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Am J Physiol Regul Integr Comp Physiol 268: R130-R134, 1995;
0363-6119/95 $5.00
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AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 1 130-R134, Copyright © 1995 by American Physiological Society

ARTICLES

Adaptive responses of GLUT-4 and citrate synthase in fast-twitch muscle of voluntary running rats

E. J. Henriksen and A. E. Halseth
Department of Exercise and Sport Sciences, University of Arizona, Tucson 85721.

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.


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