Insulin-induced Glut4 recruitment in the fatty Zucker rat heart is not associated with changes in Glut4 content in the intracellular membrane

Mechanisms of insulin-dependent glucose transport into porcine and bovine skeletal muscle

Euglycemic, hyperinsulinemic clamp tests have shown that adult ruminants are less insulin-sensitive than monogastric omnivores. The present study was carried out to elucidate possible cellular mechanisms contributing to this impaired insulin sensitivity of ruminants. Western blotting was used to measure glucose transporters 1 and 4 (GLUT1, GLUT4) in oxidative (musculus masseter and diaphragm) and glycolytic (musculus longissimus dorsi and semitendinosus) skeletal muscle in the crude membranes of pigs and cows. Muscles were characterized biochemically. To determine insulin-stimulated 3-O-D-[(3)H]-methylglucose (3-O-MG) uptake and GLUT4 translocation, porcine and bovine musculus semitendinosus strips were removed by open muscle biopsy and incubated without and with 0.1 or 20 mIU insulin/ml. GLUT4 translocation was analyzed using subcellular fractionation techniques to isolate partially purified plasma membranes and cytoplasmic vesicles and using Western blotting. GLUT4 protein contents were significantly higher in oxidative than in glycolytic muscles in pigs and cows. GLUT1 protein contents were significantly higher in glycolytic than in oxidative muscles in bovines but not in porcines. The 3-O-MG uptake into musculus semitendinosus was similar in both species. Maximum insulin-induced GLUT4 translocation into musculus semitendinosus plasma membrane was significantly lower in bovines than in porcines. These results indicate that GLUT1 is the predominant glucose transporter in bovine glycolytic muscles and that a reinforced insulin-independent glucose uptake via GLUT1 may compensate for the impaired insulin-stimulated GLUT4 translocation, resulting in a similar 3-O-MG uptake in bovine and porcine musculus semitendinosus. These findings may explain at least in part the impaired in vivo insulin sensitivity of adult ruminants compared with that of omnivorous monogastric animals.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

In vivo effects of vanadium on GLUT4 translocation in cardiac tissue of STZ-diabetic rats

The effect of vanadium treatment on insulin-stimulated glucose transporter type 4 (GLUT4) translocation was studied in cardiac tissue of streptozotocin (STZ)-induced diabetic rats by determining the subcellular distribution of GLUT4. Four groups of rats were examined: control and diabetic, with or without bis(maltolato)oxovanadium(IV) (BMOV, an organic form of vanadium) treatment for 8 weeks. The effect of vanadium on insulin-induced GLUT4 translocation was studied at 5 min as the early insulin response and at 15 min after insulin injection as the maximal insulin response. At 5 min after insulin injection, plasma membrane GLUT4 level in the diabetic-treated group was not different from the control groups and was significantly higher than that of the insulin-stimulated diabetic group, indicating an enhancement of insulin response on GLUT4 translocation brought about by vanadium treatment. In contrast to that at 5 min after insulin injection, no significant difference in the plasma membrane GLUT4 level was observed between the diabetic and the diabetic-treated groups at 15 min after insulin injection. GLUT4 mobilization from the intracellular pool in response to insulin was also investigated at 15 min after insulin injection. Basal intracellular GLUT4 content was significantly higher in the diabetic-treated group when compared to the diabetic group under the same condition. However, the increased basal intracellular GLUT4 in the diabetic-treated group did not result in more insulin-mediated GLUT4 translocation at 15 min after insulin injection. In conclusion, the finding that plasma membrane GLUT4 in the diabetic-treated group is significantly higher than that of the diabetic group at 5 min but not at 15 min post-insulin injection indicates that vanadium treatment enhances insulin-mediated GLUT4 translocation in cardiac tissue by enhancing its early response.

Validation of a competitive enzyme-linked immunosorbent assay for measuring the insulin-regulatable glucose transporter

A novel competitive enzyme-linked immunosorbent assay (ELISA) for measuring the insulin-regulatable glucose transporter (GLUT4) has been developed in our laboratory. The purpose of the present study was to verify the competitive ELISA assay. Towards this goal, a time-course study was conducted in control Wistar rat hearts to determine the points at which insulin stimulated an initial and a maximal GLUT4 translocation. Plasma and intracellular membrane fractions were purified from heart ventricles isolated from rats either in the basal state or injected with insulin. GLUT4 content in the membrane fractions was quantified with both the competitive ELISA method and also with enhanced chemiluminescence (ECL) Western blot. It was shown that after insulin injection plasma membrane GLUT4 level increased about 60% at 15 min and intracellular GLUT4 decreased about 40-50% at 5 min and remained at this level throughout the remaining 25 min by both methods. In conclusion, the data from this study demonstrate that the ELISA assay is reliable as verified by the Western blot method.