Neural influences on the distribution of troponin I isotypes in the cat

An immunohistochemical technique was used to study changes in the distribution of fast and slow forms of troponin I (TN-I) in response to alterations in the nerve supply. Hind limb muscles from normal, spinal-isolated, and cordotomized cats, one leg of which had undergone cross innervation between slow (soleus) and fast (flexor hallucis longus, FHL) muscles, were examined. At 8 months after cross innervation of normal soleus by the FHL nerve, the number of fast TN-I-positive cells had increased from 0.26 to 22.1%. At 8 months after cross innervation of normal FHL muscle with the soleus nerve, the number of fast TN-I-positive cells had decreased from 86.5 to 30.5%. The number of intermediate cells staining for both fast and slow TN-I, increased considerably after cross innervation of both soleus and FHL muscles. Spinal isolation by itself had a dramatic effect on the distribution of fast and slow TN-I, converting almost all the originally slow fibers in the FHL and 60.0% of the soleus fibers to fast TN-I-positive cells in 8 months. Cordotomy, in contrast, produced an increase of only 15.6% in the soleus, and did not change the FHL. There was no quantitative difference in the crossed-and uncrossed muscles of spinal isolated cats. In cordotomized cats, cross innervation of the soleus by the FHL nerve resulted in 32.3% fast TN-I-positive cells, with some fiber type grouping. Thus, distribution of fast and slow forms of TN-I changed after each neural manipulation which altered amounts and patterns of muscle contraction and stretch.