP (GPb), with IC 50 values of 11.6, 15.7, and 9.7 microM, respectively. Furthermore, a structure-activity relationship study revealed that the presence of the 3' and 4' OH groups in the B-ring and double bonds between C2 and C3 in flavones and flavonols are important factors for enzyme recognition and binding. Quercetagetin inhibited GPb in a noncompetitive manner, with a K i value of 3.5 microM. Multiple inhibition studies by Dixon plots suggested that quercetagetin binds to the allosteric site. In primary cultured rat hepatocytes, quercetagetin and quercetin suppressed glucagon-stimulated glycogenolysis, with IC 50 values of 66.2 and 68.7 microM, respectively. These results suggested that as a group of novel GP inhibitors, flavonoids have potential to contribute to the protection or improvement of control of diabetes type II.

Greater glycogen utilization during 1- than 2-adrenergic receptor stimulation in the isolated perfused rat heart

Differences in energy metabolism during beta(1)- and beta(2)-adrenergic receptor (AR) stimulation have been shown to translate to differences in the elicited functional responses. It has been suggested that differential access to glycogen during beta(1)- compared with beta(2)-AR stimulation may influence the peak functional response and modulation of the response during sustained adrenergic stimulation. Interleaved (13)C- and (31)P-NMR spectroscopy was used during beta(1)- and beta(2)-AR stimulation at matched peak workload (2.5 times baseline) in the isolated perfused rat heart to monitor glycogen levels, phosphorylation potential, and intracellular pH. Simultaneous measurements of left ventricular (LV) function [LV developed pressure (LVDP)], heart rate (HR), and rate-pressure product (RPP = LVDP x HR) were also performed. The heart was perfused under both substrate-free (SF) conditions and with exogenous glucose (G). The greater glycogenolysis was observed during beta(1)- than beta(2)-AR stimulation with G (54% vs. 38% reduction, P = 0.006) and SF (92% vs. 79% reduction, P = 0.04) perfusions. The greater beta(1)-AR-mediated glycogenolysis was correlated with greater ability to sustain the initial contractile response. However, with SF perfusion, the duration of this ability was limited: excessive early glycogen depletion caused an earlier decline in LVDP and phosphorylation potential during beta(1)- than beta(2)-AR stimulation. Therefore, endogenous glycogen stores are depleted earlier and to a greater extent, despite a slightly weaker overall inotropic response, during beta(1)- than beta(2)-AR stimulation. These findings are consistent with beta(1)-AR-specific PKA-dependent glycogen phosphorylase kinase signaling.

The control of hepatic glycogen metabolism in an in vitro model of sepsis

Culturing hepatocytes with a combination of LPS, TNF-alpha, IL-1beta and IFN-gamma resulted in an inhibition of glucose output from glycogen and prevented the repletion of glycogen in freshly cultured cells. The reduced glycogen mobilisation correlated with the lower cell glycogen content and reduced rate of glycogen synthesis from [U-(14)C]glucose rather than alterations in either total phosphorylase or phosphorylase a activity. There was no change in the percentage of glycogen exported as glucose nor the production of lactate plus pyruvate indicating that redistribution of the Gluc-6-P cannot explain the failure of the liver to export glucose. Although changes in glycogen mobilisation correlated with NO production, inhibition of NO synthase by inclusion of L-NMMA in the culture medium failed to prevent the inhibition of either glycogen accumulation or mobilisation by the proinflammatory cytokines, precluding the involvement of NO in this response. LPS plus cytokine treatment had no effect on total glycogen synthase activity although the activity ratio was lowered, indicative of increased phosphorylation. The inhibition of glycogen synthesis correlated with a fall in the intracellular concentrations of Gluc-6-P and UDP-glucose and in the absence of measured changes in kinase activity, it is suggested that the fall in Gluc-6-P reduces both substrate supply and glycogen synthase phosphatase activity. The fall in Gluc-6-P coincided with a reduction in total glucokinase and hexokinase activity within the cells, but no significant change in either the translocation of glucokinase or glucose-6-phosphatase activity. This demonstrates direct cytokine effects on glycogen metabolism independent of changes in glucoregulatory hormones.

A Thr94Ala mutation in human liver fatty acid-binding protein contributes to reduced hepatic glycogenolysis and blunted elevation of plasma glucose levels in lipid-exposed subjects

Liver fatty acid-binding protein (L-FABP) is a highly conserved key factor in lipid metabolism. Amino acid replacements in L-FABP might alter its function and thereby affect glucose metabolism in lipid-exposed subjects, as indicated by studies in L-FABP knockout mice. Amino acid replacements in L-FABP were investigated in a cohort of 1,453 Caucasian subjects. Endogenous glucose production (EGP), gluconeogenesis, and glycogenolysis were measured in healthy carriers of the only common Thr(94)-to-Ala amino acid replacement (Ala/Ala(94)) vs. age-, sex-, and BMI-matched wild-type (Thr/Thr(94)) controls at baseline and after 320-min lipid/heparin-somatostatin-insulin-glucagon clamps (n = 18). Whole body glucose disposal was further investigated (subset; n = 13) using euglycemic-hyperinsulinemic clamps without and with lipid/heparin infusion. In the entire cohort, the only common Ala/Ala(94) mutation was significantly associated with reduced body weight, which is in agreement with a previous report. In lipid-exposed, individually matched subjects there was a genotype vs. lipid-treatment interaction for EGP (P = 0.009) driven mainly by reduced glycogenolysis in Ala/Ala(94) carriers (0.46 +/- 0.05 vs. 0.59 +/- 0.05 mgxkg(-1)xmin(-1), P = 0.013). The lipid-induced elevation of plasma glucose levels was smaller in Ala/Ala(94) carriers compared with wild types (P < 0.0001). Whole body glucose disposal was not different between lipid-exposed L-FABP genotypes. In summary, the Ala/Ala(94)-mutation contributed significantly to reduced glycogenolysis and less severe hyperglycemia in lipid-exposed humans and was further associated with reduced body weight in a large cohort. Data clearly show that investigation of L-FABP phenotypes in the basal overnight-fasted state yielded incomplete information, and a challenge test was essential to detect phenotypical differences in glucose metabolism between L-FABP genotypes.

Glutathione-dependent reduction of arsenate by glycogen phosphorylase responsiveness to endogenous and xenobiotic inhibitors

Rabbit muscle glycogen phosphorylase-a (GPa) reduces arsenate (As(V)) to the more toxic arsenite (As(III)) in a glutathione (GSH)-dependent fashion. To determine whether reduction of As(V) by GPa is countered by compounds known to inhibit GP-catalyzed glycogenolysis, the effects of thiol reagents, endogenous compounds (glucose, ATP, ADP) as well as nonspecific glycogen phosphorylase inhibitors (GPIs; caffeine, quercetin, flavopiridol [FP]), and specific GPIs (1,4-dideoxy-1,4-imino-D-arabinitol [DAB], BAY U6751, CP320626) were tested on reduction of As(V) by rabbit muscle GPa in the presence of glycogen (substrate), AMP (activator), and GSH, and the As(III) formed from As(V) was quantified by high-performance liquid chromatography-hydride generation-atomic fluorescence spectrometry. The As(V)-reducing activity of GPa was moderately sensitive to thiol reagents. Glucose above 5mM and ADP or ATP at physiological levels diminished GPa-catalyzed As(V) reduction. All GPIs inhibited As(V) reduction by GPa in a concentration-dependent fashion; however, their effects were differentially affected by glucose (10mM) or AMP (200microM instead of 25microM), known modulators of the action of some GPIs on the GP-catalyzed glycogenolysis. Inhibition of As(V) reduction by DAB and quercetin was not influenced by glucose or AMP. Glucose that potentiates the inhibitory effects of caffeine, BAY U6751, and CP320626 on the glycogenolytic activity of GPa also enhanced the inhibitory effects of these GPIs on GPa-catalyzed As(V) reduction. AMP at high concentration alleviated the inhibition by BAY U6751 and CP320626 (whose antagonistic effect on GP-catalyzed glycogen breakdown is also AMP sensitive), whereas the inhibition in As(V) reduction by FP or caffeine was little affected by AMP. Thus, GPIs inhibit both the glycogenolytic and As(V)-reducing activities of GP, supporting that the latter is coupled to glycogenolysis. It was also shown that a GPa-rich extract of rat liver contained GSH-dependent As(V)-reducing activity that was inhibited by specific GPIs, suggesting that the liver-type GPa can also catalyze reduction of As(V).

A Novel Glucagon Receptor Antagonist, NNC 25-0926, Blunts Hepatic Glucose Production in the Conscious Dog

Elevated glucagon is associated with fasting hyperglycemia in type 2 diabetes. We assessed the effects of the glucagon receptor antagonist (2R)-N-[4-({4-(1-cyclohexen-1-yl)[(3,5-dichloroanilino)carbonyl]anilino}methyl)benzoyl]-2-hydroxy-b-alanine (NNC 25-0926) on hepatic glucose production (HPG) in vivo, using arteriovenous difference and tracer techniques in conscious dogs. The experiments consisted of equilibration (-140 to -40 min), control (40-0 min), and experimental [0-180 min, divided into P1 (0-60 min) and P2 (60-180 min)] periods. In P1, NNC 25-0926 was given intragastrically at 0 (veh), 10, 20, 40, or 100 mg/kg, and euglycemia was maintained. In P2, somatostatin, basal intraportal insulin, and 5-fold basal intraportal glucagon (2.5 ng/kg/min) were infused. Arterial plasma insulin levels remained basal throughout the study in all groups. Arterial plasma glucagon levels remained basal during the control period and P1 and then increased to approximately 70 pg/ml in P2 in all groups. Arterial plasma glucose levels were basal in the control period and P1 in all groups. In P2, the arterial glucose level increased to 245+/-22 and 172+/-15 mg/dl in the veh and 10 mg/kg groups, respectively, whereas in the 20, 40, and 100 mg/kg groups, there was no rise in glucose. Net hepatic glucose output was approximately 2 mg/kg/min in all groups during the control period. In P2, it increased by 9.4+/-2 mg/kg/min in the veh group. In the 10, 20, 40, and 100 mg/kg groups, the rise was only 4.1+/-0.9, 1.6+/-0.6, 2.4+/-0.7, and 1.5+/-0.3 mg/kg/min, respectively, due to inhibition of glycogenolysis. In conclusion, NNC 25-0926 effectively blocked the ability of glucagon to increase HGP in the dog.

Effect of subchronic exposure to malathion on metabolic parameters in the rat

This study investigates the effects of subchronic exposure to organophosphate insecticide Malathion (Fyfanon 50 EC 500 g/l) of commercial grade. It was administered intragastrically by stomach tube in the amount of 1 ml of corn oil containing 100 mg/kg body weight (BW) daily for 32 days. At the end of the experiment, acetylcholinesterase activity (AChE), haematocrit value, haemoglobin content, and blood glucose concentration were estimated. The liver and the skeletal muscle were removed to determine hepatic and muscular glycogen, hepatic proteins and lipids contents. No sign of toxicity was observed until the end of experiment. No significant change in the haematocrit value was observed, in spite of the significant increase in haemoglobin content, which can be considered as an adaptive situation in order to guarantee a good oxygenation in response to pulmonary damage induced following subchronic exposure to organophosphorus compound. Malathion intoxication decreased significantly hepatic proteins and lipid contents that could be associated to liver gluconeogenesis. This result was coupled with a significant decrease in muscular glycogen rate, which indicates a stimulated glycogenolysis in favour of glucose release into the blood until reaching hyperglycaemia. Several studies indicate that hyperglycaemia is temporary, which is probably due to a stimulated glycogenesis that increases hepatic glycogen deposition and return of glucose to control levels, as demonstrated in our study. One possible explanation for these results could be the turnover of glucose by a succession between its release via glycogenolysis and gluconeogenesis, which involves abnormal hyperglycaemia, and its storage via glycogenesis in subchronic exposure to malation.

Bioactivity of glycogen phosphorylase inhibitors that bind to the purine nucleoside site

The bioactivity in hepatocytes of glycogen phosphorylase inhibitors that bind to the active site, the allosteric activator site and the indole carboxamide site has been described. However, the pharmacological potential of the purine nucleoside inhibitor site has remained unexplored. We report the chemical synthesis and bioactivity in hepatocytes of four new olefin derivatives of flavopiridol (1-4) that bind to the purine site. Flavopiridol and 1-4 counteracted the activation of phosphorylase in hepatocytes caused by AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolised to an AMP analogue. Unlike an indole carboxamide inhibitor, the analogues 1 and 4 suppressed the basal rate of glycogenolysis in hepatocytes by allosteric inhibition rather than by inactivation of phosphorylase, and accordingly caused negligible stimulation of glycogen synthesis. However, they counteracted the stimulation of glycogenolysis by dibutyryl cAMP by both allosteric inhibition and inactivation of phosphorylase. Cumulatively, the results show key differences between purine site and indole carboxamide site inhibitors in terms of (i) relative roles of dephosphorylation of phosphorylase-a as compared with allosteric inhibition, (ii) counteraction of the efficacy of the inhibitors on glycogenolysis by dibutyryl-cAMP and (iii) stimulation of glycogen synthesis.

Glucagon challenge in the rat: A robust method for the in vivo assessment of Glycogen phosphorlyase inhibitor efficacy

INTRODUCTION

Glycogen phosphorlyase inhibitors (GPi) act on the glycogenolytic pathway decreasing hepatic glucose output, making them potential candidates for Type 2 diabetes treatment. We established a robust in vivo method to assess GPis efficacy utilising glucagon-stimulated glycogenolysis.

METHODS

Blood glucose was monitored in both male AP Wistar and AP Zucker rats using tail prick samples pre- and post intraperitoneal or subcutaneous glucagon administration. The effect of glycogen phosphorylase inhibitors GPi296 (6-60 mg kg(-1) po) and DAB (5 mg kg(-1) po) upon glucose response to subcutaneous glucagon were examined in both strains.

RESULTS

In the Wistar rat glucagon induced dose related increases in blood glucose, with the maximum increase occurring 20 min post dose (4.0+/-0.88 mmol l(-1), intraperitoneal; and 2.8+/-0.72 mmol l(-1), subcutaneous, ns). Intraperitoneal glucagon administration produced shorter duration blood glucose elevation than observed with the subcutaneous route of administration. In the Zucker rat, no differences were observed between the 10 and 13 week old rats in response to glucagon (3-200 microg kg(-1) subcutaneous). The maximum blood glucose increase was lower in the Wistar rat compared to the Zucker rats (2.9+/-0.20 vs 7.7+/-1.22 mmol l(-1), P<0.0000018). GPi296 and DAB both produced similar inhibition in each strain.

DISCUSSION

Subcutaneous glucagon administration induced more sustained increases in blood glucose than intraperitoneal administration. Blood glucose response to glucagon was higher in the Zucker rat compared to the Wistar rat; there was no difference in inhibition mediated by either GPi296 or DAB between the two strains. We believe that subcutaneous glucagon administration produces a robust model for the assessment of GPis in either rat strain.

Inhibition of glycogenolysis in astrocytes interrupts memory consolidation in young chickens

Glycolysis and glycogenolysis are involved in memory processing in day-old chickens and, aside from the provision of energy for neuronal and astrocytic energy metabolism these pathways enable astrocytes to supply neurones with precursor for transmitter glutamate by glucose-based de novo synthesis. We have previously shown that memory processing for bead discrimination learning is dependent on glycolysis; however, the metabolic inhibitor used, iodoacetate, inhibits pyruvate formation from both glucose and glycogen. At specific time points after training transient reductions in brain glycogen content occur, mirrored by increases in glutamate/glutamine content. In the present study, we used intracerebral injection of a glycogen phosphorylase inhibitor, 1,4-dideoxy-1,4-imino-D-arabinitol (DAB), which does not affect glucose breakdown, to evaluate the role of glycogen metabolism in memory consolidation. Dose-dependent inhibition of learning occurred when DAB was administered at specific time periods in relation to training: (i) 5 min before training, (ii) around 30 min posttraining, and (iii) 55 min posttraining. After injection at either of the two earlier periods, memory disappeared after consolidation 30 min postlearning, and after injection 55 min after learning memory was absent at 70 min. The memory loss caused by early administration could be prevented after training by central injection of the glutamate precursor glutamine or the astrocyte-specific substrate acetate together with aspartate, substituting for pyruvate carboxylation. Thus, glycogenolysis is essential for learning in this paradigm and, aside from energy supply considerations, we suggest that an important role for glycogenolysis is to provide neurones with glutamine as the precursor for neuronal glutamate and GABA.

Liver parenchyma heterogeneity in the response to extracellular NAD+

The perfused rat liver responds intensely to NAD+ infusion (20-100 microM). Increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption are some of the effects that were observed. The aim of the present work was to investigate the distribution of the response to extracellular NAD+ along the hepatic acinus. The bivascularly perfused rat liver was used. Various combinations of perfusion directions (antegrade and retrograde) and infusion routes (portal vein, hepatic vein and hepatic artery) were used in order to supply NAD+ to different regions of the liver parenchyma, also taking advantage of the fact that its extracellular transformation generates steep concentration gradients. Oxygen uptake was stimulated by NAD+ in retrograde perfusion (irrespective of the infusion route) and transiently inhibited in antegrade perfusion. This indicates that the signal causing oxygen uptake inhibition is generated in the periportal area. The signal responsible for oxygen uptake stimulation is homogenously distributed. Stimulation of glucose release was more intense when NAD+ was infused into the portal vein or into the hepatic artery, indicating that stimulation of glycogenolysis predominates in the periportal area. The increases in perfusion pressure were more pronounced when the periportal area was supplied with NAD+ suggesting that the vasoconstrictive elements responding to NAD+ predominate in this region. The response to extracellular NAD+ is thus unequally distributed in the liver. As a paracrine agent, NAD+ is likely to be released locally. It can be concluded that its effects will be different depending on the area where it is released.

The effect of an acute elevation of NEFA concentrations on glucagon-stimulated hepatic glucose output

To determine the effect of nonesterified fatty acids (NEFA) on glucagon action, glucagon was infused intraportally (1.65 ng.min(-1).kg(-1)) for 3 h into 18-h-fasted, pancreatic-clamped conscious dogs in the presence [NEFA + glucagon (GGN)] or absence (GGN) of peripheral Intralipid plus heparin infusion. Additionally, hyperglycemic (HG), hyperglycemic-hyperlipidemic (NEFA + HG), and glycerol plus glucagon (GLYC + GGN) controls were studied. Arterial plasma glucagon concentrations rose equally in GGN, NEFA + GGN, and GLYC + GGN but remained basal in hyperglycemic controls. Peripheral infusions of Intralipid and heparin increased arterial plasma NEFA concentrations equally in NEFA + GGN and NEFA + HG and did not change in other protocols. After 15 min, glucagon infusion resulted in a rapid, brief increase in net hepatic glycogenolysis (NHGLY, mg.min(-1).kg(-1)) of approximately 6.0 in GGN and GLYC + GGN but only increased by 3.8 +/- 1.3 in NEFA + GGN. Thus increases in NHGLY, and consequently net hepatic glucose output (NHGO), were blunted by 40%, with no difference between the groups in the last 2.5 h of the study. NHGO and NHGLY did not significantly change in HG and NEFA + HG. Net hepatic gluconeogenic flux did not change in GGN, GLYC + GGN, or HG. However, Intralipid and heparin infusion resulted in similar increases in net hepatic gluconeogenic flux in NEFA + GGN and NEFA + HG. Thus elevated NEFA limit the initial increase in glucagon-stimulated HGO by blunting glycogenolysis, without having any effect on the gluconeogenic or glycogenolytic contributions or NHGO thereafter.

The role of adrenergic agonists on glycogenolysis in rat hepatocyte cultures and possible involvement of NO

Certain liver metabolic diseases point to the presence of disturbances in glycogen deposition. Epinephrine raises the cAMP level that activates protein kinase A leading to the activation of phosphorylase and glycogen breakdown. In the present report, we sought to investigate whether NO is produced during adrenoceptor agonist-induced glycogenolysis in rat hepatocytes in cultures. Isolated glycogen rich rat hepatocytes in cultures were used. NO production (NO(2)(-)) was assessed under the effect of adrenergic agonists and adrenergic agonist/antagonist pairs, dibutyryl cyclic AMP sodium-potassium salt (db-cAMP), NO synthase (NOS) inhibitors N(omega)-nitro-L-arginine methyl ester (L-NAME), aminoguanidine (AG) and the NO donor S-nitroso-N-acetyl penicillamine (SNAP). The inducible NO synthase (iNOS) mRNA was examined by the reverse transcription-polymerase chain reaction (RT-PCR). Glycogenolysis was quantified by glucose levels released into medium. The amount of glucose and NO(2)(-) released by hepatocytes was increased as a result of epinephrine, phenylephrine or db-cAMP treatments. The increase in glucose and NO(2)(-) released by epinephrine or phenylephrine was blocked or reduced by prazosin pretreatment and by NOS inhibitors aminoguanidine and L-NAME. iNOS gene expression was up-regulated by epinephrine. It can be concluded that glycogenolysis occurs through -adrenoceptor stimulation and a signaling cascade may involve NO production.

Inosine released after hypoxia activates hepatic glucose liberation through A3 adenosine receptors

Inosine, an endogenous nucleoside, has recently been shown to exert potent effects on the immune, neural, and cardiovascular systems. This work addresses modulation of intermediary metabolism by inosine through adenosine receptors (ARs) in isolated rat hepatocytes. We conducted an in silico search in the GenBank and complete genomic sequence databases for additional adenosine/inosine receptors and for a feasible physiological role of inosine in homeostasis. Inosine stimulated glycogenolysis (approximately 40%, EC50 4.2 x 10(-9) M), gluconeogenesis (approximately 40%, EC50 7.8 x 10(-9) M), and ureagenesis (approximately 130%, EC50 7.0 x 10(-8) M) compared with basal values; these effects were blunted by the selective A3 AR antagonist 9-chloro-2-(2-furanyl)-5-[(phenylacetyl)amino][1,2,4]-triazolo[1,5-c]quinazoline (MRS 1220) but not by selective A1, A2A, and A2B AR antagonists. In addition, MRS 1220 antagonized inosine-induced transient increase (40%) in cytosolic Ca2+ and enhanced (90%) glycogen phosphorylase activity. Inosine-induced Ca2+ mobilization was desensitized by adenosine; in a reciprocal manner, inosine desensitized adenosine action. Inosine decreased the cAMP pool in hepatocytes when A1, A2A, and A2B AR were blocked by a mixture of selective antagonists. Inosine-promoted metabolic changes were unrelated to cAMP decrease but were Ca2+ dependent because they were absent in hepatocytes incubated in EGTA- or BAPTA-AM-supplemented Ca2+-free medium. After in silico analysis, no additional cognate adenosine/inosine receptors were found in human, mouse, and rat. In both perfused rat liver and isolated hepatocytes, hypoxia/reoxygenation produced an increase in inosine, adenosine, and glucose release; these actions were quantitatively greater in perfused rat liver than in isolated cells. Moreover, all of these effects were impaired by the antagonist MRS 1220. On the basis of results obtained, known higher extracellular inosine levels under ischemic conditions, and inosine's higher sensitivity for stimulating hepatic gluconeogenesis, it is suggested that, after tissular ischemia, inosine contributes to the maintenance of homeostasis by releasing glucose from the liver through stimulation of A3 ARs.

Growth hormone and prolactin actions on osmoregulation and energy metabolism of gilthead sea bream (Sparus auratus)

The gilthead sea bream (Sparus auratus) is an euryhaline fish where prolactin (PRL) and growth hormone (GH) play a role in the adaptation to different environmental salinities. To find out the role of these pituitary hormones in osmoregulation and energy metabolism, fish were implanted with slow release implants of ovine GH (oGH, 5 microg g(-1) body mass) or ovine prolactin (oPRL, 5 microg g(-1) body mass), and sampled 7 days after the start of the treatment. GH increased branchial Na(+),K(+)-ATPase activity and decreased sodium levels in line with its predicted hypoosmoregulatory action. GH had metabolic effects as indicated by lowered plasma protein and lactate levels, while glucose, triglycerides and plasma cortisol levels were not affected. Also, GH changed liver glucose and lipid metabolism, stimulated branchial and renal glucose metabolism and glycolytic activity, and enhanced glycogenolysis in brain. PRL induced hypernatremia. Furthermore, this hormone decreased liver lipid oxidation potential, and increased glucose availability in kidney and brain. Both hormones have opposite osmoregulatory effects and different metabolic effects. These metabolic changes may support a role for both hormones in the control of energy metabolism in fish that could be related to the metabolic changes occurring during osmotic acclimation.

Glucose production in response to glucagon is comparable in preterm AGA and SGA infants

BACKGROUND AND AIMS

Low plasma glucose concentrations are more often detected in small-for-gestational-age (SGA) than in appropriate-for-gestational-age (AGA) infants. This is ascribed to impaired glucose production due to presumed lower liver glycogen stores in SGA infants. The change in glucose production induced by glucagon is considered to be an indicator of liver glycogen content. We compared the effect of glucagon on glucose kinetics in preterm AGA and SGA infants.

METHODS

In 5 AGA and 5 SGA preterm infants (postnatal age: 3-6 days) glucose production and gluconeogenesis were measured using stable isotopes immediately before and for 1 h after a bolus of glucagon.

RESULTS

After glucagon the plasma glucose concentration and glucose production increased significantly over time (P<0.05 and P<0.0001, respectively). The changes were comparable between AGA and SGA infants. Glycogenolysis contributed 75-80% to the increase in glucose production.

CONCLUSION

The increase in glucose production after glucagon was similar in preterm AGA and SGA infants, and mainly due to an increase in glycogenolysis. Based on the assumption that glycogenolysis is an indicator of liver glycogen content, our data do not support the hypothesis that liver glycogen content is lower in preterm SGA compared to AGA infants after the first postnatal day.

[Effect of tranylcypromine on glycogenolysis in the rat brain]

In rat brain the tranylcypromine administration induces after two hours a 151% increase of serotonin levels and a 50% fall of glycogen concentration. The activity of phosphorylase a and b and the glycolytic flow rate are unchanged.