ELECTRICAL-STIMULATION INDUCES FIBER TYPE-SPECIFIC TRANSLOCATION OF GLUT-4 TO T-TUBULE IN SKELETAL-MUSCLE

Insulin and contraction independently stimulate glucose transport in s keletal muscle. Whereas insulin activates glucose transport more in mu scles composed of type I and IIa fibers, electrical stimulation increa ses glucose transport at least as much in type IIb fiber-enriched musc les despite the fact that the latter fiber type contains less GLUT-4 g lucose transporters. The aim of the present study was to test the hypo thesis that a greater GLUT-4 translocation to the cell surface may und erlie the higher contraction-stimulated glucose transport in type IIb myofibers. Leg muscles from rats were stimulated in situ at 100 Hz (20 0 ms) each 2 s via the sciatic nerve over a period of 20 min while the contralateral leg was kept at rest. Muscle 2-[H-3]deoxy-D-glucose upt ake (2-DG) was measured in separated red gastrocnemius (RG, type I and IIa fibers) and white gastrocnemius WG, type IIb fibers) muscles. Res ting 2-DG uptake was greater in RG than WG. Electrical stimulation inc reased 2-DG uptake over resting values similarly in WG and RG. Fractio ns enriched with either plasma membranes, transverse (T) tubules, tria ds, or GLUT-4-enriched intracellular membranes were isolated from RG a nd WG using a recently developed subcellular fractionation procedure. Electrical stimulation similarly increased GLUT-4 protein content-in p lasma membranes of RG and WG, whereas it stimulated GLUT-4 translocati on more (similar to 50%) in T tubules of WG than in RG. GLUT-4 content was not changed in triads of both muscle types. The increments in cel l surface GLUT-4 protein levels were paralleled by significant reducti ons in the amount of the transporter in the intracellular membrane fra ctions of both muscle types (by 60% in RG and 56% in WG). It is conclu ded that electrically induced contraction stimulates GLUT-4 translocat ion more in T tubules of WG than RG. The physiological implications of this finding for glucose uptake by contracting RG and WG muscles is d iscussed.