Enzymatic adaptations to treadmill training in skeletal muscle of young and old rats.

The activities of lactic dehydrogenase (LDH), hexokinase (HK), citrate synthetase (CS), and 3-hydroacyl CoA dehydrogenase (HAD), were measured in Soleus and extensor digitorum longus (EDL) muscles of four different age groups of male Wistar rats. Young rats (3 months) and old rats (24 months) were trained during 12 weeks by a 3 days a week running exercises session. Each training session was of 2 h. All the enzymatic activities increased between 3 and 6 months in both Soleus and EDL muscles. During aging (6-27 months) Soleus showed about the same percent decrease in enzymatic activities (30%) for the four enzymes studied. In contrast with Soleus, CS activity of EDL increased with age but HK, LDH, and HAD decreased. The training induced an improvement of all enzymatic activities, except for LDH, which decreased with training in same cases. In the old group the training effect was more important than in the young group. When the relative enzymatic activities were compared it was found that Soleus muscle keeps its energy supply enzyme pattern during aging, whereas EDL evolutes to a more oxidative metabolism. Moreover old skeletal muscle remains trainable and training induces an enzymatic adaptation which follows the same pattern in both muscles: an increase of HK, CS, and HAD activities and a decrease of LDH.