MECHANICAL POWER AND MYOSIN COMPOSITION OF SOLEUS AND EXTENSOR DIGITORUM LONGUS MUSCLES OF KY MICE

Muscles of ky/ky homozygote mice exhibit neonatal muscle fiber necrosi s and regeneration with subsequent motor nerve sprouting and developme nt of a prominent kyphoscoliosis from similar to 100 days onward. Sole us and extensor digitorum longus (EDL) muscles from hy mice weighed <5 0% of control muscles from age-matched NMRI mice. Maximal tetanic forc e was more reduced in soleus than in EDL. In EDL, the velocity constan t of the force-velocity relation, maximal velocity, twitch time-to-pea k, and isomyosin content were normal at all ages. The early mechanical changes seen in hy soleus muscles (47 day) were not accompanied by si gnificant alterations in isomyosin or myosin heavy- and light-chain co mposition, since hy and NMRI expressed slow-twitch native myosin 2 (SM (2), type I fibers) and intermediate-twitch native myosin (IM, type II a fibers). Adult ky soleus (172 day) showed wholesale loss of IM and s ole expression of SM(2). This is sufficient to account for the markedl y slowing of the force-velocity relation and the twitches observed in adult hy soleus. We propose that since shifts in muscle type only occu rred in soleus, this reflects the persistent requirement to withstand the force of gravity.