Adaptive responses of GLUT-4 and citrate synthase in fast-twitch muscle of voluntary running rats.
Glucose transporter (GLUT-4) protein, hexokinase, and citrate synthase (proteins involved in oxidative energy production from blood glucose catabolism) increase in response to chronically elevated neuromuscular activity. It is currently unclear whether these proteins increase in a coordinated manner in response to this stimulus. Therefore, voluntary wheel running (WR) was used to chronically overload the fast-twitch rat plantaris muscle and the myocardium, and the early time courses of adaptative responses of GLUT-4 protein and the activities of hexokinase and citrate synthase were characterized and compared. Plantaris hexokinase activity increased 51% after just 1 wk of WR, whereas GLUT-4 and citrate synthase were increased by 51 and 40%, respectively, only after 2 wk of WR. All three variables remained comparably elevated (+50-64%) through 4 wk of WR. Despite the overload of the myocardium with this protocol, no substantial elevations in these variables were observed. These findings are consistent with a coordinated upregulation of GLUT-4 and citrate synthase in the fast-twitch plantaris, but not in the myocardium, in response to this increased neuromuscular activity. Regulation of hexokinase in fast-twitch muscle appears to be uncoupled from regulation of GLUT-4 and citrate synthase, as increases in the former are detectable well before increases in the latter.
Department of Exercise and Sport Sciences, University of Arizona, Tucson 85721.
E J
EJ
Henriksen
A E
AE
Halseth
D000064:Acclimatization; D000818:Animals; D002950:Citrate (si)-Synthase / Q000378:metabolism*; D005260:Female; D051275:Glucose Transporter Type 4; D006003:Glycogen / Q000378:metabolism; D006321:Heart / Q000502:physiology*; D006593:Hexokinase / Q000378:metabolism; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D009124:Muscle Proteins; D018482:Muscle, Skeletal / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D010805:Physical Conditioning, Animal; D005082:Physical Exertion; D051381:Rats; D017207:Rats, Sprague-Dawley; D012044:Regression Analysis; D013997:Time Factors
D051275:Glucose Transporter Type 4; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495694:Slc2a4 protein, rat; D006003:Glycogen; D002950:Citrate (si)-Synthase; D006593:Hexokinase