Force-velocity relationship and isomyosin content of mouse soleus

Maréchal, Georges [UCL] Beckers-Bleukx, G. [UCL]

Soleus muscles (n=30) of NMRI mice were tetanically stimulated. Their length was controlled by an electromagnetic ergometer and recorded together with the force on a fast UV recorder. Fibre length (Lf) was evaluated by microscopic measurements of a few superficial fibres. The constants (b, in fibre length/s, and a/Fo) of the hyperbolic Hill's equation were computed using an iterative least square method. The maximum power, Pmax, was computed as b.F0.(1 + 2a/Fo - 2,[(a/F0)2 + a/F0]). The myosin heavy chains of a crude extract were separated by electrophoresis on glycerol-SDS-polyacrylamide gels and the zones were quantified by computerized by densitometry and identified as MHC1, MHC2A and MHC 2B. In spite of their large variations, there was no significant relation between the mechanical data and the myosin heavy chains. It is not clear why a slow-twitch muscle (soleus) should contain variable amounts of several isomyosins since the mechanical power or the Hill's constant do not change correlatively.