Maximum specific force depends on myosin heavy chain content in rat diaphragm muscle fibers

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 specif ic force [force per cross-sectional area (CSA)] reflect the number of cross bridges present per CSA. Studies were performed on single, Triton X-100-pe rmeabilized rat Dia(m) fibers, Maximum specific force was determined by act ivation of single Dia(m) fibers in the presence of a high-calcium solution (pCa, -log Ca2+ concentration of 4.0). SDS-PAGE and Western blot analyses w ere used to determine MHC isoform composition and MHC content per half sarc omere. Differences in maximum specific force across fast MHC isoforms were eliminated when controlled for half-sarcomere MHC content. However, the for ce produced by slow fibers remained below that of fast fibers when normaliz ed 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 cros s bridge compared with fast fibers.