Pathway of glycogen synthesis from glucose in hepatocytes maintained in primary culture

Dexamethasone enhances glucose uptake by SGLT1 and GLUT1 and boosts ATP generation through the PPP-TCA cycle in bovine neutrophils

Background

Clinical dexamethasone (DEX) treatment or stress in bovines results in extensive physiological changes with prominent hyperglycemia and neutrophils dysfunction.

Objectives

To elucidate the effects of DEX treatment in vivo on cellular energy status and the underlying mechanism in circulating neutrophils.

Methods

We selected eight-month-old male bovines and injected DEX for 3 consecutive days (1 time/d). The levels of glucose, total protein (TP), total cholesterol (TC), and the proinflammatory cytokines interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α in blood were examined, and we then detected glycogen and adenosine triphosphate (ATP) content, phosphofructosekinase-1 (PFK1) and glucose-6-phosphate dehydrogenase (G6PDH) activity, glucose transporter (GLUT)1, GLUT4, sodium/glucose cotransporter (SGLT)1 and citrate synthase (CS) protein expression and autophagy levels in circulating neutrophils.

Results

DEX injection markedly increased blood glucose, TP and TC levels, the Ca²⁺/P⁵⁺ ratio and the neutrophil/lymphocyte ratio and significantly decreased blood IL-1β, IL-6 and TNF-α levels. Particularly in neutrophils, DEX injection inhibited p65-NFκB activation and elevated glycogen and ATP contents and SGLT1, GLUT1 and GR expression while inhibiting PFK1 activity, enhancing G6PDH activity and CS expression and lowering cell autophagy levels.

Conclusions

DEX induced neutrophils glucose uptake by enhancing SGLT1 and GLUT1 expression and the transformation of energy metabolism from glycolysis to pentose phosphate pathway (PPP)-tricarboxylic acid (TCA) cycle. This finding gives us a new perspective on deeper understanding of clinical anti-inflammatory effects of DEX on bovine.

The role of insulin, glucagon, dexamethasone, and leptin in the regulation of ketogenesis and glycogen storage in primary cultures of porcine hepatocytes prepared from 60 kg pigs

A study was conducted to elucidate hormonal control of ketogenesis and glycogen deposition in primary cultures of porcine hepatocytes. Hepatocytes were isolated from pigs (54-68 kg) by collagenase perfusion and seeded into collagen-coated T-25 flasks. Monolayers were established in medium containing fetal bovine serum for 1 day and switched to a serum-free medium for the remainder of the culture period. Hepatocytes were maintained in DMEM/M199 containing 1% DMSO, dexamethasone (10(-6) or 10(-7) M), linoleic acid (3.4 x 10(-5) M), and carnitine (10(-3) M) for 3 days. On the first day of serum-free culture, insulin was added at 1 or 100 ng/ml and glucagon was added at 0, 1, or 100 ng/ml. Recombinant human leptin (200 ng/ml) was added during the final 24 h; medium and all cells were harvested on the third day. Concentrations of acetoacetate and beta-hydroxybutyrate (ketone bodies) in media and glycogen deposition in the cellular compartment were determined. Ketogenesis was highly stimulated by glucagon (1 and 100 ng/ml) and inhibited by insulin. In contrast, glycogen deposition was stimulated by insulin and attenuated by glucagon; high insulin was also associated with a reduction in the ketone body ratio (acetoacetate:beta-hydroxybutyrate). High levels of dexamethasone stimulated ketogenesis, but inhibited glycogen deposition at low insulin. Culture of cells with leptin for 24 h, over the range of insulin, glucagon, and dexamethasone concentrations had no effect on either glycogen deposition or ketogenesis. These data suggest that while adult porcine hepatocytes are indeed sensitive to hormonal manipulation, leptin has no direct influence on hepatic energy metabolism in swine.

Elucidation of the role of fructose 2,6-bisphosphate in the regulation of glucose fluxes in mice using in vivo (13)C NMR measurements of hepatic carbohydrate metabolism

Fructose 2,6-bisphosphate (Fru-2,6-P2) plays an important role in the regulation of major carbohydrate fluxes as both allosteric activator and inhibitor of target enzymes. To examine the role of Fru-2,6-P2 in the regulation of hepatic carbohydrate metabolism in vivo, Fru-2,6-P2 levels were elevated in ADM mice with adenovirus-mediated overexpression of a double mutant bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (n = 6), in comparison to normal control mice (control, n = 6). The rates of hepatic glycogen synthesis in the ADM and control mouse liver in vivo were measured using new advances in 13C NMR including 3D localization in conjunction with [1-13C]glucose infusion. In addition to glycogen C1, the C6 and C2-C5 signals were measured simultaneously for the first time in vivo, which provide the basis for the estimation of direct and indirect synthesis of glycogen in the liver. The rate of label incorporation into glycogen C1 was not different between the control and ADM group, whereas the rate of label incorporation into glycogen C6 signals was in the ADM group 5.6 +/- 0.5 micro mol.g-1.h-1, which was higher than that of the control group of 3.7 +/- 0.5 micro mol.g-1.h-1 (P < 0.02). The rates of net glycogen synthesis, determined by the glycogen C2-C5 signal changes, were twofold higher in the ADM group (P = 0.04). The results provide direct in vivo evidence that the effects of elevated Fru-2,6-P2 levels in the liver include increased glycogen storage through indirect synthesis of glycogen. These observations provide a key to understanding the mechanisms by which elevated hepatic Fru-2,6-P2 levels promote reduced hepatic glucose production and lower blood glucose in diabetes mellitus.

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

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

Regulatory role of fructose-2,6-bisP on glucose metabolism in frog oocytes: in vivo inhibition of glycogen synthesis

Glycogen synthesis following glucose microinjection in frog oocytes proceeds preferentially by an indirect pathway involving gluconeogenesis from triose compounds. Because of the known regulatory role of fructose-2,6-bisP on glucose utilization in most vertebrate tissues we coinjected [U-14C]glucose and fructose-2,6-bisP into oocytes and observed a marked inhibition of label incorporation into glycogen, with an I50 value of 2 microM, which is similar to the value measured for the in vitro inhibition of oocyte fructose-1,6-bisphosphatase. Other hexoses-bisP were tested: 2,5-anhydromannitol-1,6-bisP was as effective as inhibitor as fructose-2,6-bisP; glucose-1,6-bisP showed some effect although 50% inhibition was obtained at a concentration 10 times higher than with fructose-2,6-bisP; fructose-1,6-bisP had no effect at all. The inhibition pattern for the in vivo glycogen synthesis by these analogs closely matched the one obtained with partially purified oocyte fructose-1,6-bisphosphatase. The intracellular concentration of fructose-2,6-bisP in unperturbed oocytes was found to be between 0.1 and 0.2 microM. Fructose-6-phosphate,2-kinase levels measured in oocyte homogenates were between 0.02 and 0.06 mU per gram of ovary. After 60 min incubation, fructose-2,6-bisP microinjected into the oocytes was almost completely degraded, suggesting that fructose-2,6-bisphosphatase is active in vivo. The results presented in this paper indicate that fructose-2,6-bisP plays an important role in the in vivo regulation of glucose utilization in frog-grown oocytes.

Multiple defects of both hepatic and peripheral intracellular glucose processing contribute to the hyperglycaemia of NIDDM

Non-insulin-dependent diabetic (NIDDM) patients were studied during a modified euglycaemic state when fasting hyperglycaemia was normalized by a prior (-210 to -150 min)--and later withdrawn (-150-0 min)--intravenous insulin infusion. Glucose metabolism was assessed in NIDDM patients (n = 10) and matched control subjects (n = 10) using tritiated glucose turnover rates, indirect calorimetry and skeletal muscle glycogen synthase activity determinations. Total and non-oxidative exogenous glycolytic flux rates were measured using appearance rates of tritiated water. A + 180 min euglycaemic hyperinsulinaemic (40 mU.m-2.min-1) clamp was performed to determine the insulin responsiveness of the various metabolic pathways. Plasma glucose concentration increased spontaneously during baseline measurements in the NIDDM patients (-120 to 0 min: 4.8 +/- 0.3 to 7.0 +/- 0.3 mmol/l; p < 0.01), and was primarily due to an elevated rate of hepatic glucose production (3.16 +/- 0.13 vs 2.51 +/- 0.16 mg.kg FFM-1.min-1; p < 0.01). In the NIDDM subjects baseline glucose oxidation was decreased (0.92 +/- 0.17 vs 1.33 +/- 0.14 mg.kg FFM-1.min-1; p < 0.01) in the presence of a normal rate of total exogenous glycolytic flux and skeletal muscle glycogen synthase activity. The simultaneous finding of an increased lipid oxidation rate (1.95 +/- 0.13 vs 1.61 +/- 0.07 mg.kg FFM-1.min-1; p = 0.05) and increased plasma lactate concentrations (0.86 +/- 0.05 vs 0.66 +/- 0.03 mmol/l; p = 0.01) are consistent with a role for both the glucose-fatty acid cycle and the Cori cycle in the maintenance and development of fasting hyperglycaemia in NIDDM during decompensation. Insulin resistance was demonstrated during the hyperinsulinaemic clamp in the NIDDM patients with a decrease in the major peripheral pathways of intracellular glucose metabolism (oxidation, storage and muscle glycogen synthase activity), but not in the pathway of non-oxidative glycolytic flux which was not completely suppressed during insulin infusion in the NIDDM patients (0.55 +/- 0.15 mg.kg FFM-1.min-1; p < 0.05 vs 0; control subjects: 0.17 +/- 0.29; NS vs 0). Thus, these data also indicate that the defect(s) of peripheral (skeletal muscle) glucose processing in NIDDM goes beyond the site of glucose transport across the cell membrane.

Rate of glucose entry into hepatic uridine diphosphoglucose by the direct pathway in fasted and fed states in normal humans

We used the glucuronate (GlcUA) probe technique to measure the rate of glucose entry into hepatic uridine diphosphoglucose (UDP-glc) by the direct pathway, to quantify the rate of appearance (Ra) of hepatic UDP-glc, and to calculate hepatic glucose cycling in vivo in normal humans. The direct pathway contribution to UDP-glc as determined by the ratio of [1-d1]-GlcUA to plasma [LD1]-glucose enrichments was minor (15% to 20%) in normal men after an overnight fast. After 9 hours of refeeding with intravenous (IV) glucose or an oral liquid formula meal each at a rate of 7 mg carbohydrate/kg/min, the direct pathway increased to 66.3% +/- 6.7% and 61.6% +/- 6.0% (mean +/- SE), respectively. Plasma glucose concentrations remained below 7.8 mmol/L and could not account for most of the variability in direct pathway contribution. The dilution of labeled [L-D1]-galactose in excreted acetaminophen-GlcUA was used to measure Ra UDP-glc, on the assumption that labeled galactose passes through the liver during its assimilation. Ra UDP-glc was 1.1 +/- 0.1 mg/kg/min after an overnight fast and increased to 2.0 +/- 0.1 with i.v. glucose and 2.6 +/- 0.2 with the oral liquid mixed meal. By combining the fractional glucose contribution with the Ra of hepatic UDP-glc, the rate of direct glucose entry into hepatic UDP-glc was 0.2 mg/kg/min (fasted) and increased to 1.3 to 1.6 (fed). This represented approximately 18% to 21% of systemic glucose disposal or 19% to 23% of the administered carbohydrate load during i.v. or oral refeeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycogen synthesis from glucose by direct and indirect pathways in hepatocyte cultures from different nutritional states

The conversion of glucose to glycogen by direct and indirect pathways was determined from the incorporation of [6-3H,U-14C]glucose into glycogen in hepatocyte cultures isolated from fed, fasted or fasted-refed rats. Mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase (PEPCK) was used to determine the extent by which 6-tritium is lost by mechanisms not involving flux through PEPCK. Glucose conversion to glycogen was lower in hepatocytes from fasted and higher in hepatocytes from fasted-refed rats than in hepatocytes from rats fed ad libitum. Insulin increased glycogen synthesis in hepatocytes from all nutritional states, and it decreased the 3H/14C ratio incorporated into glycogen. This increased loss of 6-tritium was only in part mercaptopicolinate-sensitive. Lactate and pyruvate (2 mM + 0.2 mM) increased glycogen deposition, largely by stimulation of glucose conversion to glycogen by the direct pathway. Insulin-induced glucokinase mRNA expression was higher in hepatocytes from fed than from fasted or refed rats whereas PEPCK mRNA expression was lowest in hepatocytes from fasted-refed rats. Hepatocyte cultures derived from different nutritional states express differences in glycogen synthesis from glucose by direct and indirect pathways as well as differences in the extent by which pyruvate cycling accounts for loss of 6-tritium.

Quantitation of glycolysis and skeletal muscle glycogen synthesis in humans

We measured the net rates of skeletal muscle glycogen synthesis and glycolysis (conversion of [3-3H]glucose to 3H2O) in healthy overnight-fasted volunteers. Two studies were performed. In study 1, seven subjects participated in two paired infusions under basal conditions of either [2-3H]glucose (H2) or [3-3H]glucose (H3). Total glucose uptake (Rd) and rates of whole body 3H2O formation (3H2O Ra) were measured. With H2, Rd and 3H2O Ra were similar. With H3, 3H2O Ra, equal to glycolysis, was 65% of Rd. In study 2, six different subjects underwent a 3-h, 40 mU.m-2 x min-1 euglycemic insulin clamp. [6,6-2H2]glucose was infused throughout and H3 was infused during the last hour of the study. Open muscle biopsies were obtained at 150 and 180 min. Glycogen synthesis was assessed by three independent means: 1) direct measurement, as 3H disintegrations per minute in isolated muscle glycogen per plasma H3 specific activity; 2) extrapolation from the activity of glycogen synthase assayed in the presence of the concentrations of glucose 6-phosphate and UDP-glucose measured in the biopsy; and 3) the difference between Rd and glycolysis. Despite a wide range in Rd [24.5-58.8 mumol.kg fat-free mass (FFM)-1 x min-1] and glycolysis (14.2-26.1), the three methods yielded similar results of 20.0 +/- 3.9, 22.5 +/- 3.7, and 20.6 +/- 3.7 mumol.kg FFM-1 x min-1 and correlated highly with each other (r2 = 0.92-0.96). Our results (study 1) indicate that the rate of plasma tritiated water formation reflects the intracellular detritiation of tritiated glucose. Under hyperinsulinemic conditions (study 2) the net rate of muscle glycogen synthesis can be accurately estimated from the glycogen synthase activity and from the difference between total glucose uptake and glycolysis. Thus, at high physiological plasma insulin concentrations resulting in submaximal stimulation of muscle glycogen synthesis, the latter can be accurately measured in humans.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Glycogen metabolism as detected by in vivo and in vitro 13C-NMR spectroscopy using [1,2-13C2]glucose as substrate

The metabolism of glucose to glycogen in the liver of fasted and well-fed rats was investigated with 13C nuclear magnetic resonance spectroscopy using [1,2-(13)C2]glucose as the main substrate. The unique spectroscopic feature of this molecule is the 13C-13C homonuclear coupling leading to characteristic doublets for the C-1 and C-2 resonances of glucose and its breakdown products as long as the two 13C nuclei remain bonded together. The doublet resonances of [1,2-(13)C2]glucose thus provide an ideal marker to follow the fate of this exogenous substrate through the metabolic pathways. [1,2-(13)C2]Glucose was injected intraperitoneally into anesthetized rats and the in vivo 13C-NMR measurements of the intact animals revealed the transformation of the injected glucose into liver glycogen. Glycogen was extracted from the liver and high resolution 13C-NMR spectra were obtained before and after hydrolysis of glycogen. Intact [1,2-13C2]glucose molecules give rise to doublet resonances, natural abundance [13C]glucose molecules produce singlet resonances. From an analysis of the doublet-to-singlet intensities the following conclusions were derived. (i) In fasted rats virtually 100% of the glycosyl units in glycogen were 13C-NMR visible. In contrast, the 13C-NMR visibility of glycogen decreased to 30-40% in well-fed rats. (ii) In fed rats a minimum of 67 +/- 7% of the exogenous [1,2-(13)C2]glucose was incorporated into the liver glycogen via the direct pathway. No contribution of the indirect pathway could be detected. (iii) In fasted rats externally supplied glucose appeared to be consumed in different metabolic processes and less [1,2-(13)C2]glucose was found to be incorporated into glycogen (13 +/- 1%). However, the observation of [5,6-(13)C2]glucose in liver glycogen provided evidence for the operation of the so-called indirect pathway of glycogen synthesis. The activity of the indirect pathway was at least 9% but not more than 30% of the direct pathway. (vi) The pentose phosphate pathway was of little significance for glucose but became detectable upon injection of [1-(13)C]ribose.

In vitro stimulation of glucose utilization by insulin in primary cultures of rat hepatocytes

The effect of glucose concentration and insulin on glucose incorporation was studied in primary cultures of rat hepatocytes. The rate of glucose incorporation into hepatocytes was proportional to the medium glucose concentration from 100 to 800 mg/dl. At 800 mg/dl glucose the rate reached a plateau. Of the glucose taken up by hepatocytes, 16 and 18% was incorporated into glycogen and lipid, respectively, and 58% into the nucleotide fraction after incubation for 4 h. In the medium, lactate was the major product found. Insulin stimulates glucose incorporation by 20-112% into all the above pathways at glucose concentrations between 100 and 800 mg/dl. The insulin effect was noted as early as 2-4 h (early effect) and up to 24 h (delayed effect). This effect of insulin was observed to be dose dependent from 5 to 200 ng/ml insulin. While the delayed insulin effect was abolished by cycloheximide, the early effect of insulin was not affected. With respect to the key enzyme activities of glucose utilization, activation of glycogen synthase (increase of I-activity/total activity) and pyruvate kinase (activation at 0.2 mM phosphoenolpyruvate) was noted 4 h after insulin addition, and these effects were not abolished by cycloheximide. These two enzymes increased in total activity after 24 h. Both glucokinase and glucose-6-phosphate dehydrogenase activities increased by 30-35% and 65-93% at 4 and 24 h, respectively. The results indicate that hepatocytes directly utilize glucose in a dose-dependent manner with respect to glucose and insulin. A major early and delayed effect of insulin appeared due to the activation and induction of the key hepatic enzymes of glucose utilization, respectively.

Pathway and carbon sources for hepatic glycogen repletion in dogs

The present studies were undertaken to quantitate the relative contributions of the indirect and direct pathways for hepatic glycogen repletion and to determine the role of splanchnic tissues in provision of C precursors used for the indirect pathway. For this purpose, we administered oral glucose (1.4 g/kg) enriched with [1-14C]glucose to 18-h fasted dogs and measured net hepatic and net gastrointestinal glucose, lactate, and alanine balance, hepatic and gastrointestinal fractional extraction [( 3H]lactate), release and uptake of lactate, as well as the total amount of hepatic glycogen formed from the oral glucose and the 14C labeling pattern of the glycogen-glucose C. Although net hepatic glucose uptake (8.7 +/- 0.6 g, 27% of the oral load) exceeded the amount of glycogen formed from the oral glucose (6.3 +/- 1.1 g), analysis of radioactivity in C-1 of the glycogen glucose indicated that nearly 50% of the glycogen was formed by the indirect pathway. Net hepatic uptake of lactate (1.4 +/- 0.1 g) and alanine (1.5 +/- 0.1 g) could account for greater than 90% of glycogen formed by the indirect pathway if all of the lactate and alanine taken up by the liver had been incorporated into glycogen. Release of lactate and alanine by splanchnic tissues approximated the amount of lactate and alanine taken up by the liver. However, in addition to taking up lactate, the liver also produced nearly as much lactate as the gastrointestinal tract (1.8 +/- 0.2 vs. 2.0 +/- 0.3 g, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Pathways of hepatic glycogen formation in humans following ingestion of a glucose load in the fed state

The relative contributions of the direct and the indirect pathways to hepatic glycogen formation following a glucose load given to humans four hours after a substantial breakfast have been examined. Glucose loads labeled with [6-(14)C]glucose were given to six healthy volunteers along with diflunisal (1 g) or acetaminophen (1.5 g), drugs excreted in urine as glucuronides. Distribution of 14C in the glucose unit of the glucuronide was taken as a measure of the extent to which glucose was deposited directly in liver glycogen (ie, glucose----glucose-6-phosphate----glycogen) rather than indirectly (ie, glucose----C3-compound----glucose-6-phosphate----glycogen). The maximum contribution to glycogen formation by the direct pathway was estimated to be 77% +/- 4%, which is somewhat higher than previous estimates in humans fasted overnight (65% +/- 1%, P less than 0.05). Thus, the indirect pathway of liver glycogen formation following a glucose load is operative in both the overnight fasted and the fed state, although its contribution may be somewhat less in the fed state.

Disposition of a glucose load in fed rats and rats adapted to a high-carbohydrate diet

Current evidence suggests that, during the transition from the fasted to the fed state, liver glycogen is synthesized primarily from gluconeogenic precursors rather than from glucose unless the circulating glucose concentration is high. In the fed state the glucose concentration already is elevated. We aimed to determine whether administration of an additional oral glucose load (4 g/kg) to chow- or high-carbohydrate diet-adapted (CHO) rats would further increase the glucose concentration and result in increased direct glucose uptake. In the chow- and CHO-fed rats, 70 and 98% of the administered glucose was absorbed by 120 min. In the chow-fed rats the glucose concentration entering the liver increased by only 1.0 mM from 8.0 to 9.0 mM; no net hepatic glucose uptake was observed. In the CHO-adapted rats the entering glucose concentration increased transiently by 3.5 mM from 8.0 to 11.5 mM. This was associated with net glucose uptake, which continued until the entering glucose concentration fell below 9.5 mM, the entering glucose concentration threshold above which net glucose uptake was observed previously in fasted rats. Net hepatic glucose uptake could not be correlated with insulin or hepatic intracellular glucose concentrations. Net glycogen synthesis did not occur in either group. We could not account for the absorbed glucose by the rise in portal glucose concentrations or by increased muscle glycogen deposition. The fate of much of the absorbed glucose remains unknown.

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 microM 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.

The onset of liver glycogen synthesis in fasted-refed lean and genetically obese (fa/fa) rats

Lean and genetically obese (fa/fa) rats were fed ad libitum, or fasted for 17 h and then meal-fed for varying time intervals. During refeeding, glucose-6-phosphatase activity of lean rats declined to the low value that was present in livers of fasted obese rats and which remained unchanged in the obese group during the meal. Refeeding also resulted in increases in hepatic concentrations of glucose-6-phosphate and fructose-6-phosphate, fructose 1,6-bisphosphate, fructose-2,6-bisphosphate, alpha-glycerophosphate, pyruvate and lactate in lean and obese rats, absolute values being higher in the fasted obese than in the fasted lean group. Obese animals had higher postprandial portal blood insulin, glucose and lactate concentrations than lean animals. In spite of this, the rate of hepatic glycogen deposition was the same in both groups and was accompanied by similar glycogen synthase a levels. Following refeeding, phosphorylase was transiently inactivated in livers of lean but not of obese animals, while glycogen synthase was inactivated in both groups. The data suggest that in lean animals refeeding was associated with a stimulation of liver glycolysis, presumably by insulin; in fasted obese rats hepatic glycolysis was already in a stimulated state and was only slightly enhanced further after the meal, in keeping with their unaltered hyperinsulinaemia; there was an increased turnover of liver glycogen or a resistance to insulin stimulation of glycogen synthesis in fa/fa rats during refeeding.

Tracer methods and the metabolic disposal of a carbohydrate load in man

Figure 18 outlines a summary of the results obtained in our laboratory and how these might be interpreted. Following a 100-g oral glucose load, about 25 g is taken up by the liver. About 5 g or 5% of this would be removed on a first-pass basis since only about a fifth of the portal vein glucose is newly absorbed. The remainder of the glucose is disposed of in peripheral tissues. This disposal is enhanced by intestinal insulinotropic factors that stimulate insulin secretion. Lactate is produced peripherally (with the red cells as one of the most important sources) by the gut and, perhaps, by hepatocytes. It is taken up by gluconeogenetic hepatocytes to form glycogen. This pathway appears to account for half to two-thirds of glycogen synthesis, the remainder being by direct uptake of glucose. The gluconeogenetic pathway of glycogen formation may be important in that it clears the obligatory production of lactate from certain tissues. The only difference between intravenous and oral glucose loading is that there is no absorbed glucose in the portal vein when glucose is infused. The glucose concentrations here are, however, almost the same as during oral glucose loading since peripheral clearance of glucose is slower in the absence of insulinotropic intestinal factors. This helps to explain why liver handling of intravenous glucose and glycogen formation are almost identical to the case of oral loading.

Plasma glucose concentration determines direct versus indirect liver glycogen synthesis

In the present study hepatic glycogenesis by the direct versus indirect pathway was determined as a function of the glucose infusion rate. Glycogen synthesis was examined in catheterized conscious rats that had been fasted 48 h before receiving a 3-h infusion (iv) of glucose. Glucose, containing tracer quantities of [U-14C]- and [6-3H]glucose, was infused at rates ranging from 0 to 230 mumol X min-1 X kg-1. Plasma concentrations of glucose, lactate, and insulin were positively correlated with the glucose infusion rate. Despite large changes in plasma glucose, lactate, and insulin concentrations, the rate of hepatic glycogen deposition (0.46 +/- 0.03 mumol X min-1 X g-1) did not vary significantly between glucose infusion rates of 20 and 230 mumol X min-1 X kg-1. However, the percent contribution of the direct pathway to glycogen repletion gradually increased from 13 +/- 2 to 74 +/- 4% in the lowest to the highest glucose infusion rates, with prevailing plasma glucose concentrations from 9.4 +/- 0.5 to 21.5 +/- 2.1 mM. Endogenous glucose production was depressed (by up to 40%), but not abolished by the glucose infusions. Only a small fraction (7-14%) of the infused glucose load was incorporated into liver glycogen via the direct pathway irrespective of the glucose infusion rate. Our data indicate that the relative contribution of the direct and indirect pathways of hepatic glycogen synthesis are dependent on the glucose load or plasma glucose concentration and emphasize the predominance of the indirect pathway of glycogenesis at plasma glucose concentrations normally observed after feeding.

Antiketogenic effect of glucose per se in vivo in man and in vitro in isolated rat liver cells

The in vivo effect of glucose per se on blood ketone bodies, glycerol, and nonesterified fatty acids (NEFA) has been investigated in five normal (60 hours fasted) men receiving a somatostatin (SRIF) infusion (500 micrograms/h-1). When glycemia was raised over 10 mmol/L for 180 minutes by exogenous IV glucose infusion, neither insulin nor C peptide increase. NEFA and glycerol returned to fasting value in 40 minutes and remained stable. Ketone bodies decreased continuously and were significantly below the fasting values at the end of the study (1.3 +/- 0.3 mmol/L v 2.2 +/- 0.4 mmol/L, P less than 0.05). In order to ascertain whether glucose has been acting only on lipolysis or also on the liver ketogenic capacity, its effect was studied in vitro on isolated liver cells from 24-hour starved rats incubated with various amounts of palmitate. Glucose (30 mmol/L) did not affect the maximal ketogenic capacity (80 mumol/g (w/w)/h) measured with 1.6 mmol/L palmitate but increased the apparent palmitate K 0.5 for ketogenesis from 0.16 to 0.3 mmol/L. At physiologic free fatty acids concentration (0.22 mmol/L), glucose decreased ketogenesis by 90%. The effect was time-dependent, maximum after 30 minutes of incubation. Half-maximum inhibition by glucose was obtained at 6 mmol/L, a concentration at which lactate production was unaffected. These results suggest that glucose per se inhibits ketogenesis in vivo by acting probably both on lipolysis and on liver ketogenic capacity.

Purification of rat hepatic glucokinase

We have developed a rapid, reliable procedure for the purification of rat hepatic glucokinase. The purification utilizes DEAE-cellulose, two affinity chromatography steps, and high-performance liquid chromatography. Glucokinase with a specific activity of 240 units/mg, a 42 K-fold purification, and a yield of 60% is obtained. The enzyme appears as a homogeneous band, with over 99% purity as assessed by polyacrylamide gel electrophoresis. The purification procedure can be completed in 5 days.

The compartmentation of glycolytic and gluconeogenic enzymes in rat kidney and liver and its significance to renal and hepatic metabolism

An indirect immunoperoxidase procedure has been used to demonstrate sites of glycolysis and gluconeogenesis in normal rat kidney and liver. In kidney, the gluconeogenic enzyme fructose 1,6-biphosphatase was restricted to the proximal tubular epithelium, while the glycolytic enzyme hexokinase predominated in more distal segments. Intense staining for the biphosphatase in proximal convoluted tubular brush borders suggests that reabsorbed substrates may be used directly at this site in renal gluconeogenesis. In view of the high phosphofructokinase and pyruvate kinase activities present in collecting ducts, their relatively low hexokinase activities and their relatively pale immunostaining for hexokinase indicate that glycolytic substrates which feed into the pathway subsequent to the initial phosphorylation step, rather than glucose, may be the major energy source for the rat renal papilla. Immunostaining in the liver was consistent with the metabolic zonation of liver parenchyma, in that glucokinase occurred mainly in perivenous regions and fructose 1,6-bisphosphatase in periportal areas. The presence of such metabolic zonation is difficult to reconcile with the widely held view that the majority of hepatic glycogen is derived directly from glucose. A model for hepatic glycogen synthesis is proposed which links the concept of parenchymal zonal heterogeneity with recent biochemical evidence concerning the 'glucose paradox' and with microscopical studies on the dynamics of glycogen deposition after refeeding.