In the present study, we examined whether GLUT4 concentration in rat skeletal muscle is dependent on local muscle activity level or not. In ten male Sprague-Dawley rats, one side of gastrocnemius muscle was tenotomized, and the other side contralateral muscle was treated sham operation as a control. Gastrocnemius and plantaris muscles were excised from both legs at the five weeks after surgery. After the muscles were weighed, GLUT 4 concentration and citrate synthase (CS) activity were measured. The results are summarized as follows:
In gastrocnemius muscle, tenotomy induced decreases of 25% in muscle weight, 16% in CS activity, and 25% in GLUT 4 concentration as compared with the control muscle. These data suggest that although extramuscular environment is similar, different GLUT 4 concen-tration in both muscles is induced by different muscle activity level. Therefore, it is con-cluded that muscle activity level regulates GLUT 4 concentration in skeletal muscle. In over-loaded synergistic plantaris muscle, muscle weight and GLUT 4 content per whole muscle were increased by 18% and 17%, respectively, but GLUT 4 concentration and CS activity were not changed as compared with the control muscle. These data could be interpreted that GLUT 4 concentration and mitochondrial oxidative enzyme activity in skeletal muscle are coregulated.

This study was undertaken to determine the effect of neurotrophic substance on atrophy of denervated rat skeletal muscle. Hind-limb muscles of 14-21-week-old rats were denervated and/or artery-ligated for 1 week. Some muscles were also injected with saline buffer or a saline extract of porcine spinal cord (10 mg protein/ml) daily via the femoral artery. Atrophy was assessed by measurement of muscle wet weight and cross-sectional area of type I, type II A and type II B muscle fibers. The results obtained were as follows
1. Denervation produced a significant decrease in the weights of the gastrocnemius, soleus and extensor digitorum longus (EDL) muscles. It also significantly decreased the area of each fiber type in the lateral head of the gastrocnemius (deep portion) and soleus muscles.
2. Artery ligation produced a significant decrease in the weights of the gastrocnemius and EDL muscles, but did not significantly change the area of each fiber type in the lateral head of the gastrocnemius (deep portion) and soleus muscles.
3. Buffer injection did not change the weight or fiber areas of hind-limb muscles to a significant extent.
4. Injection of spinal cord extract significantly ameliorated the atrophy of denervated EDL muscle.
In conclusion, it is suggested that a substance present in the spinal cord may ameliorate the atrophy of denervated muscle in vivo.

In this study, we investigated the possibility that GLUT4 protein concentration and mitochondrial enzyme activity is coregulated by muscle contractile activity. In the first experiment, male rats were trained by treadmill running or swimming for 3 wks and training effects in upper-and hind-limb muscle were investigated. Treadmill training program induced increases of 30% in citrate synthase activity, 51% in hexokinase activity, and 35% in GLUT4 protein concentration in hind-limb soleus muscles without causing training effects in upper-limb epitrochlearis muscles. On the other hand, swimming training program induced increases of 67% in citrate synthase activity, 139% in hexokinase activity, and 89% in GLUT4 protein concentration in epitrochlearis muscle without causing training effects in soleus muscles. These results suggest that muscle contractile activity which raises mitochondrial enzyme activity increases GLUT4 protein concentration in skeletal muscle. In a second set of experiment, we investigated relationship between muscle GLUT4 protein concentration and mitochondrial enzyme activity by use of the various type of rat muscles (soleus, plantaris, red gastrocnemius, white gastrocnemius, epitrochlearis and heart), which possese different contractile activity level. Significant correlation was observed between GLUT4 protein concentration and citrate synthase activity among different muscles (r=0.936) . They suggest that muscle GLUT4 protein concentration and mitochondrial enzyme activity level may be coregulated according to muscle contractile activity level.