FAILURE OF INSULIN-REGULATED RECRUITMENT OF THE GLUCOSE-TRANSPORTER GLUT4 IN CARDIAC-MUSCLE OF OBESE ZUCKER RATS IS ASSOCIATED WITH ALTERATIONS OF SMALL-MOLECULAR-MASS GTP-BINDING PROTEINS

Cardiac ventricular tissue of lean and genetically obese (fa/fa) Zucke r rats was used to study the expression, subcellular distribution and insulin-induced recruitment of the glucose transporter GLUT4 and to el ucidate possible molecular alterations of the translocation process. H earts were removed from basal and insulin-treated (20 min) lean and ob ese Zucker rats, and processed for subcellular fractionation and Weste rn blotting of proteins. In obese rats, the total GLUT4 content in a c rude membrane fraction was reduced to 75 +/- 8% (P = 0.019) of lean co ntrols. In contrast, GLUT4 abundance in plasma membranes was not signi ficantly different between lean and obese rats with a concomitant decr ease (47 +/- 3%) in the microsomal fraction of obese animals. In plasm a membranes of lean animals insulin was found to increase the GLUT4 ab undance to 294 +/- 43% of control with a significantly (P = 0.009) red uced effect in the obese group (139 +/- 10% of control). In these anim als insulin failed to recruit GLUT4 from the microsomal fraction, wher eas the hormone induced a significant decrease (41 +/- 4%) of microsom al GLUT4 in lean controls. In GLUT4-enriched membrane vesicles, obtain ed from cardiac microsomes of lean rats, a 24 kDa GTP-binding protein could be detected, whereas no significant labelling of this species wa s observed in GLUT4 vesicles prepared from obese animals. In addition to the translocation of GLUT4, insulin was found to promote the moveme nt of the small GTP-binding protein rab4A from the cytosol (decrease t o 61 +/- 13% of control) to the plasma membrane (increase to 177 +/- 1 9% of control) in lean rats with no effect of the hormone on rab4A red istribution in the obese group. In conclusion, cardiac glucose uptake of insulin-resistant obese Zucker rats is subject to multiple cellular abnormalities involving a reduced expression, altered redistribution and defective recruitment of GLUT4. We show here an association of the latter defect with alterations at the level of small GTP-binding prot eins possibly leading to an impaired trafficking of GLUT4 in the insul in-resistant state.