PHOSPHATIDYLINOSITOL 4-KINASE, BUT NOT PHOSPHATIDYLINOSITOL 3-KINASE,IS PRESENT IN GLUT4-CONTAINING VESICLES ISOLATED FROM RAT SKELETAL-MUSCLE

Insulin stimulates the rate of glucose transport into muscle and adipo se cells by translocation of glucose transporter (GLUT4)-containing ve sicles from an intracellular storage pool to the surface membrane. Thi s event is mediated through the insulin receptor substrates (IRSs), wh ich in turn activate phosphatidylinositol (PI) 3-kinase isoforms. It h as been suggested that insulin causes attachment of PI 3-kinases to th e intracellular GLUT4-containing vesicles in rat adipose cells. Furthe rmore, it has also been shown that GLUT4-containing vesicles in adipos e cells contain a PI 4-kinase. In the present study we investigate whe ther GLUT4-containing vesicles isolated from rat skeletal muscle displ ay PI 3-kinase and/or PI 4-kinase activities. Insulin stimulation caus ed a rapid increase (5-15-fold increase compared with control) in the intracellular cytosolic IRS-1-associated PI-3 kinase activity. This PI 3-kinase activity was also present in a membrane preparation containi ng the insulin-regulatable pool of GLUT4 transporters. However, when G LUT4-containing vesicles were isolated by immunoprecipitation from bas al and insulin-stimulated (3 min) skeletal muscle, the vesicles displa yed PI 4-kinase, but not PI 3-kinase, activity. Insulin did not regula te the PI 4-kinase activity in the GLUT4-containing vesicles. In concl usion, GLUT4-containing vesicles from rat skeletal muscle contain a PI 4-kinase, but not a PI 3-kinase. It is suggested that, in skeletal mu scle, insulin causes activation of the IRS/PI 3-kinase complex in an i ntracellular membrane compartment associated closely with the GLUT4-co ntaining vesicles, but not in the GLUT4-containing vesicles themselves .