KINETICS OF 2-DEOXYGLUCOSE TRANSPORT IN SKELETAL-MUSCLE - EFFECTS OF INSULIN AND CONTRACTIONS

There is some controversy regarding whether insulin or contractile activity alters the affinity of skeletal muscle glucose transporters for glucose and its analogues. The effects of insulin and contractions on the kinetics of glucose transport were therefore reexamined in isolated rat skeletal muscles. Concentration-dependent rates of 2-deoxyglucose (2-DG) transport were measured in the absence or presence of insulin (2 mU/ml) in the epitrochlearis and split soleus muscles. The apparent half-maximal saturating substrate concentration (K-m) for basal 2-DG transport (similar to 12 mM) was similar for the split soleus and epitrochlearis, and the apparent K-m was not changed by insulin in either muscle type. The presence of 2 mU/ml insulin increased the maximal transport velocity (V-max) approximately fourfold in the epitrochlearis and approximately eightfold in the split soleus. In the epitrochlearis, in vitro muscle contractions also resulted in an approximately fourfold increase in V-max with no change in apparent K-m. The combined effects of insulin and contractions on V-max were completely additive, but the apparent K-m was not different from insulin alone. The apparent K-m values for basal and insulin-stimulated glucose transport were further characterized in the epitrochlearis isolated from transgenic mice overexpressing the GLUT-1 isoform in the sarcolemma and their nontransgenic littermates. The apparent K, for basal 2-DG transport in the transgenic muscle (9 mM) was not significantly different from the apparent K-m for insulin-stimulated transport in the control muscle (10 mM). The present study provides evidence that insulin and contractions, either alone or in combination, increase glucose transport activity in skeletal muscle by increasing V-max with no significant change in K-m. Our results also suggest that, in intact skeletal muscle, the K-m for basal glucose transport (a process mediated primarily by GLUT-1) is similar to the K-m values for stimulated transport, mediated predominantly by GLUT-4.