Large GLUT4 Vesicles Are Stationary While Locally and Reversibly Depleted During Transient Insulin Stimulation of Skeletal Muscle of Living Mice

Abstract

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 μm) 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 μm) 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 μm) or more distant (20 μm) 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 μm) takes place between perinuclear depots and sarcolemma but not at t-tubules.