FORCE-VELOCITY RELATION AND ISOMYOSINS IN SOLEUS MUSCLES FROM I STRAINS OF MICE (C57 AND NMRI)

We compared soleus muscles, from two strains of mice, NMRI and C57. So leus muscles from NMRI mice produced slower twitches and lower maximum tetanic force (F(o)) but higher maximum tetanic stress (S(o)), (owing to their smaller weight). Their Hill's velocity constant (b) was lowe r, but their force constant (a/S(o)), their maximum velocity of unload ed shortening (V(u)) and their maximal mechanical power (P(max)) were similar. All soleus muscles contained two isomyosins (SM2 and IM) and the two myosin heavy chains (MHC1 and MHC2A) corresponding to type I f ibres and type IIA fibres; however, soleus muscles from NMRI strain ha d higher proportions of isomyosin SM2 and of myosin heavy chain 2A. Re gression equations were, computed between the mechanical variables and the myosin heavy chain content. Using a simple hypothesis, the result s were used to estimate the mechanical properties of type I and type I IA fibres. We conclude that type IIA fibres from soleus muscle are mec hanically more similar to slow-twitch type I fibres than to fast-twitc h type II fibres. The results also suggest a hypothesis to account for the diversity of isomyosins, by a matching diversity of mechanical pr operties based on a separate physiological control of the three factor s that control P(max).