Enzyme activity changes in rat soleus motoneurons and muscle after synergist ablation.

Quantitative enzyme histochemical methods have been used to determine the effect of ablation of synergists on the oxidative metabolism of the alpha-motoneurons and muscle fibers of the rat soleus. Sixty days postablation, the NADH-tetrazolium reductase (NADH-TR) activity of soleus motoneurons decreased 12.5% from 0.327 +/- 0.005 (mean +/- SE; optical density units) to 0.286 +/- 0.007. In the muscle fibers, the alpha-glycerophosphate dehydrogenase activity (glycolytic enzyme) decreased from 0.114 +/- 0.010 to 0.074 +/- 0.009, a change of 35.1%, and the NADH-TR activity decreased 21.2% from 0.348 +/- 0.018 to 0.274 +/- 0.017. In both the motoneurons and the muscle fibers, the decrease was nonspecific for all cells, although a greater effect on the cells with higher enzyme activity was observed. The decreased NADH-TR activity represents a shift in the oxidative profile of the motoneurons and muscle fibers, indicating a decreased ability to use oxidative metabolism for periods of short-term high-energy demands. Furthermore, the parallel decrease in muscle fibers and motoneurons with high NADH-TR activity (fast-twitch oxidative-glycolytic fibers and presumably also motoneurons) demonstrates the tight correlation of the NADH-TR activity between these parts of the motor unit in both control and synergist-ablated muscles.