Differential effects of GLUT-1 or GLUT-4 overexpression on insulin responsiveness in transgenic mice

The effect of glucose transporter expression on insulin-stimulated whole body glucose disposal was examined in transgenic mice overexpressing GLUT-1 or GLUT-4. Transgenic mice and their control littermates were subjected to a euglycemic hyperinsulinemic clamp under pentobarbital sodium anesthesia using an insulin infusion rate of 20 mU · kg^-1 · min^-1 and a variable glucose infusion rate (GIR). Fasted mice overexpressing GLUT-1 in skeletal muscle exhibited a GIR that was only 54% that of controls (19.3 ± 1.8 vs. 36.0 ± 3.9 mg · kg^-1 · min^-1) when blood glucose was clamped at euglycemic values. In contrast, fasted mice overexpressing GLUT-4 in fat and muscle exhibited a GIR that was 40% higher than controls (53.9 ± 2.3 vs. 39.1 ± 2.5 mg · kg^-1 · min^-1). At the end of the clamp, β-hydroxybutyrate levels were 10-fold higher in the GLUT-1 transgenic mice relative to nontransgenic littermates (2.0 ± 0.6 vs. 0.2 ± 0.1 mM) but did not differ between the GLUT-4 transgenic mice and their control littermates (0.3 ± 0.1 vs. 0.3 ± 0.1 mM). These data demonstrate that the level of expression of a glucose transporter in muscle and fat can have marked effects on whole body glucose homeostasis and fuel metabolism. Insulin responsiveness was enhanced by overexpression of GLUT-4. Strikingly, however, overexpression of GLUT-1 in muscle induced a profound reduction in insulin-stimulated whole body glucose disposal. The effect of transporter overexpression appears to be isoform specific, although some of the difference in the effects of the GLUT-1 and GLUT-4 transgenes may be due to differences in the level of overexpression or the tissue distribution.