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At least 37 records · Page 2Linked to original sources
PubMed1990

Hexose metabolism in pancreatic islet cells: the coupling between hexose phosphorylation and mitochondrial respiration.

The possible relevance of D-glucose phosphorylation by mitochondria-bound hexokinase to the control of respiration was examined in mitochondria prepared from either tumoral pancreatic islet cells (RINm5F line) or normal rat liver. In both systems, ATP generated by mitochondria exposed to ADP and succinate could serve as a substrate for the phosphorylation of D-glucose. However, after exposure to exogenous ADP in the presence of succinate, only mitochondria isolated from RINm5F cells displayed a sizeable increase in O2 consumption in response to a subsequent administration of D-glucose. In this respect, the discrepancy between mitochondria from islet cells and liver, respectively, was found to be attributable to the much lower hexokinase activity, relative to respiratory rate, in liver than in RINm5F cell mitochondria. It is speculated that the coupling between hexose phosphorylation and respiration in islet cells may prime the mitochondria to generate ATP during the early metabolic and secretory response to a rise in extracellular D-glucose concentration.

Adenosine Diphosphate↗
PubMed1983

Preference of glucokinase for the alpha anomer of hexose: relation to alpha-anomer preference in hexose-induced insulin release by pancreatic islets.

The alpha anomers of glucose and mannose were more rapidly phosphorylated than the beta anomers by rat liver glucokinase, whose chromatographic and kinetic properties are known to be quite similar to those of pancreatic islet glucokinase. This result may explain our previous findings of the alpha-anomeric preference in glucose- and mannose-stimulated insulin release, and therefore suggests to support the hypothesis that glucokinase in islets functions as a crucial hexose sensing enzyme for insulin release induced by glucose and mannose.

Animals↗
PubMed1971

Effects of hexoses and anions on the erythritol permeability of human red cells.

1. The effect of hexoses and of the anions chloride, thiocyanate, and salicylate on the permeability of human red cells to [(14)C]erythritol has been studied.2. It was confirmed that erythritol competes with glucose, mannose, and galactose for the hexose transfer system of the red cell membrane. Approximately 25% of the erythritol influx was insensitive to the presence of hexoses or phloretin. Identical maximum degrees of inhibition were obtained with 0.3 M glucose and with phloretin (0.5 x 10(-3)M). In the absence of competing inhibitors the erythritol permeability, P, was 1.2 x 10(-7) cm/sec at 38 degrees C. At maximum inhibition P was 0.3 x 10(-7) cm/sec.3. Erythritol is able to penetrate the membrane by two pathways, only one of which is sensitive to hexoses. Both hexose-sensitive and hexose-insensitive erythritol influx are well described by first-order diffusion kinetics. The affinity of erythritol for the hexose transfer system is very low, and the half saturation constants of hexoses can be determined from their ability to retard erythritol permeation. The following values were found for the half saturation of the transport system with hexoses at 38 degrees C: glucose 6 mM, mannose 11 mM, and galactose 40 mM.4. Thiocyanate and salicylate reduce the hexose-sensitive fraction of erythritol influx, but the hexose-insensitive erythritol permeability is not affected when chloride is replaced by the foreign anions. This applies to the whole temperature range between 0 and 38 degrees C, where the ionic permeabilities of red cells have been shown to be profoundly changed by thiocyanate and salicylate.

Alcohols↗
PubMed1987

Forskolin inhibition of hexose transport in cardiomyocytes.

The effects of insulin, forskolin, isoproterenol, and epinephrine on 3-O-methylglucose (hexose) transport and cell cyclic AMP levels were determined in adult rat cardiomyocytes. Insulin stimulated hexose transport in these cells an average of 2.5-fold. Initial hexose transport rates at 1 mM hexose were 3.75 X 10(-2) nmol/mg cell protein/second in the absence of insulin, and 8.25 X 10(-2) nmol/mg cell protein/second in the presence of 12.3 microM insulin. Forskolin at 5 microM nearly abolished hexose transport within 3 s of exposure, but did not increase cell cyclic AMP concentrations within 9 s. The apparent Ki for hexose transport inhibition was about 0.3 microM forskolin. Epinephrine and isoproterenol at 50 microM increased cell cyclic AMP 4-fold during 9 s exposure, but did not affect hexose transport. Treatment of cells with these catecholamines of forskolin for up to 99 s increased cell cyclic AMP, but only forskolin inhibited hexose transport. We conclude from these results that forskolin acts on hexose transport independent of its action on adenyl cyclase, and that cyclic AMP does not inhibit or stimulate hexose transport.

3-O-Methylglucose↗
PubMed1991

Absorption of hexose and pentose sugars in vivo in perfused intestinal segments in the fowl.

1. Rates of absorption of two hexose (D-glucose and D-galactose) and two pentose (D-xylose and D-arabinose) sugars were measured by in vivo perfusion, in jejunum, ileum and (distal) caecum, in immature hens conditioned to either a standard (ST) or "high fibre" (ST + 20% grass) diet. 2. Each bird was tested in one intestinal segment with all four (U-14C-labelled, 10 mM) sugars, with either the hexoses preceding the pentoses or vice versa. 3. With all treatments, absorption rates of the hexoses were alike, as were those of the pentoses. Hexose absorption was twice as fast as pentose absorption in jejunum and ileum with both dietary pretreatments, whereas in caecum hexose and pentose rates were similarly high, except when pentose (and its associated fluid transfer) was apparently inhibited by prior hexose absorption with the ST diet. 4. With the ST diet, hexose absorption (per unit length and dry weight) was faster in caecum than in jejunum and ileum, and pentose absorption was also fastest in caecum when all pentose data from testing after hexose were excluded. 5. With the ST/grass diet, hexose absorption was faster in jejunum than in ileum and caecum when expressed per unit length, and pentose absorption was fastest in caecum on a dry weight basis. 6. Hexose absorption was faster in jejunum and slower in caecum with the ST/grass pretreatment than with ST. However, the dietary comparison was not conclusive because it involved birds form (two) different hatches (of similar age and weight) tested at different times.

Absorption↗
PubMed1984

Mechanism of antibody stimulation of hexose transport in rat myoblasts.

We have recently demonstrated that exposure of rat myoblasts to anti-rat myoblast antiserum results in two- to three-fold activation of hexose transport. The present communication reports the possible mechanism(s) by which specific antibody can bring about such activation. Studies with Fab and Fc fragments indicate that the binding of Fab to specific cell surface component(s) is not sufficient to trigger activation of hexose transport; the immunoglobulin G (IgG) mediated dimerization of membrane components is required for this process. Although cytochalasin D has no effect on hexose transport in control and antibody-treated cells, pretreatment of cells with this inhibitor prevents antibody-mediated activation of hexose transport. It may be inferred from this observation that proper disposition of membrane components is required for the dimerization of membrane receptors. Since this activation of hexose transport is an irreversible process, it is possible that covalent modification of membrane components may have occurred as a result of antibody treatment. Pretreatment of cells with ammonium chloride or methylamine is found to abolish the antibody-mediated activation of hexose transport, even though these inhibitors have no effect on hexose transport in control and antibody-treated cells. These inhibitors may be acting on transglutaminase and (or) on some other proteins involved in the activation process. Several lines of evidence suggest that limited proteolytic cleavage of membrane components may be involved in the antibody-mediated activation of hexose transport. First, pretreatment with several protease inhibitors prevents activation of hexose transport. Second, several cell surface proteins are missing in antibody-treated cells. Third, limited proteolysis of cell surface proteins with trypsin can also bring about activation of hexose transport. In view of the fact that proteolytic activity cannot be detected in various IgG and serum preparations, it seems likely that endogenous membrane associated proteases may be involved in this activation process.

Ammonium Chloride↗
PubMed1986

Approaches used to examine the mechanism and regulation of hexose transport in rat myoblasts.

This review discusses some of the approaches and general criteria that we have used to examine the properties of the hexose transport system in undifferentiated L6 rat myoblasts. These approaches include studying the kinetics of hexose transport in whole cells and plasma membrane vesicles, the effects of various inhibitors on hexose transport, the isolation and characterization of hexose transport mutants, and the use of cytochalasin B (CB) to identify the transport component(s). Transport kinetics indicated that two transport systems are present in these cells. 2-Deoxy-D-glucose is transported primarily by the high affinity system, whereas 3-O-methyl-D-glucose is transported by the low affinity system. Furthermore, these two transport systems are inactivated to different extents by CB. CB has a higher binding affinity for the low affinity hexose transport system. The inhibitory effect of various hexose analogues also revealed the presence of two hexose transport systems. The effects of various ionophores and energy uncouplers on hexose transport suggest that the high affinity system is an active transport process, whereas the low affinity system is of the facilitated diffusion type. The high affinity system is also sensitive to sulfhydryl reagents, whereas the low affinity system is not. Further evidence for the presence of two transport systems comes from the characterization of hexose transport mutants. Two of the mutants isolated are shown to be defective in the high affinity transport system, but not in the low affinity transport system. These mutants are also defective in the CB low affinity binding site. Based on our results a tentative working model for hexose transport in L6 rat myoblasts is presented.

Animals↗