Increase In GLUT4 Research Articles

Oncogenic H-Ras Affects on Glucose Transporter and Hexokinase RNA Stabilization

Increased glucose uptake and phosphorylation is a common event seen in most solid malignancies. Previously we have shown in an ITPG inducible oncogenic H-RasV12 rat intestinal epithelial cell line (RIE), that H-Ras increases glucose metabolism with an increase in the Vmax of glucose utilization without an appreciable change in the Km of the reaction. There is also a corresponding increase in GLUT1, GLUT3, and HK2 mRNA and protein, with little change in HK1 levels. GLUT1 and HK2 mRNA levels typically increase 4 - 8 fold after a 24hour induction of H-Ras.

Molecular mechanisms involved in Sertoli cell adaptation to glucose deprivation

Sertoli cells provide the physical support and the necessary environment for germ cell development. Among the products secreted by Sertoli cells, lactate, the preferred energy substrate for spermatocytes and spermatids, is present. Considering the essential role of lactate on germ cell metabolism, it is supposed that Sertoli cells must ensure its production even in adverse conditions, such as those that would result from a decrease in glucose levels in the extracellular milieu. The aim of the present study was to investigate 1) a possible effect of glucose deprivation on glucose uptake and on the expression of glucose transporters in rat Sertoli cells and 2) the participation of different signal transduction pathways in the above-mentioned regulation. Results obtained show that decreasing glucose levels in Sertoli cell culture medium provokes 1) an increase in glucose uptake accompanied by only a slight decrease in lactate production, 2) an increase in GLUT1 and a decrease in GLUT3 expression, and 3) an activation of AMP-activated protein kinase (AMPK)-, phosphatidylinositol 3-kinase (PI3K)/PKB-, and p38 MAPK-dependent pathways. Additionally, by using specific inhibitors of these pathways, a possible participation of AMPK- and p38MAPK-dependent pathways in the regulation of glucose uptake and GLUT1 expression is shown. These results suggest that Sertoli cells adapt to conditions of glucose deprivation to ensure an adequate lactate concentration in the microenvironment where germ cell development occurs.

Prenatal Low Protein Diet Influences Placental Gene Expression In Rats

Placental function and efficiency has been identified as a strong predictor of fetal growth and development. Because placenta acts as the only means for the fetus to obtain its nutrients, changes in placental morphology have dire consequences for fetal size and development. However, there is little data showing what physiological or external factors may cause these changes. This study intended to show that nutrition, specifically a low protein diet, contributes to altered expression of genes associated with placental health and transport capacity, including Igf2, H19, Atf3, Atf4, Dnmt1, Peg3, Glut1 and Snat2. Feeding pregnant rats a 9% low protein (LP) diet throughout gestation did not affect food intake or maternal weight. Atf3, but not Atf4 expression increased in LP placentas and there was a trend of increase in Igf2 and H19 expression. There was also a significant decrease in Peg3 and Dnmt1 expression in the LP group, and a significant increase in Glut1 and Snat2 expression. We did not detect changes in DNA methylation in many of these genes, indicating that control of gene expression for these genes is accomplished by means other than DNA methylation. Our study showed that mammalian amino acid response (AAR) pathway in placenta is up‐regulated by prenatal LP diet and this pathway can potentially cause other changes in placental gene expression.

In vitro silencing of a putative phospholipid translocase results in an increase in glucose uptake in 3T3 L1 adipocytes

Atp10c is a putative phospholipid translocase which encodes for a type IV P‐type ATPase. Atp10c gene, located on mouse chromosome 7, appears to be a strong candidate for diet‐induced obesity and diabetes mellitus type II. Our in vitro studies using a 3T3‐L1 murine adipocyte cell line revealed that when Atp10c expression was silenced or knocked down using siRNA, glucose uptake and fat accumulation increased, while insulin resistance decreased. Furthermore, the silencing effect of Atp10c caused an increase in Glut 1 and CEBP‐α gene expression and a decrease in Glut 4 and PPAR‐γ gene expression. To further investigate the role of Atp10c in insulin resistance and glucose uptake, immunolocalization of Atp10c by fluorescence microscopy was employed. Data from these experiments suggest that Atp10c not only co‐localizes with caveolin‐1 and GLUT4 at the nuclear membrane, but also can be visualized as a punctate pattern in and around the nucleus. This finding suggests that Atp10c additionally may localize to the trans‐Golgi network or endosome, strengthening its biological role in protein trafficking in the exocytic and endocytic pathways.

Erythrocyte Glut1 Triggers Dehydroascorbic Acid Uptake in Mammals Unable to Synthesize Vitamin C

Of all cells, human erythrocytes express the highest level of the Glut1 glucose transporter. However, the regulation and function of Glut1 during erythropoiesis are not known. Here, we report that glucose transport actually decreases during human erythropoiesis despite a >3-log increase in Glut1 transcripts. In contrast, Glut1-mediated transport of L-dehydroascorbic acid (DHA), an oxidized form of ascorbic acid (AA), is dramatically enhanced. We identified stomatin, an integral erythrocyte membrane protein, as regulating the switch from glucose to DHA transport. Notably though, we found that erythrocyte Glut1 and associated DHA uptake are unique traits of humans and the few other mammals that have lost the ability to synthesize AA from glucose. Accordingly, we show that mice, a species capable of synthesizing AA, express Glut4 but not Glut1 in mature erythrocytes. Thus, erythrocyte-specific coexpression of Glut1 with stomatin constitutes a compensatory mechanism in mammals that are unable to synthesize vitamin C.

Diet-induced ketosis increases capillary density without altered blood flow in rat brain

It is recognized that ketone bodies, such as R-beta-hydroxybutyrate (beta-HB) and acetoacetate, are energy sources for the brain. As with glucose metabolism, monocarboxylate uptake by the brain is dependent on the function and regulation of its own transporter system. We concurrently investigated ketone body influx, blood flow, and regulation of monocarboxylate transporter (MCT-1) and glucose transporter (GLUT-1) in diet-induced ketotic (KG) rat brain. Regional blood-to-brain beta-HB influx (micromol.g(-1).min(-1)) increased 40-fold with ketosis (4.8 +/- 1.8 plasmabeta-HB; mM) in all regions compared with the nonketotic groups (standard and no-fat diets); there were no changes in regional blood flow. Immunohistochemical staining revealed that GLUT-1 density (number/mm2) in the cortex was significantly elevated (40%) in the ketotic group compared with the standard and no-fat diet groups. MCT-1 was also markedly (3-fold) upregulated in the ketotic group compared with the standard diet group. In the standard diet group, 40% of the brain capillaries stained positive for MCT-1; this amount doubled with the ketotic diet. Western blot analysis of isolated microvessels from ketotic rat brain showed an eightfold increase in GLUT-1 and a threefold increase in MCT-1 compared with the standard diet group. These data suggest that diet-induced ketosis results in increased vascular density at the blood-brain barrier without changes in blood flow. The increase in extraction fraction and capillary density with increased plasma ketone bodies indicates a significant flux of substrates available for brain energy metabolism.

Glucose transporters in diabetic nephropathy

Changes in glucose transporter expression in glomerular cells occur early in diabetes. These changes, especially the GLUT1 increase in mesangial cells, appear to play a pathogenic role in the development of ECM expansion and perhaps other features of diabetic nephropathy. In addition, it appears that at least some diabetic patients may be predisposed to nephropathy because of polymorphisms in their GLUT1 genes. GLUT1 overexpression leads to increased glucose metabolic flux which in turn triggers the polyol pathway and activation of PKC alpha and B1. Activation of these PKC isoforms can lead directly to AP-1 induced increases in fibronectin expression and ECM accumulation. Other, more novel effects of GLUT1 on cellular hypertrophy and injury could also promote changes of diabetic nephropathy. Strategies to prevent GLUT1 overexpression could ameliorate or prevent the progression of diabetic nephropathy.

Effects of mercuric chloride on glucose transport in 3T3-L1 adipocytes

Mercury, as well as the other Group IIB metals, stimulates glucose transport in adipocytes. Here we characterize the action of mercury on adipocyte glucose transport and examine several potential mechanisms of action. Mercury exposure causes a modest (compared to insulin) 1.8-fold increase in glucose transport. This glucose transport corresponds with an increase in GLUT 1, but not GLUT 4 glucose transporters. Phosphorylation of p38 kinase and c-Jun N-terminal kinase (JNK) were examined as possible mediators of mercury induced GLUT 1 levels. Phosphorylation of p38 kinase, but not JNK, increased with mercury exposure. Activation of p38 and an increase in glucose transport corresponding to an increase in GLUT 1 are indicative the induction of a stress response, which can contribute to the induction of insulin resistance in adipocytes. However, inhibition of p38 by the p38 inhibitor SB203580 did not prevent mercury-mediated glucose uptake. While the magnitude of the action of mercury is modest, its effects were sustained over many days of exposure and impacted subsequent insulin-mediated glucose transport. Pre-treatment with HgCl 2 decreased insulin-mediated glucose transport 1.3-fold suggesting that exposure to mercury may contribute to pathologies associated with glucose homeostasis.

Alterations in hexose, amino acid and peptide transporter expression in intestinal epithelial cells during Nippostrongylus brasiliensis infection in the rat

Infection with the nematode Nippostrongylus brasiliensis induces various types of cytological alterations in the intestinal villus epithelium. The aim of this study was to analyse the expression of hexose, peptide and amino acid transporters in the small intestinal epithelium after infection. Brown-Norway rats were infected with 2000 N. brasiliensis L3 larvae and villus epithelial cells were isolated at various time points after infection. Expression of hexose transporters Na +/glucose cotransporter SGLT1 and glucose transporter GLUT-1, -2 and -5, a peptide transporter (PepT1) and an amino acid transporter (LAT2) was examined by reverse transcription-PCR, Western blotting or immunohistochemistry. Semi-quantitative reverse transcription-PCR studies of separated jejunal epithelial cells showed that expression levels of GLUT5, PepT1 and LAT2 were significantly decreased 7 and 14 days after infection, while these changes were not observed in the ileal epithelium. Although the apical surface glucose transporter SGLT1 showed no significant alteration in mRNA expression, Western blotting analyses of jejunal epithelial cell lysate showed a marked decrease. Contrary to SGLT1, GLUT5, PepT1 and LAT2, expression of GLUT1, which is essential in maintaining high rates of glucose influx, was significantly up-regulated in the jejunal epithelium 7 and 14 days after infection in reverse transcription-PCR as in Western blotting analyses. Immunohistochemical studies showed that GLUT1 immunoreactivity was localised to the basolateral membrane of intestinal epithelial cells 7 days after infection. These results show that N. brasiliensis infection results in an increase in GLUT1 and a decrease in various hexose, amino acid and peptide transporter expression in jejunal epithelial cells. Up-regulation of GLUT1 might be a compensatory response in injured epithelial cells.

Effects of selective hyperglycemia and hyperinsulinemia on glucose transporters in fetal ovine skeletal muscle.

We measured net fetal glucose uptake rate from the placenta, shown previously to be equal to total fetal glucose utilization rate (GUR(f)) and proportional to fetal hindlimb skeletal muscle glucose utilization, under normal conditions and after 1, 2.5, and 24 h of selective hyperglycemia increasing G or selective hyperinsulinemia increasing I. We simultaneously measured the amount of Glut 1 and Glut 4 glucose transporter proteins in fetal sheep skeletal muscle. With increasing G , GUR(f) was increased approximately 40% at 1 and 2.5 h but returned to the control rate by 24 h. This transient increasing G -specific increasing GUR(f) was associated with increased plasma membrane-associated Glut 1 (4-fold) and intracellular Glut 4 (3-fold) protein beginning at 1 h. With increasing I, GUR(f) was increased approximately 70% at 1, 2.5, and 24 h. This more sustained increasing I-specific increasing GUR(f) was associated with a significant increase in Glut 4 protein (2-fold) at 2.5 h but no change in Glut 1 protein. These results show that increasing G and increasing I have independent effects on the amount of Glut 1 and Glut 4 glucose transporter proteins in ovine fetal skeletal muscle. These effects are time dependent and isoform specific and may contribute to increased glucose utilization in fetal skeletal muscle. The lack of a sustained temporal correlation between the increase in transporter proteins and glucose utilization rates indicates that subcellular localization and activity of a transporter or tissues other than the skeletal muscle contribute to net GUR(f).

Glucose transport and apoptosis after gene therapy with HSV thymidine kinase.

The relation between tumour metabolism and induction of apoptosis by gene therapy was investigated in a rat Morris hepatoma (MH3924A) model expressing the HSV thymidine kinase (HSVtk) gene. In vivo the amount of glucose transporter (GLUT1 and GLUT3 isoforms) expressing cells was determined in tumours of untreated and treated animals using immunohistochemistry. In vitro uptake studies with 2-fluoro-2-deoxy-D-glucose (FDG), 3-O-methylglucose and thymidine (TdR) and a TUNEL (TdT-mediated dUTP nick end labelling) assay for the assessment of apoptosis were done immediately and 24 h after treatment of the recombinant cells with different doses of ganciclovir (GCV). Immunohistochemistry revealed a significant increase in GLUT1 in treated tumours which showed enhanced transport activity for FDG. In vitro the FDG and 3-O-methylglucose uptake increased to 186% when compared with that of the non-treated cells immediately after incubation with GCV. However, 24 h later the FDG uptake had declined to its normal level, whereas the accumulation of 3-O-methylglucose remained elevated. The uptake of TdR, which was determined simultaneously, decreased in the acid-insoluble fraction of the cells to 27% and 11%, respectively, immediately and 24 h after therapy, while in the acid-soluble fraction it increased to 229% and to 167%, respectively. Employing the TUNEL technique, 25% of cells were found to be apoptotic 24 h after the termination of GCV treatment. Inhibition of glucose transport by cytochalasin B or competition with deoxyglucose resulted in a 78% (cytochalasin B) and 88% (deoxyglucose) decrease in FDG uptake in the recombinant hepatoma cells and in an increase in the apoptotic cell fraction. It is concluded that inhibition of enhanced glucose transport in GCV-treated cells increased apoptosis. Therefore, enhanced glucose transport seems to represent a stress reaction of tumour cells dedicated for the prevention of cell death.

Insulin-mediated translocation of GLUT-4-containing vesicles is preserved in denervated muscles.

Skeletal muscle denervation decreases insulin-sensitive glucose uptake into this tissue as a result of marked GLUT-4 protein downregulation ( approximately 20% of controls). The process of insulin-stimulated glucose transport in muscle requires the movement or translocation of intracellular GLUT-4-rich vesicles to the cell surface, and it is accompanied by the translocation of several additional vesicular cargo proteins. Thus examining GLUT-4 translocation in muscles from denervated animals allows us to determine whether the loss of a major cargo protein, GLUT-4, affects the insulin-dependent behavior of the remaining cargo proteins. We find no difference, control vs. denervated, in the insulin-dependent translocation of the insulin-responsive aminopeptidase (IRAP) and the receptors for transferrin and insulin-like growth factor II/mannose 6-phosphate, proteins that completely (IRAP) or partially co-localize with GLUT-4. We conclude that 1) denervation of skeletal muscle does not block the specific branch of insulin signaling pathway that connects receptor proximal events to intracellular GLUT-4-vesicles, and 2) normal levels of GLUT-4 protein are not necessary for the structural organization and insulin-sensitive translocation of its cognate intracellular compartment. Muscle denervation also causes a twofold increase in GLUT-1. In normal muscle, all GLUT-1 is present at the cell surface, but in denervated muscle a significant fraction (25.1 +/- 6.1%) of this transporter is found in intracellular vesicles that have the same sedimentation coefficient as GLUT-4-containing vesicles but can be separated from the latter by immunoadsorption. These GLUT-1-containing vesicles respond to insulin and translocate to the cell surface. Thus the formation of insulin-sensitive GLUT-1-containing vesicles in denervated muscle may be a compensatory mechanism for the decreased level of GLUT-4.

Stimulation of glucose transport by thyroid hormone in 3T3-L1 adipocytes: increased abundance of GLUT1 and GLUT4 glucose transporter proteins.

In 3T3-L1 adipocytes we have examined the effect of tri-iodothyronine (T(3)) on glucose transport, total protein content and subcellular distribution of GLUT1 and GLUT4 glucose transporters. Cells incubated in T(3)-depleted serum were used as controls. Cells treated with T(3) (50 nM) for three days had a 3.6-fold increase in glucose uptake (P<0.05), and also presented a higher insulin sensitivity, without changes in insulin binding. The two glucose carriers, GLUT1 and GLUT4, increased by 87% (P<0.05) and 90% (P<0. 05), respectively, in cells treated with T(3). Under non-insulin-stimulated conditions, plasma membrane fractions obtained from cells exposed to T(3) were enriched with both GLUT1 (3. 29+/-0.69 vs 1.20+/-0.29 arbitrary units (A.U.)/5 microg protein, P<0.05) and GLUT4 (3.50+/-1.16 vs 0.82+/-0.28 A.U./5 microg protein, P<0.03). The incubation of cells with insulin produced the translocation of both glucose transporters to plasma membranes, and again cells treated with T(3) presented a higher amount of GLUT1 and GLUT4 in the plasma membrane fractions (P<0.05 and P<0.03 respectively). These data indicate that T(3) has a direct stimulatory effect on glucose transport in 3T3-L1 adipocytes due to an increase in GLUT1 and GLUT4, and by favouring their partitioning to plasma membranes. The effect of T(3) on glucose uptake induced by insulin can also be explained by the high expression of both glucose transporters.

Increased intracellular localization of brain GLUT-1 transporter in response to ethanol during chick embryogenesis.

Fetal exposure to ethanol is associated with growth retardation of the developing central nervous system. We have previously described a chick model to study the molecular mechanism of ethanol effects on glucose metabolism in ovo. Total membrane fractions were prepared from day 4, day 5, and day 7 chick embryos exposed in ovo to ethanol or to vehicle. By Western blotting analysis, ethanol exposure caused a mean 7- to 10-fold increase in total GLUT-1 and a 2-fold increase in total GLUT-3. However, glucose uptake by ethanol-treated cells increased by only 10%. Analysis of isolated plasma (PM) and intracellular (IM) membranes from day 5 cranial tissue revealed a mean 25% decrease in GLUT-1 in the PM and a 66% increase in the IM in the ethanol group vs. control. The amount of PM GLUT-3 was unchanged but that of IM GLUT-3 was significantly decreased. The data suggest that GLUT-3 cell surface expression may be resistant to the suppressive effects of ethanol in the developing brain of ethanol-treated embryos. The overall increase in GLUT-1 may reflect a deregulation of the transporter induced by ethanol exposure. The increased IM localization and decreased amount of PM GLUT-1 may be a mechanism used by the ethanol-treated cell to maintain normal glucose uptake despite the overall increased level of the transporter.

In L6 Skeletal Muscle Cells, Glucose Induces Cytosolic Translocation of Protein Kinase C-α and Trans-activates the Insulin Receptor Kinase

In L6 skeletal muscle cells expressing human insulin receptors (L6(hIR)), exposure to 25 mM glucose for 3 min induced a rapid 3-fold increase in GLUT1 and GLUT4 membrane translocation and glucose uptake. The high glucose concentration also activated the insulin receptor kinase toward the endogenous insulin receptor substrates (IRS)-1 and IRS-2. At variance, in L6 cells expressing kinase-deficient insulin receptors, the exposure to 25 mM glucose elicited no effect on glucose disposal. In the L6(hIR) cells, the acute effect of glucose on insulin receptor kinase was paralleled by a 2-fold decrease in both the membrane and the insulin receptor co-precipitated protein kinase C (PKC) activities and a 3-fold decrease in receptor Ser/Thr phosphorylation. Western blotting of the receptor precipitates with isoform-specific PKC antibodies revealed that the glucose-induced decrease in membrane- and receptor-associated PKC activities was accounted for by dissociation of PKCalpha but not of PKCbeta or -delta. This decrease in PKCalpha was paralleled by a similarly sized increase in cytosolic PKCalpha. In intact L6(hIR) cells, inhibition of PKCalpha expression by using a specific antisense oligonucleotide caused a 3-fold increase in IRS phosphorylation by the insulin receptor. This effect was independent of insulin and accompanied by a 2.5-fold increase in glucose disposal by the cells. Thus, in the L6 skeletal muscle cells, glucose acutely regulates its own utilization through the insulin signaling system, independent of insulin. Glucose autoregulation appears to involve PKCalpha dissociation from the insulin receptor and its cytosolic translocation.

Intra-uterine growth restriction differentially regulates perinatal brain and skeletal muscle glucose transporters

Employing Western blot analysis, we investigated the effect of maternal uterine artery ligation causing uteroplacental insufficiency with asymmetrical intrauterine growth restriction (IUGR) upon fetal (22d) and postnatal (1d, 7d, 14d and 21d) brain (Glut 1 and Glut 3) and skeletal muscle (Glut 1 and Glut 4) glucose transporter protein concentrations. IUGR was associated with a ∼42% decline in fetal plasma glucose ( p<0.05) and a ∼25% decrease in fetal body weights ( p<0.05) with no change in brain weights when compared to the sham operated controls (SHAM). In addition, IUGR caused a ∼45% increase in fetal brain Glut 1 (55 kDa) with no change in Glut 3 (50 kDa) protein concentrations. This fetal brain Glut 1 change persisted, though marginal, through postnatal suckling stages of development (1d–21d), with no concomitant change in brain Glut 3 levels at day 1. In contrast, in the absence of a change in fetal skeletal muscle Glut 1 levels (48 kDa), a 70% increase was observed in the 1d IUGR with no concomitant change in either fetal or postnatal Glut 4 levels (45 kDa). The change in skeletal muscle Glut 1 levels normalized by d7 of age. We conclude that IUGR with hypoglycemia led to a compensatory increase in brain and skeletal muscle Glut 1 concentrations with a change in the brain preceding that of the skeletal muscle. Since Glut 1 is the isoform of proliferating cells, fetal brain weight changes were not as pronounced as the decline in somatic weight. The increase in Glut 1 may be protective against glucose deprivation in proliferating fetal brain cells and postnatal skeletal myocytes which exhibit `catch-up growth', thereby preserving the specialized function mediated by Glut 3 and Glut 4 towards maintaining the intracellular glucose milieu.

Developmental regulation of genes mediating murine brain glucose uptake.

We examined the molecular mechanisms that mediate the developmental increase in murine whole brain 2-deoxyglucose uptake. Northern and Western blot analyses revealed an age-dependent increase in brain GLUT-1 (endothelial cell and glial) and GLUT-3 (neuronal) membrane-spanning facilitative glucose transporter mRNA and protein concentrations. Nuclear run-on experiments revealed that these developmental changes in GLUT-1 and -3 were regulated posttranscriptionally. In contrast, the mRNA and protein levels of the mitochondrially bound glucose phosphorylating hexokinase I enzyme were unaltered. However, hexokinase I enzyme activity increased in an age-dependent manner suggestive of a posttranslational modification that is necessary for enzymatic activation. Together, the postnatal increase in GLUT-1 and -3 concentrations and hexokinase I enzymatic activity led to a parallel increase in murine brain 2-deoxyglucose uptake. Whereas the molecular mechanisms regulating the increase in the three different gene products may vary, the age-dependent increase of all three constituents appears essential for meeting the increasing demand of the maturing brain to fuel the processes of cellular growth, differentiation, and neurotransmission.

Placental glucose transport and GLUT 1 expression in insulin-dependent diabetes

Objective: Altered transport functions in the placenta might contribute to adverse outcome of pregnancies in women with diabetes. Therefore we studied placental glucose transport in this pregnancy complication. Study Design: Syncytiotrophoblast microvillous membrane vesicles and basal membrane vesicles were isolated from women with uneventful pregnancies (control subjects, n = 21) and from women with pregnancies complicated by insulin-dependent diabetes mellitus, White class D (n = 7). Glucose uptake and GLUT 1 (glucose transporter 1) expression were studied by means of radiolabeled tracers and Western blot, respectively. Results: In the group with insulin-dependent diabetes mellitus, values for hemoglobin A 1c were moderately elevated in the first trimester (6.61 ± 0.35) but not later in pregnancy and 4 of the 7 neonates were large for gestational age. In the basal membrane vesicles, insulin-dependent diabetes mellitus was associated with a 40% increase in GLUT 1 expression and a 59% higher mediated uptake of d -glucose. No alterations could be demonstrated in microvillus membrane vesicles. Conclusion: Placental glucose transport capacity appears to be increased in insulin-dependent diabetes mellitus. These alterations might explain the occurrence of macrosomia despite well-controlled diabetes. (Am J Obstet Gynecol 1999;180:163-8.)

Distribution of Glucose Transporter - 3 and Hexokinase I in the Developing Mouse Brain. • 421

Glucose is transported into neurons by a facilitative glucose transporter - 3 (Glut 3) isoform and is phosphorylated into glucose-6-phosphate by hexokinase I. Employing the 2d, 14d, and 60d old rostrocaudal floating microtome murine brain sections (n=3 each) and immunohistochemical analysis we examined the distribution of Glut 3 and hexokinase I. An age-dependent increase in Glut 3 immunoreactivity was uniformly noted in all regions. Overall Glut 3 was present in the neuropil, neuronal processes, and nerve fiber bundles in most regions examined with the exception of a neuronal cellular distribution in the cerebellar granular layer. Prominent axonal Glut 3 labeling was noted in circumventricular regions lacking a blood-brain barrier and the periventricular hypothalamic region. In the 2d old, Glut 3 positive cells and processes were also noted in the dorsomedial subnuclei of the suprachiasmatic nucleus. In contrast, hexokinase I was observed intracellularly with a perinuclear distribution in all areas, including the cerebellar Purkinje cells and dendrites that extend into the molecular layer, the laminar zone between the brain parenchymal and ependymal cells of the third ventricle, and the choroid plexus. We conclude that 1] Glut 3 immunoreactivity increases with age while hexokinase I labeling remains unchanged, 2] Glut 3 is expressed in neuronal cellular processes, being most prominent in regions that are exposed to circulating factors and peripheral stimuli, while hexokinase I is noted intracellularly in neurons and non-neuronal cells. We speculate that Glut 3 mediates glucose transport necessary to fuel neurotransmission in response to perturbations in the metabolic/hormonal mileu, while hexokinase I phosphorylates glucose in brain cells not limited to expressing Glut 3.

Increase of glucose transporter densities (Glut1 and Glut3) during chronic administration of nicotine in rat brain

Chronic infusion of nicotine is known to result in a distinct pattern of increases in local cerebral glucose utilization (LCGU). The present study addresses the question whether this increase in LCGU is paralleled by (1) a local increase in Glut1 and/or Glut3 glucose transporter densities and (2) a local increase in capillary density in the brain. Nicotine was infused by osmotic minipumps for one week. In cryosections of rat brains local densities of Glut1 (vascular) and Glut3 (neuronal) glucose transporters were measured by immunoautoradiographic methods whereas local capillary densities were determined by an immunofluorescent method. Densities of glucose transporters Glut1 and Glut3 were increased in 12 of the 27 structures investigated. Glut1 was elevated in four additional structures and Glut3 in two more structures. Comparison of the changes in transporter densities with the changes of LCGU measured in a previous study during chronic nicotine infusion showed that LCGU was also elevated in most of these structures. In contrast, capillary density remained unchanged in all structures investigated. It is concluded that one week of nicotine infusion is sufficient to raise the densities of Glut1 and Glut3 glucose transporters predominantly in those structures in which LCGU is elevated. The unchanged capillary density under these conditions indicates an increased density of Glut1 transporters per capillary.