Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “facilitated diffusion”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15Linked to original sources
PubMed1989

Glucose transport in crabtree-positive and crabtree-negative yeasts.

The kinetic parameters of glucose transport in four Crabtree-positive and four Crabtree-negative yeasts were determined. The organisms were grown in aerobic glucose-limited chemostats at a dilution rate of 0.1 h-1. The results show a clear correlation between the presence of high-affinity glucose transport systems and the absence of aerobic fermentation upon addition of excess glucose to steady-state cultures. The presence of these H+-symport systems could be established by determination of intracellular accumulation of 6-deoxy-[3H]glucose and alkalinization of buffered cell suspensions upon addition of glucose. In contrast, the yeasts that did show aerobic alcoholic fermentation during these glucose pulse experiments had low-affinity facilitated-diffusion carriers only. In the yeasts examined the capacity of the glucose transport carriers was higher than the actual glucose consumption rates during the glucose pulse experiments. The relationship between the rate of sugar consumption and the rate of alcoholic fermentation was studied in detail with Saccharomyces cerevisiae. When S. cerevisiae was pulsed with low amounts of glucose or mannose, in order to obtain submaximal sugar consumption rates, fermentation was already occurring at sugar consumption rates just above those which were maintained in the glucose-limited steady-state culture. The results are interpreted in relation with the Crabtree effect. In Crabtree-positive yeasts, an increase in the external glucose concentration may lead to unrestricted glucose uptake by facilitated diffusion and hence, to aerobic fermentation. In contrast, Crabtree-negative yeasts may restrict the entry of glucose by their regulated H+-symport systems and thus prevent the occurrence of overflow metabolism.

Biological Transport↗
PubMed1999

Immunocytochemical Localization of Glucose Transporter-2 (GLUT-2) in Pancreatic Islets and Islet Cell Tumors.

Glucose is a major metabolic fuel in mammals and is transported into organs and cells by a facilitated diffusion which involves binding of glucose to glucose transporters (GLUTs). Among several GLUTs so far indentified, GLUT-2 is specifically localized immunocytochemV cally in beta-islet cells. Using immunocytochemical staining, normal pancreases and 27 cases of islet cell tumors, including insulinomas, gastrinomas, glucagonomas, pancreatic polypeptide-omas (PPomas), and a nonfunctioning islet cells tumor, were systematically stained for four different pancreatic hormones, gastrin, and GLUT-2. GLUT-2 staining in beta-islet cells was more diffuse than that of insulin immunostaining, and corresponded with the positive staining in the lateral segments of beta-cell plasma membrane, that faced adjacent beta-cells. Glucagon, somatostatin (SRIF) and PP cells stained weakly for GLUT-2, weaker than that of beta-cells. Some nonbeta cells, especially extra-islet PP cells were not stained for GLUT-2. Among islet cell tumors, insulinomas stained less strongly for GLUT-2 than normal beta-cells from the adjacent normal pancreas. Gastrinomas, glucagonomas, and PPomas stained weaker than insulinomas. Even nonfunctioning islet cell tumors were weakly stained for GLUT-2. The positive staining for GLUT-2 observed for islets cells and all islet tumors is consistent with the notion that all pancreatic islet cells and islet cell tumors utilize glucose as a major fuel, requiring transporter-facilitated diffusion of glucose into the cells of normal organ and their tumors.

Journal Article↗
PubMed2000

Deciphering the mysteries of myoglobin in striated muscle.

Myoglobin (Mb) is a large protein that reversibly binds oxygen in the muscle cell and is thought to be critical for O2 supply to the mitochondria during exercise. The role of Mb in aerobic function is evaluated based on the physical properties of Mb as an O2 carrier and experimental evidence of Mb function in vivo. This role depends on the reversible binding of O2 by Mb depending on PO2, which results in: (1) storage of O2; (2) buffering of PO2 in the cell to prevent mitochondrial anoxia; and (3) parallel diffusion of O2 (so-called, 'facilitated diffusion'). The storage role is well established in diving mammals and buffering of cell PO2 above anoxic levels is shown here by in vivo magnetic resonance spectroscopy (MRS). However, the quantitative role of Mb in 'facilitated' or parallel diffusion of O2 is controversial. Evidence in support of this role is from MRS analyses, which reveal rapid Mb desaturation with exercise, and from the proportionality of Mb content of a muscle to the O2 diffusion limitation. Recent experiments with myoglobin knockout mice demonstrating high levels of aerobic function in normal and myoglobin-free mice argue against a link between Mb and oxidative phosphorylation. Thus, the current evidence supports the role of Mb in the physical diffusion of O2; however, the unimpaired aerobic function of Mb knockout mice indicates that this role may not be critical to O2 supply in active muscle.

Animals↗
PubMed2000

Glucose modulates vitamin C transport in adult human small intestinal brush border membrane vesicles.

The uptake of L-ascorbate (vitamin C) and its oxidized form, dehydro-L-ascorbic acid (DHAA), was evaluated in brush border membrane vesicles isolated from adult human duodenum, jejunum and ileum. Ascorbate was taken up along the entire length of the small intestine with a threefold higher initial uptake rate in distal than proximal segments. Ascorbate uptake was Na(+)-dependent, potential-sensitive and saturable (K(m), 200 micromol/L), whereas DHAA transport involved facilitated diffusion (K(m), 800 micromol/L). Pharmacologic experiments were conducted to characterize further these transport mechanisms. DHAA uptake was not mediated by the fructose carrier GLUT5, the uridine transporter or the 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive anion exchanger of the apical membrane. DIDS and sulfinpyrazone, an inhibitor of the urate/lactate exchanger, both significantly reduced the initial rate of ascorbate uptake. Acidic pH inhibited ascorbate uptake, and this effect was not due to a transmembrane proton gradient. Increasing concentrations of glucose in the transport media also significantly inhibited ascorbate uptake, but no effect of glucose was seen when glucose internalization was blocked by phlorizin. Preloading the vesicles with glucose inhibited ascorbate uptake similarly, indicating that glucose interferes with the ascorbate transporter from the internal side of the membrane. The results of this study suggest that DHAA crosses the apical membrane by facilitated diffusion, whereas ascorbate transport is a Na(+)-dependent, electrogenic process modulated by glucose.

Adult↗
PubMed1983

Cellular and lysosomal uptake of methylamine in isolated rat hepatocytes.

Upon addition of methylamine to intact cells, this lysosomotropic weak base accumulates intracellularly as the result of at least two different mechanisms: (1) facilitated diffusion across the plasma membrane, i.e. a process which is carrier-mediated and subject to both trans-stimulation (accelerative exchange) and cis-inhibition (competition) by other amines (e.g. ammonia, methylamine and triethylamine); this transport process is furthermore non-concentrative, energy-independent, and (although moderately temperature-sensitive) operative even at 0 degrees C; (2) active uptake, i.e. an energy-dependent concentrative process which is inhibited by anoxia and energy inhibitors. With time, methylamine accumulates in lysosomes and gives rise to a lysosomal swelling which is easily visible by optical microscopy, and which causes the cells to appear coarsely granular. After a 1h incubation with 10mM-methylamine, the total cell volume is increased by about 12%. Under anoxic conditions or in the presence of energy inhibitors, lysosomal swelling is abolished regardless of there being a high concentration of methylamine intracellularly (taken up by facilitated diffusion). The continuous accumulation of methylamine in lysosomes therefore seems to depend on an energy-requiring process (such as continuous proton pumping), and not only on trapping by Donnan-equilibrium-generated protons.

Amines↗
PubMedDate not supplied

Lactate transport and transporters: general principles and functional roles in brain cells.

Lactate is transported across cell membranes by diffusional, saturable cotransport with protons, mediated by monocarboxylate transporters (MCTs). This transport is bidirectional and in the absence of a transcellular H(+) gradient, it can increase the intracellular concentration of lactate up to but not beyond the extracellular level (or vice versa). If extra- and intracellular pH differ, however, the equilibrium level is determined by the gradients of both lactate anions and protons. Rates of lactate uptake are determined most often by measuring uptake of labeled lactate, e.g., [U-14C]lactate. In the case of lactate and other compounds that are metabolized, errors are introduced easily because continuing inwardly directed diffusional net transport of label can be achieved by intracellular metabolism, reducing the intracellular level of the nonmetabolized lactate and thus maintaining a concentration gradient between extra- and intracellular concentrations of the nonmetabolized compound (metabolism-driven uptake). For measurement of facilitated diffusion kinetics, it is essential that the period during which the uptake is measured is short enough that little or no metabolism-driven uptake contributes to the measured uptake (or that first-order regression analysis is carried out to obtain initial uptake rates from nonlinear traces). To achieve initial uptake rates, incubation periods well below 1 min are generally required. Lactate uptake is fast in astrocytes, which express powerful, low-affinity MCTs, i.e., MCT1 and MCT4. Due to the low affinity of these transporters, they respond to increased lactate gradients with enhanced transporter activity. The predominant MCT in neurons is the high-affinity MCT2, which can only increase its activity to a limited extent in the face of an increased lactate gradient. This is reflected by a high-affinity lactate uptake, although most investigators also have demonstrated a component of lactate uptake with lower affinity. In both neurons and astrocytes, however, facilitated diffusion is fast enough that under most conditions lactate fluxes will be determined mainly by the rate of metabolism-driven uptake, and MCT-mediated transport only will be rate-limiting after establishment of large transmembrane gradients.

Animals↗
PubMed1972

Inhibition of cellular transport processes by 5-thio-D-glucopyranose.

5-Thio-d-glucopyranose, the nearest analogue of normal d-glucose, which is proving a useful tool in examinations of d-glucose biochemistry, affects active and facilitated-diffusion transport processes. 5-Thio-d-glucose is readily transported in rabbit kidney-cortex slices and reaches a tissue/medium ratio of 6.5 within 40min. The sulphur analogue shows typical saturation kinetics with a K(m) value of 2.4mm and V(max.) value of 70mumol/h per g of cell water. Uptake of 5-thio-d-glucose is phlorrhizin-sensitive, Na(+)-dependent and energy-dependent. d-Galactose and methyl alpha-d-glucopyranoside transport is competitively inhibited by 5-thio-d-glucose with K(i) values of 4.8 and 9.7mm respectively. 5-Thio-d-glucose thus shows all of the characteristics of active transport in kidney cortex. Transport of neutral amino acids in rat kidney cortex is inhibited by 5-thio-d-glucose. Thus 5.6mm-5-thio-d-glucose causes a 25-30% inhibition of the transport of glycine and the non-metabolized amino acids cycloleucine and alpha-aminoisobutyric acid. 5-Thio-d-glucose is freely taken up by the facilitated-diffusion transport system in rat diaphragm muscle. The sulphur analogue inhibits the transport of d-xylose in this tissue but has no effect on the uptake of d-arabinose. It is concluded that the ring heteroatom is not an effector of binding in the transport processes examined and causes no important alteration in the conformation of the sugar. The diabetogenic action produced by 5-thio-d-glucose is due, in part, to the ability of the analogue to interfere with cellular transport processes that use d-glucose.

Amino Acids↗
PubMed2003

Functional characterization of the Frt1 sugar transporter and of fructose uptake in Kluyveromyces lactis.

Most yeast hexose transporters studied so far at the molecular level mediate facilitated diffusion of glucose and fructose. Here, we report that a novel Kluyveromyces lactis gene, FRT1, encodes a proton-coupled fructose-uptake transporter. Frt1, when expressed in a Saccharomyces cerevisiae hxt null mutant strain that is unable to take up monosaccharides, restored growth on fructose. Determination of substrate specificities and kinetic parameters revealed Frt1 as a fructose transporter with a K(m) of 0.16+/-0.02 mM. Uptake of fructose was accompanied by an initial alkalization of the medium, indicating a proton-coupled uptake mechanism. Deletion of the FRT1 gene in a K. lactis strain already deleted for its RAG1 and HGT1 hexose transporter genes completely prevented uptake of and growth with fructose but not with glucose. Kinetic parameters of Frt1 in K. lactis, as assessed in a rag1 hgt1 mutant strain, were comparable with those obtained after heterologous expression in S. cerevisiae. Transcription of the FRT1 gene, which was undetectable when cells were grown in ethanol, was induced by various sugars. Our results indicate that, unlike S. cerevisiae, K. lactis exhibits proton symport systems for the uptake of hexoses, in addition to their facilitated diffusion.

Biological Transport↗
PubMed1995

Phosphatidylethanolamine is required for in vivo function of the membrane-associated lactose permease of Escherichia coli.

Experiments with mutant Escherichia coli cells lacking phosphatidylethanolamine (PE) as a membrane component (DeChavigny, A., Heacock, P. N., and Dowhan, W. (1991) J. Biol. Chem. 266, 5323-5332) were carried out to establish whether or not PE is necessary for full function of the lac permease in vivo. The Vmax for active transport of both lactose (in cells lacking beta-galactosidase, lacZ) and the unhydrolyzable lactose analog, methyl-beta-D-galactopyranoside (TMG), by mutant cells lacking PE was reduced 5-10-fold relative to cells containing PE, while the Km for the uptake of both substrates was the same in both types of cells. The low rate of TMG and lactose uptake by PE-deficient cells was unaffected by the presence of a protonophore (uncoupler) and for TMG uptake was on the order of the greatly reduced rate of uptake in uncoupler-treated cells containing PE. The rate of entry of lactose into lacZ+ derivatives of both types of cells, as a measure of facilitated diffusion, was nearly the same. The Km for lactose (lacZ cells) and TMG transport in PE-deficient cells was unaffected by the presence of an uncoupler which had a small effect on Vmax. In PE-containing cells these kinetic parameters for TMG transport were reduced by an uncoupler to the level found with PE-deficient cells while an uncoupler reduced lactose uptake by PE-containing (lacZ) cells to below measureable levels. Inverted membrane vesicles made from both types of cells could be loaded with TMG, but energizing TMG-loaded vesicles by ATP only induced rapid, uphill, permease-dependent efflux of TMG from PE-containing vesicles. The decrease in apparent active transport activity of cells with no PE was not due to a change in membrane permeability, to a reduced delta microH+ (proton electrochemical gradient) across the cell membrane, or to a reduced level of membrane-associated lac permease protein. These results suggest that in the absence of PE the lac permease cannot couple substrate uptake to delta microH+ in order to effect accumulation of substrate and as a result only carries out facilitated diffusion.

Biological Transport, Active↗
PubMed1984

Measurement of facilitated calcium diffusion by a soluble calcium-binding protein.

The flux of calcium through an aqueous compartment was determined in a flow-dialysis cell in which two dialysis membranes separated the middle aqueous compartment from two outer compartments. The contribution of convection to the total calcium flux was large but could be removed by addition of 1% agar. The flux of calcium through the gelled aqueous compartment agreed with theoretical expectations. The self-diffusion coefficient for calcium from these results was calculated to be 0.81 X 10(-5) cm2 X s-1. Carp parvalbumin significantly enhanced the calcium flux at 2.3 X 10(-6)M free calcium. The calcium flux increased linearly with parvalbumin concentration. These observations are consistent with the hypothesis that the overall unidirectional calcium flux J is the sum of free calcium diffusion and protein-calcium diffusion: J = D[Ca] + D'[CaPr]. The value of D', the self-diffusion coefficient for parvalbumin, was calculated from the flux data to be 13.7 X 10(-7) cm2 X s-1.

Animals↗
PubMed1987

A numerical model for blood oxygenation in the pulmonary capillaries--effect of pulmonary membrane resistance.

A study of the blood oxygenation in pulmonary capillaries is made by considering the transport mechanisms of molecular diffusion, convection and the facilitated diffusion due to the presence of haemoglobin. The resistance offered by the pulmonary membrane on the transport of gases has been incorporated. The resulting system of coupled, non-linear partial differential equations is solved numerically. It is found that, in the immediate neighbourhood of the entry, the amount of dissolved O2 decreases. This decreases further as the resistance offered by the pulmonary membrane increases. The rate of oxygenation of blood increases as the permeability coefficient for O2 (PO) increases. It is shown that the ideally permeable case for both O2 and CO2 can be approximated by taking PO approximately 10 cm/s. Further, it is shown that the oxygen takes longest and CO2 is the fastest to attain equilibration. The equilibration length increases as the resistance offered by the membrane increases. Finally, some of the pulmonary diseases such as pulmonary oedema and fibrosis have been analyzed.

Capillaries↗
PubMed1990

A numerical model for the oxygenation of blood in lung capillaries--effect of nth order one-step kinetics of oxygen uptake by haemoglobin.

A numerical model is described for the oxygenation of blood in lung capillaries by considering the transport mechanisms of molecular diffusion, convection and the facilitated diffusion due to the presence of haemoglobin. In order to represent the oxygen dissociation curve accurately in the model, the nth order one-step kinetics of oxygen uptake by haemoglobin has been used. The resulting system of coupled, non-linear partial differential equations is solved numerically. It is shown that the blood is required to traverse a larger distance in the capillary before becoming fully oxygenated with nth order one-step kinetics in comparison to first-order one-step kinetics.

Capillaries↗
PubMed1986

Catalytic facilitation by diffusion of adsorbed substrate on membrane surface.

Membrane-alkaline phosphatase shows greater velocity of reaction than solubilized enzyme at low substrate concentration, whereas at saturation-concentration the opposite is true. The catalytic rate enhancement with the membrane-enzyme, when substrate availability is limiting, is attributed to non-specific adsorption of substrate to the membrane followed by its surface-diffusion to the active site resulting in an enhanced collision rate for the substrate with the enzyme. Experimental evidence for the adsorption-diffusion is provided by the dynamic quenching of 1-anilino-naphthalene-8-sulphonate, a membrane-bound probe's fluorescence by the substrate, 4-nitrophenyl-phosphate.

Adsorption↗
PubMed2006

Specific aquaporins facilitate the diffusion of hydrogen peroxide across membranes.

The metabolism of aerobic organisms continuously produces reactive oxygen species. Although potentially toxic, these compounds also function in signaling. One important feature of signaling compounds is their ability to move between different compartments, e.g. to cross membranes. Here we present evidence that aquaporins can channel hydrogen peroxide (H2O2). Twenty-four aquaporins from plants and mammals were screened in five yeast strains differing in sensitivity toward oxidative stress. Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H2O2. Further evidence for aquaporin-mediated H2O2 diffusion was obtained by a fluorescence assay with intact yeast cells using an intracellular reactive oxygen species-sensitive fluorescent dye. Application of silver ions (Ag+), which block aquaporin-mediated water diffusion in a fast kinetics swelling assay, also reversed both the aquaporin-dependent growth repression and the H2O2-induced fluorescence. Our results present the first molecular genetic evidence for the diffusion of H2O2 through specific members of the aquaporin family.

Animals↗
PubMed1991

A mathematical model for the rate of oxygenation of blood in pulmonary capillaries using nth-order one-step kinetics of oxygen uptake by haemoglobin.

A mathematical model is described for the process of gas exchange in pulmonary capillaries by taking into account the transport mechanisms of molecular diffusion, convection, and the facilitated diffusion due to haemoglobin. The nth-order one-step kinetics of oxygen uptake by haemoglobin has been incorporated. The rate k at which blood becomes oxygenated is determined by setting up an appropriate eigenvalue problem. This method eventually leads to a transcendental equation in k. A multiprecision technique due to Verma and Sharan (1980) is employed to obtain a physically acceptable solution. It is shown that, at equilibrium, the saturation of haemoglobin with oxygen computed from the analysis is fairly close to the data of Severinghaus (1966). It was found that 97.15% of the total haemoglobin combined with oxygen. The blood is oxygenated well before it leaves the pulmonary capillary. The dissolved oxygen takes longer to achieve equilibration whereas the carbon dioxide traverses a comparatively smaller distance in the capillary.

Biological Transport↗
PubMed1995

Use of ultrasound to facilitate antibiotic diffusion through Pseudomonas aeruginosa alginate.

The effect of ultrasonic treatment of two pseudomonal alginate samples was studied by investigation of their rheological characteristics and interaction with tobramycin, piperacillin and ciprofloxacin. Whilst exposure to ultrasound at an amplitude of 20 microns for 8 min caused a significant decrease in viscosity of both alginates, molecular fragmentation, as demonstrated by PAGE and gel-permeation chromatography, was minimal. Antibiotic penetration was improved substantially however with sonication, leading to a 100% increase in tobramycin and piperacillin diffusion from the alginate compared with an untreated control polymer.

Alginates↗