Regulation of glucose transport in Clone 9 cells by thyroid hormone.

Triiodothyronine (T3) is found to stimulate cytochalasin B-inhibitable glucose transport in Clone 9 cells, a 'non-transformed' rat liver cell line. After an initial lag period of more than 3 h, glucose transport rate is significantly increased at 6 h and reaches more than 3-times the control rate at 24 h. The enhancement of glucose transport by T3 is due to an increase in transport Vmax and occurs in the absence of a change in either the Km for glucose transport (approximately 3 mM) or the Ki for inhibition of transport by cytochalasin B ((1-2).10(-7) M). Consistent with the observed Ki for cytochalasin B, Northern blot analysis of RNA from control and T3-treated cells employing cDNA probes encoding GTs of the human erythrocyte/rat brain/HepG2 cell transporter (GLUT-1), rat muscle/fat cell transporter (GLUT-4), and rat liver transporter (GLUT-2) types indicates expression of only the GLUT-1 mRNA isoform in these cells. The abundance of GLUT-1 mRNA increases approx. 1.9-fold after 24 h of T3 treatment and is accompanied by an approx. 1.3-fold increase in the abundance of GLUT-1 in whole-cell extracts as demonstrated by Western blot analysis employing a polyclonal antibody directed against the 13 amino acid C-terminal peptide of GLUT-1. The more than 3-fold stimulation of glucose transport at 24 h substantially exceeds the fractional increment in transporter abundance suggesting that, in addition to increasing total GLUT-1 abundance, exposure to T3 may result in a translocation of transporters to the plasma membrane or an activation of pre-existing membrane transporter sites.     

Hypoxia increases expression of selective facilitative glucose transporters (GLUT) and 2-deoxy-D-glucose uptake in human adipocytes

Hypoxia modulates the production of key inflammation-related adipokines and may underlie adipose tissue dysfunction in obesity. Here we have examined the effects of hypoxia on glucose transport by human adipocytes. Exposure of adipocytes to hypoxia (1% O₂) for up to 24 h resulted in increases in GLUT-1 (9.2-fold), GLUT-3 (9.6-fold peak at 8 h), and GLUT-5 (8.9-fold) mRNA level compared to adipocytes in normoxia (21% O₂). In contrast, there was no change in GLUT-4, GLUT-10 or GLUT-12 expression. The rise in GLUT-1 mRNA was accompanied by a substantial increase in GLUT-1 protein (10-fold), but there was no change in GLUT-5; GLUT-3 protein was not detected. Functional studies with [³H]2-deoxy-d-glucose showed that hypoxia led to a stimulation of glucose transport (4.4-fold) which was blocked by cytochalasin B. These results indicate that hypoxia increases monosaccharide uptake capacity in human adipocytes; this may contribute to adipose tissue dysregulation in obesity.     

Regulation of the functional expression of hexose transporter GLUT-1 by glucose in murine fibroblasts: role of lysosomal degradation.

The nature of the membrane compartments involved in the regulation by glucose of hexose transport is not well defined. The effect of inhibitors of lysosomal protein degradation on hexose transport (i.e., uptake of [3H]-2-deoxy-D-glucose) and hexose transporter protein GLUT-1 (i.e., immunoblotting with antipeptide serum) in glucose-fed and -deprived cultured murine fibroblasts (3T3-C2 cells) was studied. The acidotropic amines chloroquine (20 microM) and ammonium chloride (10 mM) cause accumulation (both approximately 4-fold) of GLUT-1 protein and a small increase (both approximately 25%) in hexose transport in glucose-fed fibroblasts (24 h). The endopeptidase inhibitor, leupeptin (100 microM) causes accumulation (approximately 4-fold) of GLUT-1 protein in glucose-fed fibroblasts (24 h) without changing hexose transport (less than or equal to 5%). These agents do not greatly alter the electrophoretic mobility of GLUT-1. Neither chloroquine nor leupeptin augment the glucose deprivation (24 h) induced increases in hexose transport (approximately 4-fold) and GLUT-1 content (approximately 7-fold). In contrast, chloroquine or leupeptin diminish the reversal by glucose refeeding of the glucose deprivation induced accumulation of GLUT-1 protein but fail to alter the return of hexose transport to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential accumulation of Glut1 in the non-DRM domain of the plasma membrane in response to the inhibition of oxidative phosphorylation

Approximately 50% of Glut1 in the plasma membrane of Clone 9 cells is localized to the detergent-resistant membrane (DRM) fraction. Acute exposure (90 min) to 5mM azide stimulated glucose transport by approximately 4.7-fold and increased the abundance of Glut1 in the non-DRM fraction of the plasma membrane by approximately 2.9-fold while the abundance of Glut1 in the DRMs was not changed. In parallel experiments, approximately 17 h exposure to azide further increased the rate of glucose transport over that observed at 90 min by approximately 33% and increased plasma membrane Glut1 content by approximately 3.5-fold over control. The increase in total plasma membrane Glut1 reflected a approximately 4.7-fold increase of Glut1 content in the non-DRM fraction and a approximately 2.6-fold increase in the DRMs. We conclude that acute exposure to azide increases Glut1 content in the non-DRM fractions, while prolonged exposure to azide increases the Glut1 content in both non-DRM and DRM fractions. These changes may play an important role in the stimulation of glucose transport in response to the inhibition of oxidative phosphorylation.     

Differential regulation of the GLUT-1 and GLUT-4 glucose transport systems by glucose and insulin in L6 muscle cells in culture

The regulation by glucose and insulin of the muscle-specific facilitative glucose transport system GLUT-4 was investigated in L6 muscle cells in culture. Hexose transport activity, mRNA expression, and the subcellular localization of the GLUT-4 protein were analyzed. As observed previously (Walker, P. S., Ramlal, T., Sarabia, V., Koivisto, U.-M., Bilan, P. J., Pessin, J. E., and Klip, A. (1990) J. Biol. Chem. 265, 1516-1523), 24 h of glucose starvation and 24 h of insulin treatment each increase glucose transport activity severalfold. Here we report a differential regulation of the GLUT-4 and GLUT-1 transport systems under these conditions. (a) The level of GLUT-4 mRNA was not affected by glucose starvation and was diminished by prolonged (24 h) administration of insulin; in contrast, the level of GLUT-1 mRNA was elevated under both conditions. (b) Glucose starvation and prolonged insulin administration increased the amount of both GLUT-4 and GLUT-1 proteins in the plasma membrane. (c) In intracellular membranes, glucose starvation elevated, and prolonged insulin administration reduced, the GLUT-4 protein content. In contrast, the GLUT-1 protein content in these membranes decreased with glucose starvation and increased with insulin treatment. Glucose transport was rapidly curbed upon refeeding glucose to glucose-starved cells, with half-maximal reversal after 30 min and maximal reversal after 4 h. This was followed by a marked decrease in the levels of GLUT-1 mRNA without major changes in GLUT-4 mRNA. Neither 2-deoxy-D-glucose nor 3-O-methyl-D-glucose could substitute for D-glucose in these effects. It is proposed that glucose and insulin differentially regulate the two glucose transport systems in L6 muscle cells and that the rapid down-regulation of hexose transport activity by glucose is regulated by post-translational mechanisms.     

Regulation of glucose transporter messenger RNA levels in rat adipose tissue by insulin

Analysis of glucose transporter mRNA levels in adipose tissue from streptozotocin (STZ)-induced diabetic rats demonstrated a specific decrease (10-fold) in adipose tissue GLUT-4 mRNA with no significant effect on GLUT-1 mRNA levels. Treatment of STZ-diabetic rats with twice daily injections of insulin for 1-3 days resulted in a 16-fold increase in the relative amount of GLUT-4 mRNA to levels approximately 2-fold greater than those in control animals. However, after 7 days of insulin therapy the amount of GLUT-4 mRNA decreased approximately 2-fold back to the levels in the control animals. Normalization of the STZ-induced serum hyperglycemia by phlorizin treatment, which inhibits renal tubular reabsorption of glucose, had no effect on GLUT-4 mRNA in the absence of insulin. Similar to STZ-diabetes, fasting for 48 h also reduced adipose GLUT-4 mRNA levels. Parenteral administration of insulin with glucose over 7.5 h, but not glucose alone, increased the levels of the GLUT-4 mRNA 3- to 4-fold. These studies demonstrate that the relative glycemic state does not influence GLUT-4 glucose transporter mRNA expression in vivo and strongly suggests that insulin is a major factor regulating the levels of GLUT-4 mRNA in adipose tissue.     

Increased expression of GLUT-4 and hexokinase in rat epitrochlearis muscles exposed to AICAR in vitro.

Exercise acutely stimulates muscle glucose transport and also brings about an adaptive increase in the capacity of muscle for glucose uptake by inducing increases in GLUT-4 and hexokinase.(1) Recent studies have provided evidence that activation of AMP protein kinase (AMPK) is involved in the stimulation of glucose transport by exercise. The purpose of this study was to determine whether activation of AMPK is also involved in mediating the adaptive increases in GLUT-4 and hexokinase. To this end, we examined the effect of incubating rat epitrochlearis muscles in culture medium for 18 h in the presence or absence of 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), which enters cells and is converted to the AMP analog ZMP, thus activating AMPK. Exposure of muscles to 0.5 mM AICAR in vitro for 18 h resulted in an approximately 50% increase in GLUT-4 protein and an approximately 80% increase in hexokinase. This finding provides strong evidence in support of the hypothesis that the activation of AMPK that occurs in muscle during exercise is involved in mediating the adaptive increases in GLUT-4 and hexokinase.     

Regulation of glucose transporters by connective tissue activating peptide-III isoforms.

Connective tissue activating peptide-III (CTAP-III) is a component of platelet alpha-granules which elicits a series of responses in connective tissue cells referred to as activation, including increased glucose consumption and mitogenesis and increased secretion of hyaluronic acid and glycosaminoglycans. As anticipated by a requirement for glucose or glucose precursors in the activation process, an early event following CTAP-III activation of connective tissue cells is an increase in glucose transport. The present study investigates the molecular basis for this increase in glucose transport. Murine 3T3-F442A fibroblasts were found to respond to CTAP-III in a manner similar to human connective tissue cells (synovial cells, chondrocytes, skin fibroblasts). CTAP-III increases the rate of glucose transport to similar extents at 4 and 24 h, and at physiologic (micrograms/ml) concentrations of CTAP-III. A proteolytic cleavage product of recombinant CTAP-III (rCTAP-III-Leu-21 (des-1-15)), also known as neutrophil-activating peptide-2 (NAP-2), was found to be equally effective as CTAP-III, whereas NAP-1/interleukin-8, another member of the CTAP-III super-family, was ineffective in stimulating glucose transport. This contrasts with neutrophil chemotaxis, in which CTAP-III (des-1-15)/NAP-2 acts similarly to NAP-1/interleukin 8 while CTAP-III is ineffective. CTAP-III appears to elicit a different type of glucose transport response than many other growth factors in that its response is sustained (greater than or equal to 24 h) rather than transient (peak approximately 4 h) in confluent as well as in subconfluent cells. Western blot analysis using antibodies to the GLUT-1 glucose transporter revealed an increased level of GLUT-1 protein in response to CTAP-III isoforms that corresponded in magnitude (on a percentage basis) to the increased level of glucose transport. The increased levels of GLUT-1 protein in response to CTAP-III and rCTAP-III-Leu-21 (des-1-15)/NAP-2 were accompanied by an increase in levels of GLUT-1 mRNA of a magnitude sufficient to account for observed increased levels of GLUT-1. These results are consistent with CTAP-III isoforms stimulating glucose transport in connective tissue cells by increasing levels of GLUT-1 mRNA and is one of the few known instances in which increases in levels of GLUT-1 mRNA and protein are sufficient to account for observed increases in glucose transport. They also provide further evidence that CTAP-III (des-1-15)/NAP-2 binds to more than one type of receptor and that CTAP-III acts in a manner different than other well characterized growth factors (e.g. platelet-derived growth factor, transforming growth factor-beta) in that it causes a sustained (greater than or equal to 24 h) elevation in glucose transport in confluent as well as subconfluent cells.     

Transient enhancement of GLUT-4 levels in rat epitrochlearis muscle after exercise training.

The purpose of the present study was to examine the effect of detraining on the glucose transport system after short-term swim training (5 days), long-term swim training (5 wk), and treadmill run training (5 wk). Skeletal muscles were isolated from female Wistar rats at 24 or 48 h posttraining. SST produces a 48% increase in GLUT-4 mRNA, a 30% increase in GLUT-4 protein, and a 60% increase in insulin-stimulated glucose transport activity at 24 h posttraining but not at 48 h posttraining. Similar to SST, long-term swim training produces a 60% increase in GLUT-4 mRNA and a 30% increase in GLUT-4 protein content at 24 h posttraining but not at 48 h posttraining. Finally, treadmill run training produces a transient 35% increase in GLUT-4 protein content that is completely reversed at 48 h after the last bout of exercise. These results demonstrate that the increase in GLUT-4 mRNA and GLUT-4 protein occurs during the first week of exercise training and is rapidly lost after training cessation. We believe that the transient enhancement in GLUT-4 protein after exercise training is due to a short GLUT-4 half-life, a process that is primarily regulated by pretranslational mechanisms.     

Chronic elevated calcium blocks AMPK-induced GLUT-4 expression in skeletal muscle

Muscle contraction stimulates glucose transport independent of insulin. Glucose uptake into muscle cells is positively related to skeletal muscle-specific glucose transporter (GLUT-4) expression. Therefore, our objective was to determine the effects of the contraction-mediated signals, calcium and AMP-activated protein kinase (AMPK), on glucose uptake and GLUT-4 expression under acute and chronic conditions. To accomplish this, we used pharmacological agents, cell culture, and pigs possessing genetic mutations for increased cytosolic calcium and constitutively active AMPK. In C2C12 myotubes, caffeine, a sarcoplasmic reticulum calcium-releasing agent, had a biphasic effect on GLUT-4 expression and glucose uptake. Low-concentration (1.25 to 2 mM) or short-term (4 h) caffeine treatment together with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR), had an additive effect on GLUT-4 expression. However, high-concentration (2.5 to 5 mM) or long-term (4 to 30 h) caffeine treatment decreased AMPK-induced GLUT-4 expression without affecting cell viability. The negative effect of caffeine on AICAR-induced GLUT-4 expression was reduced by dantrolene, which desensitizes the ryanodine receptor. Consistent with cell culture data, increases in GLUT-4 mRNA and protein expression induced by AMPK were blunted in pigs possessing genetic mutations for both increased cytosolic calcium and constitutively active AMPK. Altogether, these data suggest that chronic exposure to elevated cytosolic calcium concentration blocks AMPK-induced GLUT-4 expression in skeletal muscle.     

Glucose transporter protein responses to selective hyperglycemia or hyperinsulinemia in fetal sheep

The acute effect of selective hyperglycemia or hyperinsulinemia on late gestation fetal ovine glucose transporter protein (GLUT-1, GLUT-3, and GLUT-4) concentrations was examined in insulin-insensitive (brain and liver) and insulin-sensitive (myocardium and fat) tissues at 1, 2.5, and 24 h. Hyperglycemia with euinsulinemia caused a two- to threefold increase in brain GLUT-3, liver GLUT-1, and myocardial GLUT-1 concentrations only at 1 h. There was no change in GLUT-4 protein amounts at any time during the selective hyperglycemia. In contrast, selective hyperinsulinemia with euglycemia led to an immediate and persistent twofold increase in liver GLUT-1, which lasted from 1 until 24 h with a concomitant decline in myocardial tissue GLUT-4 amounts, reaching statistical significance at 24 h. No other significant change in response to hyperinsulinemia was noted in any of the other isoforms in any of the other tissues. Simultaneous assessment of total fetal glucose utilization rate (GURf) during selective hyperglycemia demonstrated a transient 40% increase at 1 and 2.5 h, corresponding temporally with a transient increase in brain GLUT-3 and liver and myocardial GLUT-1 protein amounts. In contrast, selective hyperinsulinemia led to a sustained increase in GURf, corresponding temporally with the persistent increase in hepatic GLUT-1 concentrations. We conclude that excess substrate acutely increases GURf associated with an increase in various tissues of the transporter isoforms GLUT-1 and GLUT-3 that mediate fetal basal glucose transport without an effect on the GLUT-4 isoform that mediates insulin action. This contrasts with the tissue-specific effects of selective hyperinsulinemia with a sustained increase in GURf associated with a sustained increase in hepatic basal glucose transporter (GLUT-1) amounts and a myocardial-specific emergence of mild insulin resistance associated with a downregulation of GLUT-4.     

Differential role of insulin receptor autophosphorylation sites 1162 and 1163 in the long-term insulin stimulation of glucose transport, glycogenesis, and protein synthesis.

The long-term regulatory effect of insulin on glucose transport activity and glucose transporter expression was examined in Chinese hamster ovary (CHO) transfectants that overexpress either human insulin receptors of the wild type (CHO-R cells) or human insulin receptors mutated at two major autophosphorylation sites, Tyr1162 and Tyr1163 (CHO-Y2 cells). Previous studies showed that, when acutely stimulated by insulin, CHO-Y2 cells exhibit decreased receptor kinase activity along with decreased signaling of several pathways, including that for glucose transport, as compared with CHO-R cells. We now report the following. (i) When treated for 24 h with insulin (10(-10) to 10(-6) M), CHO-R and CHO-Y2 cells displayed closely similar concentration-dependent increases in 2-deoxyglucose uptake. In both transfectants, the maximal insulin-induced increase (approximately 3.5-fold) in uptake was cycloheximide-sensitive and was paralleled by equivalent increases in the levels of GLUT-1 immunoreactive protein and mRNA. (ii) By contrast, under similar conditions, CHO-Y2 cells exhibited a marked decrease in their response to insulin for [U-14C]glucose incorporation into glycogen (decreased sensitivity and maximal responsiveness) and for [U-14C]leucine incorporation into protein (decreased sensitivity) as compared with CHO-R cells. (iii) After a 24-h treatment with 10(-7) M insulin, CHO-R (but not CHO-Y2) cells showed a decreased ability to respond to a subsequent acute insulin stimulation of either receptor exogenous kinase activity or 2-deoxyglucose uptake as compared with respective untreated controls. These results indicate that (i) insulin receptors mutated at Tyr1162 and Tyr1163 retain normal signaling of the long-term stimulatory effect of insulin on glucose transport activity and GLUT-1 expression, but not on glycogenesis and overall protein synthesis; (ii) these three insulin signaling pathways may be triggered by distinct domains of the insulin receptor beta-subunit; and (iii) wild-type (but not twin-tyrosine mutant) receptors undergo negative regulation by chronic insulin treatment for subsequent signaling of acute biological actions of insulin.     

Regulation of hexose transport in aortic endothelial cells by vascular permeability factor and tumor necrosis factor-alpha, but not by insulin

Vascular permeability factor (VPF) is mitogenic for bovine aortic endothelial (BAE) cells, whereas tumor necrosis factor (TNF) is cytostatic and was found to completely block the mitogenic response to VPF. In contrast to the apparently antagonistic mitogenic effects that these two factors elicit, chronic exposure of BAE cells to either VPF of TNF resulted in significant (about 3-fold) increases in the rates of hexose transport. The concentrations required for half-maximal stimulation were 2 ng/ml (40 pM) for TNF and 4 ng/ml (100 pM) for VPF. Exposure to both factors simultaneously resulted in a greater stimulation of transport (about 7-fold) than exposure to either factor alone. Northern blot analysis indicated that the amount of message for the GLUT-1/erythrocyte form of the glucose transporter was specifically increased by treatment with VPF (5-fold), TNF (25-fold), or to both cytokines together (35-fold). Expression of mRNAs for the insulin-sensitive muscle/adipose transporter (GLUT-4), brain/fetal skeletal muscle transporter (GLUT-3), or the hepatic transporter (GLUT-2) were not detected in either control or treated cells. Acute or chronic exposure to insulin (10(-9) to 10(-6) M) did not activate hexose transport in BAE cells. Thus, glucose transport in aortic endothelial cells can be up-regulated by either VPF, a growth stimulator, or by TNF, a growth inhibitor, but not by insulin. The additive effect of the two cytokines together may be important in the control of increased glucose metabolism at sites of inflammation.     

Activation of syntaxin 1C, an alternative splice variant of HPC-1/syntaxin 1A, by phorbol 12-myristate 13-acetate (PMA) suppresses glucose transport into astroglioma cells via the glucose transporter-1 (GLUT-1)

Syntaxin 1C is an alternative splice variant lacking the transmembrane domain of HPC-1/syntaxin 1A. We found previously that syntaxin 1C is expressed as a soluble protein in human astroglioma (T98G) cells, and syntaxin 1C expression is enhanced by stimulation with phorbol 12-myristate 13-acetate (PMA). However, the physiological function of syntaxin 1C is not known. In this study, we examined the relationship between syntaxin 1C and glucose transport. First, we discovered that glucose transporter-1 (GLUT-1) was the primary isoform in T98G cells. Second, we demonstrated that glucose uptake in T98G cells was suppressed following an increase in endogenous syntaxin 1C after stimulation with PMA, which did not alter the expression levels of other plasma membrane syntaxins. We further examined glucose uptake and intracellular localization of GLUT-1 in cells that overexpressed exogenous syntaxin 1C; glucose uptake via GLUT-1 was inhibited without affecting sodium-dependent glucose transport. The value of Vmax for the dose-dependent uptake of glucose was reduced in syntaxin 1C-expressing cells, whereas there was no change in Km. Immunofluorescence studies revealed a reduction in the amount of GLUT-1 in the plasma membrane in cells that expressed syntaxin 1C. Based on these results, we postulate that syntaxin 1C regulates glucose transport in astroglioma cells by changing the intracellular trafficking of GLUT-1. This is the first report to indicate that a syntaxin isoform that lacks a transmembrane domain can regulate the intracellular transport of a plasma membrane protein.     

Glucose transporters and transport kinetics in retinoic acid-differentiated T47D human breast cancer cells

The rates of glucose transport and of glycolysis and the expression of the glucose transporters GLUT-1 through GLUT-4 were measured in T47D human breast cancer cells that underwent differentiation by retinoic acid. Glucose transport was found to be the rate-limiting step of glycolysis in control and differentiated cells. The transporters GLUT-1, GLUT-3, and GLUT-4 were present in the cell membrane and in the cytoplasm, and GLUT-2 was present solely in the cytoplasm. Differentiation led to a reduction in GLUT-1 and to an increase in cytoplasmic GLUT-2 and GLUT-3 with no change in GLUT-4. Differentiation also caused a reduction in the maximal velocity of glucose transport by approximately 40% without affecting the Michaelis-Menten constant of glucose transport. These changes did not alter the steady-state concentration of the phosphate metabolites regulating cell energetics but increased the content of phospholipid breakdown phosphodiesters. In conclusion, differentiation of human breast cancer cells appears to be associated with decreased glycolysis by a mechanism that involves a reduction in GLUT-1 and a slowdown of glucose transport.     

Hexose transport stimulation and membrane redistribution of glucose transporter isoforms in response to cholera toxin, dibutyryl cyclic AMP, and insulin in 3T3-L1 adipocytes

Exposure of 3T3-L1 adipocytes to 100 ng/ml of cholera toxin or 1 mM dibutyryl cyclic AMP caused a marked stimulation of deoxyglucose transport. A maximal increase of 10- to 15-fold was observed after 12-24 h of exposure, while 100 nM insulin elicited an increase of similar magnitude within 30 min. A short term exposure (4 h) of cells to cholera toxin or dibutyryl cyclic AMP resulted in a 3- to 4-fold increase in deoxyglucose transport which was associated with significant redistribution of both the HepG2/erythrocyte (GLUT1) and muscle/adipocyte (GLUT4) glucose transporters from low density microsomes to the plasma membrane fraction. Total cellular amounts of both transporter proteins remained constant. In contrast, cells exposed to cholera toxin or dibutyryl cyclic AMP for 12 h exhibited elevations in total cellular contents of GLUT1 (but not GLUT4) protein to about 1.5- and 2.5-fold above controls, respectively. Although such treatments of cells with cholera toxin (12 h) versus insulin (30 min) caused similar 10-fold enhancements of deoxyglucose transport, a striking discrepancy was observed with respect to the content of glucose transporter proteins in the plasma membrane fraction. While insulin elicited a 2.6-fold increase in the levels of GLUT4 protein in the plasma membrane fraction, cholera toxin increased the amount of this transporter by only 30%. Insulin or cholera toxin increased the levels of GLUT1 protein in the plasma membrane fraction equally (1.6-fold). Thus, a greater number of glucose transporters in the plasma membrane fraction is associated with transport stimulation by insulin compared to cholera toxin. We conclude that: 1) at early times (4 h) after the addition of cholera toxin or dibutyryl cyclic AMP to 3T3-L1 adipocytes, redistribution of glucose transporters to the plasma membrane appears to contribute to elevated deoxyglucose uptake rates, and 2) the stimulation of hexose uptake after prolonged treatment (12-18 h) of cells with cholera toxin may involve an additional increase in the intrinsic activity of one or both glucose transporter isoforms.