The role of the motor neuron in the maintenance of contractile properties and myosin expression in skeletal muscle of the mouse.
Creators: name: Desypris, George.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1992
Subjects: subject: Biology, Neuroscience.

description: This study was undertaken to further elucidate the relative contributions of activity and myotrophic influences in maintaining certain contractile and histochemical properties of skeletal muscle. Four month old male C57BL mice were subjected to unilateral hindlimb denervation for either 10 days of 6 weeks. The denervation-induced changes in contractile properties as well as fiber-type distribution and myosin isoforms of the soleus (SOL) and extensor digitorum longus (EDL) muscles were investigated. To determine whether the denervation-induced alterations seen were due to withdrawal of putative myotrophic influences or activity, colchicine containing cuffs wrapped around the sciatic nerve were used to block fast axoplasmic transport thus restricting the access of myotrophic substances to the muscles, and tetrodotoxin (TTX) cuffs were used to block action potential propagation and thus activation of the muscles. When stimulated directly, denervated SOL and EDL muscles contracted slower than their innervated counterparts. This was reflected in an increase in both the time to peak tension (TTP) and time to half-relaxation (1/2RT) of the isometric twitch. In the case of denervated EDL muscles a decrease in the velocity of unloaded shortening (Vus) was also seen for both denervated groups. EDL muscles that were paralysed by TTX-treatment for 14 days also showed a reduction in the Vus. For both denervated and TTX-treated groups the reduction in Vus was accompanied by a loss of myosin light-chain 3f (LC3f). No change in velocity from control SOL was noted in any of the SOL experimental groups. Using 6% PAGE a loss of IIB myosin heavy chain and an increase in IIA myosin heavy chain was seen in the case of EDL muscles denervated for 6 weeks. This was corroborated by a decrease in the proportion of fibers expressing IIB myosin and an increase in the proportion of fibers expressing IIA myosin that was seen using immunohistochemistry with monoclonal antibodies directed against the various myosin heavy chain isoforms. Aside from 6 weeks of denervation, only paralysis with TTX was effective in precipitating a decrease in the proportion of fibers containing IIB myosin heavy chain in EDL muscles. An increase in fibers expressing type I myosin was seen in 6 weeks denervated mouse SOL muscles. Paralysis with TTX resulted in an increase in the proportion of fibers expressing type I myosin in SOL muscles. Also, treatment with TTX led to the appearance of fibers expressing type I myosin in EDL muscles. It is concluded that loss of activity is the main precipitating factor for the alterations seen in contractile and histochemical properties of denervated EDL and SOL muscles. Furthermore, the early reduction in Vus seen in EDL muscles denervated for 10 days or treated for 14 days with TTX and which was accompanied by a loss of myosin LC3f argues that loss of activity is an important stimulus for velocity transformation and that velocity characteristics can be significantly altered in a relatively short period of time. The results are also consistent with the idea that LC3f is an important regulator of Vus.