Glucose regulates its transport in L8 myocytes by modulating cellular trafficking of the transporter GLUT-1

The effect of culture conditions simulating hypo- and hyper-glycaemia on glucose transport and on the subcellular localization of the glucose transporter GLUT-1 was studied in L8 myocytes. Incubation of the cells with 20 mM-glucose for 25 h decreased the rate of 2-deoxy-D-[3H]glucose (dGlc) uptake to 0.106 +/- 0.016 nmol/min per 10(6) cells compared with 0.212 +/- 0.025 in cells maintained at 2 mM-glucose (final glucose concentrations at the end of the incubation period were 16-17 mM and 0.7-1.0 mM respectively). An additional 5 h incubation of these cells with medium containing the opposite glucose concentration (i.e. change from 17 mM to 1 mM and from 1 mM to 17 mM) increased the transport rate to 0.172 +/- 0.033 nmol/min per 10(6) cells in cultures initially conditioned at high glucose, and decreased the transport to 0.125 +/- 0.029 in those conditioned at low glucose. Plasma-membrane- and microsomal-membrane-enriched fractions were prepared from these cells for [3H]cytochalasin B (CB) binding and Western-blot analysis with antibodies against GLUT-1 and GLUT-4. A decrease in glucose concentration increased the number of D-glucose-displaceable CB-binding sites and GLUT-1 protein in the plasma-membrane fraction to the same extent as the increase in dGlc transport. Under downregulatory conditions, the lower dGlc-transport capacity could be accounted for by a decreased number of transporters in the plasma membrane of the cells. No apparent modification of the intrinsic activity of the glucose transporters was observed in up- or down-regulated cells. Under downregulatory conditions, the CB-binding data indicated a large increase in the number of transporters in the intracellular membranes of the myocytes. Western blots of the same membranes also indicated an increase in GLUT-1 content. However, the interaction of the intracellular GLUT-1 protein with the polyclonal antibodies was much weaker than that of the plasma-membrane-associated GLUT-1. The GLUT-4 concentration was too low to permit quantification in membrane fractions. Our findings suggest that autoregulation of glucose transport in L8 myocytes is accompanied by parallel changes in the number of GLUT-1 transporters in the plasma membrane, and that the rate of transporter degradation may be augmented in the upregulated myocytes. These glucose-induced changes are fully reversible.

Effect of a ?-adrenergic agonist on glucose transport and insulin-responsive glucose transporters (GLUT4) in brown adipose tissue of control and obese fa/fa rats

A beta-adrenergic agonist specific for brown adipose tissue, Ro 16-8714, was administered to control and obese insulin-resistant fa/fa rats and glucose utilisation measured in brown adipose tissue using the euglycaemic hyperinsulinaemic clamp combined with the injection of 2-deoxyglucose. Treatment with the beta-agonist increased basal and insulin-stimulated glucose utilization in both groups, resulting in an increased effect of the hormone in treated animals. This effect is specific for brown adipose tissue and is not found in other insulin-sensitive tissue. The total number of insulin-responsive glucose transporters (GLUT4) measured in crude membrane preparations was similar in the two groups when expressed per total tissue. They were, however, decreased in the fa/fa group when expressed per milligram of tissue. Acute treatment with the beta-adrenergic agonist increased the total number of GLUT4 in both groups. The agonist also increased the amount of mRNA coding for GLUT4 suggesting an effect on the transcription and/or on the stability of GLUT4 mRNA.

Central corticotropin-releasing factor administration prevents the excessive body weight gain of genetically obese (fa/fa) rats

The genetic obesity of the fa/fa rat is due to or accompanied by perturbances in the autonomic nervous control of different target tissues (e.g. endocrine pancreas, brown adipose tissue). These disorders are likely to be secondary to central dysregulation(s), which could lie somewhere within or in relationship with the hypothalamus. In view of the reported effects of CRF in stimulating sympathetic nerve-mediated mechanisms, while inhibiting vagus nerve-mediated ones, ovine CRF (oCRF) was administered for 7 days into the cerebral ventricles of fa/fa rats at a dose (5 micrograms/day) that did not affect the pituitary-adrenal axis. oCRF treatment stopped the excessive weight gain of the obese animals; oCRF-treated animals gained only 1 g over 6 days, while the vehicle-treated ones gained 29 g (P = 0.044). The oCRF effect was unrelated to changes in food intake, as the two groups were pair-fed. oCRF-treated obese rats were characterized by a decrease in basal hyperinsulinemia, increases in brown adipose tissue weight and activity, and decreases in hepatic glycogen content and epididymal fat pad weight. It is suggested that intracerebroventricular oCRF administration to obese fa/fa rats prevents the 10-15% increase in body weight observed in vehicle-infused obese rats within 1 week by modulating the impaired autonomic nervous control of different target tissues. This does not occur in lean rats.

Dysregulation of glucose transport and transporters in perfused hearts of genetically obese (fa/fa) rats

The regulation of glucose transport in normal and insulin-resistant obese rat hearts have been studied by measuring glucose transport via the efflux of labelled 3-0-methyl-D-glucose. Glucose transporters in obese rat hearts were also investigated using the labelled cytochalasin B-binding assay. Basal, and insulin- or increasing workload-induced stimulation of glucose transport was decreased in obese rat hearts compared to those of normal ones. Total number of glucose transporters (plasma membrane plus microsomal ones) was about half that previously reported for normal rat hearts. Insulin or workload favoured the translocation of glucose transporters from an intercellular pool (microsomes) to the plasma membrane, as they do in normal rats. Due to the measured decrease in total number of transporters of obese rat hearts, those present in the plasma membrane (under basal conditions, or following stimulation by insulin or workload) were less than those previously found in normal rat hearts tested under identical conditions. In obese rat hearts, regulation of plasma membrane transporters was perturbed. The Hill coefficient (an index of positive cooperativity amongst glucose transporters) was paradoxically decreased by insulin while leaving affinity values unaltered. The Hill coefficient was unaltered by workload, although the affinity values were increased compared to respective controls. To sum up, obese rat hearts have decreased total transporter number, and although the two stimuli studied favour the translocation of available transporters, they fail to "activate" them adequately once present in the plasma membrane.

Effects of insulin on glucose transport and glucose transporters in rat heart

The effect of insulin on glucose transport and glucose transporters was studied in perfused rat heart. Glucose transport was measured by the efflux of labelled 3-O-methylglucose from hearts preloaded with this hexose. Insulin stimulated 3-O-methylglucose transport by: (a) doubling the maximal velocity (Vmax); (b) decreasing the Kd from 6.9 to 2.7 mM; (c) increasing the Hill coefficient toward 3-O-methylglucose from 1.9 to 3.1; (d) increasing the efficiency of the transport process (k constant). Glucose transporters in enriched plasma and microsomal membranes from heart were quantified by the [3H]cytochalasin-B-binding assay. When added to normal hearts, insulin produced the following changes in the glucose transporters: (a) it increased the translocation of transporters from an intracellular pool to the plasma membranes; (b) it increased (from 1.6 to 2.7) the Hill coefficient of the transporters translocated into the plasma membranes toward cytochalasin B, suggesting the existence of a positive co-operativity among the transporters appearing in these membranes; (c) it increased the affinity of the transporters (and hence, possibly, of glucose) for cytochalasin B. The data provide evidence that the stimulatory effect of insulin on glucose transport may be due not to the sole translocation of intracellular glucose transporters to the plasma membrane, but to changes in the functional properties thereof.

Heart glucose transport and transporters in rat heart: regulation by insulin, workload and glucose

Aspects of the regulation of the glucose transport by perfused hearts of normal rats have been studied by measuring glucose transport (via the efflux of labelled 3-O-methyl-D-glucose) and glucose transporters (via the labelled cytochalasin B binding assay). Similarly to what is observed with insulin, increasing workload (by raising perfusion pressure from 50 to 100 mm Hg) stimulated glucose transport 7 to 8-fold. Glucose (via its analog 3-O-methylglucose, used at 15 mmol/l) stimulated its own transport 4-fold. The three stimuli favored the translocation of glucose transporters from an intracellular pool (microsomes) to the plasma membrane. Insulin increased the apparent affinity (decreased dissociation constant values) of plasma membrane transporters for cytochalasin, as well as the Hill coefficient, indicating the occurrence of a positive cooperativity amongst plasma membrane transporters. Workload increased only the Hill coefficient, glucose only the apparent affinity for cytochalasin of plasma membrane transporters. This study shows that insulin, workload and glucose itself stimulate glucose transport by favouring the translocation process of glucose transporter as well as by changing, albeit by a different mechanism, the functional properties of the transporters once translocated to the plasma membrane.

Insulin modifies the properties of glucose transporters in rat brown adipose tissue

The properties of glucose transporters associated with plasma and microsomal membranes have been studied in brown adipose tissue of rats after treatment by saline infusion or hyperinsulinaemic/euglycaemic clamp. In this tissue, insulin produces a 40-fold increase in glucose utilization as measured by the 2-deoxy-D-glucose technique, and therefore a 40-fold increase in the rate-limiting glucose transport. This increase, promoted by insulin, is associated with: (a) translocation of the transporters from a pool associated with the microsomal fraction to the plasma membrane without modification of the total number of transporters; (b) an increase in the Hill coefficient of the plasma-membrane glucose transporters for cytochalasin B from 1.1 to 2.5, indicating the presence of positive co-operativity; (c) a decrease in the Kd (apparent dissociation constant) of the transporters towards cytochalasin B from 148 to 82 nM; (d) no change in the Hill coefficient or Kd for the transporters associated with the microsomal membranes. These data indicate that, in addition to causing translocation of the glucose transporters, insulin modifies their properties and behaviour towards cytochalasin B. This may reflect modifications in their properties and behaviour towards glucose, and by this contribute to bringing about the marked effect of this hormone on glucose transport in brown adipose tissue.

Stimulatory effect of cold adaptation on glucose utilization by brown adipose tissue. Relationship with changes in the glucose transporter system

The effect of cold adaptation (4 degrees C) on the in vivo glucose utilization and on the number and properties of the glucose transporters has been studied in brown adipose tissue of normal rats. Glucose utilization was assessed in vivo by the 2-deoxyglucose method. Glucose transporters in plasma and microsomal membranes were quantified by the [3H]cytochalasin B-binding assay. After cold adaptation the in vivo glucose utilization by brown adipose tissue increased 21-fold compared to controls (22 degrees C). The number of glucose transporters in plasma membranes of brown adipose tissue increased from 75 to 436 pmol/g tissue and that of total glucose transporters (plasma + microsomal membranes) from 438 to 754 pmol/g tissue. In addition, cold adaptation increased the Hill coefficient of the plasma membrane transporter for cytochalasin B from 0.90 to 2.03 and decreased the Kd from 100 to 54 nM. This study shows that cold adaptation promotes: a translocation of glucose transporters from an intracellular pool to plasma membranes; an increased number of plasma membrane glucose transporters unaccounted for by the translocation process (e.g. "de novo" synthesis); an increase in the Hill coefficient for cytochalasin B that could also represent changes in the properties of the transporters vis-à-vis glucose, (e.g. positive cooperativity); and a decrease in the Kd value for cytochalasin B.