OBJECTIVE:
Insulin stimulates
glucose transport in
skeletal muscle by
GLUT4 translocation from
intracellular compartments to
sarcolemma and t-tubules. We studied in living animals the recruitment of
GLUT4 vesicles in more detail than previously done and, for the first time, analyzed the steady-state recycling and subsequent re-internalization of
GLUT4 on an
insulin bolus. RESEARCH DESIGN AND METHODS: A confocal imaging technique was used in GLUT4-enhanced
green fluorescent protein-transfected superficial
muscle fibers in living
mice. RESULTS: During the first 30 min of
insulin stimulation, very few superficially or deeply located
GLUT4 storage vesicles (>1 microm) moved in toto. Rather, big vesicles were stationary in their original position at
sarcolemma or t-tubules and were locally depleted of
GLUT4 by budding off of smaller vesicles.
Photobleaching experiments revealed that during initial
translocation and steady-state recycling,
GLUT4 microvesicles (<1 microm) move from perinuclear
GLUT4 depots out along the
plasma membrane. Furthermore, after
photobleaching of t-tubule areas, recovery of
GLUT4 was slow or absent, indicating no recycling of
GLUT4 from perinuclear or adjacent (1 microm) or more distant (20 microm) t-tubule areas. During waning of
insulin effect,
GLUT4 was re-internalized to basal stores with a delay in t-tubules compared with
sarcolemma, probably reflecting delayed disappearance of
insulin from t-tubules. CONCLUSIONS: In
skeletal muscle,
insulin reversibly stimulates local depletion of
GLUT4 storage vesicles at
sarcolemma and t-tubules rather than inducing movement of intact storage vesicles. During steady-state stimulation, recycling of GLUT4-containing
microvesicles over longer distances (10-20 microm) takes place between perinuclear depots and
sarcolemma but not at t-tubules.
