MUSCLE SUBCELLULAR-LOCALIZATION AND RECRUITMENT BY INSULIN OF GLUCOSETRANSPORTERS AND NA-K+-ATPASE SUBUNITS IN TRANSGENIC MICE OVEREXPRESSING THE GLUT4 GLUCOSE-TRANSPORTER()

Insulin-stimulated glucose uptake in skeletal muscle is mediated throu gh the GLUT4 glucose transporter. Transgenic (TG) mice overexpressing human GLUT4 in skeletal muscle show an increased ability to handle a g lucose load. Here, the participation of the overexpressed GLUT4 in the response to insulin tvas examined. In TG mouse muscle, the GLUT4 prot ein content was 10-fold higher in crude membrane (CM), sevenfold highe r in internal membrane (IM), and 15-fold higher in a plasma membrane ( PM)-rich fraction, relative to non-TG littermates. This suggested part ial saturation of the normal sorting mechanisms. The distribution and abundance of the GLUT1 glucose transporter teas not affected. Insulin injection (4.3 U/kg body wt) increased GLUT4 in the PM-rich fraction; the increase was threefold higher in TG than in non-TG mice. Insulin d ecreased the GLUT4 content of the IM in both animal groups and of a se cond, heavier intracellular membrane fraction only in TG mice. The net content of Na+-K+-pump subunits was 40-65% lower in CM from TG compar ed with non-TG littermates. In spite of this, insulin caused a three- to sixfold higher translocation of the alpha 2 and beta 1 subunits of the Na+-K+-pump in TG compared with non-TG animals. The results sugges t that overexpression of GLUT4 confers to the muscle increased ability to translocate subunits of the Na+-K+-pump either as a direct consequ ence of the recruitment of glucose transporters or as an adapta tion t o the more demanding metabolic state.