In vitro glucose and 2-aminoisobutyric acid uptake by rat interscapular brown adipose tissue

The dependence upon substrate and insulin concentrations, as well as on sodium and potassium concentrations in the medium of the uptake of glucose and 2-aminoisobutyric acid, was determined for fragments of brown and white adipose tissues incubated in vitro. Brown adipose tissue showed a high capacity for glucose uptake at high glucose concentrations, this uptake being dependent on both glucose and insulin concentration. White adipose tissue showed much more limited uptake capabilities. The presence of Na+ and K+ had little effect on the uptake. The uptake of 2-aminoisobutyric acid was similar in both adipose tissues, being enhanced by physiological levels of insulin and depressed by ouabain. This amino acid transport was dependent on Na+ and K+ concentrations, and the overall transporting capability was two to three orders of magnitude lower than that for glucose. It was concluded that amino acids could not play a significant role as bulk thermogenic substrates for brown adipose tissue, as their transporters lack the plasticity of response to high substrate and insulin concentrations which characterize brown adipose tissue uptake of glucose.     

The allosteric regulation of hexokinase C from amphibian liver.

A type C hexokinase (ATP:D-hexose-6-phosphotransferase EC 2.7.1.1) was partially purified from the liver of the frog Calyptocephalella caudiverbera. The enzyme is inhibited by glucose levels in the range of normal blood sugar concentrations. The extent of the inhibition by glucose depends on the concentration of ATP, being most marked between 1 and 5 mM ATP. Fructose, although a substrate, was not inhibitory of its own phosphorylation. The inhibitory effect of high glucose levels exhibited a strong, reversible pH dependence being most marked at pH 6.5. At pH 7.5 the inhibition by high glucose levels was a function of the enzyme concentration, the effect being stronger at high enzyme concentrations, whereas no inhibition was observed when assaying very diluted preparations. At all enzyme concentrations studied, high levels of glucose caused no inhibition at pH 8.5, whereas at pH 6.5 strong inhibition was always observed. Short times of photooxidation of hexokinase C as well as incubation with low concentrations of p-chloromercuribenzoate resulted in the loss of the inhibition by excess of glucose. Glucose-6-phosphate was found to be a strong inhibitor of hexokinase C but only at high glucose levels. The inhibitory effect of glucose-6-P follows sigmoidal kinetics at low (about 0.02 mM) glucose concentrations, the Hill coefficient being 2.3. The kinetics of the inhibition became hyperbolic at high (greater than 0.2 mM) glucose levels. These results suggest that the inhibition of hexokinase C by excess glucose is due to the interaction of glucose with a second, aldose-specific, regulatory site on the enzyme. The modification of the inhibitory effect by ATP, glucose-6-P, enzyme concentration, and pH, all of them at physiological levels, indicates a major role for hexokinase C in the regulation of glucose utilization by the liver.     

Glucose tolerance during long term treatment with a somatostatin analogue.

Seven patients with active acromegaly were treated with SMS 201-995, an analogue of somatostatin, for one year, the maximum dose being 100 micrograms three times a day. Three patients had impaired glucose tolerance before treatment, due to insulin resistance in two and insulin deficiency in one. In all patients treatment with the analogue slightly increased postprandial glucose concentrations and suppressed insulin concentrations for two to two and a half hours after each injection; growth hormone concentrations decreased progressively with treatment. The patient with impaired glucose tolerance due to insulin deficiency developed diabetes mellitus after four months'' treatment; concomitant treatment with glibenclamide resulted in a decreased glucose concentration and increased insulin concentration. This analogue of somatostatin had only minor side effects on glucose tolerance in patients with acromegaly and may be used in patients with impaired glucose tolerance provided that glucose concentrations are monitored closely.     

Diurnal changes in the glucose and glycogen levels of the blood and liver of rats in chronic consumption of alcohol and after its withdrawal

Chronic ethanol ingestion by rats exerts almost no effect on the diurnal rhythms of the blood and hepatic glucose concentrations. The rhythm of liver glycogen alters substantially in ethanol-fed animals, the phase of the rhythm being shifted and the daily mean level of glycogen being reduced by a factor of 2. Much more drastic disturbances in carbohydrate metabolism occur after ethanol withdrawal than with ethanol consumption. The diurnal rhythm of liver glycogen becomes inverted in phasing, and the rhythmic amplitude reduced greatly as compared with controls. Both the blood and hepatic glucose concentrations are maintained at nearly constant levels for 18-21 h after ethanol withdrawal, but then the level of blood glucose sharply falls, while that of hepatic glucose somewhat increases. The liver cytosolic water/blood plasma water gradient of glucose 24 h after ethanol withdrawal achieves a value of 4 and remains low 24 h later. The liver glycogen level remains relatively high over the 24 h period after ethanol withdrawal despite the severe hypoglycemia, that can be a result of a limitation of the liver cell membrane permeability for glucose.     

Experimental alteration of litter sex ratios in a mammal

Adaptive theory predicts that mothers would be advantaged by adjusting the sex ratio of their offspring in relation to their offspring's future reproductive success. Studies investigating sex ratio variation in mammals, including humans, have obtained notoriously inconsistent results, except when maternal condition is measured around conception. Several mechanisms for sex ratio adjustment have been proposed. Here, we test the hypothesis that glucose concentrations around conception influence sex ratios. The change in glucose levels resulted in a change in sex ratios, with more daughters being born to females with experimentally lowered glucose, and with the change in glucose levels being more predictive than the glucose levels per se. We provide evidence for a mechanism, which, in tandem with other mechanisms, could explain observed sex ratio variation in mammals.     

Home blood glucose concentrations in maturity-onset diabetes.

Blood glucose concentrations during normal daily activities were measured in 106 patients with maturity-onset diabetes from capillary blood samples collected on to filter paper. Samples were taken before and two hours after main meals, before going to bed, and, in 51 cases, during the night. Fasting and mid-morning values were closely correlated with the mean values over 24 hours irrespective of the type of anti-diabetic treatment being given. Postprandial blood glucose concentrations remained below 11.5 mmol/l (207 mg/100 ml) when the fasting blood glucose value was 7.0 mmol/l (126 mg/100 ml) or less, and repeated fasting blood glucose values exceeding 7.0 mmol/l were associated with raised blood glycosylated haemoglobin concentrations. Diabetic control in maturity-onset diabetes may be satisfactorily monitored by regular measurement of fasting or mid-morning blood glucose values.     

Comparison of glucose and insulin concentrations in macaque sera and plasma

The glucose and insulin concentrations in blood from Macaca nigra and M. mulatta were determined after an overnight fast and 3 min after a glucose infusion. Blood treatment included clotting for serum or additions of fluoride, heparin, or heparin plus fluoride. Samples were centrifuged at 0 min or after being held at 22 degrees C for 20, 40, or 60 min. Levels of glucose and insulin generally agreed within 1 SD at all times examined for samples removed at 0 or 3 min.     

In vivo investigations of glucose transport in Saccharomyces cerevisiae

In the present study, the glucose transport into the yeast Saccharomyces cerevisiae has been investigated. The approach suggested is based on a rapid sampling technique for studying the dynamic response of the yeast to rapid changes in extracellular glucose concentrations. For this purpose a concentrated glucose solution has been injected into a continuous culture at steady state growth conditions resulting in a shift of the extracellular glucose level. Samples have been taken every 5 s for determination of extracellular glucose and intracellular glucose-6-phosphate concentrations. Attempts to fit the experimental observations with simulations from existing models failed. The mechanism then proposed is based on a facilitated diffusion of glucose superimposed by an inhibition of glucose-6-phosphate. The use of the so-called in vivo approach suggested in this article appears to be proper, because the investigations can be performed at defined physiological states of the microbial cultures. Furthermore, the experimental observations are not being corrupted by the preparation of the samples for the transport studies as it happens during radioactive measurements. (c) 1996 John Wiley & Sons, Inc.     

Transport and metabolism of D-glucose in human adipocytes. Studies of the dependence on medium glucose and insulin concentrations

Uptake and metabolism of the physiologically labelled D-glucose (D-[U-14C]glucose) has been characterized in human adipocytes at several unlabelled D-glucose concentrations in the absence and presence of insulin. Following a 90 min incubation, about 80% of the intracellular radioactivity was incorporated into total lipids at tracer glucose concentration, as well as at higher glucose concentrations in basal and insulin-stimulated cells, whereas 20% was recovered as hydrophilic metabolites. The only 14C-labelled metabolite escaping the cells in detectable amounts was CO2, which accounted about 4%. At trace glucose concentrations (5 mumol/l), the rate of glucose uptake was linear with time. Comparative studies of initial glucose uptake after 10 s and tracer D-glucose conversion to total lipids after 90 min showed high coefficients of correlation between basal rates (r = 0.87), maximal response above basal level to insulin (r = 0.92) and insulin sensitivity (r = 0.78). Thus, under these conditions glucose transport is rate-limiting for net glucose uptake, and measurements over long time intervals of rates for total cell-associated radioactivity or lipogenesis may serve as reliable estimates of initial glucose influx rates. However, the conversion rate of tracer glucose to metabolites decreased progressively with the glucose concentration and with an apparent Km of about 0.2 mmol/l. The three metabolic pathways exhibited similar percentage decreases in their activities, suggesting that a common enzymatic step is rate-limiting. In comparison, the Km for initial D-glucose uptake rate was about 7 mmol/l. Hence, the capacity for total glucose metabolism comprised only a small fraction of the glucose transport capacity at medium glucose concentrations above tracer concentrations. Both basal, half-maximal and maximal insulin-stimulated rates of adipocyte glucose utilization were dependent on the glucose concentration. Thus, comparing lipogenesis at tracer and at 0.5 mmol/l medium glucose concentration, it was shown that the higher medium glucose concentration was associated with a 60% lowering of the basal rate, a 35% reduction in the percentage response above baseline to maximal insulin stimulation and a 4-fold increase in the insulin sensitivity. Obviously, these findings reflect some intracellular step(s) being rate-limiting at medium glucose levels above tracer values.     

Nonaggregating mutant of recombinant human hexokinase I exhibits wild-type kinetics and rod-like conformations in solution.

Hexokinase I governs the rate-limiting step of glycolysis in brain tissue, being inhibited by its product, glucose 6-phosphate, and allosterically relieved of product inhibition by phosphate. On the basis of small-angle X-ray scattering, the wild-type enzyme is a monomer in the presence of glucose and phosphate at protein concentrations up to 10 mg/mL, but in the presence of glucose 6-phosphate, is a dimer down to protein concentrations as low as 1 mg/mL. A mutant form of hexokinase I, specifically engineered by directed mutation to block dimerization, remains monomeric at high protein concentration under all conditions of ligation. This nondimerizing mutant exhibits wild-type activity, potent inhibition by glucose 6-phosphate, and phosphate reversal of product inhibition. Small-angle X-ray scattering data from the mutant hexokinase I in the presence of glucose/phosphate, glucose/glucose 6-phosphate, and glucose/ADP/Mg2+/AlF3 are consistent with a rodlike conformation for the monomer similar to that observed in crystal structures of the hexokinase I dimer. Hence, any mechanism for allosteric regulation of hexokinase I should maintain a global conformation of the polypeptide similar to that observed in crystallographic structures.     

Effect of liver glycogen content on glucose production in running rats

The influence of supranormal compared with normal hepatic glycogen levels on hepatic glucose production (Ra) during exercise was investigated in chronically catheterized rats. Supranormal hepatic glycogen levels were obtained by a 24-h fast-24-h refeeding regimen. During treadmill running for 35 min at a speed of 21 m/min, Ra and plasma glucose increased more (P less than 0.05) and liver glucogen breakdown was larger in fasted-refed compared with control rats, although the stimuli for Ra were higher in control rats, the plasma concentrations of insulin and glucose being lower (P less than 0.05) in control compared with fasted-refed rats. Also, plasma concentrations of glucagon and both catecholamines tended to be higher and muscle glycogenolysis lower in control compared with fasted-refed rats. Lipid metabolism was similar in the two groups. The results indicate that hepatic glycogenolysis during exercise is directly related to hepatic glycogen content. The smaller endocrine glycogenolytic signal in face of higher plasma glucose concentrations in fasted-refed compared with control rats is indicative of metabolic feedback control of glucose mobilization during exercise. However, the higher exercise-induced increase in Ra, plasma glucose, and liver glycogen breakdown in fasted-refed compared with control rats indicates that metabolic feedback mechanisms are not able to accurately match Ra to the metabolic needs of working muscles.     

Uptake of Glucose and Phosphorus by Growing Colonies of Fusarium oxysporum as Quantified by Image Analysis

Olsson, S. 1994. Uptake of glucose and phosphorus by growing colonies of Fusarium oxysporum as quantified by image analysis. Experimental Mycology 18, 33-47. The simplest of all heterogeneous environments for fungal colony growth is the petri dish with an agar medium. As the colony grows there will be a depression of nutrient concentrations under the colony caused by the uptake of nutrients by the growing colony. Image analysis methods have been developed for measuring medium concentrations of glucose and phosphorus with simultaneous biomass density determinations in agar systems. Maps of the concentrations in the agar medium under the colony and of colony biomass density were produced. A new method for weighing fungal colonies grown on agar is also presented. For Fusarium oxysporum phosphorus and glucose uptake from the medium was the same irrespective of the C/mineral ratios in the medium within the measured range of ratios. Even the concentration profiles of the nutrients under the colony were the same irrespective of nutrient ratios. Distribution of biomass density was affected by differences in glucose concentrations, being highest at the colony margin at the lower concentrations. The results indicate that the fungal colony is able to take up nutrients at the margin in excess of the local needs.     

Effect of glucose and ammonium nitrogen concentrations on riboxine biosynthesis

The effect of glucose and ammonium nitrogen concentrations on the riboxine biosynthesis by the culture Bacillus subtilis GEN 265 was investigated. The purpose of the investigation was to optimize the process with respect to these parameters. The investigation was carried out in a 1 m3 fermenter. Glucose and ammonium nitrate were added for various fermentation times. It was shown that riboxine biosynthesis did not require glucose dosing in the course of the process, with the optimal concentration being 13-15%. Optimal concentrations of ammonium nitrogen were determined. It was found that the dose of 0.35 mg/ml provided a 24% increase in the yield.     

Characteristics of Ba2+-stimulated insulin release with special reference to pancreatic beta-cells sensitized by cyclic AMP

Ca2+-dependent processes are activated by Ba2+ in a variety of biological systems. When Ca2+ was replaced by equimolar amounts of Ba2+ there was a marked increase in insulin secretion from beta-cell-rich pancreatic islets microdissected from ob/ob-mice. At both 3 and 20 mM glucose Ba2+ stimulated insulin release in a concentration-dependent manner, being less stimulatory at high concentrations. The stimulatory effect of Ba2+ on insulin release is similar to that of Ca2+ in being more pronounced and reached at lower concentrations when the beta-cells were sensitized by cyclic AMP. However, both glucose oxidation and utilization were suppressed when Ca2+ was replaced by equimolar amounts of Ba2+. Ba2+-stimulated insulin release resembled physiological secretion initiated by Ca2+ in being inhibited by L-epinephrine, pentobarbital and a low oxygen tension.     

A model for glucose-induced wave propagation in pancreatic islets of Langerhans

A reaction-diffusion type model is constructed, describing the spatio-temporal dynamics of the basic intracellular variables assumed to be involved in the initiation of the insulin secretion process by beta -cells in the pancreatic islets of Langerhans. The model includes equations for the electric membrane potential of the cells, with respective kinetics for ionic currents, for concentrations of both free and stored intracellular calcium, and for the intra- and extracellular concentrations of glucose. An empirical expression connecting the equation for the intracellular glucose concentration to the electrical equation is introduced. The model reproduces the events observed in experiments in vitro upon external glucose application to the islets of Langerhans, such as usual bursting oscillations of the membrane potential and corresponding oscillations of the intracellular calcium concentration. It also allows simulation of electric wave propagation through the islet, initiated by the spatial gradient of glucose concentration within the islet. The gradient emerges due to glucose diffusing into the islets from the external medium, being high at the edges. The latter results show that glucose diffusion presents a means for wave initiation in the islets, which supports our previous assumption (Aslanidi et al., 2001).     

UPTAKE OF α-METHYL-D-GLUCOSIDE BY SYNAPTOSOMES FROM RAT BRAIN

Influx of α-methylglucoside into synaptosomes prepared by differential and Ficoll density gradient centrifugation was studied to determine whether this sugar could be used as a model for glucose transport in nerve endings. The rate of uptake of a-methylglucoside was linear over a wide range of substrate concentrations. Influx was only slightly inhibited (12–15%) in the presence of glucose, 2-deoxyglncose, phloretin and 2,4-dinitrophenol and was unaffected by galactose or phlorizin. Conver- sion of a-methylglucoside to phosphorylated intermediates in synaptosomes was negligible. The data are consislent with the influx of a-methylglucoside being primarily a diffusion process or being mediated by a system with an extremely high K_m. However, it is possible that a small portion of the sugar may be transported by the low affinity glucose transport system. The results indicate that a-methylgluco- side is not a good model for glucose transport in synaptosomes as it is in other tissues.     

Effect of chronic hypoglycaemia on glucose concentration and glycogen content in rat brain: A localized 13C NMR study

While chronic hypoglycaemia has been reported to increase unidirectional glucose transport across the blood-brain barrier (BBB) and to increase GLUT1 expression at the endothelium, the effect on steady-state brain d-glucose and brain glycogen content is currently unknown. Brain glucose and glycogen concentrations were directly measured in vivo using localized 13C magnetic resonance spectroscopy (MRS) following 12-14 days of hypoglycaemia. Brain glucose content was significantly increased by 48%, which is consistent with an increase in the maximal glucose transport rate, Tmax, by 58% compared with the sham-treated animals. The localized 13C NMR measurements of brain glucose were directly validated by comparison with biochemically determined brain glucose content after rapid focused microwave fixation (1.4 s at 4 kW). Both in vivo MRS and biochemical measurements implied that brain glycogen content was not affected by chronic hypoglycaemia, consistent with brain glucose being a major factor controlling brain glycogen content. We conclude that the increased glucose transporter expression in chronic hypoglycaemia leads to increased brain glucose content at a given level of glycaemia. Such increased brain glucose concentrations can result in a lowered glycaemic threshold of counter-regulation observed in chronic hypoglycaemia.     

Carbohydrate metabolism in thyrotoxicosis: studies on insulin secretion before and after remission from the hyperthyroid state.

The effects of thyrotoxicosis on insulin secretion were studied in 11 patients with Graves' disease and compared with the results obtained in 6 of the patients after they had attained clinical remission. Constant glucose infusion (CGI) and acute tolbutamide infusion (AIT) tests were chosen to investigate beta-cell function. For similar means of fasting plasma glucose, basal plasma insulin mean was significantly higher during thyrotoxicosis. During AIT, mean peak insulin was obtained earlier in the hyperthyroid state (2 min) than after remission (4 min), being its level higher in the hyperthyroid state. During CGI, early insulin responses to similar plasma glucose increments, were comparable for both hyper and euthyroid subjects. After 10 minutes of CGI, insulin concentrations in the hyperthyroid state did not increase as in the euthyroid state in spite of comparable increments of plasma glucose, being similar plateau insulin levels attained thereafter. These results suggest the presence of an insulin-resistant state in hyperthyroidism which may either disappear during a chronic glucose infusion and/or be accompanied by a deficient late glucose-induced insulin release.     

Glucose metabolism in insulin-producing tumoral cells

Homogenates of insulin-producing tumoral cells catalyzed the phosphorylation of glucose, mannose, and fructose. The kinetics of phosphorylation at increasing glucose concentrations, the inhibitory effect of glucose 6-phosphate, and the comparison of results obtained with distinct hexoses indicated the presence of both low-Km hexokinase-like and high-Km enzymatic activities, the results being grossly comparable to those collected in normal pancreatic islets. Relative to protein content, the glucose-phosphorylating enzymatic activity was higher in tumoral than normal islet cells. The activity of other enzymes was either lower (glutamate dehydrogenase), moderately higher (phosphoglucomutase, lactate dehydrogenase) or considerably greater (ornithine decarboxylase) in tumoral than in normal islet cells. In intact tumoral cells, incubated under increasing glucose concentrations, the oxidation of D-[U-14C]glucose and the output of lactic and pyruvic acids reached a close-to-maximal value at 2.8 mM glucose. The ratios for glucose oxidation/utilization and lactate/pyruvate output were much lower in tumoral than in normal islet cells. Although glucose caused a modest increase in insulin output from the tumoral cells, this effect was saturated at a low glucose concentration (2.8 mM) and less marked than that of other secretagogues (e.g., L-leucine, L-ornithine, or forskolin). Thus, despite a close-to-normal enzymatic equipment for glucose phosphorylation, the tumoral cells displayed severe abnormalities in the metabolism and secretory response to this hexose. These findings point to regulatory mechanisms distal to glucose phosphorylation in the control of glucose metabolism in insulin-producing cells.