Disorders of gluconeogenesis

Summary Gluconeogenesis, or the formation of glucose from mainly lactate/pyruvate, glycerol and alanine, plays an essential role in the maintenance of normoglycaemia during fasting. Inborn deficiencies are known of each of the four enzymes of the glycolytic — gluconeogenic pathway that ensure a unidirectional flux from pyruvate to glucose: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. In this paper, the clinical picture, pathophysiology, diagnostic tests, genetics, treatment and prognosis of the deficiencies of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase are reviewed.     

The intravenous L-alanine tolerance test as a means for investigating gluconeogenesis

Gluconeogenesis was tested in eight children with abnormalities of carbohydrate metabolism by means of the intravenous injection of 0.5 g L-alanine per kilogram body weight. The test was performed on two girls with debranching enzyme-deficient glycogenosis, two brothers with phosphorylase-deficient glycogenosis, three boys with ketotic hypoglycemia, and one boy with fructose-1,6-diphosphatase deficiency. The former seven patients showed a marked increase of the initially low fasting blood glucose levels after alanine injection, whereas the last patient merely responded with a further increase of the initially high blood lactate concentration, glucose levels remaining as low as before the test. An intravenous glycerol tolerance test was characterized by the same response. The blood alanine concentrations, determined in three patients of the former group, decreased rapidly after alanine injection, whereas the patient with a fructose-1,6-diphosphatase deficiency showed a delayed alanine elimination. The results pointed to unimpaired gluconeogenesis of the first seven patients and a defective glucose formation in the eighth patient. It is discussed that the intravenous alanine tolerance test permits gluconeogenesis to be regarded as normal or impaired only, if the previous fast of the patient has been sufficiently long. In the case of impaired gluconeogenesis, an additional tolerance test, with a glucogenic substrate that enters the gluconeogenic pathway at a level different from alanine, facilitates the localization of the enzyme defect underlying the impaired gluconeogenesis.     

Relationship between gluconeogenesis and glutathione redox state in rabbit kidney-cortex tubules

The intracellular glutathione redox state and the rate of glucose formation were studied in rabbit kidney-cortex tubules. In the presence of substrates effectively utilized for glucose formation, ie, aspartate + glycerol + octanoate, alanine + glycerol + octanoate, malate, or pyruvate, the intracellular reduced glutathione/oxidized glutathione (GSH/GSSG) ratios were significantly higher than those under conditions of negligible glucose production. Changes in the intracellular GSH/GSSG ratio corresponded to those in glucose-6-phosphate content and reduced nicotinamide adenine dinucleotide phosphate/oxidized nicotinamide adenine dinucleotide phosphate (NADPH/NADP(+)) ratio obtained from malate/pyruvate measurements. Gluconeogenesis stimulation by extracellular adenosine triphosphate (ATP) or inosine caused an elevation of the intracellular GSH/GSSG and NADPH/NADP(+) ratios, as well as glucose-6-phosphate level. Surprisingly, in the presence of 5 mmol/L glucose, both the intracellular GSH/GSSG and NADPH/NADP(+) ratios and glucose-6-phosphate content were almost as low as under conditions of negligible glucose synthesis. L-buthionine sulfoximine (BSO)-induced decline in both the intracellular glutathione level and redox state resulted in inhibition of gluconeogenesis accompanied by accumulation of phosphotrioses and a decrease in fructose-1,6-bisphosphate content, while cysteine precursors altered neither GSH redox state nor the rate of glucose formation. In view of the data, it seems likely that: (1) intensive gluconeogenesis rather than extracellular glucose is responsible for maintaining a high intracellular GSH/GSSG ratio due to effective glucose-6-phosphate delivery for NADPH generation via the pentose phosphate pathway; (2) a decline in the intracellular glutathione level and/or redox state causes a decrease in glucose synthesis resulting from a diminished flux through aldolase; (3) induced by cysteine precursors, elevation of the intracellular GSH level does not affect the rate of glucose formation, probably due to no changes in the intracellular GSH/GSSG ratio.     

Inhibition by 2,5-anhydromannitol of glycolysis in isolated rat hepatocytes and in Ehrlich ascites cells.

2,5-Anhydromannitol decreases lactate formation and 3H2O formation from [5-3H]glucose in isolated rat hepatocytes metabolizing high concentrations of glucose. The inhibition of glycolysis is accompanied by a slight decrease in the cellular content of fructose-6-P and a more substantial decrease in the cellular content of fructose-1,6-P2, with no change in the content of glucose-6-P. The 3H2O release data and changes in hexosephosphate distribution indicate possible inhibitions at phosphofructokinase-1 and phosphoglucose isomerase. 2,5-Anhydromannitol also inhibits glycolysis in Ehrlich ascites cells, but the tumor cells, unlike hepatocytes, must be treated with 2,5-anhydromannitol prior to exposure to glucose to obtain the inhibition. The decrease in 3H2O formation from [5-3H]glucose and the metabolite pattern that results from the addition of low concentrations (less than or equal to 0.25 mM) of 2,5-anhydromannitol indicate an inhibition at phosphofructokinase-1 that cannot be attributed to a decrease in the cellular content of fructose-2,6-P2. Higher concentrations (greater than or equal to 0.5 mM) of 2,5-anhydromannitol cause a substantial decrease in the cellular content of ATP that is accompanied by decreases in the content of glucose-6-P and fructose-6-P and transient increases in fructose-1,6-P2. In Ehrlich ascites cells, 2,5-anhydromannitol is metabolized to 2,5-anhydromannitol mono- and bisphosphate. The inhibition of glycolysis caused by 2,5-anhydromanitol decreases with time, because the phosphorylated metabolites formed during the preliminary incubation in the absence of glucose are rapidly dephosphorylated during the incubation in the presence of glucose.     

Investigations on glucose uptake in isolated human leucocytes from normal and diabetic subjects

Summary 1. Leucocyte preparations have been obtained from the blood of 135 healthy control persons and 10 poorly-controlled insulin-dependent diabetics according to a procedure described earlier. Cells were suspended in a Krebs-Ringer-Bicarbonate-Buffer. Glucose uptake, lactate production and cell glucose space were determined after incubating the cells for 1 h at 37° C in an atmosphere of O₂/CO₂ (95/5). Extracellular water-space of the cell sediment was corrected by measuring U-¹⁴C-sucrose levels in the medium before and after addition to the cell sediment. — 2. The cells showed intact structures and amoeboid motility under the light-microscope as well as under the phase-contrast-microscope. During an incubation lasting 1 h, the ATP/ADP quotient diminished by about 20%; the following metabolites: glucose-6-phosphate, fructose-1,6-diphosphate, 1,3-phosphoglycerate, pyruvate and lactate increased slightly up to markedly, especially glucose-6-phosphate and lactate. — 3. Sources of methodological errors were investigated in preliminary experiments. Disregarding corrections for the extracellular space of the cell sediments led to a dilution effect which imitates an apparent glucose uptake. For measurements of intracellular glucose and cell glucose space the procedure of Crofford and Renold provided the best results. Falsely high values for cell glucose have been found using the glucose oxidase reaction. — 4. Glucose uptake of healthy leucocytes increases at first steeply, later on less with rising medium glucose concentrations. Addition of insulin (50–500 mU/ml) gave no consistent effects. At medium glucose concentrations of 400 and 600 mg/100 ml intracellular glucose could be demonstrated with statistical significance. Intracellular glucose and cell glucose space were not affected by insulin to any marked degree. Determinations of lactate production indicated that human leucocytes utilize the largest portion of glucose via glycolysis. — 5. At medium glucose concentrations of 600 mg/100 ml leucocytes from diabetics who had received no insulin for 14 h prior to incubation showed a significant diminution of glucose uptake. — 6. Theoretical curves for inward transport and phosphorylation have been calculated from the data for glucose uptake and intracellular glucose concentration.The authors are greatly indepted to Prof. Wilbrandt, Department of Pharmacology, University Bern, for advice and discussion.     

Use of a heated superficial hand vein as an alternative site for the measurement of amino acid concentrations and for the study of glucose and alanine kinetics in man

Comparisons were made between the artery and a heated superficial hand vein (HSHV) for the measurements of amino acids, lactate, glycerol, free fatty acids, insulin and glucagon and the measurements of glucose and alanine kinetics in man. Normal subjects (n = 8) were studied after an overnight fast (12–14 hr). U-14C-alanine and 3, ³H glucose were administered by a constant infusion and blood was sampled from catheters placed in a radial artery and a superficial dorsal vein of a heated hand (68°C environment), during a control period and a period of a steady state hyperaminoacidemia achieved by a constant infusion of an L-amino acid solution. The blood concentrations of all substrates and hormones measured and the concentrations of cold and radioactive glucose and alanine were comparable in the two vessels during both study periods. In contrast, measurements obtained in a deep forearm vein (DV) showed the concentrations of plasma glucose to be lower (3% in the control period and 5% during the experimental period) and those of plasma alanine to be higher (13% and 5% during control and experimental periods respectively) than the artery or the HSHV. The difference in glucose specific activity between the artery or the HSHV and the DV were however slight but non-significant, while plasma alanine specific activity was significantly lower in the DV as compared to the artery or the HSHV (32% in the control period versus 14% in the experimental period) suggesting a process of exchange of alanine and glucose occuring during the transit of blood across the forearm. As a result blood samples obtained from a DV will overestimate the derived total body glucose and alanine turnover rates. Thus the heated superficial hand vein can adequately replace the artery for the measurements of whole blood amino acids, lactate and glycerol and for plasma FFA, insulin and glucagon; its use can obviate the risks associated with arterial catheterization and can be a suitable site for the measurements of total body glucose and alanine kinetics in man.     

Combined deficiency of glucose-6-phosphatase and fructose-1, 6-diphosphatase. Studies of glucagon secretion and fuel utilization.

An adult woman with hypoglycemia, hyperlactatemia, hyperuricemia, hypertriglyceridemia, hyperketonemia and inability to make new glucose from galactose, fructose, glycerol and alanine was found to have no hepatic glucose-6-phosphatase and deficient fructose-1,6-diphosphatase. Nonautonomous hyperglucagonemia was demonstrated and shown to contribute to the hyperlactatemia and hyperketonemia. A paradoxic hyperlactatemic response to glucose and galactose was observed. Studies of substrate utilization showed prompt adaptation to changes in dietary supply of energy which probably accounted for her never having experienced symptoms of hypoglycemia.     

Effect of exogenous fructose-1,6-bisphosphate on glycolysis in the isolated perfused rat heart.

To test the mechanism of action of fructose-1,6-bisphosphate (F-1,6-P2), experiments were conducted on isolated perfused rat hearts to measure the glycolytic rate supported by exogenous glucose with simultaneous measurement of oxygen consumption and the release of lactate and pyruvate. Glycolysis was assayed in terms of the release of tritiated water from [5-3H] glucose, a measure of the rate through the aldolase step. It was found that 5 mmol/L F-1,6-P2 reduced the glycolytic rate parallel to the decrease in oxygen consumption. The results suggest that the cardioprotective action of F-1,6-P2 is related to a substrate effect and a decrease in adenosine triphosphate consumption as indicated by a decrease in oxygen consumption in accordance with the recent demonstration of Ca2+ binding by F-1,6-P2.     

Hormonal control of glucose production by Amphiuma means liver in organ culture.

The effects of glucagon, insulin, and adrenalin on glucose production by Amphiuma means liver organ cultures maintained at 25° were studied. Glucagon stimulated gluconeogenesis from pyruvate and alanine, and adrenalin treatment resulted in a more rapid glycogenolysis than glucagon treatment. Glucagon increased the tissue levels of glutamate-oxaloacetate transaminase (GOT), glutamate-pyruvate transaminase (GPT), and fructose-1,6-diphosphatase after 40 hr, while insulin decreased GOT and GPT levels after 72 hr. The presence of pyruvate in the medium significantly reduced nitrogenous excretion by the liver fragments. We conclude that cultured A. means liver responds to glucagon, insulin, and adrenalin in ways that are compatible with the known effects of these hormones in mammals.     

Regulation of splanchnic and renal substrate supply by insulin in humans

To determine the effects of peripheral insulin infusion on total, hepatic, and renal glucose production and on the percent contribution to glucose production of gluconeogenesis versus glycogenolysis, 10 healthy subjects had arterialized hand and hepatic vein catheterization after an overnight fast and the results were compared with data from 12 age- and weight-matched subjects with renal vein catheterization during a 180-minute infusion of either insulin (0.25 mU/kg x min) with dextrose, or saline. Endogenous, hepatic, and renal glucose production was measured with [6,6(-2)H2]glucose, regional lactate, alanine, and glycerol balance by arteriovenous difference; hepatic blood flow by indocyanine green clearance; and renal blood flow by p-aminohippurate clearance, before and every 30 minutes during each infusion period. Insulin increased from about 42 to 98 pmol/L and blood glucose remained constant in all studies (3.8 +/- 0.2 v4.4 +/- 0.1 micromol/ml, hepatic vrenal vein). In response to insulin infusion, endogenous, hepatic, and renal glucose production decreased immediately (30 minutes) and reached a lower plateau value (10.8 +/- 0.8 v6.4 +/- 0.7, 10.4 +/- 1.1 v7.8 +/- 1.0, and 2.8 +/- 0.6 v 1.5 +/- 0.6 micromol/kg x min, respectively) between 120 and 180 minutes (all P < .05). Net renal uptake of lactate (2.4 +/- 0.4 v0.9 +/- 0.6) decreased earlier (30 minutes) and returned to baseline between 120 and 180 minutes (2.4 +/- 0.5 micromol/kg x min), whereas net splanchnic uptake of lactate (5.7 +/- 0.7 v 0.7 +/- 0.6) and alanine (1.8 +/- 0.1 v 1.0 +/- 0.5 micromol/kg x min) decreased later (120 to 180 minutes). Net renal (0.3 +/- 0.1 v 0.1 +/- 0.1) and splanchnic (0.7 +/- 0.3 v 0.4 +/- 0.2 micromol/kg x min) glycerol uptake decreased 90 to 180 minutes after insulin and increased (P < .05) with saline infusion (0.4 +/- 0.1 v0.6 +/- 0.3 and 1.0 +/- 0.5 v1.8 +/- 0.4 micromol/kg x min, respectively). These data indicate that the rapid suppression of endogenous glucose production by insulin reflects primarily a decrease in hepatic glucose release, most likely due to inhibition of net glycogenolysis, combined with suppression of renal gluconeogenesis. Inhibition of hepatic gluconeogenesis presumably occurs later during hyperinsulinemia. We conclude that peripheral insulin, in addition to its inhibition of glycogen degradation, regulates endogenous glucose production, in part, by modifying the splanchnic and renal substrate supply.     

The glycolytic cascade in pancreatic islets

Conclusion The present report emphasizes the view that the acceleration of glycolysis occurring in islets stimulated by a rise in the extracellular concentration of glucose involves increases in both the availability of glycolytic intermediates, such as glucose-6-P and fructose-6-P, and the activity of the key glycolytic enzyme phosphofructokinase. We initially thought that such a dual mechanism could help to reconcile the substrate-site and regulatory-site hypotheses for the process of glucose-induced insulin release [33]. However, our more recent study on the properties of fructose-6-P,2-kinase now suggests that the increase in the rate of fructose-2,6-P₂ synthesis may be mainly due to an elevation in the fructose-6-P content of the islet cells, an elevation itself attributable to a mass action phenomenon. We were also unable to detect any direct effect of glucose on the activity of phosphoglucomutase, the enzyme catalyzing the synthesis of glucose-1,6P₂, another activator of phosphofructokinase. Therefore, there is as yet no convincing evidence that the molecule of glucose, whether located in the extracellular or intracellular fluid, interacts with a specific receptor to induce activation of a hypothetical glucosensor system. Obviously, the latter statement is not meant to deny that glucose binds to the stereospecific carrier mediating glucose transport across the B cell plasma membrane and to those enzymes (hexokinase, glucokinase, aldose reductase) catalyzing the first steps of its intracellular metabolism. Such binding phenomena, however, cannot be equated with the situation in which the insulin secretory response depends solely on the allosteric activation of an enzyme by a given secretagogue, as appears to be the case in islets stimulated by the nonmetabolized analogue of L-leucine, 2-aminobicyclo [2,2,12] heptane-2-carboxylic acid [34,35].     

Products of Leishmania braziliensis glucose catabolism: release of D-lactate and, under anaerobic conditions, glycerol.

Leishmania braziliensis panamensis promastigotes were incubated with glucose as the sole carbon source. About one-fifth of the glucose consumed under aerobic conditions was oxidized to CO2. Nuclear magnetic resonance studies with [1-13C]glucose showed that the other products released were succinate, acetate, alanine, pyruvate, and lactate. Under anaerobic conditions, lactate output increased, glycerol became a major product, and, surprisingly, glucose consumption decreased. Enzymatic assays showed that the lactate formed was D(-)-lactate. The release of alanine during incubation with glucose as the sole carbon source suggested that appreciable proteolysis occurred, consistent with our observation that a large amount of ammonia was released under these conditions. The discoveries that D-lactate is a product of L. braziliensis glucose catabolism, that glycerol is produced under anaerobic conditions, and that the cells exhibit a "reverse" Pasteur effect open the way for detailed studies of the pathways of glucose metabolism and their regulation in this organism.     

The effects of HDV-insulin on carbohydrate metabolism in Type 1 diabetic patients

The aim of this study was to compare the metabolic effects of a single equimolar subcutaneous injection of hepatic directed vesicle-insulin (HDV-insulin) and regular insulin on glucose levels and intermediary metabolism during a 75-g oral glucose tolerance test (OGTT). Nine Type 1 diabetic patients underwent two experiments separated by 4 weeks. Each experimental protocol consisted of an identical evening meal followed by overnight euglycemic control achieved by a continuous low-dose insulin infusion. The next morning a subcutaneous injection (0.1 U/kg) of HDV-insulin or regular insulin was administered 30 min before a 75-g OGTT. The overnight basal insulin infusion was maintained unaltered throughout the 150-min OGTT. Plasma glucose, glucoregulatory hormones (insulin, glucagon, cortisol), and intermediary metabolites (lactate, alanine, glycerol, NEFA, beta-hydroxybutyrate) were measured to assess the metabolic effects of the two insulin preparations. Compared to regular insulin, an equivalent subcutaneous dose of HDV-insulin significantly lowered glucose levels during OGTT (mean reduction 2.2+/-0.4 mmol/l; P<.005). Plasma levels of insulin and glucagon were equivalent during both series of experiments. Blood lactate, glycerol and plasma NEFA levels were not different during OGTT indicating similar peripheral action of the insulins. beta-Hydroxybutyrate levels were significantly reduced (P<.05) following HDV-insulin supporting a preferential hepatic action of the preparation. We conclude that HDV-insulin can significantly lower plasma glucose excursions compared to an equivalent dose of regular insulin during an OGTT in Type 1 diabetic patients. The metabolic profile of equivalent peripheral insulin, glucagon and glycerol levels but reduced beta-hydroxybutyrate values support a hepatospecific effect of HDV-insulin.     

Age-dependent changes of hormone-stimulated gluconeogenesis in isolated rat hepatocytes

Effects of glucagon or catecholamine stimulation on glucose synthesis from lactate/pyruvate, alanine, fructose or glycerol in isolated hepatocytes from young and old rats were compared. It was found that production of glucose by non-stimulated hepatocytes was almost the same in hepatocytes both from young and old rats. The concentrations of hormones which caused the maximal effect were the same for both compared groups of rats. However, the effect of stimulation by glucagon was significantly greater in hepatocytes of young rats than of old ones, when lactate/pyruvate or alanine were used as substrates, while no such difference was observed when fructose or glycerol were used. Epinephrine and norepinephrine stimulated glucose synthesis from all substrates investigated to the same extent in hepatocytes isolated from both compared groups of rats. The biochemical and physiological aspects of these findings are discussed.     

The effect of insulin on the leg and splanchnic balance of glucose and gluconeogenic substrates after cardiac surgery

The effect of insulin and glucose infusion on the leg and splanchnic balance of glucose and the gluconeogenic substrates lactate, pyruvate, alanine and glycerol was studied in 13 patients directly after cardiac surgery. Insulin was infused continuously at a rate of 1.0 Unit/kg/hr during 1 hour. Sixteen patients served as a control group and received no insulin and glucose infusion. A significant increase in arterial lactate concentration in both the control group (from 1.40 +/- 0.19 to 1.68 +/- 0.24 mmol/l p less than 0.01) and the insulin group (from 1.58 +/- 0.27 to 2.07 +/- 0.22 mmol/l, p less than 0.05) was observed. The arterial pyruvate concentration was significantly increased (from 0.075 +/- 0.019 to 0.105 +/- 0.021 mmol/l, p less than 0.01) and glycerol was significantly decreased (from 0.15 +/- 0.03 to 0.12 +/- 0.03 mmol/l, p less than 0.05) during the insulin and glucose infusion. The alanine concentration was unchanged in both groups. Insulin and glucose infusion was followed by an increased leg uptake of glucose (from 0.15 +/- 0.14 to 1.06 +/- 0.16 mmol/min/100 ml, p less than 0.05), and by a changed splanchnic glucose balance (from -8.2 +/- 2.8 to +4.0 +/- 1.2 mmol/kg/min, p less than 0.01). A net leg release and at the same time a net splanchnic uptake of all gluconeogenic substrates was observed. The insulin and glucose infusion did not significantly change either the splanchnic balance or the leg balance of the gluconeogenic substrates.     

Impaired glucose tolerance is characterized by multiple abnormalities in the regulation of intermediary metabolism.

The responses of circulating intermediary metabolites to a low-dose sequential insulin infusion (basal, 0.005, 0.01, and 0.05 U kg-1 h-1) were assessed in eight non-obese men with Impaired Glucose Tolerance (IGT), and in eight healthy control subjects with normal glucose tolerance matched for age, gender, and body mass index. Fasting hyperinsulinaemia was observed in the subjects with IGT (7.4 +/- 1.0 vs 2.9 +/- 0.3 mU I-1, p less than 0.001). While there was no significant difference (p greater than 0.1) in fasting venous glucose levels between the groups, fasting concentrations of lactate (p less than 0.02), alanine (p less than 0.01), and glycerol (p less than 0.05) were significantly elevated in the subjects with IGT. During the incremental insulin infusion, overall concentrations of glucose (p less than 0.05), lactate (p less than 0.05), alanine (p less than 0.05), glycerol (p less than 0.05), immunoreactive insulin (p less than 0.001), and C-peptide (p less than 0.01) were significantly higher in the subjects with IGT. Linear dose-response relationships (p less than 0.005) for circulating immunoreactive insulin (log) vs metabolite concentrations were demonstrated by analysis of variance for glucose, non-esterified fatty acids (NEFA), glycerol, and total ketone bodies. For glucose, glycerol, and NEFA, group dose-response regression lines for the subjects with IGT were displaced significantly to the right (p less than 0.001 for each) of those for the normal control subjects, implying insulin insensitivity. In addition to the recognized defect in glucose homeostasis, these results indicate impaired regulation of multiple aspects of intermediary metabolism including lipolysis in IGT.     

Urinary sugar phosphates and related organic acids in fructose-1,6-diphosphatase deficiency

Summary Two sisters with fructose-1,6-diphosphatase deficiency are reported. They presented with ketonuria, elevated plasma transaminase activity and severe metabolic acidosis during hypoglycaemic crises, which resembled Reye syndrome. Intravenous fructose tolerance tests provoked severe hypoglycaemia and metabolic acidosis. Fructose-1,6-diphosphatase activities in both peripheral leukocytes and cultured lymphocytes were below the limit of detection. Urinary organic acid analysis during crises revealed markedly increased excretion of lactate, ketone bodies, glycerol and glycerol-3-phosphate. We newly identified other glycolytic intermediates, glyceraldehyde, 3-phosphoglycerate and fructose-1,6-diphosphate, in the urine during hypoglycaemic attacks or after fructose tolerance tests. Identification of such compounds may be useful in the early diagnosis of this disease.     

Diurnal plasma profiles of metabolite and hormone concentration in insulin-dependent diabetic patients during conventional insulin treatment and continuous subcutaneous insulin infusion. A controlled study

In addition to hyperglycaemia, derangement of metabolic and hormonal control may play an important role in the development of microvascular complications in diabetes. Little, however, is known about the impact of insulin pump treatment on metabolic and hormonal parameters. In a 6-month prospective randomized study in insulin-dependent diabetics we therefore investigated the effects of continuous subcutaneous insulin infusion by pump (10 patients) and conventional insulin treatment (10 patients) on the 24-h profiles of blood glucose, glycerol, lactate, 3-hydroxybutyrate, insulin, glucagon and growth hormone by measuring the respective concentrations every 2 h. We found that average blood glucose levels and HbA1c were significantly lower in the group treated by continuous subcutaneous insulin infusion as compared with the group on conventional insulin treatment. Furthermore, we observed an improvement in diurnal levels of lactate and 3-hydroxybutyrate in the pump-treated group which was not seen in the conventionally treated group. A slight increment in alanine was seen in the group treated with insulin pump. Serum growth hormone, glycerol, plasma free insulin as well as the daily insulin supply were unchanged and identical in the two groups. It is noteworthy that in the pump group, the decrease in blood glucose and 3-hydroxybutyrate takes place concomitantly with a significant suppression of glucagon.     

Chronically increased glucose uptake by adipose tissue leads to lactate production and improved insulin sensitivity rather than obesity in the mouse

Aims/hypothesis

In adipocytes, triacylglycerol synthesis depends on the formation of glycerol 3-phosphate, which originates either from glucose, through glycolysis, or from lactate, through glyceroneogenesis. However, glucose is traditionally viewed as the main precursor of the glycerol backbone and thus, enhanced glucose uptake would be expected to result in increased triacylglycerol synthesis and contribute to obesity.

Methods

To further explore this issue, we generated a mouse model with chronically increased glucose uptake in adipose tissue by expressing Gck, which encodes the glucokinase enzyme.

Results

Here we show that the production of high levels of glucokinase led to increased adipose tissue glucose uptake and lactate production, improved glucose tolerance and higher whole-body and skeletal muscle insulin sensitivity. There was no parallel increase in glycerol 3-phosphate synthesis in vivo, fat accumulation or obesity. Moreover, at high glucose concentrations, in cultured fat cells overproducing glucokinase, glycerol 3-phosphate synthesis from pyruvate decreased, while glyceroneogenesis increased in fat cells overproducing hexokinase II.

Conclusions/interpretations

These findings indicate that the absence of glucokinase inhibition by glucose 6-phosphate probably led to increased glycolysis and blocked glyceroneogenesis in the mouse model. Furthermore, this study suggests that under physiological conditions, when blood glucose increases, glyceroneogenesis may prevail over glycolysis for triacylglycerol formation because of the inhibition of hexokinase II by glucose 6-phosphate. Together these results point to the indirect pathway (glucose to lactate to glycerol 3-phosphate) being key for fat deposition in adipose tissue.     

Diurnal profiles of intermediary metabolites in insulin-dependent diabetes and their relationship to different degrees of residual B-cell function

Summary Diurnal profiles of blood metabolites, insulin and C-peptide were measured in 18 insulin dependent diabetics with less than 2-year duration of disease. Seventeen of the 18 patients had endogenous insulin secretion. A significant inverse correlation was found between mean C-peptide and mean blood glucose. Patients with maximal C-peptide equal to or above 0.30 pmol/ml had a better degree of control evaluated from mean blood glucose in spite of being treated with less insulin compared to patients with less B-cell function. The mean concentrations of alanine, lactate, glycerol and -hydroxybutyrate were not different in patients with CPR equal to or above 0.30 pmol/ml compared to patients with less CPR. The highest concentration in blood hydroxybutyrate for all the patients was seen in the fasting state, and the disappearance rate of blood hydroxybutyrate after breakfast was inversely correlated with the residual B-cell function. During the study period the mean insulin concentration in the diabetics and the normals was the same, but the brisk increase in insulin concentration related to meals was decreased or lost in the diabetics. Compared to normals the mean blood glucose, -hydroxybutyrate and glycerol were higher in the diabetics and the diurnal changes were grossly abnormal. No difference was found in alanine concentration and mean lactate concentration, but the increase in lactate seen after meals in normals was lost in the diabetics. The results confirm that other metabolites than glucose are abnormal in diabetics and that residual B-cell function has metabolic consequences for other metabolites than glucose during daily life conditions.