Effect of chronic electrical stimulation on GLUT-4 protein content in fast-twitch muscle

Insulin- and contraction-stimulated skeletal muscle glucose transport is governed largely by the GLUT-4 isoform of the glucose transporter. Recently, it has been demonstrated that denervated muscle has decreased GLUT-4 protein content, suggesting that regulation of GLUT-4 protein is related to neuromuscular activity. However, until now the effects of the opposite situation, enhanced neuromuscular activity, could only be speculated on from exercise training studies. In the present investigation the effect of chronic low-frequency electrical stimulation (10 Hz, 8 h/day) on GLUT-4 protein content and citrate synthase activity was determined in the predominantly fast-twitch plantaris. Chronic electrical stimulation enhanced GLUT-4 protein content and citrate synthase activity in the muscles stimulated for 10-20 days. Electrical stimulation lasting 30-40 days resulted in no further enhancement of GLUT-4 protein content while citrate synthase activity continued to increase. Further prolongation of electrical stimulation (60-90 days) resulted in a plateauing of citrate synthase activity. The results suggest that increased neuromuscular activity can act independently of systemic changes to increase total GLUT-4 protein content. They also suggest that both GLUT-4 protein content and citrate synthase activity are positively related to increased neuromuscular activity but that their rates of increase differ substantially.