INSULIN UNMASKS A COOH-TERMINAL GLUT4 EPITOPE AND INCREASES GLUCOSE-TRANSPORT ACROSS T-TUBULES IN SKELETAL-MUSCLE

An improved immunogold labeling procedure was used to examine the subc ellular distribution of glucose transporters in Lowricryl HM20-embedde d skeletal muscle from transgenic mice overexpressing either Glut1 or Glut4. In basal muscle, Glut4 was highly enriched in membranes of the transverse tubules and the terminal cisternae of the triadic junctions . Less than 10% of total muscle Glut4 was present in the vicinity of t he sarcolemmal membrane. Insulin treatment increased the number of gol d particles associated with the transverse tubules and the sarcolemma by threefold. However, insulin also increased the total Glut4 immunogo ld reactivity in muscle ultrathin sections by up to 1.8-fold and drama tically increased the amount of Glut4 in muscle sections as observed b y laser confocal immunofluorescence microscopy. The average diameter o f transverse tubules observed in longitudinal sections increased by 50 % after insulin treatment. Glut1 was highly enriched in the sarcolemma , both in the basal state and after insulin treatment. Disruption of t ransverse tubule morphology by in vitro glycerol shock completely abol ished insulin-stimulated glucose transport in isolated rat epitrochlea ris muscles. These data indicate that: (a) Glut1 and Glut4 are targete d to distinct plasma membrane domains in skeletal muscle; (b) Glut1 co ntributes to basal transport at the sarcolemma and the bulk of insulin -stimulated transport is mediated by Glut4 localized in the transverse tubules; (c) insulin increases the apparent surface area of transvers e tubules in skeletal muscle; and (d) insulin causes the unmasking of a COOH-terminal antigenic epitope in skeletal muscle in much the same fashion as it does in rat adipocytes.