EXERCISE INDUCES RAPID INCREASES IN GLUT4 EXPRESSION, GLUCOSE-TRANSPORT CAPACITY, AND INSULIN-STIMULATED GLYCOGEN-STORAGE IN MUSCLE

GLUT4 glucose transporter content and glucose transport capacity are c losely correlated in skeletal muscle. In this study, we tested the hyp othesis that a rapid increase in GLUT4 expression occurs as part of th e early adaptive response of muscle to exercise and serves to enhance glycogen storage. Rats exercised by swimming had a similar to 2-fold i ncrease in GLUT4 mRNA and a 50% increase in GLUT4 protein expression i n epitrochlearis muscle 16 h after one prolonged exercise session. Aft er a 2nd day of exercise, muscle GLUT4 protein was increased further t o similar to 2-fold while there was no additional increase in GLUT4 mR NA. Muscle hexokinase activity also doubled in response to 2 days of e xercise. Glucose transport activity maximally stimulated with insulin, contractions, or hypoxia was increased roughly in proportion to the a daptive increase in GLUT4 protein in epitrochlearis muscles. Treatment with insulin prior to subcellular fractionation of muscle resulted in a similar to 2-fold greater increase in GLUT4 content of a plasma mem brane fraction in the 2-day swimmers than in controls. When epitrochle aris muscles were incubated with glucose and insulin, glycogen accumul ation over 3 h was twice as great in muscles from 2-day swimmers as in control muscles. Our results show that a rapid increase in GLUT4 expr ession is an early adaptive response of muscle to exercise. This adapt ation appears to be mediated by pretranslational mechanisms. We hypoth esize that the physiological role of this adaptation is to enhance rep lenishment of muscle glycogen stores.