Fractionation of endocytic vesicles and glucose-transporter-containing vesicles in rat adipocytes

We subfractionated intracellular vesicles from rat adipocytes in order to examine the subcellular distribution of endocytic vesicles or endosomes with respect to insulin-regulatable glucose-transporter (GT)-containing vesicles [James, Lederman & Pilch (1987) J. Biol. Chem. 262, 11817-11824]. Vesicles mediating fluid-phase endocytosis sedimented as a single major peak of greater density than the single distinct peak of GT-containing vesicles. This difference was also apparent during cellular insulin exposure and after insulin removal. Endocytosis of insulin and IGF (insulin-like growth factor) II was also examined. In sucrose gradients, IGF II-containing vesicles were less dense than those containing internalized insulin. Receptor-mediated endocytic vesicles were distinct from fluid-phase endocytic vesicles, but overlapped with the GT-containing vesicles. Vesicles containing internalized ligand were further fractionated by agarose-gel electrophoresis after various times of internalization. At least three different vesicle subpopulations containing the iodinated ligands were resolved after 5 min of internalization. Endocytic vesicles containing rapidly internalized insulin (1.5 min at 37 degrees C) consistently co-migrated with GT-containing vesicles. These data indicate that fluid-phase and receptor-mediated endocytosis occur via different pathways in adipocytes. Furthermore, whereas the intracellular GT-containing vesicles are distinct from fluid-phase vesicles, a rapidly labelled pool of insulin-containing vesicles consistently co-fractionated with GT-containing vesicles when separation techniques based on size, density and charge were used. This suggests that the insulin receptor may directly interact with the intracellular GT-containing vesicles after insulin-induced endocytosis.