Number Of Glucose Transporters Research Articles

Resistance Training in Cardiovascular Diseases: A Review on Its Effectiveness in Controlling Risk Factors.

Cardiovascular Disease (CVD), a term encompassing various disorders affecting the heart and blood vessels, includes coronary artery disease (CAD). CAD is primarily due to the development of atherosclerotic plaques that disrupt blood flow, oxygenation, and nutrient delivery to the myocardium. Risk factors contributing to CAD progression include smoking, hypertension, diabetes mellitus (DM), dyslipidaemia, and obesity. While aerobic exercise (AE) has shown promising results in controlling CVD risk factors, the impact of resistance training (RT) has not been extensively investigated. This review aims to describe the effects of RT on CVD risk factors based on studies retrieved from PubMed and Google Scholar databases. Both isometric and isotonic RT have been found to decrease systolic blood pressure (SBP), diastolic blood pressure, or mean arterial pressure, with SBP showing a more significant reduction. Hypertensive patients engaging in RT alongside a calorie-restricted diet demonstrated significant improvements in blood pressure. RT is associated with increased nitric oxide bioavailability, sympathetic modulation, and enhanced endothelial function. In type-2 DM patients, 8-12 weeks of RT led to improvements in fasting blood glucose levels, insulin secretion, metabolic syndrome risk, and glucose transporter numbers. Combining AE with RT had a more significant impact in reducing insulin resistance and enhancing blood glucose compared to performing exercises separately. It also significantly decreased total cholesterol, triglycerides, and low-density lipoprotein levels while increasing high-density lipoprotein within 12 weeks of application. However, improvements are considered insignificant when lipid levels are already low to normal at baseline. The administration of RT resulted in weight loss and improved body mass index, with more pronounced effects seen when combining AE with RT and a calorie-restricted diet. Considering these results, the administration of RT, either alone or in combination with AE, proves beneficial in rehabilitating CAD patients by improving various risk factors.

Glucose transporters as markers of diagnosis and prognosis in cancer diseases.

The primary metabolic substrate for cells is glucose, which acts as both a source of energy and a substrate in several processes. However, being lipophilic, the cell membrane is impermeable to glucose and specific carrier proteins are needed to allow transport. In contrast to normal cells, cancer cells are more likely to generate energy by glycolysis; as this process generates fewer molecules of adenosine triphosphate (ATP) than complete oxidative breakdown, more glucose molecules are needed. The increased demand for glucose in cancer cells is satisfied by overexpression of a number of glucose transporters, and decreased levels of others. As specific correlations have been observed between the occurrence of cancer and the expression of glucose carrier proteins, the presence of changes in expression of glucose transporters may be treated as a marker of diagnosis and/or prognosis for cancer patients.

The Appearance of a Leptin Effect on Glucose Absorption in Caco2 Cells Depends on Their Differentiation Level

Backdround/Aims: The aim of this work was to study the effect and mechanism of action of leptin added apically, on glucose absorption, using Caco-2 cells as a model. Methods: Cells were grown on inserts and treated with leptin, at different time points after confluence. Radiolabelled glucose was added to the upper chamber and samples from the lower chamber were collected and assayed for radioactivity. Results: Glucose absorption increased with an increase in the level of differentiation and was associated with an increase in the protein expression level of glucose transporters. Leptin reduced glucose absorption only by day 16 after confluence, the time at which apical leptin receptors started appearing. This inhibitory effect became higher the longer the post confluence period, and was prominent on day 23. The hormone effect was found to be mediated via a decrease in the number of glucose transporters (SGLT1 and GLUT2) and a decrease in the activity of the Na⁺/K⁺ ATPase which was assayed by measuring the amount of inorganic phosphate liberated in presence and absence of enzyme activators. Conclusion: It was concluded that by day 23 post confluence, Caco-2 cells are differentiated and are appropriate to use as a model for intestinal transport studies.

Advection, diffusion, and delivery over a network.

Many biological, geophysical, and technological systems involve the transport of a resource over a network. In this paper, we present an efficient method for calculating the exact quantity of the resource in each part of an arbitrary network, where the resource is lost or delivered out of the network at a given rate, while being subject to advection and diffusion. The key conceptual step is to partition the resource into material that does or does not reach a node over a given time step. As an example application, we consider resource allocation within fungal networks, and analyze the spatial distribution of the resource that emerges as such networks grow over time. Fungal growth involves the expansion of fluid filled vessels, and such growth necessarily involves the movement of fluid. We develop a model of delivery in growing fungal networks, and find good empirical agreement between our model and experimental data gathered using radio-labeled tracers. Our results lead us to suggest that in foraging fungi, growth-induced mass flow is sufficient to account for long-distance transport, if the system is well insulated. We conclude that active transport mechanisms may only be required at the very end of the transport pathway, near the growing tips.

Relationship between erythrocyte GLUT1 function and membrane glycation in type 2 diabetes

This paper investigates the effect of glycation on glucose transport in erythrocytes. Glucose transporter function, numbers and erythrocyte phosphorylation rates are simultaneously studied using 30 Caucasian patients with diabetes and 30 Caucasian control volunteers (mean±SD where P≤0.05; age 48±8 vs. 45±8 years [ns]; body mass index [BMI] 31±7 vs. 27±5 [P=0.035]; blood glucose 12±7 vs. 5±0.6 mmol/L [P=0.001]; HbA1c 8±2 vs. 5±0.3% [P=0.0001]; fructosamine 336±64 vs. 237±16 µmol/L [P=0.0001]; disease duration 13±11 years, respectively). Significant differences were found for glucose transporter function, with 3-O-methylglucose uptake rates (108±49 vs. 146±55 µmol/L/sec/1012 cells [P=0.010]); D-glucose influx (64±30 vs. 117±45 µmol/L/sec/1012 cells [P=0.0001]); and D-glucose net transport (31±22 vs. 74±55 µmol/L/sec/ 1012 cells [P = 0.0001]). No differences were found for phosphorylation rates using 2-deoxyglucose (33±17 vs. 38±12 µmol/L/sec/1012 cells [P=0.194]). The number of functional transporters using cytochalasin B studies measured via Bmax, was not found to be significantly different between the groups (195±139 vs. 264±174 [P=0.206]). However, Kd was lower for those with diabetes, suggesting higher binding affinity (12±11 vs. 32±25 nmol/L [P=0.006]). A negative correlation between HbA1c and D-glucose influx involving both groups was found (r=–0.670, P=0.0001). Glucose transport is shown to be decreased in people who have diabetes compared to normoglycaemic volunteers, whereas the number of glucose transporters is apparently unchanged; however, affinity for binding is increased.

Pine bark extract inhibits glucose transport in enterocytes via mitogen-activated kinase and phosphoinositol 3-kinase

Objective Pine bark extract (PBE) has been reported to have hypoglycemic effects but its mode of action is still unclear. This work studied the effect of PBE on glucose uptake by Caco-2 cells in isolation of its effect on insulin, which may appear if ingested by the animal. Methods Caco-2 cells were incubated in the presence of PBE and [ 14C] 3-O-methyl-D-glucose as a tracer and the change in radioactivity of the incubation medium was taken as a measurement of glucose uptake. To determine the mechanism of action of the extract and type of transporters involved, Na +-coupled glucose transporter-1 (SGLT1) and glucose transporter-2 (GLUT2) and different signaling mediators known to be involved in glucose transport were inactivated by specific inhibitors. Changes in the protein expression of glucose transporters were studied by western blotting. Results The extract significantly decreased glucose transport but did not affect the activity or expression of Na +/K + adenosine triphosphatase. It was concluded that PBE affects the number of glucose transporters in the brush-border membrane. This conclusion was confirmed by western blot analysis. The results showed that the extract acts by activating p38 mitogen-activated kinase, which in turn activates SGLT1 transporters and two different pathways that target GLUT2: an inhibitory pathway involving phosphoinositol 3-kinase and a stimulatory pathway involving mitogen activated protein kinase/extracellular signal-regulated kinase kinase. The activity of the two pathways is orchestrated by SGLT1. Conclusion Pine bark extract inhibits glucose absorption by p38 mitogen-activated kinase and constitutes a potential complementary therapeutic or prophylactic agent for diabetes and its complications.

Insulin down-regulates the Na +/K + ATPase in enterocytes but increases intestinal glucose absorption

The effect of insulin on [ 14C] 3-O-methyl- d-glucose (3OMG) absorption in the rat jejunum was studied using an in situ perfusion technique. Insulin increased apical glucose entry into the cells and decreased intestinal retention suggesting that serosal glucose transport was enhanced by the hormone. This enhanced uptake was ascribed to an increase in the expression of glucose transporters as confirmed by Western blot analysis and not to a higher sodium gradient, since insulin reduced the activity and protein expression of the Na +/K + ATPase. To separate the glycemic from the insulinemic effect on glucose transport, the effect of the hormone was investigated in vitro using cultured Caco-2 cells. The cells also showed an increase in [ 14C] 3OMG uptake and intracellular glucose levels when treated with insulin and a lower Na +/K + ATPase activity. Phloretin, an inhibitor of GLUT2 was used to determine if these transporters are targeted by the hormone. The results showed that the effect of insulin on glucose uptake and intracellular glucose was still enhanced in presence of phloretin. Considering the inhibitory effect of the hormone on the Na +/K + ATPase, it was concluded that insulin acts by increasing the number of glucose transporters, a hypothesis that was confirmed by Western blot analysis.

Phosphatidylinostitol-3 kinase and phospholipase C enhance CSF-1-dependent macrophage survival by controlling glucose uptake

Colony stimulating factor-1 (CSF-1)-dependent macrophages play crucial roles in the development and progression of several pathological conditions including atherosclerosis and breast cancer metastasis. Macrophages in both of these pathologies take up increased amounts of glucose. Since we had previously shown that CSF-1 stimulates glucose uptake by macrophages, we have now investigated whether glucose metabolism is required for the survival of CSF-1-dependent macrophages as well as examined the mechanism by which CSF-1 stimulates glucose uptake. Importantly, we found that CSF-1-induced macrophage survival required metabolism of the glucose taken up in response to CSF-1 stimulation. Kinetic studies showed that CSF-1 stimulated an increase in the number of glucose transporters at the plasma membrane, including Glut1. The uptake of glucose induced by CSF-1 required intact PI3K and PLC signalling pathways, as well as the downstream effectors Akt and PKC, together with a dynamic actin cytoskeleton. Expression of constitutively active Akt partially restored glucose uptake and macrophage survival in the absence of CSF-1, suggesting that Akt is necessary but not sufficient for optimal glucose uptake and macrophage survival. Taken together, these results suggest that CSF-1 regulates macrophage survival, in part, by stimulating glucose uptake via Glut1, and PI3K and PLC signalling pathways.

Nephrotoxicity of uranyl acetate: effect on rat kidney brush border membrane vesicles

Since the Gulf war exposure to depleted uranium, a known nephrotoxic agent, there is a renewed interest in the toxic effects of uranium in general and its mechanism of nephrotoxicity which is still largely unknown in particular. In order to investigate the mechanism responsible for uranium nephrotoxicity and the therapeutic effect of urine alkalization, we utilized rat renal brush border membrane vesicles (BBMV). Uranyl acetate (UA) caused a decrease in glucose transport in BBMV. The apparent K (i) of uranyl was 139+/-30 microg uranyl/mg protein of BBMV. Uranyl at 140 microg/mg protein of BBMV reduced the maximal capacity of the system to transport glucose [V (max) 2.2+/-0.2 and 0.96+/-0.16 nmol/mg protein for control and uranyl treated BBMV (P<0.001), respectively] with no effect on the apparent K (m) (1.54+/-0.33 and 1.54+/-0.51 mM for control, and uranyl treated BBMV, respectively). This reduction in V(max) is at least partially due to a decrease in the number of sodium-coupled glucose transporters as apparent from the reduction in phlorizin binding to the uranyl treated membranes, V (max) was reduced from 247+/-13 pmol/mg protein in control BBMV to 119+/-3 pmol/mg protein in treated vesicles (P<0.001). The pH of the medium has a profound effect on the toxicity of UA on sodium-coupled glucose transport in BBMV: higher toxicity at neutral pH (around pH 7.0), and practically no toxicity at alkaline pH (7.6). This is the first report showing a direct inhibitory dose and pH dependent effect of uranyl on the glucose transport system in isolated apical membrane from kidney cortex.

Gene expression of glucose transporter (GLUT) 1, 3 and 4 in bovine follicle and corpus luteum

Glucose is the main energy substrate in the bovine ovary, and a sufficient supply of it is necessary to sustain the ovarian activity. Glucose cannot permeate the plasma membrane, and its uptake is mediated by a number of glucose transporters (GLUT). In the present study, we investigated the gene expression of GLUT1, 3 and 4 in the bovine follicle and corpus luteum (CL). Ovaries were obtained from Holstein x Japanese Black F1 heifers. Granulosa cells and theca interna layers were harvested from follicles classified into five categories by their physiologic status: follicular size (>or= 8.5 mm: dominant; < 8.5 mm: subordinate), ratio of estradiol (E(2)) to progesterone in follicular fluid (>or= 1: E(2) active;<1: E(2) inactive), and stage of estrous cycle (luteal phase, follicular phase). CL were also classified by the stage of estrous cycle. Expression levels of GLUT1, 3 and 4 mRNA were quantified by a real-time PCR. The mRNA for GLUT1 and 3 were detected in the bovine follicle and CL at comparable levels to those in classic GLUT-expressing organs such as brain and heart. Much lower but appreciable levels of GLUT4 were also detected in these tissues. The gene expression of these GLUT showed tissue- and stage-specific patterns. Despite considerable differences in physiologic conditions, similar levels of GLUT1, 3 and 4 mRNA were expressed in subordinate follicles as well as dominant E(2)-active follicles in both luteal and follicular phases, whereas a notable increase in the gene expression of these GLUT was observed in dominant E(2)-inactive follicles undergoing the atretic process. In these follicles, highly significant negative correlations were observed between the concentrations of glucose in follicular fluid and the levels of GLUT1 and 3 mRNA in granulosa cells, implying that the local glucose environment affects glucose uptake of follicles. These results indicate that GLUT1 and 3 act as major transporters of glucose while GLUT4 may play a supporting role in the bovine follicle and CL.

An In vivo Tumor Model Exploiting Metabolic Response as a Biomarker for Targeted Drug Development

In vivo models that recapitulate oncogene-dependent tumorigenesis will greatly facilitate development of molecularly targeted anticancer therapies. We have developed a model based on activating mutations in c-KIT in gastrointestinal stromal tumors (GISTs). This model comprises murine tumors of FDC-P1 cell lines expressing c-KIT mutations that render the tumors either responsive (V560G) or resistant (D816V) to the small-molecule c-KIT inhibitor, imatinib. Clinically, GIST response to imatinib is associated with rapid reduction in fluorodeoxyglucose (FDG) uptake on positron emission tomography (PET), preceding changes in conventional response criteria by several weeks. Using the FDC-P1 model in small animal PET, FDG uptake into tumors expressing the c-KIT V560G mutation was significantly reduced as early as 4 hours after imatinib treatment. In contrast, no change in FDG uptake was observed in resistant c-KIT D816V-expressing tumors after 48 hours of imatinib treatment. Consistent with the PET results, expression of the glucose transporter, GLUT1, was significantly reduced in V560G tumors at 4 hours, preceding changes in markers of proliferation by several hours. In vitro, imatinib treatment of V560G cells resulted in a reduction of glucose transporter numbers at the cell surface and decreased glucose uptake well before changes in cell viability. Notably, decreased ambient glucose concentrations enhanced the cytotoxic effect of imatinib. Taken together, these data account for the rapidity and significance of the PET response to imatinib and suggest that metabolic effects may contribute to imatinib cytotoxicity. Further, the FDC-P1 model represents a very useful paradigm for molecularly targeted drug development.

Glucose transporters (GLUT and SGLT): expanded families of sugar transport proteins.

The number of known glucose transporters has expanded considerably over the past 2 years. At least three, and up to six, Na+-dependent glucose transporters (SGLT1-SGLT6; gene name SLC5A) have been identified. Similarly, thirteen members of the family of facilitative sugar transporters (GLUT1-GLUT12 and HMIT; gene name SLC2A) are now recognised. These various transporters exhibit different substrate specificities, kinetic properties and tissue expression profiles. The number of distinct gene products, together with the presence of several different transporters in certain tissues and cells (for example, GLUT1, GLUT4, GLUT5, GLUT8, GLUT12 and HMIT in white adipose tissue), indicates that glucose delivery into cells is a process of considerable complexity.

Regulation of lactate production and glucose transport as well as of glucose transporter 1 and lactate dehydrogenase A mRNA levels by basic fibroblast growth factor in rat Sertoli cells.

By using cultured rat Sertoli cells as a model, both the action of basic fibroblast growth factor (bFGF) on lactate production and the site of this action were studied. bFGF stimulated Sertoli cell lactate production in a dose-dependent manner (basal: 7.3+/-0.5; 0.1 ng/ml bFGF: 7.5+/-0.5; 1 ng/ml bFGF: 7.5+/-0.6; 10 ng/ml bFGF: 10.3+/-1.0; 30 ng/ml bFGF: 15.2+/-1.5; 50 ng/ml bFGF: 15.4+/-1.6 microg/microg DNA). Two major sites for the action of this growth factor were identified. First, bFGF was shown to exert short- and long-term stimulatory effects on glucose transport (basal: 1170+/-102; 30 ng/ml bFGF for 120 min: 1718+/-152 and basal: 718+/-64; 30 ng/ml bFGF for 48 h: 1069+/-69 d.p.m./microg DNA respectively). Short-term bFGF stimulation of glucose transport was not inhibited by the protein synthesis inhibitor cycloheximide. These results indicate that short-term bFGF stimulation of glucose uptake does not involve an increase in the number of glucose transporters. On the other hand, stimulation with bFGF for periods of time longer than 12 h increased glucose transporter 1 (GLUT1) mRNA levels. These increased mRNA levels were probably ultimately responsible for the increments in glucose uptake that are observed in long-term treated cultures. Secondly, bFGF increased lactate dehydrogenase (LDH) activity (basal: 31.0+/-1.4; 30 ng/ml bFGF: 45.7+/- 2.4 mIU/microg DNA). The principal subunit component of those LDH isozymes that favors the transformation of pyruvate to lactate is subunit A. bFGF increased LDH A mRNA levels in a dose- and time-dependent manner. In summary, the results presented herein show that glucose transport, LDH activity and GLUT1 and LDH A mRNA levels are regulated by bFGF to achieve an increase in lactate production. These observed regulatory actions provide unequivocal evidence of the participation of bFGF in Sertoli cell lactate production which may be related to normal germ cell development.

Effects of Resalination on Intestinal Glucose Transport in Chickens Adapted to Low Na+ Intakes

In chickens, we have shown that intestinal absorption of glucose via apical SGLT1 and basolateral GLUT2 transport systems is affected by dietary Na+; low-Na+ adapted birds show a dramatic reduction of glucose transporters in both membranes in the rectum, an intermediate response in the ileum and no effects in the jejunum. We have now studied the effect of resalination of low-Na+ adapted chickens on glucose kinetics across SGLT1 (using -methyl-D-glucoside as substrate) and GLUT2 (using D-glucose) and on the specific binding of phlorizin and cytochalasin B, respectively. Twelve-week-old male Leghorn chickens were fed wheat and barley with drinking water containing either 150 mM NaCl (high-Na+ group) or 0. 015 mM (low-Na+ group) for 14 days (serum aldosterone: 242 +/- 6 pg ml-1 in the low-Na+ and 46 +/- 4 pg ml-1 in the high-Na+ group). On day 14, the low-Na+ group was either resalinated with an oral dose of NaCl (9 g (kg body wt)-1) or switched to the high-Na+ condition, for 1 week. Serum aldosterone measured 4 h, 1 day and 7 days after the change in NaCl intake fell to between 30 and 39 pg ml-1. The changes in apical -methyl-D-glucoside and basolateral D-glucose transport observed in the ileum and rectum of low-Na+ adapted animals were completely reversed by resalination within 4 h of NaCl administration to the level of values observed for high-Na+ adapted birds. The good correlation between the -methyl-D-glucoside and D-glucose Vmax and the SGLT1 and GLUT2 density, respectively, supports the view that the increase in apical and basolateral hexose transport found in the ileum and rectum of both groups of resalinated birds is due to an increase in the number of protein transporters. The rapid changes in the number of glucose transporters observed suggest that the target of the regulatory signal(s) involved are the mature enterocytes present in the villi rather than the developing enterocytes in the crypt.

GLUT-1 reduces hypoxia-induced apoptosis and JNK pathway activation.

Many studies have suggested that enhanced glucose uptake protects cells from hypoxic injury. More recently, it has become clear that hypoxia induces apoptosis as well as necrotic cell death. We have previously shown that hypoxia-induced apoptosis can be prevented by glucose uptake and glycolytic metabolism in cardiac myocytes. To test whether increasing the number of glucose transporters on the plasma membrane of cells could elicit a similar protective response, independent of the levels of extracellular glucose, we overexpressed the facilitative glucose transporter GLUT-1 in a vascular smooth muscle cell line. After 4 h of hypoxia, the percentage of cells that showed morphological changes of apoptosis was 30.5 +/- 2.6% in control cells and only 6.0 +/- 1.1 and 3.9 +/- 0.3% in GLUT-1-overexpressing cells. Similar protection against cell death and apoptosis was seen in GLUT-1-overexpressing cells treated for 6 h with the electron transport inhibitor rotenone. In addition, hypoxia and rotenone stimulated c-Jun-NH(2)-terminal kinase (JNK) activity >10-fold in control cell lines, and this activation was markedly reduced in GLUT-1-overexpressing cell lines. A catalytically inactive mutant of MEKK1, an upstream kinase in the JNK pathway, reduced hypoxia-induced apoptosis by 39%. These findings show that GLUT-1 overexpression prevents hypoxia-induced apoptosis possibly via inhibition of stress-activated protein kinase pathway activation.

Prolactin induction of insulin gene expression: the roles of glucose and glucose transporter-2.

Previous studies have shown that lactogenic hormones stimulate beta-cell proliferation and insulin production in pancreatic islets. However, all such studies have been conducted in cells incubated in medium containing glucose. Since glucose independently stimulates beta-cell replication and insulin production, it is unclear whether the effects of prolactin (PRL) on insulin gene expression are exerted directly or through the uptake and/or metabolism of glucose. We examined the interactions between glucose and PRL in the regulation of insulin gene transcription and the expression of glucose transporter-2 (glut-2) and glucokinase mRNAs in rat insulinoma (INS-1) cells. In the presence of 5.5 mM glucose, the levels of preproinsulin and glut-2 mRNAs in PRL-treated cells exceeded the levels in control cells (1.7-fold, P<0.05 and 2-fold, P<0.05 respectively). The maximal effects of PRL were noted at 24-48 h of incubation. PRL had no effect on the levels of glucokinase mRNA. The higher levels of glut-2 mRNA were accompanied by an increase in the number of cellular glucose transporters, as demonstrated by a 1. 4- to 2.4-fold increase in the uptake of 2-deoxy-d-[(3)H]glucose in PRL-treated INS-1 cells (P<0.001). These findings suggested that the insulinotropic effect of PRL is mediated, in part, by induction of glucose transport and/or glucose metabolism. Nevertheless, even in the absence of glucose, PRL stimulated increases in the levels of preproinsulin mRNA (3.4-fold higher than controls, P<0.0001) and glut-2 mRNA (2-fold higher than controls, P<0.01). These observations suggested that PRL exerts glucose-independent as well as glucose-dependent effects on insulin gene expression. Support for this hypothesis was provided by studies of insulin gene transcription using INS-1 cells transfected with a plasmid containing the rat insulin 1 promoter linked to a luciferase reporter gene. Glucose and PRL, alone and in combination, stimulated increases in cellular luciferase activity. The relative potencies of glucose (5.5 mM) alone, PRL alone, and glucose plus PRL in combination were 2.2 (P<0.001), 3.4 (P<0.01), and 7.9 (P<0.0001) respectively. Our findings suggest that glucose and PRL act synergistically to induce insulin gene transcription.

Hexose transport in the apical and basolateral membranes of enterocytes in chickens adapted to high and low NaCl intakes.

1. The effect of a low-NaCl diet (LS diet) on the properties of hexose transport across the brush-border and basolateral membranes of enterocytes from jejunum, ileum and rectum of the chicken was investigated. 2. In the brush-border membrane, LS adaptation had no effect on Km for alpha-methyl-D-glucoside while Vmax values were significantly reduced in the ileum and in the rectum. All Scatchard plots of specific [3H]phlorizin binding give a straight line, consistent with a single population of binding sites. Phlorizin binding vs. alpha-methyl-D-glucoside maximal transport rates showed a linear correlation. 3. In the basolateral membrane, the LS diet did not modify the Km for D-glucose but reduced the Vmax in the ileum and in the rectum. Scatchard plots of [3H]cytochalasin B binding support the view that there is a single transport system in this membrane. There was a linear correlation between cytochalasin B binding and D-glucose Vmax values. 4. The response of the chicken intestine to LS intake consists of a dramatic reduction in the number of glucose transporters in both apical and basolateral membranes of the rectum, an intermediate response in the ileum and no significant effects in the jejunum.

Insulin action in growth hormone-deficient and age-matched control rats: effect of growth hormone treatment.

The isolated effect of growth hormone on carbohydrate metabolism in rat skeletal muscle was studied in growth hormone-deficient dwarf rats (dw/dw) treated with either recombinant human growth hormone or saline for 10 days. In addition, age-matched heterozygous (DW/dw) (normal weight and plasma IGF-I) control rats were treated with saline. Growth hormone increased weight gain from 0.1+/-0.1 (s.e.m) to 3.6+/-0.1 g/day and plasma IGF-I concentration from 364+/-23 to 451+/-32 ng/ml. Glucose metabolism in skeletal muscle perfused with basal, submaximal and maximal concentrations (0, 600 and 60 000 pmol/l respectively) of insulin was not changed by growth hormone. No change could be detected in the total number of glucose transporters (GLUT1 and GLUT4) in the skeletal muscles, except from a lower amount of GLUT4 in the soleus muscle in the heterozygous control group. However, at submaximal insulin concentrations, skeletal muscle glucose uptake and transport were significantly lower in the heterozygous control group compared with the growth hormone-deficient group. This could indicate either a direct long-term effect of growth hormone or more likely a secondary effect attributable to the difference in body weight (205.2+/-3.1 vs 361. 6+/-5.9 g for dwarf rats and heterozygous controls respectively), and thereby muscle fibre size, between the groups probably resulting in lower average interstitial insulin and glucose concentrations at a given plasma concentration in the heterozygous rats. It is concluded that restoration of subnormal growth hormone concentrations for 10 days has no effect on insulin-stimulated glucose metabolism in skeletal muscle in vitro.

An overview of muscle glucose uptake during exercise. Sites of regulation.

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

Inositol phospholipid 3-kinase is activated by cellular stress but is not required for the stress-induced activation of glucose transport in L6 rat skeletal muscle cells.

A characteristic response of cells subjected to a stress stimulus is a rapid activation of cellular glucose transport. The mechanisms governing this increase in glucose transport are poorly understood, but it has been suggested that the response may involve the intracellular-signaling components that also participate in the hormonal activation of glucose transport. In skeletal muscle and fat tissue, inositol phospholipid 3-kinase plays an integral role in the regulation of both basal and insulin-stimulated glucose transport. In this study, we have investigated whether inositol phospholipid 3-kinase is activated by chemical stress and, if so, whether it has a role to play in the stress-induced increase in glucose transport in L6 muscle cells. Furthermore, we have attempted to assess the basis by which inositol phospholipid 3-kinase may participate in the regulation of basal glucose transport. Acute exposure (30 min) of L6 muscle cells to 0.5 mM arsenite induced an 80% stimulation in glucose transport. This activation was due to a rise in the number of cell-surface glucose transporters, based on an increase in the Vmax of glucose transport and the observation that arsenite increases the plasma membrane content of GLUT1 and GLUT4 glucose transporters by 95% and 60%, respectively, from an intracellular compartment. Arsenite induced rapid activation (< 2 min) of inositol phospholipid 3-kinase with an approximately fourfold increase in phosphatidylinositol 3,4,5-trisphosphate (PtdIns3,4,5P3). In contrast, phosphatidylinositol 3-phosphate (PtdIns3P) levels were unaffected. Prior treatment of L6 cells with 100 nM wortmannin suppressed the arsenite-induced increase in PtdIns3,4,5P3 and reduced the cellular content of PtdIns3P by 50%. Under these conditions however, wortmannin failed to prevent the stress-induced activation of glucose transport, but suppressed basal glucose transport by 60% with an IC50 of about 10 nM. In the absence of arsenite, wortmannin caused a dose-dependent inhibition in the cellular levels of PtdIns3P and PtdIns3,4,5P3 with IC50 values of about 10 nM and 100 nM, respectively. In summary, the present results demonstrate that chemical stress activates inositol phospholipid 3-kinase and glucose transport in L6 muscle cells, but unlike the hormonal responses of these cells the activation of inositol phospholipid 3-kinase is not responsible for the stress-induced increase in glucose transport. This implies that stress-induced and hormonal stimulated increases in PtdIns3,4,5P3 levels are functionally distinct. By contrast, the maintenance of PtdIns3P levels, presumably involving a PtdIns-specific, wortmannin-sensitive inositol phospholipid 3-kinase may be required to support basal glucose transport.