title: Electrical stimulation induces fiber type-specific translocation of GLUT-4 to T tubules in skeletal muscle.
abstract: Insulin and contraction independently stimulate glucose transport in skeletal muscle. Whereas insulin activates glucose transport more in muscles composed of type I and IIa fibers, electrical stimulation increases glucose transport at least as much in type IIb fiber-enriched muscles despite the fact that the latter fiber type contains less GLUT-4 glucose transporters. The aim of the present study was to test the hypothesis that a greater GLUT-4 translocation to the cell surface may underlie the higher contraction-stimulated glucose transport in type IIb myofibers. Leg muscles from rats were stimulated in situ at 100 Hz (200 ms) each 2 s via the sciatic nerve over a period of 20 min while the contralateral leg was kept at rest. Muscle 2-[3H]deoxy-D-glucose uptake (2-DG) was measured in separated red gastrocnemius (RG, type I and IIa fibers) and white gastrocnemius (WG, type IIb fibers) muscles. Resting 2-DG uptake was greater in RG than WG. Electrical stimulation increased 2-DG uptake over resting values similarly in WG and RG. Fractions enriched with either plasma membranes, transverse (T) tubules, triads, or GLUT-4-enriched intracellular membranes were isolated from RG and WG using a recently developed subcellular fractionation procedure. Electrical stimulation similarly increased GLUT-4 protein content in plasma membranes of RG and WG, whereas it stimulated GLUT-4 translocation more (approximately 50%) in T tubules of WG than in RG. GLUT-4 content was not changed in triads of both muscle types. The increments in cell surface GLUT-4 protein levels were paralleled by significant reductions in the amount of the transporter in the intracellular membrane fractions of both muscle types (by 60% in RG and 56% in WG). It is concluded that electrically induced contraction stimulates GLUT-4 translocation more in T tubules of WG than RG. The physiological implications of this finding for glucose uptake by contracting RG and WG muscles is discussed.
authors.affiliation: Department of Physiology, Laval University Hospital Research Center, Ste-Foy, Québec, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Roy
authors.forename: E
authors.initials: E
authors.lastname: Jóhannsson
authors.forename: A
authors.initials: A
authors.lastname: Bonen
authors.forename: A
authors.initials: A
authors.lastname: Marette
mesh_terms: D000818:Animals; D015415:Biomarkers; D002462:Cell Membrane / Q000502:physiology; D003847:Deoxyglucose / Q000378:metabolism; D004558:Electric Stimulation; D051275:Glucose Transporter Type 4; D006003:Glycogen / Q000378:metabolism; D007773:Lactates / Q000378:metabolism; D008297:Male; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D009124:Muscle Proteins; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D012584:Sciatic Nerve / Q000502:physiology
chemical_list: D015415:Biomarkers; D051275:Glucose Transporter Type 4; D007773:Lactates; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495694:Slc2a4 protein, rat; D006003:Glycogen; D003847:Deoxyglucose