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Am J Physiol Endocrinol Metab (May 30, 2006). doi:10.1152/ajpendo.00194.2006
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Submitted on April 25, 2006
Accepted on May 24, 2006

Regulations of Glucose Transporters by Insulin and Extracellular Glucose in C2C12 Myotubes

Taku Nedachi1 and Makoto Kanzaki2*

1 TUBERO, Tohoku University, Sendai, Miyagi, Japan
2 Tubaro, Tohoku University, Sendai, Miyagi, Japan

* To whom correspondence should be addressed. E-mail: kanzaki{at}tubero.tohoku.ac.jp.

It is well established that insulin stimulation of glucose uptake into skeletal muscle cells is mediated through translocation of GLUT4 from intracellular storage sites to the cell surface. However, the established skeletal muscle cell lines, with the exception of L6 myocytes, reportedly show minimal insulin-dependent glucose uptake and GLUT4 translocation. Using C2C12 myocytes expressing exofacial-Myc-GLUT4-ECFP, we herein show that differentiated C2C12 myotubes are equipped with basic GLUT4 translocation machinery that can be activated by insulin stimulation (approximately 3-fold increase as assessed by anti-Myc antibody uptake and immunostaining assay). However, this insulin-stimulation of GLUT4 translocation was difficult to demonstrate with a conventional 2-deoxyglucose uptake assay, due to markedly elevated basal glucose uptake via other glucose transporter(s). Intriguingly, the basal glucose transport activity in C2C12 myotubes appeared to be acutely suppressed within 5 minutes by pre-incubation with a pathophysiologically high level of extracellular glucose (25 mM). In contrast, this activity was augmented by acute glucose deprivation, via an unidentified mechanism which is independent of GLUT4 translocation but is dependent on phosphatidylinositol 3-kinase activity. Taken together, these findings indicate that regulation of the facilitative glucose transport system in differentiated C2C12 myotubes can be achieved through surprisingly acute glucose-dependent modulation of the activity of glucose transporter(s), which apparently contributes to obscuring the insulin-augmentation of glucose uptake elicited by GLUT4 translocation. We herein also describe several methods of monitoring insulin-dependent glucose uptake in C2C12 myotubes, and propose this cell line to be a useful model for analyzing GLUT4 translocation in skeletal muscle.




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