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At least 685 records · Page 38Linked to original sources
PubMed2003

Isoform selectivity of 3-125I-iodo-alpha-methyl-L-tyrosine membrane transport in human L-type amino acid transporters.

UNLABELLED: 3-(123)I-Iodo-alpha-methyl-L-tyrosine ((123)I-IMT) has been developed for SPECT of amino acid transport imaging. We examined the isoform selectivity of (125)I-IMT transport of the 2 human L-type amino acid transporters, hLAT1 and hLAT2, with human 4F2hc-coexpressed Xenopus laevis oocytes. METHODS: An uptake study of (125)I-IMT was performed using transporter-expressed X. laevis oocytes. Oocytes were injected with 17.6 ng of hLAT1 or hLAT2 complementary RNA (cRNA) and 7.4 ng of h4F2hc cRNA in a molar ratio of 1:1. Two days after injection, the uptake of (125)I-IMT was measured in the Na(+)-free uptake solution containing 18.5 kBq of noncarrier-added (125)I-IMT. After incubation for 30 min at room temperature, radioactivity of the oocytes was determined. RESULTS: Of the 2 hLAT isoforms and h4F2hc-coexpressed X. laevis oocytes, (125)I-IMT uptake via hLAT1 was 5.95-fold higher than that via hLAT2 (P < 0.005). CONCLUSION: (125)I-IMT transport was hLAT1 selective. Investigations on the isoform selectivity of (125)I-IMT transport with other transporters are anticipated.

Amino Acid Transport System y+↗
PubMed1998

[Hepatocellular transport of bile acids and organic anions in infection and SIRS--evidence for different mechanisms for regulating membrane transport proteins].

The alteration of proinflammatory mediators during sepsis and SIRS results in a large variety of adaptive changes of metabolic and physiologic variables. This study investigated the alterations of hepatocellular transport in a rat sepsis model (LPS i.p.) as well as in a model inducing SIRS by sterile abscess formation (turpentine i.m.). Two bile acids (Cholyltaurine and Chemodeoxycholyltaurine) and one organic anion (Sulfolithocholyltaurine) were used as marker substrates to investigate the time course of hepatocellular transport function. Experiments were performed in isolated perfused rat livers and plasma membrane vesicles. During sepsis, both, the transport of bile acids and that of the organic anion was markedly reduced. In contrast no alteration of transport was detected during SIRS. However, biliary secretion of glutathione (+90%) and bile acid independent bile flow (%) were increased. mRNA levels of bile acid and organic anion transport proteins were reduced. The lowest values were noted 12 h after injection of LPS or turpentine. Almost unchanged kinetic parameters during SIRS pointed to a normal population of transporters with regard to quantity and substrate affinity. Therefore it seems that transcriptional regulation plays an important role for the expression of transport proteins during sepsis, whereas posttranscriptional regulation may be of importance during SIRS. The clinical phenomenon of septic cholestasis including jaundice implies endotoxemia and differenciates against SIRS.

Animals↗
PubMed1990

Two-channel transporter versus a single-channel Na+-dependent transporter for glucose and amino acids in rat and turtle.

Conditions in rat and turtle small intestine tissue where glucose and glycine transport are inhibited while glucose-induced Na+ transport is preserved are described. The generally accepted model for the Na(+)-dependent transporter (a single channel for the Na+ and nutrient) does not account for the data obtained from an analysis of the interaction between the transport of glucose, glycine, and Na+ at different temperatures and the effect of inhibitors on these processes. The phenomenon of uncoupling of Na+ and nutrient transport can best be described by a two-channel model with a gate mechanism. According to this model, the Na(+)-dependent transporter has at least two channels: one for Na+ and another for nutrients. The model provides for the passage of Na+ in both directions along a channel opened by glucose, and accounts for changes in the stoichiometric ratio of Na+: glucose transport. Experiments are reported that confirm these theoretical predictions.

Amino Acids↗
PubMed1990

Cell surface labeling of glucose transporter isoform GLUT4 by bis-mannose photolabel. Correlation with stimulation of glucose transport in rat adipose cells by insulin and phorbol ester.

A new impermeant photoaffinity label has been used for identifying cell surface glucose transporters in isolated rat adipose cells. This compound is 2-N-4(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis(D-mannos-4- yloxy)-2- propylamine. We have used this reagent in combination with immunoprecipitation by specific antibodies against the GLUT4 and GLUT1 glucose transporter isoforms to estimate the relative abundance of these two transporters on the surface of the intact adipose cell following stimulation by insulin and phorbol 12-myristate 13-acetate (PMA). In the basal state, GLUT4 and GLUT1 are both present at the cell surface but GLUT4 is more abundant than GLUT1. In response to insulin, GLUT4 increases 15-20-fold and GLUT1 increases approximately 5-fold while 3-O-methyl-D-glucose transport is stimulated 20-30-fold. By contrast, PMA only induces a approximately 4-fold increase in GLUT4 while GLUT1 increases approximately 5-fold to the same level as seen with insulin. In addition, PMA stimulates 3-O-methyl-D-glucose transport approximately 3-fold to only 13% of the insulin-stimulated state. Thus GLUT4 is the major glucose transporter isoform under all conditions, and it is selectively and markedly enriched in response to insulin but not PMA which increases GLUT1 and GLUT4 equally. Furthermore, stimulation of glucose transport activity correlates closely with the appearance of GLUT4 on the cell surface in response to both insulin and PMA but does not correlate with the sum of GLUT1 and GLUT4 appearance. These results suggest that GLUT4 may be inherently more active than GLUT1 due to a higher TK (turnover/Km).

Adipose Tissue↗
PubMed1994

The di- and tripeptide transport protein of Lactococcus lactis. A new type of bacterial peptide transporter.

Lactococcus lactis takes up di- and tripeptides via a proton motive force-dependent carrier protein. The gene (dtpT) encoding the di-tripeptide transport protein of L. lactis was cloned by complementation of a dipeptide transport-deficient and proline auxotrophic Escherichia coli strain. Functional expression of the dipeptide transport gene was demonstrated by uptake studies of alanyl-[14C]glutamate and other peptides in E. coli cells. The di-tripeptide transport protein catalyzes proton motive force-driven peptide uptake and dipeptide exchange activity. The nucleotide sequence of dtpT was determined and the translated sequence corresponds with a protein of 463 amino acid residues. Hydropathy profiling indicates that the protein could form 12 membrane-spanning segments with the amino and carboxyl termini at the outer surface of the membrane. A secondary structure model is presented which is substantiated by analysis of DtpT-PhoA fusion constructs. Amino acid sequence comparisons showed no significant homology with other bacterial peptide transport systems nor with any other known protein. Flanking regions of the di-tripeptide transport gene were used to delete dtpT from the chromosome of L. lactis. Genetic and biochemical characterization of this mutant indicates that DtpT is the only transport protein in L. lactis for hydrophilic di- and tripeptides.

Amino Acid Sequence↗
PubMed1998

Transport of cholesterol from the endoplasmic reticulum to the plasma membrane is constitutive in CaCo-2 cells and differs from the transport of plasma membrane cholesterol to the endoplasmic reticulum.

The transport of newly synthesized cholesterol from its site of synthesis, the endoplasmic reticulum, to the plasma membrane was studied in CaCo-2 cells. The appearance of newly synthesized cholesterol on the cell surface was rapid. By 30 min, 50% of the total labeled cholesterol was observed in the plasma membrane. The arrival of cholesterol at the plasma membrane was independent of new protein synthesis, a functional Golgi apparatus, or microtubular function. Progesterone, verapamil, and trifluoperazine, inhibitors of p-glycoprotein which are known to inhibit cholesterol transport from the plasma membrane to the endoplasmic reticulum, reduced the amount of newly synthesized cholesterol reaching the plasma membrane. The p-glycoprotein inhibitors, however, caused the accumulation of sterol intermediates in the plasma membrane, suggesting that sterol trafficking to the plasma membrane remained intact, but that trafficking from the plasma membrane to the endoplasmic reticulum was disrupted. In contrast, nigericin, another potent inhibitor of cholesterol movement from the plasma membrane to the endoplasmic reticulum, did not alter the transport of newly synthesized cholesterol to the plasma membrane. Moreover, promoting cholesterol transport from the plasma membrane to the endoplasmic reticulum by sphingomyelin hydrolysis or by micellar cholesterol influx did not alter the percent of newly synthesized cholesterol transported to the plasma membrane. Likewise, preventing plasma membrane cholesterol from reaching the endoplasmic reticulum by incubating cells with lysophosphatidylcholine, filipin, or digitonin did not alter the arrival of newly synthesized cholesterol to the plasma membrane. The results suggest that the amount of cholesterol moving to the plasma membrane from the endoplasmic reticulum is constitutive and regulated at the level of cholesterol synthesis and not at the level of the transport process. The pathways of cholesterol transport to and from the plasma membrane are distinct.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2004

Mutation of amino acid 475 of rat organic cation transporter 2 (rOCT2) impairs organic cation transport.

Protein sequence alignments revealed one amino acid position, where organic cation transporters (OCTs, aspartate (D) at position 475 of rOCT2) and organic anion transporters (OATs, arginine (R) at position 466 of rOAT1) are charged oppositely. To address the impact of this amino acid for protein function we cloned rat organic cation transporter 2 (rOCT2), the renal electrogenic cation transporter of the basolateral side of proximal tubule cells. Site-directed mutagenesis was used to generate rOCT2-D475R (rOCT2-mut). Heterologous expression of rOCT2 wild-type (rOCT2-wt) in A6 cells resulted in a significant uptake of the fluorescent organic cation 4-(4-dimethylaminostyryl)-N-methylpyridinium (ASP(+)). Accordingly, rOCT2-wt-transfected COS 7 cells showed an almost fourfold uptake of 25 microM [(14)C]-TEA, whereas rOCT2-mut did not exhibit any uptake of [(14)C]-TEA. These data indicate that rOCT2 transports both ASP(+) and TEA and that aspartate at position 475 of rOCT2 plays a critical role in transport function.

Amino Acid Substitution↗
PubMed1997

Characterization of rat Glut4 glucose transporter expressed in the yeast Saccharomyces cerevisiae: comparison with Glut1 glucose transporter.

Rat Glut4 glucose transporter was expressed in the yeast Saccharomyces cerevisiae, but was retained in an intracellular membranous compartment and did not contribute to glucose uptake by intact cells. A crude membrane fraction was prepared and reconstituted in liposome with the use of the freeze-thaw/sonication method. D-glucose-specific, cytochalasin B inhibitable glucose transport activity was observed. Kinetic analysis of D-glucose transport was performed by an integrated rate equation approach. The K(m) under zero-trans influx condition was 12 +/- 1 mM (mean +/- S.E., n = 3) and that under equilibrium exchange condition was 22 +/- 3 mM (n = 4). D-glucose transport was inhibited by 2-deoxy-D-glucose or 3-O-methyl-D-glucose, but not by D-allose, D-fructose or L-glucose. Cytochalasin B, phloretin and phlorizin inhibited D-glucose transport, but neither p-chloromercuribenzoic acid (pCMB) (0-0.1 mM) nor p-chloromercuribenzene sulfonic acid (pCMBS) (0-1.0 mM) inhibited this activity. High concentrations of HgCl2 were required to inhibit D-glucose transport (IC50, 370 microM). Comparing these properties to those of rat Glut1 we found two notable differences; (1) in Glut1, K(m) under zero-trans influx was significantly smaller than that under equilibrium exchange but in Glut4 less than two-fold difference was seen between these two K(m) values; and (2) Glut1 was inhibited with pCMB, pCMBS and low concentrations of HgCl2 (IC50, 3.5 microM), whereas Glut4 was almost insensitive to SH reagents. To examine the role of the exofacial cysteine, we replaced Met-455 of Glut4 (corresponding to Cys-429 of Glut1) with cysteine. The mutated Glut4 was inhibited by pCMB or pCMBS and the IC50 of HgCl2 decreased to 47 microM, whereas K(m), substrate specificity and the sensitivity to cytochalasin B were not significantly changed, indicating that the existence of exofacial cysteine contributed only to increase SH sensitivity in Glut4.

Animals↗
PubMed1999

Characterization of glucose transport and glucose transporters in the human choriocarcinoma cell line, BeWo.

In this study we have characterized 2-deoxyglucose (2DG) transport and hexose transporter expression in the human choriocarcinoma cell line, BeWo. 2DG uptake in BeWo cells displayed saturable kinetics (V(max), 29+/-1.5 nmol/min/mg protein;K(m), 1.5+/-0.02 m m) and was significantly inhibited in the presence of 2-deoxyglucose, mannose and 3- O -methyl glucose (all at a competing concentration of 30 m m) by up to 97 per cent, but not by galactose or fructose. Glucose uptake was not Na(+)-dependent, but was inhibited by cytochalasin B (by approx 85 per cent) indicating that hexose uptake was mediated via a facilitative glucose transport mechanism. Northern and immunoblot analyses revealed that BeWo cells expressed GLUT1 and GLUT5, but not GLUT2 or GLUT3. On immunoblots, GLUT1 migrated as a broad protein band on SDS-gels (average M(r)of 55 kDa) and treatment with N -glycanase resulted in a significant shift in its electrophoretic mobility; the core protein migrating as a 40 kDa band indicating that the carrier was heavily glycosylated. GLUT5 was detected as a discrete 60 kDa band and like GLUT1, the observed immunoreactive signal was lost when using antiserum that had been pre-adsorbed with the antigenic peptide. Our findings indicate that BeWo cells express a facilitative glucose transport system with characteristics broadly similar to those reported in isolated human placental membrane vesicles and that they are likely to serve as a useful experimental system for studying the regulation of placental glucose transport and transporter expression.

Biological Transport↗
PubMed2000

Oligomerization of the human serotonin transporter and of the rat GABA transporter 1 visualized by fluorescence resonance energy transfer microscopy in living cells.

Recent biochemical studies indicate that the serotonin transporter can form oligomers. We investigated whether the human serotonin transporter (hSERT) can be visualized as an oligomer in the plasma membrane of intact cells. For this purpose, we generated fusion proteins of hSERT and spectral variants of the green fluorescent protein (cyan and yellow fluorescent proteins, CFP and YFP, respectively). When expressed in human embryonic kidney 293 cells, the resulting fusion proteins (CFP-hSERT and YFP-hSERT) were efficiently inserted into the plasma membrane and were functionally indistinguishable from wild-type hSERT. Oligomers were visualized by fluorescence resonance energy transfer microscopy in living cells using two complementary methods, i.e. ratio imaging and donor photobleaching. Interestingly, oligomerization was not confined to hSERT; fluorescence resonance energy transfer was also observed between CFP- and YFP-labeled rat gamma-aminobutyric acid transporter. The bulk of serotonin transporters was recovered as high molecular weight complexes upon gel filtration in detergent solution. In contrast, the monomers of CFP-hSERT and YFP-hSERT were essentially undetectable. This indicates that the homo-oligomeric form is the favored state of hSERT in living cells, which is not significantly affected by coincubation with transporter substrates or blockers. Based on our observations, we conclude that constitutive oligomer formation might be a general property of Na(+)/Cl(-)-dependent neurotransmitter transporters.

Animals↗
PubMed1999

TRAP transporters: an ancient family of extracytoplasmic solute-receptor-dependent secondary active transporters.

Tripartite ATP-independent periplasmic transporters (TRAP-T) represent a novel type of secondary active transporter that functions in conjunction with an extracytoplasmic solute-binding receptor. The best characterized TRAP-T family member is from Rhodobacter capsulatus and is specific for C4-dicarboxylates [Forward, J. A., Behrendt, M. C., Wyborn, N. R., Cross, R. & Kelly, D. J. (1997). J Bacteriol 179, 5482-5493]. It consists of three essential proteins, DctP, a periplasmic C4-dicarboxylate-binding receptor, and two integral membrane proteins, DctM and DctQ, which probably span the membrane 12 and 4 times, respectively. Homologues of DctM, DctP and DctQ were identified in all major bacterial subdivisions as well as in archaea. An orphan DctP homologue in the Gram-positive bacterium Bacillus subtilis may serve as a receptor for a two-component transcriptional regulatory system rather than as a constituent of a TRAP-T system. Phylogenetic data suggest that all present day TRAP-T systems probably evolved from a single ancestral transporter with minimal shuffling of constituents between systems. Homologous TRAP-T constituents exhibit decreasing degrees of sequence identity in the order DctM > DctP > DctQ. DctM appears to belong to a large superfamily of transporters, the ion transporter (IT) superfamily, one member of which can function by either protonmotive force- or ATP-dependent energization. It is proposed that IT superfamily members exhibit the unusual capacity to function in conjunction with auxiliary proteins that modify the transport process by providing (i) high-affinity solute reception, (ii) altered energy coupling and (iii) additional yet to be defined functions.

Amino Acid Sequence↗
PubMed1995

Action of metformin on glucose transport and glucose transporter GLUT1 and GLUT4 in heart muscle cells from healthy and diabetic rats.

The effects of the antidiabetic drug metformin on glucose transport were investigated in freshly isolated heart muscle cells from healthy and streptozotocin-diabetic rats. In vivo treatment of diabetic rats with metformin failed to affect the basal and insulin-stimulated rate of glucose transport measured in isolated cells. In vitro exposure to therapeutic concentrations (< or = 10(-4) M) of metformin did not influence glucose transport, even upon incubation times up to 5 h or in the presence of high glucose (20 nM). In contrast, higher metformin concentrations produced an 8- to 12-fold increase in glucose uptake (with a lag of 90 min, and a maximum at 180 min and approximately 5 mM). In the presence of submaximal insulin concentrations (< or = 3.10(-10) M), the effects of metformin (5 mM) and of insulin were more than additive, whereas, at saturating insulin concentrations (10(-8) M), partial additivity was observed. Like insulin, metformin caused an approximately 1.6-fold increase in the content of both glucose transporter isoforms GLUT1 and GLUT4 in the plasma membrane of cardiac myocytes, with a corresponding decrease in an intracellular membrane fraction. cAMP-elevating treatments depressed the metformin-, but not the insulin-dependent glucose uptake, by 20-30%. In myocytes from diabetic rats, the rate of metformin-activated glucose transport was similar to that of cells from control animals, whereas basal and insulin-stimulated transport were substantially diminished. Finally, metformin (5 mM) induced a slight depression of oxygen consumption and energy metabolism of myocytes (as determined by measuring their level of energy-rich phosphates) comparable to the effects of hypoxia in rat hearts. In conclusion, these data do not provide evidence in favor of the hypothesis that glucose uptake by muscle tissue represents the site of metformin's therapeutic action in vivo. On the other hand, the large, insulin-independent effect of metformin at high concentrations (approximately mM) in vitro may be related to the action of hypoxia and occurs through a redistribution of glucose carriers from an intracellular locus to the plasma membrane. The mechanism (or signal) involved in metformin's action is likely to differ from that triggered by insulin and is not impaired in the diabetic state.

Animals↗
PubMed1995

Direct stimulation of myocardial glucose transport and glucose transporter-1 (GLUT1) and GLUT4 protein expression by the sulfonylurea glimepiride.

Freshly isolated and primary cultured cardiac myocytes from adult rats were used to elucidate acute and chronic effects of the sulfonylurea drug glimepiride on basal and insulin-stimulated glucose transport and on expression of the transporter isoforms glucose transporter-1 (GLUT1) and GLUT4. A 30-min incubation with glimepiride (100 microM) was unable to modify the initial rates of 3-O-methylglucose transport in freshly isolated cardiocytes, both in the absence or presence of insulin (10(-7) M). Cells were then kept in serum-free culture for 20 h in the presence of glimepiride (10 microM) and a physiological insulin dose. Under these conditions, the sulfonylurea induced an increase in 2-deoxyglucose uptake to 186% of control. This drug effect was dose dependent and could also be demonstrated in the absence of insulin during the culture period. The acute action of insulin on glucose transport was additive to the effect of glimepiride, and the insulin responsiveness of glucose transport remained unaltered in sulfonylurea-treated cultures. Western blot analysis of crude membrane fractions obtained from cultured cardiocytes showed that glimepiride increased the expression of both GLUT1 and GLUT4 to 164% +/- 21% and 148% +/- 5% of control, respectively. We concluded that glimepiride increases cardiac glucose uptake by an insulin-independent pathway, probably involving an increased protein expression of GLUT1 and GLUT4. The increased expression of GLUT4 may have a therapeutic impact on the treatment of insulin-resistant states.

3-O-Methylglucose↗
PubMed1998

Cloning of the Candida albicans nucleoside transporter by complementation of nucleoside transport-deficient Saccharomyces.

The nucleoside permease gene (i.e. NUP) from Candida albicans was cloned by complementation of Saccharomyces cerevisiae deficient in nucleoside transport capability. The permease transported adenosine and guanosine and was sensitive to the mammalian nucleoside transport inhibitors: dipyridamole and NBMPR. It did not transport uridine, cytidine, adenine, guanine or uracil. The inability to transport uridine indicated that the NUP gene product was different from the Candida uridine permease, which also transported cytosine and adenosine. The NUP gene coded for a protein of 407 amino acids in size which was approximately the size of the human, Giardia and E. coli nucleoside permeases. It did not, however, exhibit any significant degree of homology with these transporters.

Adenosine↗
PubMed1997

17Beta-estradiol attenuates oxidative impairment of synaptic Na+/K+-ATPase activity, glucose transport, and glutamate transport induced by amyloid beta-peptide and iron.

Synapse loss, deposits of amyloid beta-peptide (Abeta), impaired energy metabolism, and cognitive deficits are defining features of Alzheimer's disease (AD). Estrogen replacement therapy reduces the risk of developing AD in postmenopausal women. Because synapses are likely sites for initiation of neurodegenerative cascades in AD, we tested the hypothesis that estrogens act directly on synapses to suppress oxidative impairment of membrane transport systems. Exposure of rat cortical synaptosomes to Abeta25-35 (Abeta) and FeSO4 induced membrane lipid peroxidation and impaired the function of the plasma membrane Na+/K+-ATPase, glutamate transporter, and glucose transporter. Pretreatment of synaptosomes with 17beta-estradiol or estriol largely prevented impairment of Na+/K+-ATPase activity, glutamate transport, and glucose transport; other steroids were relatively ineffective. 17Beta-estradiol suppressed membrane lipid peroxidation induced by Abeta and FeSO4, but did not prevent impairment of membrane transport systems by 4-hydroxynonenal (a toxic lipid peroxidation product), suggesting that an antioxidant property of 17beta-estradiol was responsible for its protective effects. By suppressing membrane lipid peroxidation in synaptic membranes, estrogens may prevent impairment of transport systems that maintain ion homeostasis and energy metabolism, and thereby forestall excitotoxic synaptic degeneration and neuronal loss in disorders such as AD and ischemic stroke.

Amyloid beta-Peptides↗
PubMed1998

Deficient transport of dehydroascorbic acid in the glucose transporter protein syndrome.

The glucose transporter protein syndrome (GTPS) is caused by defective transport of glucose across the blood-brain barrier via the glucose transporter GLUT1, resulting in hypoglycorrhachia, infantile seizures, and developmental delay. Recent reports indicated that GLUT1 is a multifunctional transporter. We investigated the transport of vitamin C in its oxidized form (dehydroascorbic acid) via GLUT1 into erythrocytes of 2 patients with GTPS. In both patients, uptake of oxidized vitamin C was 61% of the mothers' values. Our findings are consistent with recent observations that vitamin C is transported in its oxidized form via GLUT1. We speculate that impaired transport of this substrate and perhaps other substrates in GTPS might contribute to the pathophysiology of this condition.

Adult↗
PubMed1994

Evidence that modulation of glucose transporter intrinsic activity is the mechanism involved in the allose-mediated depression of hexose transport in mammalian cells.

In serum starved V79 Chinese hamster lung fibroblast cells, replacement of D-glucose with D-allose resulted in a significant 38 +/- 18% (P < 0.05) reduction of 2-deoxy-D-glucose (2-DG) transport. Similarly, in a respiration-deficient mutant cell line (V79-G14), which has elevated 2-DG transport activity, D-allose reduced 2-DG transport by 59 +/- 18% (P < 0.05). [3H]D-allose uptake by V79 cells occurred slowly and was not inhibited by cytochalasin B, suggesting diffusion as the mode of D-allose entry. Western blot analysis using a rabbit polyclonal antibody to the human erythrocyte glucose transporter (GT) demonstrated that, in both cell lines, GT content and GT subcellular distribution were not significantly different in D-glucose vs. D-allose-treated cells. delta-Antibody, which has been shown to bind to exofacial epitopes of the GT (Harrison et al., 1990, J. Biol. Chem., 265:5793-5801), did not demonstrate any differences in surface binding to D-glucose vs. D-allose-treated intact V79 cells. D-allose treatment of 3T3 fibroblasts resulted in a similar decrease (72%) of 2-DG transport, however D-allose had no apparent effect on basal sugar transport in 3T3 adipocytes. These results suggest that D-allose reduces sugar transport through a modulation of the intrinsic activity of the GT, and that D-allose may act in a tissue-specific manner.

3T3 Cells↗
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↗