The transferrin receptor defines two distinct contraction-responsive GLUT4vesicle populations in skeletal muscle

Insulin and contraction increase glucose transport in an additive fashion i n skeletal muscle. However, it is still unclear whether they do so by induc ing the recruitment of GLUT4 transporters from the same or distinct intrace llular compartments to the plasma membrane and the T-tubules. Using the tra nsferrin receptor as a recognized marker of recycling endosomes, we have ex amined whether insulin and/or contraction recruit GLUT4 from this pool to e ither the plasma membranes or T-tubules, isolated by subcellular fractionat ion of perfused hindlimb muscles. Either stimulus independently increased G LUT4 translocation from an intracellular fraction to both the plasma membra ne and T-tubules. The combination of insulin and contraction induced a mark ed (approximately threefold) and almost fully additive increase in GLUT4 co ntent, but only in the plasma membrane. Insulin did not stimulate transferr in receptor recruitment from the GLUT4-containing intracellular fraction to either the plasma membrane or the T-tubules. In contrast, contraction stim ulated the recruitment of the transferrin receptor from the same GLUT4-cont aining intracellular fraction to the plasma membrane but not to the T-tubul es. Contraction-induced recruitment of the transferrin receptor was also ob served from immunopurified GLUT4 vesicles. It is concluded that muscle cont raction stimulates translocation of GLUT4 from two distinct intracellular c ompartments: 1) a population of recycling endosomes that is selectively rec ruited to the plasma membrane and 2) from GLUT4 storage vesicles that are a lso insulin-responsive and recruited to both the plasma membrane and the T- tubules. The lack of additive translocation of GLUT4 to the T-tubules may b e linked to the failure of GLUT4-containing recycling endosomes to be recru ited to these structures.