Effect of hindlimb suspension on the functional properties of slow and fast soleus fibers from three strains of mice

Cross-sectionaI area (CSA), peak Ca2+-activated force (P-o), fiber specific force (P-o/CSA), and unloaded shortening velocity (V.) were measured in slow-twitch [containing type I myosin heavy chain (MHC)] and fast-twitch (containing type II MHC) chemically skinned soleus muscle fiber segments obtained from three strains of weight-bearing and 7-day hindlimb-suspended (HS) mice. HS reduced soleus slow MHC content (from similar to50 to similar to33%) in CBA/J and ICR strains without affecting slow MHC content in C57BL/6 mice (similar to20% of total MHC). Two-way ANOVA revealed HS-induced reductions in CSA, P-o, and P-o/CSA of slow and fast fibers from all strains. Fiber V-o was elevated post-HS, but not consistently across strains. No MHC X HS treatment interactions were observed for any variable for C57BL/6 and CBA/J mice, and the two significant interactions found for the ICR strain (CSA, P-o) appeared related to inherent pre-HS differences in slow vs. fast fiber CSA. In the mouse HS models studied here, fiber atrophy and contractile dysfunction were partially dependent on animal strain and generally independent of fiber MHC isoform content.