Glucose Flux Rates Research Articles

Increased GABA shunt activity in VMN of three hyperphagic rat models.

Metabolic signals have been proposed as controls of energy balance. Glucose utilization for gamma-aminobutyric acid (GABA) synthesis was evaluated in diet-restricted and streptozotocin-diabetic rats. Rates for glucose flux through the GABA shunt and glutamate decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis, were measured in ventro-medial nucleus (VMN) and lateral hypothalamic (LHA) homogenates. Rates of GAD activity were elevated in the VMN of both restricted and diabetic rats but did not change in the LHA of either model. The rate of glucose flux through the GABA shunt in the VMN was increased in restricted rats but were decreased in the VMN of diabetic rats. Restricted rats allowed to feed ad libitum initially ate 160% of the amount the control rats ate. GAD activity rates in the VMN returned to near control levels after 2 days of refeeding. Obese Zucker rats (fa/fa) also had higher rates of GAD activity and glucose flux through the GABA shunt in the VMN but not in the LHA. The increased rate of GABA synthesis may represent increased GABAergic activity in the VMN and may possibly be involved in feeding behavior and energy balance.

Effect of maternal glucose concentration on uteroplacental glucose consumption and transfer in pregnant sheep.

The present study was designed to measure the relationships between maternal arterial glucose concentration [( GI]A) and fetal arterial glucose concentration [( GI]a), uteroplacental glucose consumption (UPGC), and the rate of uteroplacental glucose transfer to the fetus (UPGT) in pregnant sheep in late gestation. [GI]A was controlled by a glucose clamp technique and the glucose flux rates of the uteroplacenta were quantified by the Fick principle. [GI]A varied from 1.81 to 154.7 mg/dl; [GI]a was directly related to [GI]A: [GI]a = 0.374 [GI]A + 1.81, r = 0.873, P less than 0.001. Fetal arterial blood oxygen content decreased with [GI]A (P less than 0.05) and fetal arterial blood lactate concentration increased with [GI]A (P less than 0.001). There was no significant effect of [GI]A on the rates of uteroplacental lactate production, uteroplacental oxygen consumption, fetal oxygen consumption, or uterine or umbilical blood flow. Both UPGC and UPGT were directly related to [GI]A: UPGC = -2.221 x 10(-3) chi 2 + 0.646 x -6.016, r = 0.80; UPGT = -1.208 x 10(-3) chi 2 + 0.405 x -2.416, r = 0.90. UPGC and UPGT were approximately parallel over the range of [GI]A studied (UPGC = 1.19 UPGT + 3.79, r = 0.764). These results demonstrate the importance of UPGC to maternal-fetal glucose homeostasis and indicate that factors regulating uteroplacental glucose consumption and transfer to the fetus become limiting at comparable levels of [GI]A and [GI]a. The estimated kinetic constants for UPGC represent the metabolism of glucose by the uteroplacental tissues, but the estimated kinetic constants for UPGT represent the metabolism of glucose by the fetus as well as the transfer of glucose by the uteroplacenta to the fetus.

The effect of prehepatic insulin administration on alanine flux rates in diabetic dogs.

The in vivo flux rates of glucose (6-3H-glucose) and of alanine (U-14C-alanine) were measured in insulin-dependent chronically diabetic dogs which were infused with insulin employing a bedside-type artificial B cell and either the peripheral or the portal venous route. In comparison with non-diabetic control animals the diabetic dogs had near-normal patterns of glucose metabolism and pancreatic glucagon regardless of the route of insulin administration. They also showed reduced basal portal but moderately elevated peripheral insulin levels on peripheral and near-normal peripheral values on portal insulin infusion. Both concentration and production rates of alanine were reduced on peripheral (0.142 +/- 0.016 mmol/l, 4.73 +/- 0.49 mumol.kg-1.min-1, p less than 0.05) but normal on portal insulin (0.206 +/- 0.030 mmol/l, 6.33 +/- 0.63 mumol.kg-1.min-1). The alanine clearance was slightly elevated or normal in the diabetic dogs, and the glucose production from alanine showed a strongly delayed response to an exogenous glucose load on either route of insulin administration. It is concluded that the peripheral hyperinsulinism during posthepatic insulin administration stimulates glucose utilisation to a normal extent, but inhibits the provision of amino groups in resting muscle. Alanine synthesis is thereby reduced, and the carbon moieties are shunted from glucose into circulating lactate. Long-term studies are needed to elucidate the role of the liver under these conditions.

Enzymes of glucose metabolism in cultured human gliomas: neoplasia is accompanied by altered hexokinase, phosphofructokinase, and glucose-6-phosphate dehydrogenase levels.

The enzymes of glycolysis and selected enzymes of the pentose phosphate pathways were measured by fluorometric methods in extracts prepared from cultures of normal cortical human astrocytes and from cultures derived from low-grade (II) or high-grade (IV) gliomas. The hexokinase and phosphofructokinase levels of the low-grade glioma-derived line were not significantly different from those of the normal astrocyte cultures. However, the activities of hexokinase and phosphofructokinase were consistently and significantly increased in the high-grade glioma-derived lines. The activity of glucose-6-phosphate dehydrogenase was significantly decreased in all glioma-derived lines and by more than 90% in the high-grade-derived lines. Other enzymes of the glycolytic pathway were not significantly different from those of normal astrocytes, or they showed a variation inconsistently related to the neoplastic state. Glucose flux is not apparently regulated to a significant degree of hexokinase in glioma-derived lines, since the measured Vmax values are in substantial excess over the measured flux rates. Reversible binding of hexokinase to the particulate fraction was observed in both the normal astrocytes cultures and the high-grade glioma-derived lines. A twofold displacement of particulate hexokinase by ATP, ADP, 1-O-methylglucose, sorbitol-6-phosphate, and dibutyryl cyclic AMP was observed in the high-grade glioma-derived lines. The degree of displacement by various agents and the basal ratio of free/bound was not significantly different between the transformers and the nontransformants. The hexokinase from both the gliomas and the normal astrocytes was noncompetitively inhibited by the glucose analogue 2-deoxy-d-glucose. Phosphofructokinase activity is close to the observed glucose flux rates in both the normal astrocyte and the glioma-derived cultures. The phosphofructokinase activity of normal astrocytes is activated twofold or more by ADP, AMP, fructose-2,6-diphosphate, and Pi. However, these same ligands activate phosphofructokinase by less than twofold in a typical high-grade glioma-derived line. ATP, dibutyryl cyclic AMP, and citrate inhibit glioma and normal astrocytic phosphofructokinase, but the magnitude of the inhibition is much less than in the glioma-derived lines.

Alterations in alanine metabolism in diabetic dogs during short-term treatment with an artificial B cell.

The flux rates of plasma glucose and alanine were studied isotopically (6-3H-glucose and U-14C-alanine simultaneously) in resting chronically diabetic dogs during short-term treatment with an artificial B cell where the insulin was infused into a peripheral vein. Despite perfect blood glucose control and normal glucose flux rates, the concentration and rates of appearance and disappearance of alanine were significantly elevated in the diabetic animals before, during and after an exogenous glucose load. The incorporation of the carbon moiety of alanine into circulating glucose was also increased, but diminished to a near-normal extent when exogenous glucose was given. The plasma clearance rates for alanine in the diabetic dogs were normal throughout the study. It is concluded that normal blood glucose control in diabetes does not necessarily mean normalization of the entire metabolic network. On the basis of peripheral hyperinsulinaemia alanine formation from glucose and branched chain amino acids is elevated in muscle. This may explain increased flux of alanine despite normal blood glucose control.

Sucralfate prevents experimental peptic esophagitis in rabbits

Sucralfate was tested in a rabbit model for its ability to prevent experimental esophagitis. Esophagitis was assessed by gross appearance and microscopic examination by an uninformed observer. In addition, the permeability of the esophagus to a number of probe molecules was measured to assess barrier function. Animals were exposed for 1 h to either acid alone (HCl at pH 2), acid plus pepsin (0.8 mg/ml), or acid plus taurocholic acid (5 mM), as well as to the same injurious agents with the addition of 1 g of sucralfate. At the completion of this hour, the perfusate was removed and all animals were again perfused for 1 h with HCl at pH 2 while mucosal permeability was assessed by measuring erythritol, glucose, potassium, and sodium fluxes. The animals were then killed. Sucralfate significantly diminished esophagitis and the attendant mucosal permeability changes induced by pepsin. The viscous sucralfate gel was shown to adhere tenaciously to the esophageal mucosa, but this characteristic of sucralfate was found not to be critical for its protective action because a clear sucrose sulfate solution with no gel present was also protective. Hence, it was not necessary for the gel to be present for the drug to be effective. Several in vitro tests suggested that the clear sucrose sulfate solution, like the sucralfate gel, probably acts through a topical protectant effect, rather than through pepsin inactivation. Although the degree of esophagitis induced by the bile acid was significantly less than that observed with pepsin, the mucosal permeability changes were comparable. Sucralfate did not significantly reduce the flux rates of glucose, potassium, and sodium nor did it affect the morphology of the mucosa after exposure to taurocholic acid. In conclusion, the binding of sucralfate to pepsin substrates in tissue results in this agent being very effective in preventing experimental peptic esophagitis.

Glucose metabolism by K+-limitedKlebsiella aerogenes: Evidence for the involvement of a quinoprotein glucose dehydrogenase

Like other members of the Enterobacteriaceae, Klebsiel la aerogenes transports glucose into the cell by means of a PEP-dependent phosphotransferase system (PTS; for review see [1]). This system has a high affinity for glucose and, not surprisingly, was found to be exceedingly active in cells of a slowly growing glucose-limited chemostat culture (apparent specific activity > 250 nmo l /min . (mg dry wt cells)-1 at D = 0 .1 /h [2]). Interestingly, the apparent activity of this system declined sharply as the growth rate was increased such that, at growth rates greater than 0.7/h, its activity was lower than that necessary to account for the actual rate of glucose uptake by the growing cells. More significantly, glucose-sufficient cultures (that were K +-, phosphateor ammonia-limited) consumed glucose at substantially higher rates than the corresponding glucose-limited culture, yet seemingly possessed uniformly low glucose-PTS activities. These discrepancies suggested that, at the higher growth rates a n d / o r with glucose-sufficient conditions, ancillary systems might contribute substantially to the overall rate of glucose flux, though such a supposition rested heavily on the assumption that the apparent glucose-PTS activity measured in vitro approximated closely to the actual activity of the undisturbed cells. However, the further observation that some glucose-sufficient cultures excreted gluconic and 2-ketoghiconic acid in substantial amounts [3] again pointed to the possibility of there being present in K. aerogenes processes of glucose catabolism that might not directly involve the glucose-PTS. Hence we undertook a search for this putative alternative glucosemetabolizing system which resulted in the finding of a quinoprotein glucose dehydrogenase (EC 1.1.99.17). To the best of our knowledge, this enzyme has not previously been reported to be present in K. aerogenes, or in related species.

Foetal and maternal rates of glucose production and utilization in chronically catheterized ditocous ewes.

1. Five well-nourished Scottish Blackface ewes carrying twin lambs were used. An umbilical artery and vein of each foetus in two sheep and one foetus in three sheep were catheterized at 109-112 days of gestation and both maternal jugular veins were catheterized 2 days before experiment. Two-day experiments were conducted between days 124 and 141 of gestation when [2-3H,U-14C]glucose was infused intravenously over 9 h into either the mother or one foetus, separate days being used for each infusion. 2. Plasma glucose specific radioactivities of the mother and infused foetus at plateau were used to determine the glucose flux rates within and between mother and foetus. The corresponding values for the uninfused foetus were calculated by using the relative weights of each twin foetus and were used to correct the maternal flux rates for the presence of the second foetus. 3. The net rate of glucose utilization by the foetus was 11.2 +/- 1.3 mg/min per kg of foetus (mean +/- S.E.M; n = 11). The corresponding rate in the mother was 1.1 +/- 0.1 mg/min per kg of live weight (n = 11). 4. The net rate of foetal uptake of glucose from the mother accounted for only 56 +/- 4% of the glucose used by the foetus. The remainder was provided by an input of 4.9 +/- 0.7 mg/min per kg of foetus to the foetal glucose pool via foetal gluconeogenesis. 5. The rate of glucose futile cycling in the foetus was high, equal to 95 +/- 19% of the net foetal glucose-utilization rate. 6. The rate of glucose utilization (mg/min per kg of foetus) decreased progressively over the observational period of 124-141 days of gestation. There was no increase in the rates of net foetal glucose uptake or of foetal gluconeogenesis to counter this decline.

Comparison of measurements of glucose flux rates in wethers and in pregnant and lactating ewes using 14C and 3H labelled tracers

Glucose flux rates (GFR) were determined, from plateau glucose specific radioactivity values obtained during continuous infusions of [U - 14C] + [2 - 3H], [U - 14C] + [3 - 3H] and [U - 14C] + [6 - 3H] glucose tracers, in wethers fed either 800 g per day of poor quality hay plus 400 g barley per day. GFR determined with [U - 14C] glucose ranged from 15 to 24 mmoles per hour on the hay diet and from 22 to 34 mmoles per hour on the hay plus barley diet. The variation in GRF within animals and between days accounted for most of the total variability. GFR determined with [6 - 3H] glucose were similar to those estimated with [U - 14C] glucose but those determined with [2 - 3H] and [3 - 3H] glucoses were 14 per cent higher. GFR were also determined with [U - 14C] + [6 - 3H] glucose tracers in pregnant ewes fed 1000 g medium quality hay plus 400 g concentrate and in lactating ewes fed different amounts of medium quality hay plus concentrate. GFR, determined with [U - 14C] glucose, ranged from 27 to 49 mmoles per hour in pregnancy and from 54 to 83 mmoles per hour in lactation. At approximately 125 days' gestation there was a significant positive relationship between GFR and total lamb birth weight. Estimates of GFR with [6 - 3H] glucose were 21 per cent higher in pregnancy and 15 per cent higher in lactation than the simultaneous estimates with [U - 14C] glucose.

Role of Glucagon in Diabetes

The glucagon-secreting A cell is a vital component of the organ system which regulates the distribution of fuel--the islets of Langerhans. Bihormonal control of glucoregulation through a push-pull system maintains the glucose concentration of extracellular fluid within narrow limits irrespective of glucose flux rates through relative equality of glucose influx and efflux. This equality requires appropriate secretion mixtures of the biologic antagonists, insulin and glucagon, directed by a glucose sensor. In severe diabetes, there are virtually no B cells and A cells are in contact largely with other A cells and their glucose-sensing capacity is lost. The A cell hypersecretes and in most juvenile type diabetics aggressive therapy with insulin fails to restore it to normal. Glucagon is a factor in the development of endogenous hyperglycemia, and ketoacidosis. Its suppression may provide a possible approach in the future pharmacologic management of diabetic hyperglycemia.

The exchange of glucose across the liver cell membrane.

A set of equations was worked out to estimate the rapidity of the outward penetration of glucose from the hepatic intracellular compartment into the plasma compartment. In dynamic steady states at different blood sugar levels the average rate constant was found to be 0.46 min−1. Thus the rate at which glucose leaves the liver cell is rapid, but not instantaneous. The flux rates of glucose across the liver cell membrane were calculated and were found to be approximately eight times higher than the rate of glucose production. Two non-metabolizable glucose analogues, L-glucose and 3-O-methylglucose, were found to be distributed in 85% of the total water content of the liver.

Coordination among Rate-limiting Steps of Glycolysis and Respiration in Intact Ascites Tumor Cells

Abstract The accumulation of lactate and phosphorylated glycolytic intermediates, as well as the variation in adenosine triphosphate, diphosphate, and 5'-monophosphate, were measured over the first 4 min after addition of 0.77 mm glucose to Ehrlich ascites carcinoma cells in 54 mm phosphate buffer (pH 7.4) at 370 and 300. The rates of glucose flux through the phosphofructokinase step (Vpfk) and the lactate dehydrogenase step (Vldh) were calculated and compared with each other and the rate of oxygen consumption (Vr), measured by means of a bare, vibrating reed platinum electrode. During the period from 10 to 90 set or more after glucose addition, both Vr and Vpfk are linearly related to Vldh; at 370, Vpfk = 2.5 (Vldh - 1.1) and Vr = 0.38 (Vldh), where velocities are measured in micromoles per ml of cells per min; at 300, Vpfk = 4.1 (Vldh - 1.3) and Vr = 0.32 (Vldh). These results suggest a common controlling mechanism for all three rates. During the period from 10 to 60 sec approximately linear relationships between Vpfk, Vldh, and Vr and the ADP concentration are observed, but the slopes and intercepts of the lines vary considerably from one experiment to another unlike the relationships among the velocities themselves, which are almost identical in all experiments at the same temperature. In a given experiment, both Vldh and Vr extrapolate to the same ADP concentration (0 to 1.0 µmole per ml of cells) at zero velocity, which implies that if ADP is governing these rates only a fraction of the ADP may be involved in the control, but the same fraction or pool controls both respiration and glycolysis. The velocities are also approximately linear functions of esterified phosphate concentration. Since the increase in esters such as fructose diphosphate probably occurs at the expense of intracellular inorganic phosphate, this observation could be consistent with control of the rates by inorganic phosphate. The kinetic data do not identify either ADP or inorganic phosphate as the major controlling agent, but they do indicate a close coordination among the initial segment of glycolysis, the terminal segment of glycolysis, and respiration which is not fully explainable in terms of variations in either the ADP or phosphate levels in the whole cell.