GLUCOSE TRANSPORTERS IN SINGLE SKELETAL-MUSCLE FIBERS - RELATIONSHIP TO HEXOKINASE AND REGULATION BY CONTRACTILE ACTIVITY

Glucose transport and phosphorylation are the first steps in the utili zation of extracellular glucose by skeletal muscle. We have examined t he relationships between proteins mediating these steps in single fibe rs of identified type dissected from rabbit skeletal muscle. The level of the glucose transporter isoform GLUT4, measured by immunoblotting, varied among fibers by a factor of 20 (slow oxidative > fast oxidativ e glycolytic > fast glycolytic). In fibers from the tibialis anterior muscle, GLUT4 was correlated (r(2) = 0.75) with the activity of malate dehydrogenase, an enzyme representative of oxidative energy metabolis m. In these fibers a strong correlation (r(2) = 0.70) was also observe d between GLUT4 and hexokinase activity. GLUT1 levels were barely dete ctable, regardless of fiber type. To investigate the possible role of muscle activity in controlling the expression of transporters, tibiali s anterior muscles were activated by chronic electrical stimulation of the peroneal nerves. GLUT1 levels increased after 1 day of stimulatio n to a plateau that was severalfold higher than the level in non-stimu lated cells. Hexokinase activity and the GLUT4 level changed in parall el: both were increased by approximately 2.5-fold after 1 day and by 1 4-fold after 21 days. Thus, while both GLUT1 and GLUT4 were regulated by muscle activity, only GLUT4 expression was coordinated with the exp ression of hexokinase.