Cycle training increased GLUT4 and activation of mammalian target of rapamycin in fast twitch muscle fibers.
OBJECTIVE
To determine whether cycle training of sedentary subjects would increase the expression of the principle muscle glucose transporters, six volunteers completed 6 wk of progressively increasing intensity stationary cycle cycling.


METHODS
In vastus lateralis muscle biopsies, changes in expression of GLUT1, GLUT4, GLUT5, and GLUT12 were compared using quantitative immunoblots with specific protein standards. Regulatory pathway components were evaluated by immunoblots of muscle homogenates and immunohistochemistry of microscopic sections.


RESULTS
GLUT1 was unchanged, GLUT4 increased 66%, GLUT12 increased 104%, and GLUT5 decreased 72%. A mitochondrial marker (cytochrome c) and regulators of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator 1 alpha and phospho-5'-adenosine monophosphate-activated protein kinase) were unchanged, but the muscle hypertrophy pathway component, phospho-mammalian target of rapamycin (mTOR), increased 83% after the exercise program. In baseline biopsies, GLUT4 by immunohistochemical techniques was 37% greater in Type I (slow twitch, red) muscle fibers, but the exercise training increased GLUT4 expression in Type II (fast twitch, white) fibers by 50%, achieving parity with the Type I fibers. Baseline phospho-mTOR expression was 50% higher in Type II fibers and increased more in Type II fibers (62%) with training but also increased in Type I fibers (34%).


CONCLUSIONS
Progressive intensity stationary cycle training of previously sedentary subjects increased muscle insulin-responsive glucose transporters (GLUT4 and GLUT12) and decreased the fructose transporter (GLUT5). The increase in GLUT4 occurred primarily in Type II muscle fibers, and this coincided with activation of the mTOR muscle hypertrophy pathway. There was little impact on Type I fiber GLUT4 expression and no evidence of change in mitochondrial biogenesis.
Stuart
Charles A
CA
Department of Internal Medicine, Quillen College of Medicine, East Tennessee State University, Johnson City, TN 37614, USA. stuartc@etsu.edu
Howell
Mary E A
ME
Baker
Jonathan D
JD
Dykes
Rhesa J
RJ
Duffourc
Michelle M
MM
Ramsey
Michael W
MW
Stone
Michael H
MH
D055372:AMP-Activated Protein Kinases / Q000378:metabolism; D000328:Adult; D001642:Bicycling / Q000502:physiology*; D001706:Biopsy; D045304:Cytochromes c / Q000378:metabolism; D005260:Female; D051246:Glucose Transport Proteins, Facilitative / Q000378:metabolism; D051272:Glucose Transporter Type 1 / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000378:metabolism*; D051296:Glucose Transporter Type 5 / Q000378:metabolism; D006360:Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism*; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D016133:Polymerase Chain Reaction; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors / Q000378:metabolism
D051246:Glucose Transport Proteins, Facilitative; D051272:Glucose Transporter Type 1; D051275:Glucose Transporter Type 4; D051296:Glucose Transporter Type 5; D006360:Heat-Shock Proteins; D047908:Intracellular Signaling Peptides and Proteins; C486497:PPARGC1A protein, human; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D012333:RNA, Messenger; C495677:SLC2A1 protein, human; C449579:SLC2A12 protein, human; C495692:SLC2A4 protein, human; D014157:Transcription Factors; D045304:Cytochromes c; C546842:MTOR protein, human; D017346:Protein Serine-Threonine Kinases; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases