Changes in glucose transporters in muscle in response to exercise

The mechanism underlying the increase in glucose uptake in response to muscular contraction is not known, although it has been established that the change does not require insulin. It is our hypothesis that exercise, like insulin, stimulates translocation of glucose transporters to the plasma membrane. To test this hypothesis an experiment was performed to determine whether glucose transporters are translocated from an intracellular membrane to the plasma membrane during exercise. Untrained male rats weighing approximately 250 g were exercised by treadmill running for 2 h at 25 m/min. They were killed immediately after completion of exercise, and the gastrocnemius and quadriceps muscles were quickly removed. Sedentary animals were treated in the same way. Plasma and intracellular membranes were isolated by sucrose density gradient centrifugation and cytochalasin B binding assays were performed. Exercise resulted in a redistribution of glucose transporters from the intracellular membrane to the plasma membrane. The ratio of cytochalasin B binding sites in the membrane fractions (intracellular/plasma membrane) was 3.2 +/- 0.6 in rested animals and 1.3 +/- 0.3 after exercise. The concentration of glucose transporters was increased in the plasma membrane (from 19.8 +/- 1.8 to 30.4 +/- 3.9 pmol/mg protein) and decreased in the intracellular membrane (from 20.7 +/- 3.0 to 10.8 +/- 1.1 pmol/mg protein) in response to exercise. These results suggest that at least part of the increase in glucose uptake that occurs during exercise is the result of a redistribution of glucose transporters to the plasma membrane.