Effects of hypophysectomy on soleus muscle fibers and spinal motoneurons in rats.

The fiber type distribution of the soleus muscle in male and female rats was investigated 4 weeks after hypophysectomy. Oxidative enzyme activity of the soleus motoneurons in the spinal cord was also examined by enzyme histochemical assay. In male rats, the total number of fibers in the soleus muscle was not changed after hypophysectomy, but the percentage of intermediate (INT) fibers (with intermediate adenosine triphosphatase activity following alkaline preincubation, and high succinate dehydrogenase and alpha-glycerophosphate dehydrogenase activities) was increased. All types of fibers in the soleus muscle of hypophysectomized rats showed high adenosine triphosphatase activity following acid preincubation. Oxidative enzyme activity of the motoneurons innervating the soleus muscle was not changed after hypophysectomy. Similar results were obtained in female rats. It is suggested that the increased percentage of INT fibers in the rat soleus muscle after hypophysectomy is due to a lack or reduced levels of growth hormones, and that the metabolic capacities of the muscle fibers and of the innervating motoneurons are affected independently by hypophysectomy.