Differences in maximum specific force across fast MHC isoforms were eliminated when controlled for half-sarcomere MHC content, but slow fibers produced less force per cross bridge than fast fibers.
The lower force produced by slow muscle fibers compared to fast fibers may be due to less force per cross bridge rather than just the number of cross bridges available.
In the present study, myosin heavy chain (MHC) content per half sarcomere, an estimate of the number of cross bridges available for force generation, was determined in rat diaphragm muscle (Dia(m)) fibers expressing different MHC isoforms. We hypothesize that fiber-type differences in maximum specific force force per cross-sectional area (CSA) reflect the number of cross bridges present per CSA. Studies were performed on single, Triton X-100-permeabilized rat Dia(m) fibers. Maximum specific force was determined by activation of single Dia(m) fibers in the presence of a high-calcium solution (pCa, -log Ca(2+) concentration of 4.0). SDS-PAGE and Western blot analyses were used to determine MHC isoform composition and MHC content per half sarcomere. Differences in maximum specific force across fast MHC isoforms were eliminated when controlled for half-sarcomere MHC content. However, the force produced by slow fibers remained below that of fast fibers when normalized for the number of cross bridges available. On the basis of these results, the lower force produced by slow fibers may be due to less force per cross bridge compared with fast fibers.
Geiger et al. (Tue,) reported a other. MHC isoform composition (fast vs slow) vs. Different MHC isoforms was evaluated on Maximum specific force (force per cross-sectional area). Differences in maximum specific force across fast MHC isoforms were eliminated when controlled for half-sarcomere MHC content, but slow fibers produced less force per cross bridge than fast fibers.