Glyconeogenesis from L-proline involves metabolite inhibition of the glucose-6-phosphatase system.

L-Proline's glycogenic action is unlike that of other amino acids in that it produces effects beyond those explainable by a simple increase in osmolarity (Baquet, A., Hue, L., Meijer, A. J., van Woerkom, G. M., and Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959). We postulate that this effect may relate to inhibition of hepatic glucose-6-P hydrolysis by a proline-derived metabolite. We tested this hypothesis with isolated livers from rats fasted 48 h which were perfused with L-proline or L-glutamine. Net glucose and net glycogen production and levels of glucose-6-P and certain other hepatic metabolites were measured. The data obtained support our hypothesis by demonstrating fundamental differences in the metabolic fates of proline and glutamine in the liver. Both pass through alpha-ketoglutarate in the initial stage of gluconeogenesis, but proline supports hepatic glycogen formation while glutamine does not. The concomitant increase in hepatic glucose-6-P and proline-associated glyconeogenesis suggests that inhibition of glucose-6-P hydrolysis by a proline-derived metabolite may divert glucose-6-P produced from proline from glucose production and to glycogen synthesis. This conclusion is supported by the effects of perfusions with and without proline (3-mercaptopicolinate present) on (a) glyconeogenesis and glucose formation from dihydroxyacetone, (b) net glucose uptake and glycogen formation with 30 mM glucose as substrate, and (c) glucose production from endogenous glycogen in perfused livers from fed rats.     

Synergism of glucose and fructose in net glycogen synthesis in perfused rat livers

Synergism of glucose and fructose in net glycogen synthesis was studied in perfused livers from 24-h fasted rats. With either glucose or fructose alone, net glycogen deposition did not occur (p greater than 0.10 for each), whereas the addition of both together resulted in significant glycogen accumulation (net glycogen accumulation was 0.21 +/- 0.03 mumol of glucose/g of liver/min at 2 mM fructose and 30 mM glucose, p less than 0.001). To better understand this synergism, intermediary substrate levels were compared at steady state with various glucose levels in the absence and in the presence of 2 mM fructose. Independent of fructose, hepatic glucose and glucose 6-phosphate increased proportionally when glucose level in the medium was raised (r = 0.86, p less than 0.001). Unlike glucose 6-phosphate, UDP-glucose did not consistently increase with glucose (p greater than 0.10); in fact, there was a small decrease at a very high glucose level (30 mM), a result consistent with the well-established activation of glycogen synthase by glucose. With elevated glucose, the level of glucose 6-phosphate was strongly correlated with glycogen content (r = 0.71, p less than 0.01, slope = 32). Adding fructose increased the "efficiency" of glucose 6-phosphate to glycogen conversion: the effect of a given increment in glucose 6-phosphate upon glycogen accumulation was increased 2.6-fold (r = 0.73, p less than 0.01, slope = 86). A kinetic modeling approach was used to investigate the mechanisms by which fructose synergized glycogen accumulation when glucose was elevated. Based on steady-state hepatic substrate levels, net hepatic glucose output, and net glycogen synthesis rate, the model estimated the rate constants of major enzymes and individual fluxes in the glycogen metabolic pathway. Modeling analysis is consistent with the following scenario: glycogen synthase is activated by glucose, whereas glucose-6-phosphatase was inhibited. In addition, the model supports the hypothesis that fructose synergizes net glycogen accumulation due to suppression of phosphorylase. Overall, our analysis suggests that glucose enhances the metabolic flux to glycogen by inducing a build up of glucose 6-phosphate via combined effects of mass action and glucose-6-phosphatase inhibition and activating glycogen synthase and that fructose enhances glycogen accumulation by retaining glycogen via phosphorylase inhibition.     

Metabolic response to anisoosmolarity of rat skeletal muscle in vitro.

Isolated rat hepatocytes exhibit an insulin-like anabolic response to hypoosmotic incubation and a glucagon-like catabolic response to hyperosmotic incubation. Recently, a distinct glycogenic response to hypoosmotic treatment was likewise reported for cultured rat myotubes. The present study examines the effects of anisoosmolar exposure on glucose metabolism in freshly isolated rat soleus muscle strips. Under the same experimental conditions as used for cultured myotubes, hypoosmolarity reduced net glycogen synthesis to 52%, while hyperosmolarity stimulated glycogen storage to 231% of isoosmolar control (nmol glucose incorporated into glycogen g(-1) x h(-1): hypoosmolar, 34+/-3; isoosmolar, 65+/-8; hyperosmolar, 150+/-11; p<0.01 each vs. isoosmolar). The responses of native skeletal muscle to anisoosmolarity are therefore in opposition to what has been described for hepatocytes and cultured myotubes. Further experiments on rat skeletal muscle revealed that the observed lack of a glycogenic response to hypoosmolarity persisted independent of medium composition, specifically with regard to prevailing glucose and K+ concentrations. In conclusion, hypoosmotic exposure inhibits glycogen synthesis in isolated rat soleus muscle, which clearly argues against the hypothesis that osmotic changes and cell swelling may be physiologically relevant stimulators of muscle glycogen synthesis.     

Constraints on the models of carbohydrate metabolism in the two cell types of Dictyostelium discoideum

The constraints in the parameters in models of the spore and stalk cells in Dictyostelium discoideum have been examined. It was found that the relative sizes of the two cellular glucose pools are not very critical, i.e. they can be varied in the models over a fairly wide range and still allow simulations which are compatible with the data. In contrast, the following model parameters are highly constrained, and must fall within narrow limits: flux through the glycogen cycle; the fraction of glycogen present which actually participates in glycogen turnover; the net rate of glycogen degradation; the concentration of exogenous labelled glucose which actually participates in cellular metabolism; the rates of exchange of this exogenous glucose with the two cellular glucose pools; the concentration of the spore glucose-6-phosphate pool, and the rate of exchange of stalk glucose-1-phosphate and stalk glucose-6-phosphate.     

Enhancement of glycogen concentrations in primary cultures of rat hepatocytes exposed to glucose and fructose.

Glycogen synthesis in isolated hepatocytes can occur from glucose both by a direct mechanism and by an indirect process in which glucose is first metabolized to C3 intermediates before use for glycogenesis via gluconeogenesis. We studied the incorporation into glycogen of glucose and the gluconeogenic substrate, fructose, in primary cultures of hepatocytes from fasted rats. In the presence of insulin, both glucose and fructose promoted net deposition of glycogen; however, fructose carbon was incorporated into glycogen to a greater extent than that from glucose. When glucose and fructose were administered simultaneously, the glycogenic utilization of glucose was stimulated 2-3-fold, and that of fructose was increased by about 50%. At constant hexose concentrations, the total incorporation of carbon, and the total accumulation of glycogen mass, from glucose and fructose when present together exceeded that from either substrate alone. Fructose did not change the relative proportion of glucose carbon incorporated into glycogen via the indirect (gluconeogenic) mechanism. The synergism of glucose and fructose in glycogen synthesis in isolated rat hepatocytes in primary culture appears to result from a decrease in the rate of degradation of newly deposited glycogen, owing to (i) decreased amount of phosphorylase a mediated by glucose and (ii) noncovalent inhibition of residual phosphorylase activity by some intermediate arising from the metabolism of fructose, presumably fructose 1-phosphate.     

Thermodynamic-based computational profiling of cellular regulatory control in hepatocyte metabolism

Thermodynamic-based constraints on biochemical fluxes and concentrations are applied in concert with mass balance of fluxes in glycogenesis and glycogenolysis in a model of hepatic cell metabolism. Constraint-based modeling methods that facilitate predictions of reactant concentrations, reaction potentials, and enzyme activities are introduced to identify putative regulatory and control sites in biological networks by computing the minimal control scheme necessary to switch between metabolic modes. Computational predictions of control sites in glycogenic and glycogenolytic operational modes in the hepatocyte network compare favorably with known regulatory mechanisms. The developed hepatic metabolic model is used to computationally analyze the impairment of glucose production in von Gierke's and Hers' diseases, two metabolic diseases impacting glycogen metabolism. The computational methodology introduced here can be generalized to identify downstream targets of agonists, to systematically probe possible drug targets, and to predict the effects of specific inhibitors (or activators) on integrated network function.     

Differences between glucose and insulin stimulation of glycogenesis in cultured fetal hepatocytes

The glycogenic effects of a glucose load (15 mM) and/or insulin (10 nM) were studied in 18-day-old fetal rat hepatocytes after 2 days of culture when medium contained 4 mM glucose. A glucose load led to a stimulation of [¹⁴C]glucose glycogen labelling (20 min) earlier than with insulin (30–40 min); maximal stimulations were 3-fold after 1 h for the glucose load and 5-fold after 2–3 h for insulin. Simultaneous addition of the two agents produced synergic effects. When insulin was added 4 h after a glucose load (or vice versa), a second glycogenic response was elicited: a further addition of the same glycogenic agent was ineffective. The early glycogenic effects (up to 2 h) also occurred in the presence of 10 μM cycloheximide, with, however, some decrease of insulin stimulation. The contribution of medium glucose to the glycogen formed for 2 days (67% in the absence of glycogenic agent) was clearly enhanced by a glucose load and to a lesser degree by insulin after a 4-h exposure (83 and 71%, respectively). This was accompanied by a related modification of the participation of glucogenic precursors such as fructose and galactose. Thus, acute glycogenic response to glucose and insulin appeared both synergic and independent, and quite different in several aspects in cultured fetal hepatocytes.     

Quantifying the contribution of the liver to glucose homeostasis: a detailed kinetic model of human hepatic glucose metabolism

Despite the crucial role of the liver in glucose homeostasis, a detailed mathematical model of human hepatic glucose metabolism is lacking so far. Here we present a detailed kinetic model of glycolysis, gluconeogenesis and glycogen metabolism in human hepatocytes integrated with the hormonal control of these pathways by insulin, glucagon and epinephrine. Model simulations are in good agreement with experimental data on (i) the quantitative contributions of glycolysis, gluconeogenesis, and glycogen metabolism to hepatic glucose production and hepatic glucose utilization under varying physiological states. (ii) the time courses of postprandial glycogen storage as well as glycogen depletion in overnight fasting and short term fasting (iii) the switch from net hepatic glucose production under hypoglycemia to net hepatic glucose utilization under hyperglycemia essential for glucose homeostasis (iv) hormone perturbations of hepatic glucose metabolism. Response analysis reveals an extra high capacity of the liver to counteract changes of plasma glucose level below 5 mM (hypoglycemia) and above 7.5 mM (hyperglycemia). Our model may serve as an important module of a whole-body model of human glucose metabolism and as a valuable tool for understanding the role of the liver in glucose homeostasis under normal conditions and in diseases like diabetes or glycogen storage diseases.     

Implications of distinct inhibitory effects of N-acetylglucosamine on glucose uptake by an isolated perfusion system incorporating erythrocytes with livers from fed and 48-hour fasted rats

Net glucose uptake in a perfusion system including erythrocytes and isolated livers from fed rats was inhibited by N-acetylglucosamine (GlcNAc), a competitive inhibitor of glucokinase. Net glucose uptake also occurred in the system incorporating livers from 48-h fasted rats, but its inhibition by GlcNAc did not. This distinction could not be explained on the basis of a different sensitivity of glucokinase from fasted compared with fed rats to inhibition by GlcNAc. Nor could it be rationalized based on several other hepatic enzymes possibly involved in glucose utilization or production. Because erythrocytes were included in our system, other explanations were sought related to the total enzymic environment. The involvement of an indirect pathway including glycolysis of glucose to lactate in erythrocytes followed by conversion of this lactate to glucose-6-P and then glycogen in liver was considered. This pathway contributed no more than 17% to total net glucose uptake in the system incorporating livers from fed rats. This per cent contribution increased when hepatic glucokinase was reduced by fasting or through inhibition by GlcNAc. However, it was too small to explain observed overall rates of net glucose uptake. We propose that the presence of erythrocytes may also promote a greater net glucose uptake by the direct hepatic pathway. An enhanced inhibition of hepatic glucose-6-P hydrolysis by some intermediate metabolite generated in the presence of lactate infusion from erythrocytes may promote net glucose uptake independently of the mechanism of residual hepatic glucose phosphorylation. This may explain why we and others who have employed liver perfusion systems including erythrocytes have seen greater net glucose uptake than have workers using systems devoid of erythrocytes.     

Correlation between brain glycogen and convulsive state in mice submitted to methionine sulfoximine

It is now well established that in epileptic patients, hypometabolic foci appear during interictal periods. The meaning and the mechanism of such an hypometabolism are as yet unclear. The aim of the present investigation was to look for a putative relationship between glucose metabolism in the brain and the genesis of seizures in mice using administration of the convulsant, methionine sulfoximine. Besides its epileptic action, methionine sulfoximine is a powerful glycogenic agent. We analyzed the epileptogenic and glycogenic effects of methionine sulfoximine in two inbred mouse strains with different susceptibility towards the convulsant. CBA/J mice displayed high response to methionine sulfoximine. The tonic convulsions appeared 5-6 h after MSO administration, without brain glycogen content variations during the preconvulsive period. These mice died of status epilepticus during the first seizure(s). Conversely, C57BL/6J mice displayed low response to MSO. The tonic and clonic seizures appeared 8 to 14 h after MSO administration with only 2% mortality. The seizures were preceded by an increase in brain glycogen content during the preconvulsive period. Moreover, during seizures, C57BL/6J mice were able to mobilize this accumulated brain glycogen, that returned to high value after seizures. The epileptic and glycogenic responses of the parental strains were also observed in mice of the F2 generation. The F2 mice that convulsed early (16%) did not utilize their small increase in brain glycogen content, and resembled CBA/J mice; while the F2 mice that seized tardily (24%) increased their brain glycogen content before convulsion, utilized it during convulsions, and resembled C57BL/6J mice. Sixty percent of the F2 mice presented an intermediate pattern in epileptogenic responses to the convulsant. These data suggest a possible genetic link between the two MSO effects, epileptiform seizures and increase in brain glycogen content. The increase in brain glycogen content and the capability of its mobilization during seizures could delay the seizure's onset and could be considered a "resistance factor" against the seizures.     

Role of nuclear glycogen synthase and cytoplasmic UDP glucose pyrophosphorylase in the biosynthesis of nuclear glycogen in HD33 Ehrlich-Lettre ascites tumor cells

Biochemical and autoradiographic evidence show both glycogen synthesis and the presence of glycogen synthase (UDP glucose [UDPG]: glycogen 4-alpha-D-glucosyltransferase; EC 2.4.1.11) in isolated nuclei of Ehrlich-Lettré mouse ascites tumor cells of the mutant subline HD33. 5 d after tumor transplantation, glycogen (average 5-7 pg/cell) is stored mainly in the cell nuclei. The activity of glycogen synthase in isolated nuclei is 14.5 mU/mg protein. At least half of the total cellular glycogen synthase activity is present in the nuclei. The nuclear glycogen synthase activity exists almost exclusively in its b form. The Km value for (a + b) glycogen synthase is 1 x 10(-3) M UDPG, the activation constant is 5 x 10(-3) M glucose-6-phosphate (Glc-6-P). Light and electron microscopic autoradiographs of isolated nuclei incubated with UDP-[1-3H]glucose show the highest activity of glycogen synthesis not only in the periphery of glycogen deposits but also in interchromatin regions unrelated to detectable glycogen particles. Together with earlier findings on nuclear glycogen synthesis in intact HD33 ascites tumor cells (Zimmermann, H.-P., V. Granzow, and C. Granzow. 1976. J. Ultrastruct. Res. 54:115-123), the results of tests on isolated nuclei suggest a predominantly appositional mode of nuclear glycogen deposition, without participation of the nuclear membrane system. In intact cells, synthesis of UDPG for nuclear glycogen synthesis depends on the activity of the exclusively cytoplasmic UDPG pyrophosphorylase (UTP: alpha-D-glucose-1-phosphate uridylyltransferase; EC 2.7.7.9). However, we conclude that glycogen synthesis is not exclusively a cytoplasmic function and that the mammalian cell nucleus is capable of synthesizing glycogen.     

Hepatic carbon flux after re-feeding in the glycogen-storage-disease (gsd/gsd) rat.

In this study we utilized the phosphorylase b kinase-deficient (gsd/gsd) rat as a model of hepatic substrate utilization where there is a constraint on glycogenesis imposed by the maintenance of high glycogen concentrations. Glucose re-feeding of 48 h-starved gsd/gsd rats led to suppression of hepatic glucose output. In contrast with the situation in normal rats, activation of the pyruvate dehydrogenase complex and lipogenesis was observed. It is suggested that impeding glycogenic flux may divert substrate into lipogenesis, possibly via activation of the pyruvate dehydrogenase complex.     

Effect of ligands on Drosophila phosphorylase a as monitored by its enzymic inactivation

The dephosphorylation of Drosophila phosphorylase a with the catalytic subunit of fruit-fly protein phosphatase-1 was inhibited by AMP, IMP, ADP, ATP, glucose-6-P, glucose-1-P and UDPG. Glucose, caffeine and glycogen did not influence the reaction. The inhibitory effect of AMP was reduced by glucose and caffeine. The above ligands acted through the modification of phosphorylase a conformation. This conclusion was drawn from the ligands' effect on the dephosphorylation of phosphohistone by Drosophila phosphatase-1 and on the tryptic digestion of fruit-fly phosphorylase a.     

Do non-equilibrium reactions in the glucose-to-triose phosphate pathway exist in the liver?

The circadian changes in the contents of intermediates of the initial reactions of the glycolytic pathway in pigeon liver were studied. the concentrations of glucose, glucose-6-phosphate, fructose-6-phosphate, fructose-1,6-diphosphate and triose phosphates were found to change synchronously, being maximal at the dark time and minimal during the light daytime. The glycogen content in the liver decreased steadily between 12.00 and 09.00. The diurnal variations in the concentrations of metabolite pairs (glucose and glucose-6-phosphate, glucose-6-phosphate and fructose-6-phosphate, fructose-6-phosphate and fructose-1.6-diphosphate, fructose-1.6-diphosphate and triose phosphates) appeared to correlate significantly. The results obtained suggest that in the liver at least there are no limiting i. e. physiologically non-equilibrium reactions in the carbohydrate metabolic pathway from glucose to triose phosphates.     

Non-enzymatic synthesis of the coenzymes,uridine diphosphate glucose and cytidine diphosphate choline,and other phosphorylated metabolic intermediates

The synthesis of uridine diphosphate glucose (UDPG), cytidine diphosphate choline (CDP-choline), glucose-1-phosphate (G1P) and glucose-6-phosphate (G6P) has been accomplished under simulated prebiotic conditions using urea and cyanamide, two condensing agents considered to have been present on the primitive Earth. The synthesis of UDPG was carried out by reacting G1P and UTP at 70 °C for 24 hours in the presence of the condensing agents in an aqueous medium. CDP-choline was obtained under the same conditions by reacting choline phosphate and CTP. G1P and G6P were synthesized from glucose and inorganic phosphate at 70 °C for 16 hours. Separation and identification of the reaction products have been performed by paper chromatography, thin layer chromatography, enzymatic analysis and ion pair reverse phase high performance liquid chromatography. These results suggest that metabolic intermediates could have been synthesized on the primitive Earth from simple precursors by means of prebiotic condensing agents.     

Glycogen metabolism in rat liver during transition form the fed to fasted states

Hepatic glycogen metabolism was studied in rats during the period of transition from the fed to fasted states. Glycogenic activity was measured in vivo based on the incorporation of [¹⁴C]glucose into liver glycogen. Its changes were almost parallel to the changes in glucogen synthase activity. Progressive accumulation of liver glycogen that occurred in the fed state was associated with a proportional increase in glycogenic activity. Within 4 h after the cessation of food intake, glycogenic activity showd a precipitous fall from the peak to its nadir without significant changes in glycogen content. Meanwhile, the glucose concentration in the portal vein decreased. Upon further development of fasting, glycogenic activity displayed a progressive regain, reciprocally as glycogen contents gradually decreased. The precipitous fall of glycogenic activity during the transition from the fed to fasted states was associated with a transient increase in plasma glucagon, and was partly overcome by the injection of anti-glucagon serum. It is concluded that the fall of portal venous concentration of glucose and secretion of glucagon act as a signal to initiate liver glycogen metabolism characteristics of the fasted or postabsorptive state.     

Regulation of Endogenous Glucose Production in Glucose Transporter 4 Over-Expressing Mice

Strategies to amplify whole-body glucose disposal are key therapies to treat type 2 diabetes. Mice that over-express glucose transporter 4 (Glut4) in skeletal muscle, heart, and adipose tissue (G4Tg) exhibit increased fasting glucose disposal and thus lowered blood glucose. Intriguingly, G4Tg mice also exhibit improved insulin-stimulated suppression of endogenous glucose production even though Glut4 is not present in the liver. It is unclear, however, if hepatic gluco-regulation is altered in G4Tg mice in the basal, non-insulin-stimulated state. The current studies were performed to examine fasting hepatic glucose metabolism in G4Tg mice and to determine whether gluco-regulatory adaptations exist in the non-insulin-stimulated condition. To test this question, phloridzin-glucose clamps were used to match blood glucose and pancreatic hormone levels while tracer dilution techniques were used to measure glucose flux. These techniques were performed in chronically-catheterized, conscious, and un-stressed 5h-fasted G4Tg and wild-type (WT) littermates. Results show reduced blood glucose, hepatic glycogen content, and hepatic glucokinase (GK) activity/expression as well as higher endogenous glucose production, glucose disposal, arterial glucagon, and hepatic glucose-6-phosphatase (G6Pase) activity/expression in G4Tg mice versus WT controls. Clamping blood glucose for 90 min at ∼115 mg/dLin G4Tg and WT mice normalized nearly all variables. Notably, however, net hepatic glycogen synthetic rates were disproportionately elevated compared to changes in blood glucose. In conclusion, these studies demonstrate that basal improvements in glucose tolerance due to increased uptake in extra-hepatic sites provoke important gluco-regulatory adaptations in the liver. Although changes in blood glucose underlie the majority of these adaptations, net hepatic glycogen synthesis is sensitized. These data emphasize that anti-diabetic therapies that target skeletal muscle, heart, and/or adipose tissue likely positively impact the liver.     

Mechanisms of antihyperglycemic effect of p-methoxycinnamic acid in normal and streptozotocin-induced diabetic rats

We investigated the antihyperglycemic effect of p-methoxycinnamic acid (p-MCA), a cinnamic acid derivative, on plasma glucose and insulin concentrations, activities of hepatic glucose-regulating enzymes and hepatic glycogen content in normal and streptozotocin (STZ)-induced diabetic rats. p-MCA (10-100 mg/kg, PO) dose-dependently decreased plasma glucose concentration in both normal and diabetic rats in the oral glucose tolerance test. To investigate the chronic effects of p-MCA on glucose metabolism, p-MCA (40 mg/kg, PO) was administered to normal and diabetic rats once a day for 4 weeks. p-MCA reduced plasma glucose concentration in diabetic rats, which was observed during the 4-week study. However, p-MCA treatment did not change plasma glucose concentrations in normal rats during the 4-week study. p-MCA also reduced the excessive activities of hepatic glucose-6-phosphatase, hepatic hexokinase, glucokinase and phosphofructokinase in diabetic rats and increased hepatic glycogen in these rats. In p-MCA-treated normal rats, there were no changes in the activities of hepatic glucose-regulating enzymes, hepatic glycogen and glucose-6-phosphate. Our findings suggested that p-MCA exert its antihyperglycemic effect by increasing insulin secretion and glycolysis, and by decreasing gluconeogenesis.     

Dietary carbohydrate effects on some plasma organic acids and aspects of glucose metabolism in turkey poults

The effects on newly-hatched turkey poults of feeding diets with varying levels of carbohydrate and of oral gavage with suspensions of corn starch were studied. Feeding lowered hepatic glucose-6-phosphatase activity and raised blood glucose and hepatic glycogen concentrations. In Nicholas strain turkeys, increases of dietary levels of carbohydrate enhanced hepatic glycogen stores without affecting blood glucose concentration or glucose-6-phosphatase activity. Oral gavage of poults with suspensions of corn starch in water raised blood glucose and hepatic glycogen concentrations and lowered glucose-6-phosphatase activity in dose- and time-dependent manners. Changes were noted at 1 hr post-gavage. Oral gavage with starch lowered lactate concentrations in muscle and plasma and lowered plasma concentrations of β-hydroxybutyrate and urate. Plasma concentrations of pyruvate appeared to decline with post-hatch holding without feed. Thus, the apparent effect of starch gavage on plasma pyruvate (high concentration) is dependent upon the length of the holding period for the controls. The data show that poults can alter their metabolism (decrease lipid oxidation and gluconeogenesis and increase carbohydrate stores) almost immediately (1 hr) after oral administration of carbohydrate.     

Pathogenesis of hyperglycemia in genetically obese-hyperglycemic rats, Wistar fatty: presence of hepatic insulin resistance

The present studies were designed to clarify the contribution of the liver to the development of hyperglycemia in Wistar fatty rats. The hepatic activities of insulin-inducible enzymes involved in glycolysis (glucokinase; GK and pyruvate kinase) and lipogenesis (glucose-6-phosphate dehydrogenase), were higher in fatty rats than in lean rats at 4 and 8 weeks of age because of the higher insulin levels in the former. Thereafter, the GK activities of fatty rats decreased slightly in spite of severe hyperinsulinemia, and did not differ from those of lean rats. In addition, fatty rats had higher levels of insulin-suppressible gluconeogenic enzymes, glucose-6-phosphatase (G6Pase) and fructose-1, 6-diphosphatase. These findings indicate that the hepatic enzymes of fatty rats are resistant to insulin. This postulation was supported by the fact that the hepatic enzyme activities of fatty rats showed a lower response to changes in plasma insulin levels produced by fasting and refeeding. The G6Pase/GK ratio, which indicates net glucose handling in the liver, increased in fatty rats and decreased in lean rats with advancing age, suggesting that hepatic glucose production in fatty rats becomes dominant with advancing age. The changes in hepatic glycolytic intermediates supported this suggestion; the glycolytic steps both from glucose to glucose-6-phosphate and from phospho-enolpyruvate to pyruvate in fatty rats were accelerated at 5 weeks of age, but suppressed at 12 weeks of age. These results indicate that insulin resistance in the hepatic enzyme regulation may contribute to the development of hyperglycemia in Wistar fatty rats.