GLUT-3 expression in human skeletal muscle

Muscle biopsy homogenates contain GLUT-3 mRNA and protein. Before these stu dies, it was unclear where GLUT-3 was located in muscle tissue. In situ hyb ridization using a midmolecule probe demonstrated GLUT-3 within all muscle fibers. Fluorescent-tagged antibody reacting with affinity-purified antibod y directed at the carboxy-terminus demonstrated GLUT-3 protein in all fiber s. Slow-twitch muscle fibers, identified by NADH-tetrazolium reductase stai ning, possessed more GLUT-3 protein than fast-twitch fibers. Electron micro scopy using affinity-purified primary antibody and gold particle-tagged sec ond antibody showed that the majority of GLUT-3 was in association with tri ads and transverse tubules inside the fiber. Strong GLUT-3 signals were see n in association with the few nerves that traversed muscle sections. Electr on microscopic evaluation of human peripheral nerve demonstrated GLUT-3 wit hin the axon, with many of the particles related to mitochondria. GLUT-3 pr otein was found in myelin but not in Schwann cells. GLUT-1 protein was not present in nerve cells, axons, myelin, or Schwann cells but was seen at the surface of the peripheral nerve in the perineurium. These studies demonstr ated that GLUT-3 mRNA and protein are expressed throughout normal human ske letal muscle, but the protein is predominantly found in the triads of slow- twitch muscle fibers.