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A Carruthers

Publications and source records attributed to A Carruthers.

At least 91 records · Page 5Linked to original sources
PubMed1987

Equilibrium ligand binding to the human erythrocyte sugar transporter. Evidence for two sugar-binding sites per carrier.

Equilibrium [3H]cytochalasin B binding to class I sites of human red cell membranes (the sugar transporter) was examined in the presence and absence of intracellular or extracellular sugars known to interact with the transport system. D-Glucose, a transported sugar, is without effect on cytochalasin B binding when present in the extracellular medium but is an effective inhibitor of binding when present within the cell. Ethylidene glucose and maltose (reactive but nontransported sugars) inhibit cytochalasin B (CCB) binding when present either outside or inside the red cell. Inhibition by intracellular sugar (Si) is of the simple, linear competitive type. Inhibition by extracellular sugars (So) is more complex; the Kd(app) for cytochalasin B binding increases in a saturable fashion with [So]. These observations are compared with the predictions of the one-site, alternating conformer model and the two-site model for substrate binding to the sugar transporter, X. The experimental results are inconsistent with the one-site model but are explained by a two-site model in which the ternary complexes of So . X . Si or So . X . CCBi exist and where the binding sites for So and Si display negative cooperativity when occupied by nontransported substrate and little or no cooperativity when occupied by the transported species, D-glucose.

Binding Sites↗
PubMed1986

ATP regulation of the human red cell sugar transporter.

Purified human red blood cell sugar transport protein intrinsic tryptophan fluorescence is quenched by D-glucose and 4,6-ethylidene glucose (sugars that bind to the transport), phloretin and cytochalasin B (transport inhibitors), and ATP. Cytochalasin B-induced quenching is a simple saturable phenomenon with Kd app of 0.15 microM and maximum capacity of 0.85 cytochalasin B binding sites per transporter. Sugar-induced quenching consists of two saturable components characterized by low and high Kd app binding parameters. These binding sites appear to correspond to influx and efflux transport sites, respectively, and coexist within the transporter molecule. ATP-induced quenching is also a simple saturable process with Kd app of 50 microM. Indirect estimates suggest that the ratio of ATP-binding sites per transporter is 0.87:1. ATP reduces the low Kd app and increases the high Kd app for sugar-induced fluorescence quenching. This effect is half-maximal at 45 microM ATP. ATP produces a 4-fold reduction in Km and 2.4-fold reduction in Vmax for cytochalasin B-inhibitable D-glucose efflux from inside-out red cell membrane vesicles (IOVs). This effect on transport is half-maximal at 45 microM ATP. AMP, ADP, alpha, beta-methyleneadenosine 5'-triphosphate, and beta, gamma-methyleneadenosine 5'-triphosphate at 1 mM are without effect on efflux of D-glucose from IOVs. ATP modulation of Km for D-glucose efflux from IOVs is immediate in onset and recovery. ATP inhibition of Vmax for D-glucose exit is complete within 5-15 min and is only partly reversed following 30-min incubation in ATP-free medium. These findings suggest that the human red cell sugar transport protein contains a nucleotide-binding site(s) through which ATP modifies the catalytic properties of the transporter.

Adenosine Diphosphate↗
PubMed1986

Direct evidence for ATP modulation of sugar transport in human erythrocyte ghosts.

Sugar transport in human erythrocyte ghosts is modulated by low molecular weight factors present in red cell cytosol that induce an asymmetry in Michaelis and velocity constants for sugar entry and exit (Carruthers, A., and Melchior, D. L. (1983) Biochim. Biophys. Acta 728, 254-266). This study examines the possibility that ATP is the transport-modulating factor. The intracellular factor must satisfy at least three criteria. It must reduce Km and Vmax for sugar efflux from inside-out red cell membrane vesicles. It should increase Km for efflux from red cell ghosts. It should have a molecular weight of less than 10 kDa. These criteria are satisfied by ATP. AMP, ADP, GTP, UTP, and ITP are without effect on sugar transport. The following results support the view that the cytosolic factor is ATP. Red cell lysate (obtained by hypotonic lysis of red cells) is unable to modify transport following dialysis against ATP-free medium. The ability of lysate to modify transport is retained following acid extraction. ATP depletion of acid-extracted lysate by treatment with apyrase results in the loss of transport-modulating potency. Myokinase partly restores both the ATP content and the ability of ATP-depleted (apyrase-treated) lysate to modify transport. Addition of ATP to ATP-depleted lysate mimics the ability to myokinase to restore the transport-modulating potency of lysate. ATP is without effect on the number and molecular size of D-glucose-sensitive cytochalasin B-binding proteins in the red cell membrane. These findings demonstrate that the transport-modulating potency of red cell cytosol is quantitatively accounted for by intracellular ATP which acts to modify the catalytic activity of plasmalemmal transporters.

Adenosine Diphosphate↗
PubMed1986

Anomalous asymmetric kinetics of human red cell hexose transfer: role of cytosolic adenosine 5'-triphosphate.

Cytosolic adenosine 5'-triphosphate (ATP) modifies the properties of human red cell sugar transport. This interaction has been examined by analysis of substrate-induced sugar transporter intrinsic fluorescence quenching and by determination of Michaelis and velocity constants for D-glucose transport in red cell ghosts and inside-out vesicles lacking and containing ATP. When excited at 295 nm, human erythrocyte ghosts stripped of peripheral proteins display an emission spectrum characterized by a scattering peak and a single emission peak centered at about 333 nm. Addition of sugar transport substrate or cytochalasin B and phloretin (sugar transport inhibitors) reduces emission peak height by 10% and 5%, respectively. Cytochalasin B induced quenching is a simple saturable phenomenon with an apparent Kd (app Kd) of 60 nM and a capacity of 1.4 nmol of sites/mg of membrane protein. Quenching by D-glucose (and other transported sugars) is characterized by at least two (high and low) app Kd parameters. Inhibitor studies indicate that these sites correspond to sugar efflux and influx sites, respectively, and that both sites can exist simultaneously. ATP induces quenching of stripped ghost fluorescence with half-maximal effects at 20-30 microM ATP. ATP reduces the low app Kd and increases the high app Kd for sugar-induced fluorescence quenching. D-Glucose transport in intact red cells is asymmetric (Km and Vmax for influx less than Km and Vmax for efflux). In addition, two operational Km parameters for efflux are detected in zero- and infinite-trans efflux conditions. Protein-mediated sugar transport in ghosts and inside-out vesicles (IOVs) is symmetric with respect to Km and Vmax for entry and exit, and only one Km for exit is detected. Addition of millimolar levels of ATP to the interior of ghosts or to the exterior of IOVs restores both transport asymmetry and two operational Km parameters for native efflux. A model for red cell hexose transport is proposed in which ATP modifies the catalytic properties of the transport system. This model mimics the behavior of the sugar transport systems of intact cells, ghosts, and inside-out vesicles.

Adenine Nucleotides↗
PubMed1986

Reconstituted human erythrocyte sugar transporter activity is determined by bilayer lipid head groups.

The effects of bilayer lipid head group on human erythrocyte passive sugar transport protein activity were examined by reconstituting the transporter into bilayers of large unilamellar vesicles (LUVs) formed from lipid classes of identical (or similar) acyl chain composition. Two reconstituted transport parameters were measured as a function of temperature. These were Km and turnover number [Tn = Vmax per reconstituted D-glucose-sensitive cytochalasin B binding site (transport molecule)]. Tn for sugar transport was found to be almost entirely a function of the properties of the bulk lipid composition of the reconstituted LUVs. It was found to be independent of both reconstituted transporter density and small amounts (less than or equal to 3%) of endogenous red cell lipids. With the dimyristoylphospholipids, Tn increases at all temperatures in the order phosphatidylcholine less than phosphatidylglycerol less than phosphatidic acid less than phosphatidylserine (at 50 degrees C, Tn for transport in dimyristoylphosphatidylcholine is 100-fold lower than Tn for transport in dimyristoylphosphatidylserine). Similar results are found with egg yolk derived lipids. Only dimyristoyl- and dipalmitoylphosphatidylcholine bilayers are incapable of supporting detectable transport activity at temperatures below the bilayer phase transition, and only the phosphatidylcholines show a clear increase in Tn during the bilayer melt. All other bilayer systems studied (phosphatidic acid, phosphatidylglycerol, phosphatidylserine, and sphingomyelin) support a small or negligible increase in Tn during the bilayer melt, the major change in transport being restricted to altered Km. With the disaturated phosphatidylglycerols (C14-C18), Tn and the activation energy (Ea) for reconstituted transport increase with acyl chain carbon number. Similar results are found with the phosphatidylcholines. Transport in bilayers formed from egg yolk sphingomyelin (a lipid containing a sphingosine rather than a glycerol backbone) is characterized by very high Km and low Tn parameters. Moreover, protein-mediated transport in sphingomyelin bilayers "spikes" during the bilayer phase transition. These and previous findings [Carruthers, A., & Melchior, D.L. (1984) Biochemistry 23, 6901-6911; Connolly, T.J., Carruthers, A., & Melchior, D. L. (1985) Biochemistry 24, 2865-2873] indicate that those bilayer factors influencing reconstituted sugar transporter activity are, in order of importance, lipid head group greater than lipid acyl chain length and saturation/unsaturation greater than lipid backbone greater than bilayer "fluidity".

Erythrocyte Membrane↗
PubMed1985

Transport of alpha- and beta-D-glucose by the intact human red cell.

The kinetics of alpha- and beta-D-glucose mutarotation and the transport of these anomers by intact human red cells were determined at 0.6 and 36.6 degrees C. The mutarotation coefficients for alpha- and beta-D-glucose in cell-free tris(hydroxymethyl)aminomethane medium (pH 7.4) at 0.6 degrees C are (2.25 +/- 0.2) and (1.73 +/- 0.42) X 10(-3) min-1, respectively, and at 36.6 degrees C are (69 +/- 12) and (75 +/- 5) X 10(-3) min-1, respectively. These values are in good agreement with previous estimates. At 0.6 degrees C, the red cell contains no detectable mutarotase activity. Initial rates of sugar uptake were measured by using radiolabeled D-glucose and time courses of uptake by turbidimetry. The time courses of alpha- and beta-D-glucose and an equilibrium mixture of alpha- and beta-D-glucose infinite-cis entry are identical at 0.66 degrees C (n = 41) where negligible mutarotation is observed. The apparent Ki values for inhibition of radiolabeled D-glucose initial uptake by unlabeled alpha- or beta-D-glucose at 0.6 degrees C are identical (1.6 mM). The calculated Vmax parameters for uptake of the radiolabeled anomers at this temperature are also indistinguishable. The time courses of infinite-cis alpha- and beta-D-glucose uptake at 36.66 degrees C are identical (n = 40). While D-glucose mutarotation is more rapid at this temperature, the anomers of D-glucose are not transported differently by the red cell hexose transfer system.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological Transport, Active↗
PubMed1985

Effects of bilayer cholesterol on human erythrocyte hexose transport protein activity in synthetic lecithin bilayers.

In this study, we describe the effects of altered bilayer cholesterol content on reconstituted, protein-mediated sugar transport. The system used was the human erythrocyte sugar transporter (band 4.5) reconstituted into the bilayers of large unilamellar vesicles. Vesicle preparations were formed from synthetic lecithins whose bilayer cholesterol content ranged from 0 to 50 mol %. Transport was measured by microturbidimetric analysis over the temperature range of 0-65 degrees C while bilayer physical state was characterized by differential scanning calorimetry. Reconstituted transport activity was irreversibly lost between 62 and 65 degrees C. The Km for reconstituted transport was found to increase only slightly with increasing temperature and was not systematically affected by bilayer cholesterol content. The most striking observation of this study is that over certain critical cholesterol concentrations, as little as a 2.5% change in bilayer cholesterol can result in as much as a 100-fold change in Vmax per reconstituted protein. Our findings run counter to the view that increasing bilayer cholesterol content monotonically transforms a membrane into a state of "intermediate fluidity". Abrupt, cholesterol-induced bilayer reorganizations occurring at 15-20 and 30 mol % bilayer cholesterol are markedly reflected in altered sugar transport rates. Increasing the cholesterol content of crystalline distearoyllecithin bilayers inhibits the activity of the reconstituted transporter. It is apparent from these studies that bilayer "fluidity" is neither the sole nor a major determinant of the Indeed, we find the effect of cholesterol on transport activity is independent of its ability to fluidize membranes.

Blood Glucose↗
PubMed1985

Effect of bilayer cholesterol content on reconstituted human erythrocyte sugar transporter activity.

The influence of altered bilayer cholesterol content on the catalytic activity of the human red cell hexose transporter was examined by reconstitution of the transport protein (band 4.5) into bilayers of large unilamellar vesicles formed from dipalmitoyl lecithin and varying amounts of cholesterol. The physical state of the bilayers was characterized by differential scanning calorimetry. The major findings are as follows: changes in bilayer phase behavior occur at membrane cholesterol levels of 15 to 20 mol % and 30 to 40 mol %; and the catalytic activity of the reconstituted transporter (Vmax/transporter) correlates with bilayer phase behavior. In crystalline bilayers, this is seen as an abrupt, stimulation of activity at 15 mol % cholesterol (which is reversed at 17.5 mol %) and a gradual acceleration of activity between 30 to 40 mol % cholesterol. In fluid bilayers (where activity is high), activity is unaffected by 10, 20, and 30 mol % cholesterol. However, 12.5 and 17.5 mol % cholesterol reduce activity by 100-fold. These studies demonstrate that small changes in bilayer cholesterol content result in drastic alterations in transporter activity. Transporter sensitivity to cholesterol is a complex rather than monotonic function of bilayer cholesterol content and appears to be primarily determined by bilayer composition rather than by bilayer "fluidity."

Blood Proteins↗
PubMed1984

Human erythrocyte hexose transporter activity is governed by bilayer lipid composition in reconstituted vesicles.

Purified membrane protein band 4.5 (a sugar transport protein) from human erythrocytes was reconstituted by reverse-phase evaporation into a variety of bilayers formed from the synthetic lecithins. The number of proteins reconstituted was estimated by determination of D-glucose-sensitive cytochalasin B binding sites. D-Glucose transport activity in reconstituted vesicles was assessed by monitoring cytochalasin B sensitive D-glucose fluxes using microturbidimetric analysis. The major points are as follows: The sugar transport activity of the reconstituted system is directly proportional to the number of cytochalasin B binding sites reconstituted. The ratio of cytochalasin B binding sites per band 4.5 protein is 0.8. These data suggest that the functional protein unit is a monomer. Inhibitor studies of reconstituted hexose transfer support the notion that the kinetics of reconstituted transport are intrinsically symmetric. The turnover number for transport is not consistent with transport proceeding via water-filled pores. The absolute activity of the reconstituted system (Vmax per reconstituted cytochalasin B binding protein) is governed by the bulk lipid composition of the synthetic membrane. At temperatures where bilayers formed from dimyristoyl- or dipalmitoyllecithin (DML and DPL, respectively) are "crystalline", hexose transport activity is not observed. Over the same temperature range, however, crystalline bilayers formed from the longer chain lecithins, distearoyl-, diarachidonoyl-, and dielaidoyllecithin (DSL, DAL, and DEL, respectively), support significant protein-mediated transport activity. In a given synthetic membrane, the bilayer transition from the liquid-crystalline to the fluid state results in increased protein-mediated sugar transport activity. In the one synthetic membrane (DEL) in which the activation energy (Ea) for transport could be measured both above and below the bilayer phase transition, Ea was unaffected by the phase change. Ea and the Arrhenius constant (A) for transport are dependent on lecithin acyl chain length and saturation. For both parameters, the order of increase is DML less than DPL = dipalmitoleoyllecithin (DPOL) less than DSL less than DAL less than DEL = dioleoyllecithin (DOL). This means that at 59-60 degrees C, the order of catalytic activity follows the lipid sequence DML less than DPOL less than DAL less than DOL much less than DPL = DEL less than DSL. Cholesterol (48 mol%) restores protein-mediated transport activity to crystalline DPL bilayers and reduces the activity supported by fluid DPL bilayers. This effect is not simply related to the effects of cholesterol on the bilayer partial specific volume. T

Binding Sites↗
PubMed1984

A rapid method of reconstituting human erythrocyte sugar transport proteins.

A rapid reconstitution procedure for human erythrocyte hexose transfer activity is described. The procedure (reverse-phase evaporation) avoids exposure of the isolated proteins to detergent, organic solvent, sonication, or freeze-thaw steps during insertion into synthetic membranes and may be effected within 15 min. The so-formed vesicles are unilamellar structures with a large encapsulated volume, narrow size range, and low passive permeabilities. Contamination by carry-through of endogenous (red cell) lipids is less than 1%. Reconstituted hexose transfer activity was examined by using unfractionated proteins (bands 3, 4.5, and 6) and purified proteins (bands 4.5 and 3). With unfractionated proteins, hexose transport activity is low [0.34 mumol X (mg of protein)-1 X min-1], is inhibited by cytochalasin B, and increases monotonically with protein concentration. Kinetic analysis indicates that Vmax values for both influx and efflux of D-glucose are identical. Reconstitution of the cytochalasin B binding protein (band 4.5) results in hexose transport with high specific activity [5 mumol X (mg of protein)-1 X min-1] and symmetry in transfer kinetics. Band 3 proteins also appear to mediate cytochalasin B sensitive D-glucose transport activity.

Blood Glucose↗
PubMed1983

Asymmetric or symmetric? Cytosolic modulation of human erythrocyte hexose transfer.

(1) The Michaelis-Menten parameters for hexose transfer in erythrocytes, erythrocyte ghosts and inside-out vesicles at 20 degrees C were determined using the light scattering method of Sen and Widdas ((1962) J. Physiol. 160, 392-403). (2) The external Km for infinite-cis exit of D-glucose in cells and ghosts is 3.6 +/- 0.5 mM. (3) Dilution of cellular solute (up to X 90 dilution) by lysing and resealing cells in varying volumes of lysate is without effect on the Vm for net D-glucose exit. The Km for net exit, however, falls from 32.4 +/- 3.7 mM in intact cells to 12.9 +/- 2.3 mM in ghosts. This effect is reversible. (4) Infinite-cis net D-glucose uptake measurements in cells and ghosts reveal the presence of a low Km, high affinity internal site of 5.9 +/- 0.8 mM. The Vm for net glucose entry increases from 23.2 +/- 3.7 mmol/1 per min in intact cells to 55.4 +/- 6.3 mmol/l per min in ghosts. (5) The external Km for infinite-cis D-glucose exit in inside-out vesicles is 6.8 +/- 2.7 mM. The kinetics of zero-trans D-glucose exit from inside-out vesicles are changed markedly when cellular solute (obtained by lysis of intact cells) is applied to either surface of inside-out vesicles. When solute is present externally, the Km and Vmax for zero-trans exit are decreased by up to 10-fold. When solute is present at the interior of inside-out vesicles, Vmax for zero-trans exit is reduced; Km for exit is unaffected. In the nominal absence of cell solute, transfer is symmetric in inside-out vesicles. The orientation of transporter in the bilayer is unaffected by the vesiculation procedure. (6) External application of cellular solute to ghosts reduces Vmax for D-glucose exit but is without effect on the external Km for infinite-cis exit. (7) The inhibitory potency of cell lysate on hexose transfer is lost following dialysis indicating that the factors responsible for transfer modulation are low molecular weight species. (8) We consider the hexose transfer in human erythrocytes is intrinsically symmetric and that asymmetry of transfer is conferred by interaction of the system with low molecular weight cytosolic factors.

Biological Transport, Active↗
PubMed1983

UDP-galactose:ceramide galactosyl transferase of isolated oligodendroglia.

The activity of UDP-galactose:ceramide galactosyl transferase (CGalT) has been studied in isolated oligodendroglia from bovine brain white matter and myelinating rat brain. The specific activity and activity per mg DNA are 4- and 10-fold higher in rat oligodendroglia compared with neuronal perikarya from rat brain, and is higher in oligodendroglia from myelinating rat brain compared with bovine oligodendroglia. In membranes isolated from oligodendroglia, the specific activity decreased in the order endoplasmic reticulum greater than plasma membrane greater than myelin.

Animals↗
PubMed1983

Sugar transport in giant barnacle muscle fibres.

The kinetics of 3-O-methylglucose transport in the giant muscle cells of Balanus nubilus have been studied both in intact fibres and in fibres subjected to intracellular solute control using internal dialysis. 3-O-methylglucose is not metabolized by barnacle muscle and at equilibrium the 3-O-methylglucose space of the tissue does not differ significantly from the water content of the muscle. These results indicate that 3-O-methylglucose transfer in barnacle muscle is mediated by a passive process. 3-O-methylglucose transport is facilitated by a saturable, symmetric transfer mechanism inhibited by cis but not trans sugars and by low concentrations of phloretin and cytochalasin B. The kinetic constants for uptake and exit are identical. These features indicate that sugar transport in barnacle muscle is mediated by a limited number of membrane transport sites. The number of sugar-displaceable cytochalasin B binding sites in barnacle muscle is 3 X 10(13) cm-2. Indirect kinetic estimates indicate that the number of sugar transport sites is in the order of 1.6 X 10(12) cm-2. This passive, facilitated, selective, saturable transport system is consistent with both symmetric mobile carrier (one-site) and symmetric simultaneous carrier (two-site) models for transport.

3-O-Methylglucose↗
PubMed1983

Insulin regulation of sugar transport in giant muscle fibres of the barnacle.

1. Sugar transport in the giant muscle cells of Balanus nubilus is accelerated during contractile activity and exposure to porcine insulin. The characteristics of hexose-transfer regulation in the giant muscle cells have been examined by studying the transport of 3-O-methylglucose (a non-metabolized sugar) in both intact giant fibres and fibres subjected to internal solute control by internal dialysis.2. Sugar transport in barnacle muscle is mediated by a saturable process which is inhibited by both phloretin and cytochalasin B. Insulin increases the capacity of the transport system with little effect on its apparent affinity for sugar. Under the same conditions insulin increases 3-O-methylglucose-displaceable cytochalasin B binding. The effects of insulin on transport are half-maximal at 5 muM-insulin and are abolished by both insulin antibody and phloretin. The intact barnacle releases an insulin-like material in response to a rise in blood glucose levels.3. Insulin increases the cyclic GMP (cGMP) content and reduces the cyclic AMP (cAMP) content of barnacle muscle. Experiments with fibres injected with aequorin show that insulin also lowers cytosolic ionized Ca levels. The changes in cyclic nucleotide levels induced by insulin precede the effects on sugar transport and cytosolic ionized Ca. During repetitive contractile activity, cAMP, cGMP and ionized Ca levels are raised.4. Agents which raise the cAMP content of barnacle muscle normally inhibit sugar transport. Dibutyryl cAMP also inhibits transport. Alterations in cytosolic ionized Ca levels in intact fibres are without effect on sugar transport. Nevertheless, stimulation of transport by insulin is blunted when cytosolic ionized Ca is lowered by intracellular injection of the Ca-chelating agent, EGTA.5. Sugar uptake in the internally dialysed fibre is inhibited by intracellular application of cAMP. Internal application of Ca and cGMP stimulate sugar uptake in the dialysed fibre. Cyclic AMP reduces the capacity of the transport system whereas Ca and cGMP increase the capacity of the saturable transfer system. Cyclic AMP and cGMP act at kinetically independent sites. Internal ATP (2 mM) inhibits sugar uptake in the dialysed fibre by some 40%, possibly through the production of cAMP.6. External insulin stimulates sugar uptake in the dialysed fibre even when ionized Ca levels are buffered using EGTA. Stimulation by insulin requires the presence of cytosolic ATP and is potentiated by internal application of 1 mM-GTP. In the dialysed fibre stimulation of transport by insulin is greater than that brought about by Ca and cGMP.7. The stimulation of transport by insulin in the intact fibre and its inhibition by dibutyryl cAMP are abolished by intracellular injection of Gpp(NH)p. Injection of intact fibres with GTPgammaS potentiates the stimulation of transport by insulin and renders insulin-activation of transport irreversible. Injection of intact fibres with ATPgammaS leads to the irreversible inhibition of transport.8. Injection of intact fibres with cAMP phosphodiesterase lowers cAMP levels close to zero and stimulates sugar transport. Application of insulin to diesterase-injected fibres still stimulates transport in the absence of altered cytosolic cAMP.

3-O-Methylglucose↗
PubMed1981

Sugar transport in giant axons of Loligo.

1. The transport of glucose and a number of other sugars has been investigated in the giant axons of Loligo forbesi. 2. Glucose and 2-deoxy-D-glucose are phosphorylated by squid axons, alpha-methyl-D-glucopyranoside and 3-O-methylglucose are not metabolized. All four sugars can diffuse freely in axoplasm. 3. Sugar uptake in squid axons is a passive, saturable process. The maximum rate of sugar uptake increases in the order 3-O-methylglucose less than 2-deoxy-D-glucose less than D-glucose. Competition between these sugars suggests a common uptake mechanism. 4. The uptake of D-glucose but not 3-O-methylglucose or 2-deoxy-D-glucose is reduced when the external Na concentration is lowered. 5. Glucose uptake is sensitive to temperature with a Q10 for saturated uptake of 1.9 between 14.5 and 5 degrees C. Uptake is unaffected by external pH in the range 5-10 but is reduced by cyanide (2 mM). 6. Glucose and 2-deoxy-D-glucose uptake and metabolism are increased by electrical stimulation. These effects are prevented by ouabain. The uptake of 3-O-methylglucose is unaffected by stimulation. 7. The maximum rate of 3-O-methylglucose efflux is higher than the maximum rate of uptake of the sugar suggesting that transport of this sugar is asymmetric. 8. 3-O-methylglucose efflux is reduced by external sugars with order of potency 3-O-methylglucose greater than 2-deoxy-D-glucose greater than D-glucose. These effects persist when the internal 3-O-methylglucose concentration is as high as 50 mM. 9. 3-O-methylglucose efflux is inhibited reversibly by cytochalasin B and phloridzin but irreversibly by phloretin. Efflux is reduced reversibly by cyanide (2 mM). 10. 3-O-methylglucose efflux is sensitive to temperature with a Q10 of 3.2 over the range 10-20 degrees C. Efflux is unaffected by external pH in the range 6-9 but is reduced reversibly by internal acidification.

Animals↗