Additive effect of islet amyloid polypeptide (IAPP/amylin) and insulin on 2-deoxyglucose uptake in mouse pancreatic acini

The effect of islet amyloid polypeptide (IAPP/amylin) on 2-deoxyglucose (2-DG) uptake was studied in isolated mouse pancreatic acini in the absence or presence of insulin. Synthetic rat IAPP-NH2 caused a dose-dependent stimulation of 2-DG uptake by mouse acini with a half-maximal concentration at 70 nM. The increase in 2-DG uptake by 1 microM IAPP-NH2 or 100 nM insulin was 68% or 60% above basal, respectively. In the presence of both 1 microM IAPP-NH2 and 100 nM insulin, the increase in 2-DG uptake was 145% above basal, indicating that the effects of IAPP-NH2 and insulin on 2-DG uptake were additive. The results suggest that IAPP stimulates glucose uptake in mouse acini probably by a different mechanism from that of insulin.     

Sympathetic activation of glucose utilization in brown adipose tissue in rats.

The effects of norepinephrine (NE) infusion and surgical denervation or electrical stimulation of the sympathetic nerves on 2-deoxyglucose (2-DG) uptake in interscapular brown adipose tissue (BAT) were investigated in vivo in rats to obtain direct evidence for sympathetic control of glucose utilization in this tissue. 2-DG uptake was rather low in fasted rats, but after refeeding it increased in the BAT as well as the heart, skeletal muscle, and white adipose tissue, in parallel with an increase in plasma insulin level. Cold exposure also enhanced 2-DG uptake in the BAT without the increase in plasma insulin level, while it had no appreciable effect on 2-DG uptake in other tissues. Sympathetic denervation greatly attenuated the stimulatory effect of cold exposure on 2-DG uptake in BAT, but it did not affect the increased 2-DG uptake after refeeding. Electrical stimulation of the sympathetic nerves entering BAT or NE infusion produced a marked increase in 2-DG uptake in BAT without noticeable effects in other tissues. beta-Adrenergic blockade, but not alpha-blockade, abolished the increased 2-DG uptake in BAT. It was concluded that glucose utilization in BAT is activated directly, independently of the action of insulin, by sympathetic nerves via the beta-adrenergic pathway.     

Regulation by Insulin and Insulin-Like Growth Factor of 2-Deoxyglucose Uptake in Primary Ependymal Cell Cultures

Ependymal cells have been reported to express the facilitative glucose carriers GLUT1, GLUT2, and GLUT4, as well as glucokinase. They are therefore speculated to be part of the cerebral glucose sensing system and may also respond to insulin with alterations in their glucose uptake rate. A cell culture model was employed to study the functional status of ependymal insulin-regulated glucose uptake in vitro. Insulin increased the uptake of the model substrate 2-deoxyglucose (2-DG) dependent on the insulin concentration. This was due to a near doubling of the maximal 2-DG uptake rate. Insulin-like growth factor (IGF-1) was at least 10 times more potent than insulin in stimulating the rate of ependymal 2-DG uptake, suggesting that IGF-1, rather than insulin, is the physiological agonist regulating glucose transport in ependymal cells. The predominant glucose transporter in ependymal cell cultures was found to be GLUT1, which is apparently regulated by IGF-1 in ependymal cells.     

Leucine and mTORc1 act independently to regulate 2-deoxyglucose uptake in L6 myotubes

Chronic mTORc1 hyperactivation via obesity-induced hyperleucinaemia has been implicated in the development of insulin resistance, yet the direct impact of leucine on insulin-stimulated glucose uptake in muscle cells remains unclear. To address this, differentiated L6 myotubes were subjected to various compounds designed to either inhibit mTORc1 activity (rapamycin), blunt leucine intracellular import (BCH), or activate mTORc1 signalling (3BDO), prior to the determination of the uptake of the glucose analogue, 2-deoxyglucose (2-DG), in response to 1 mM insulin. In separate experiments, L6 myotubes were subject to various media concentrations of leucine (0–0.8 mM) for 24 h before 2-DG uptake in response to insulin was assessed. Both rapamycin and BCH blunted 2-DG uptake, irrespective of insulin administration, and this occurred in parallel with a decline in mTOR, 4E-BP1, and p70S6K phosphorylation status, but little effect on AKT phosphorylation. In contrast, reducing leucine media concentrations suppressed 2-DG uptake, both under insulin- and non-insulin-stimulated conditions, but did not alter the phosphorylation state of AKT-mTORc1 components examined. Unexpectedly, 3BDO failed to stimulate mTORc1 signalling, but, nonetheless, caused a significant increase in 2-DG uptake under non-insulin-stimulated conditions. Both leucine and mTORc1 influence glucose uptake in muscle cells independent of insulin administration, and this likely occurs via distinct but overlapping mechanisms.

     

Insulin-stimulated glucose transport regulated by adenylate cyclase system in rat adipocytes

1. In rat adipocytes, there was an inverse correlation between insulin-stimulated 2-deoxyglucose (2-DG) uptake and cAMP levels, indicating that cAMP suppressed the 2-DG uptake stimulated by insulin. 2. This inhibitory effect of cAMP was due to suppression of translocation of glucose transporters rather than that of insulin-binding to its receptors. 3. Phosphodiesterase inhibitors (IBMX, Ro 20-1724, and cilostamide) inhibited the 2-DG uptake, which was brought about by direct interaction with glucose transporters in the plasma membranes.     

ACTH1-24 stimulates muscle cell glucose uptake

3H-2-deoxyglucose (2-DG) uptake was measured in L6A-1 rat skeletal muscle cells (a rapidly fusing subclone of L6), following addition of several concentrations (10(-16) to 10(-9)M) of the N-terminal fragment of ACTH1-24 to cells deprived of serum and insulin for 21 hours, but maintained in the presence of (5 micrograms/ml) insulin (stimulated state). There was a marked dose-dependent increase of 2-DG uptake at the various ACTH1-24 (P less than 0.001). There was no correlation between the time of exposure of the cells to serum-free conditions and the rate of uptake of 2-DG at the various ACTH1-24 concentrations both in the basal and insulin-stimulated states. Addition of catochalasin B (50 microM) to the cells, which inhibited both basal and insulin-stimulated uptake of 2-DG (by 70% and 91%, respectively) completely eliminated the enhancement of both of these uptake rates to 10(-12)M ACTH1-24. The results suggest that: 1) ACTH1-24 stimulates carrier-mediated uptake of glucose in skeletal muscle cells. 2) The site of action of ACTH1-24 is on the non-insulin mediated glucose uptake (NIMGU) system. 3) ACTH1-24 may be a useful probe to delineate some of the events associated with the NIMGU pathway.     

Uniaxial cyclic stretch increases glucose uptake into C2C12 myotubes through a signaling pathway independent of insulin-like growth factor I

Insulin-like growth factor I (IGF-I), an autocrine/paracrine growth factor involved in myogenesis, has rapid effects on muscle metabolism. In a manner analogous to insulin and mechanical stimuli such as stretch, IGF-I stimulates glucose transport through recruitment of glucose transporters to surface membranes in skeletal muscles. It is known that IGF-I is secreted from skeletal muscle cells in response to stretch. Therefore, we examined whether IGF-I is involved in the mechanism by which mechanical stretch regulates glucose transport using cultured C2C12 myotubes. IGF-I increased 2-deoxy- D-glucose (2-DG) uptake, and this created an additive effect with mechanical stretch, suggesting that these stimuli enhance glucose transport through different mechanisms. In fact, IGF-I-stimulated 2-DG uptake was not blocked by dantrolene (an inhibitor of Ca (2+)release from sarcoplasmic reticulum), whereas the stretch-stimulated effect was abolished. Conversely, the IGF-I-stimulated 2-DG uptake was prevented by phosphatidylinositol 3-kinase inhibitor wortmannin, which did not prevent the stretch-stimulated 2-DG uptake. In addition, experiments using media conditioned by stretched myotubes indicated that a mechanically induced release of locally acting autocrine/paracrine growth factors was not sufficient for induction of 2-DG uptake. Thus, our results demonstrate that mechanical stretch signaling for glucose transport is independent of the mechanism through which IGF-I increases this transport.     

The beta2-adrenergic modulator celiprolol reduces insulin resistance in obese Zucker rats.

Essential hypertension is associated with an increased incidence of insulin resistance of skeletal muscle glucose transport. The present study determined if celiprolol, an antihypertensive agent with selective beta1-adrenoceptor antagonist and additional beta2-agonistic properties, administered by gavage either acutely (3 hr) or chronically (14 d), had a direct effect on improving glucose tolerance and insulin-stimulated glucose transport activity (using 2-deoxyglucose (2-DG) uptake) in isolated epitrochlearis muscles of the insulin-resistant obese Zucker rat. The effects of a selective beta1-blocker, metoprolol, were also assessed. Acute administration of celiprolol, but not metoprolol, increased insulin-stimulated 2-DG uptake in muscle by 22% (p<0.05). Chronic celiprolol treatment significantly lowered fasting plasma insulin (22%) and free fatty acids (40%) in comparison to obese control values. Moreover, chronic celiprolol administration decreased the glucose-insulin index (calculated as the product of the glucose and insulin areas under the curve during an oral glucose tolerance test), by 32% (p<0.05) compared to obese controls, indicating that peripheral insulin action was increased. Indeed, insulin-stimulated skeletal muscle 2-DG uptake was enhanced by 49% (p<0.05) in these celiprolol-treated obese animals. Metoprolol was without significant effect on any of these variables following chronic administration. These findings indicate that, in this animal model of insulin resistance, the beta1-antagonist/beta2-agonist celiprolol has a specific effect of improving insulin-stimulated skeletal muscle glucose transport that is independent of any hemodynamic alterations.     

AICAR reverses ketone body mediated insulin resistance in isolated oxidative muscle

Recently it was demonstrated that the ketone body β-hydroxybutyrate (BOH) inhibits insulin-mediated glucose transport in isolated oxidative muscle, which was associated with decreased phosphorylation of Akt/protein kinase B. The purpose of the present study was to determine if activation of AMP-dependent protein kinase by the pharmacological activator AICAR could reverse the insulin resistance induced by BOH. Isolated mouse soleus muscle was incubated in vitro in the absence or presence of 5 mM BOH for ∼20 h. Following prolonged incubation, insulin increased 2-deoxyglucose glucose (2-DG) uptake 3-fold, but in the presence of BOH most of the insulin response was lost (only ∼30% remained). Addition of 2 mM AICAR during the last 2 h of prolonged incubation increased the insulin response in the presence of BOH to ∼80% of the normal insulin effect on 2-DG uptake. The AICAR-mediated reversal of the insulin resistance was not associated with a restoration of the insulin effect on Akt/protein kinase B phosphorylation. However, AICAR enhanced the insulin-induced phosphorylation of the Akt substrate, AS160. In conclusion, these data demonstrate that AICAR reverses the negative effect of BOH on insulin-mediated glucose uptake and this is attributed to activation of a late step in insulin signaling.     

Metabolic and mitogenic effects of IGF-I and insulin on muscle cells of rainbow trout

The relative function of IGF-I and insulin on fish muscle metabolism and growth has been investigated by the isolation and culture at different stages (myoblasts at day 1, myocytes at day 4, and myotubes at day 10) of rainbow trout muscle cells. This in vitro model avoids interactions with endogenous peptides, which could interfere with the muscle response. In these cells, the effects of IGF-I and insulin on cell proliferation, 2-deoxyglucose (2-DG), and l-alanine uptake at different development stages, and the use of inhibitors were studied and quantified. Insulin (10-1,000 nM) and IGF-I (10-100 nM) stimulated 2-DG uptake in trout myocytes at day 4 in a similar manner (maximum of 124% for insulin and of 142% for IGF-I), and this stimulation increased when cells differentiated to myotubes (maximum for IGF-I of 193%). When incubating the cells with PD-98059 and especially cytochalasin B, a reduction in 2-DG uptake was observed, suggesting that glucose transport takes place through specific facilitative transporters. IGF-I (1-100 nM) stimulated the l-alanine uptake in myocytes at day 4 (maximum of 239%), reaching higher values of stimulation than insulin (100-1,000 nM) (maximum of 160%). This stimulation decreased when cells developed to myotubes at day 10 (118% for IGF-I and 114% for insulin). IGF-I (0.125-25 nM) had a significant effect on myoblast proliferation, measured by thymidine incorporation (maximum of 170%), and required the presence of 2-5% fetal serum (FBS) to promote thymidine uptake. On the other hand, insulin was totally ineffective in stimulating thymidine uptake. We conclude that IGF-I is more effective than insulin in stimulating glucose and alanine uptake in rainbow trout myosatellite cells and that the degree of stimulation changes when cells differentiate to myotubes. IGF-I stimulates cell proliferation in this model of muscle in vitro and insulin does not. These results indicate the important role of IGF-I on growth and metabolism of fish muscle.     

Heterogeneous Effects of Calorie Restriction on In Vivo Glucose Uptake and Insulin Signaling of Individual Rat Skeletal Muscles

Calorie restriction (CR) (consuming ∼60% of ad libitum, AL, intake) improves whole body insulin sensitivity and enhances insulin-stimulated glucose uptake by isolated skeletal muscles. However, little is known about CR-effects on in vivo glucose uptake and insulin signaling in muscle. Accordingly, 9-month-old male AL and CR (initiated when 3-months-old) Fischer 344xBrown Norway rats were studied using a euglycemic-hyperinsulinemic clamp with plasma insulin elevated to a similar level (∼140 µU/ml) in each diet group. Glucose uptake (assessed by infusion of [¹⁴C]-2-deoxyglucose, 2-DG), phosphorylation of key insulin signaling proteins (insulin receptor, Akt and Akt substrate of 160kDa, AS160), abundance of GLUT4 and hexokinase proteins, and muscle fiber type composition (myosin heavy chain, MHC, isoform percentages) were determined in four predominantly fast-twitch (epitrochlearis, gastrocnemius, tibialis anterior, plantaris) and two predominantly slow-twitch (soleus, adductor longus) muscles. CR did not result in greater GLUT4 or hexokinase abundance in any of the muscles, and there were no significant diet-related effects on percentages of MHC isoforms. Glucose infusion was greater for CR versus AL rats (P<0.05) concomitant with significantly (P<0.05) elevated 2-DG uptake in 3 of the 4 fast-twitch muscles (epitrochlearis, gastrocnemius, tibialis anterior), without a significant diet-effect on 2-DG uptake by the plantaris or either slow-twitch muscle. Each of the muscles with a CR-related increase in 2-DG uptake was also characterized by significant (P<0.05) increases in phosphorylation of both Akt and AS160. Among the 3 muscles without a CR-related increase in glucose uptake, only the soleus had significant (P<0.05) CR-related increases in Akt and AS160 phosphorylation. The current data revealed that CR leads to greater whole body glucose disposal in part attributable to elevated in vivo insulin-stimulated glucose uptake by fast-twitch muscles. The results also demonstrated that CR does not uniformly enhance either insulin signaling or insulin-stimulated glucose uptake in all muscles in vivo.     

Relationship between insulin stimulation and endogenous regulation of 2-deoxyglucose uptake in 3T3-L1 adipocytes

The occurrence of the endogenous regulatory response to high rates of 2-deoxyglucose (2-DG) uptake, as previously described for C6 glioma cells during incubation with 2 mM 2-DG (Lange et al.: J. Cell. Physiol., 1989), was studied in 3T3-L1 preadipocytes and adipocytes, and the influence of insulin on this endogenous uptake regulation was examined. In contrast to 3T3-L1 preadipocytes, insulin-sensitive differentiated 3T3-L1 adipocytes displayed the time-dependent cyclic pattern of 2-DG uptake rates characteristic of the membrane-limited and endogenously regulated cellular state of hexose utilization. Although insulin induced a threefold stimulation of 2-DG tracer uptake in adipocytes, the hormone did not additionally stimulate the uptake rates or affect the periodic response: maximum and minimum levels of uptake remained unchanged. Scanning electron microscopy (SEM) revealed that the acquirement of the differentiated state is accompanied by a conspicuous transformation of the smooth surface of undifferentiated 3T3-L1 cells into a surface covered by numerous microvilli of uniform size and appearance. Treatment with insulin (10 mU/ml; 10 minutes) converted these microvilli into voluminous saccular membrane protrusions of the same type as had been formed during incubation of 3T3-L1 adipocytes with 2 mM 2-DG, and which have previously been shown to be involved in the endogenous uptake regulation of C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989). These insulin-induced saccated membrane areas appeared to become integrated into the cell surface. Accordingly, insulin treatment caused a twofold increase of the intracellular distribution space of 3-O-methylglucose (3-OMG) in 3T3-L1 adipocytes. This insulin-induced increase of the 3-OMG distribution space exhibited the same time (t1/2 = 2-2.5 minutes) and dose dependence (EC50 = 20 nM) as the insulin-induced stimulation of 3-OMG transport. Glucose deprivation during the differentiation period inhibited the outgrowth of microvilli from the cell surface. Glucose starvation (18 hours at less than 0.5 mM) induced a conspicuous reduction of the length of microvilli on differentiated 3T3-L1 cells. In this state, the stalks of the microvilli are almost invisible and the enlarged spherical tips of the microvilli (with an average diameter of 370 nm compared to 230 nm of fed cells) appeared to protrude directly out of the cell surface. Starvation-induced shortening of microvilli was accompanied by a threefold increase of the basal 3-OMG transport rate and a greater than twofold increase of the intracellular 3-OMG distribution space as compared to fed cells (10 mM; 18 hours).(ABSTRACT TRUNCATED AT 400 WORDS)     

Interleukin-6 gene expression is increased in insulin-resistant rat skeletal muscle following insulin stimulation

IL-6 expression in skeletal muscle is stimulated by contractions. We sought to examine whether hyperinsulinaemia increases IL-6 mRNA in skeletal muscle and whether any increase is modified in insulin resistant muscle. We hypothesized that intramuscular IL-6 mRNA would be increased in response to insulin, but such an affect would be unaffected by insulin resistance because the primary insulin sensitive signalling protein responsible for activating IL-6 functions normally in insulin resistant muscle. Transgenic rats over-expressing the gluconeogenic regulatory enzyme phosphoenolpyruvate carboxykinase (PEPCK) were studied. White gastrocnemius muscle samples were obtained under hyperinsulinaemic, euglycaemic clamp (4 mU kg(-1)min(-1) insulin, plasma glucose concentration 4-6 mmol L(-1)) and basal conditions in both PEPCK (basal n=4; insulin n=5) and wild-type (CON) (basal n=5; insulin n=4) rats, which were previously injected with a bolus of 2-[1-14C]deoxyglucose (2-DG) into the carotid artery. Muscle samples were assayed for 2-DG uptake and IL-6 mRNA. No differences in 2-DG uptake or IL-6 mRNA were observed when comparing groups under basal conditions. Under clamp conditions, 2-DG uptake was lower (P<0.05) in PEPCK compared with CON. Insulin stimulation in CON did not change IL-6 mRNA compared with basal levels. In contrast, there was an approximately 8-fold increase (P<0.05) in IL-6 mRNA in insulin-stimulated PEPCK compared with CON basal levels. Insulin stimulation increases IL-6 gene expression in insulin resistant, but not healthy, skeletal muscle, suggesting that IL-6 expression in skeletal muscle is sensitive to changes in insulin in circumstances of insulin resistance. It is likely that the differences observed when comparing healthy with insulin resistant muscle are due to the differential activation of insulin sensitive signalling proteins responsible for activating IL-6.     

An amino acid mixture enhances insulin-stimulated glucose uptake in isolated rat epitrochlearis muscle

Protein and certain amino acids (AA) have been found to lower blood glucose. Although these glucose-lowering AA are important modulators of skeletal muscle metabolism, their impact on muscle glucose uptake remains unclear. We therefore examined how an AA mixture consisting of 2 mM isoleucine, 0.012 mM cysteine, 0.006 mM methionine, 0.0016 mM valine, and 0.014 mM leucine impacts skeletal muscle glucose uptake in the absence or presence of a submaximal (sINS) or maximal insulin (mINS) concentration. The AA mixture, sINS, and mINS significantly increased 2-[(3)H]deoxyglucose (2-DG) uptake by 63, 79, and 298% above basal, respectively. When the AA mixture was combined with sINS and mINS, 2-DG uptake was further increased significantly by 26% (P = 0.028) and 14% (P = 0.032), respectively. Western blotting analysis revealed that the AA mixture increased basal and sINS Akt substrate of 160 kDa (AS160) phosphorylation, while AA mixture did not change phosphorylation of Akt or mammalian target of rapamycin (mTOR) under these conditions. Interestingly, addition of the AA mixture to mINS increased phosphorylation of mTOR, Akt as well as AS160, compared with mINS alone. These data suggest that certain AA increase glucose uptake in the absence of insulin and augment insulin-stimulated glucose uptake in an additive manner. Furthermore, these effects appear to be mediated via a pathway that is independent from the canonical insulin cascade and therefore may prove effective as an alternative therapeutic treatment for insulin resistance.     

Effect of starvation and diabetes on glucose transport in the lung

Transport of glucose by the isolated and perfused rat lung was studied using 2-deoxy-D-(1-14C)-glucose, 2-(1-14C)-DG, and 3-O-methyl-(U-14C)-glucose, 3-(U-14C)-MG. Uptake of 3-(U-14C)-MG was reduced by 20% in the lungs of fasting and diabetic rats, the uptake was restored by refeeding and insulin treatment, respectively. Although the intracellular accumulation of unphosphorylated 2-(1-14C)-DG in lungs was altered by a small amount in diabetes and fasting, the intracellular accumulation of phosphorylated 2-(1-14C)-DG was significantly reduced and restored by insulin treatment and refeeding, suggesting that phosphorylation was inhibited in these conditions. The reduction of glucose utilization in fasting or diabetic state was shown to be due to the inactivation of hexokinase II enzyme. Thus, a specific glucose-carrier system is present in the rat lung which appears to be insulin-sensitive and is under the nutritional and hormonal control.     

Use of RNA interference-mediated gene silencing and adenoviral overexpression to elucidate the roles of AKT/protein kinase B isoforms in insulin actions

Insulin plays a central role in the regulation of glucose homeostasis in part by stimulating glucose uptake and glycogen synthesis. The serine/threonine protein kinase Akt has been proposed to mediate insulin signaling in several processes. However, it is unclear whether Akt is involved in insulin-stimulated glucose uptake and which isoforms of Akt are responsible for each insulin action. We confirmed that expression of a constitutively active Akt, using an adenoviral expression vector, promoted translocation of glucose transporter 4 (GLUT4) to plasma membrane, 2-deoxyglucose (2-DG) uptake, and glycogen synthesis in both Chinese hamster ovary cells and 3T3-L1 adipocytes. Inhibition of Akt either by adenoviral expression of a dominant negative Akt or by the introduction of synthetic 21-mer short interference RNA against Akt markedly reduced insulin-stimulated GLUT4 translocation, 2-DG uptake, and glycogen synthesis. Experiments with isoform-specific short interference RNA revealed that Akt2, and Akt1 to a lesser extent, has an essential role in insulin-stimulated GLUT4 translocation and 2-DG uptake in both cell lines, whereas Akt1 and Akt2 contribute equally to insulin-stimulated glycogen synthesis. These data suggest a prerequisite role of Akt in insulin-stimulated glucose uptake and distinct functions among Akt isoforms.     

Resistin inhibits glucose uptake in L6 cells independently of changes in insulin signaling and GLUT4 translocation

Elevated levels of resistin have been proposed to cause insulin resistance and therefore may serve as a link between obesity and type 2 diabetes. However, its role in skeletal muscle metabolism is unknown. In this study, we examined the effect of resistin on insulin-stimulated glucose uptake and the upstream insulin-signaling components in L6 rat skeletal muscle cells that were either incubated with recombinant resistin or stably transfected with a vector containing the myc-tagged mouse resistin gene. Transfected clones expressed intracellular resistin, which was released in the medium. Incubation with recombinant resistin resulted in a dose-dependent inhibition of insulin-stimulated 2-deoxyglucose (2-DG) uptake. The inhibitory effect of resistin on insulin-stimulated 2-DG uptake was not the result of impaired GLUT4 translocation to the plasma membrane. Furthermore, resistin did not alter the insulin receptor (IR) content and its phosphorylation, nor did it affect insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation, its association with the p85 subunit of phosphatidylinositol (PI) 3-kinase, or IRS-1-associated PI 3-kinase enzymatic activity. Insulin-stimulated phosphorylation of Akt/protein kinase B-alpha, one of the downstream targets of PI 3-kinase and p38 MAPK phosphorylation, was also not affected by resistin. Expression of resistin also inhibited insulin-stimulated 2-DG uptake when compared with cells expressing the empty vector (L6Neo) without affecting GLUT4 translocation, GLUT1 content, and IRS-1/PI 3-kinase signaling. We conclude that resistin does not alter IR signaling but does affect insulin-stimulated glucose uptake, presumably by decreasing the intrinsic activity of cell surface glucose transporters.     

CARRIER MEDIATED GLUCOSE TRANSPORT IN CAPILLARIES ISOLATED FROM RAT BRAIN1

Abstract— Uptake of 2-deoxy-d-glucose (2-DG) was investigated in capillaries isolated from rat brain. A high affinity, carrier-mediated transport system was defined with an apparent Km for 2-DG of 93 μM. Uptake was temperature-dependent and markedly inhibited by phloretin and selected hexose isomers. The apparent K_i for d-glucose inhibition of 2-DG uptake was 98 μM. Essentially all of the 2-DG retained by the capillary preparation could be released by sonication and 95% was recovered as free unphosphorylated 2-DG. Uptake was not sodium-dependent and not altered by insulin. These results suggest that movement of glucose across the blood-brain barrier through endothelial cells probably is not rate-limiting unless blood glucose levels are extremely low.     

Acacetin enhances glucose uptake through insulin-independent GLUT4 translocation in L6 myotubes

BackgroundLowering blood glucose levels by increasing glucose uptake in insulin target tissues, such as skeletal muscle and adipose tissue, is one strategy to discover and develop antidiabetic drugs from natural products used as traditional medicines.PurposeOur goal was to reveal the mechanism and activity of acacetin (5,7-dihydroxy-4′-methoxyflavone), one of the major compounds in Agastache rugose, in stimulating glucose uptake in muscle cells.MethodsTo determine whether acacetin promotes GLUT4-dependent glucose uptake in cultured L6 skeletal muscle cells, we performed a [¹⁴C] 2-deoxy-_D-glucose (2-DG) uptake assay after treating differentiated L6-GLUT4myc cells with acacetin.ResultsAcacetin dose-dependently increased 2-DG uptake by enhancing GLUT4 translocation to the plasma membrane. Our results revealed that acacetin activated the CaMKII-AMPK pathway by increasing intracellular calcium concentrations. We also found that aPKCλ/ζ phosphorylation and intracellular reactive oxygen species (ROS) production were involved in acacetin-induced GLUT4 translocation. Moreover, acacetin-activated AMPK inhibited intracellular lipid accumulation and increased 2-DG uptake in HepG2 cells.ConclusionTaken together, these results suggest that acacetin might be useful as an antidiabetic functional ingredient. Subsequent experiments using disease model animals are needed to verify our results.     

Accumulation of 2-deoxy-D-glucose by primate oocytes fertilized in vitro

The present study was designed to investigate the accumulation of 2-deoxy-D-glucose (2-DG) by squirrel monkey oocytes fertilized in vitro; to assess the effects of insulin addition to the medium on 2-DG accumulation; and, finally, to evaluate the use of 2-DG in viability determinations of oocytes. Accumulation of 2-DG by unfertilized oocytes from squirrel monkeys was 13.94 fmol/oocyte/3 h and was not affected by the addition of either 10 nM or 1 μM insulin. There was no change in 2-DG accumulation with fertilization in vitro; 2-DG accumulation by degenerate ova was reduced to background levels. These results suggest low utilization of glucose by early primate embryos similar to that demonstrated for other mammalian species; 2-DG appears to be a good viability indicator of early primate embryos.