It has been found that maximum specific force (F(max); force per cross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change in F(max) reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridges in parallel) and/or a greater force per cross bridge in fibers expressing fast MHC isoforms compared with slow and neonatal MHC isoforms (MHC(slow) and MHC(neo), respectively). Single Triton 100-X-permeabilized fibers were activated at a pCa of 4.0. MHC isoform expression was determined by SDS-PAGE. MHC content per half-sarcomere was determined by densitometric analysis and comparison to a standard curve of known MHC concentrations. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibers expressing MHC(slow) and coexpressing MHC(neo) produced less force than fibers expressing fast MHC isoforms. We conclude that lower force per cross bridge in fibers expressing MHC(slow) and MHC(neo) contributes to the lower F(max) seen in early postnatal development.

We hypothesize that 1) the effect of denervation (DNV) is more pronounced in fibers expressing fast myosin heavy chain (MHC) isoforms and 2) the effect of DNV on maximum specific force reflects a reduction in MHC content per half sarcomere or the number of cross bridges in parallel. Studies were performed on single Triton X-100-permeabilized fibers activated at a pCa (-log Ca2+ concentration) of 4.0. MHC content per half sarcomere was determined by densitometric analysis of SDS-PAGE gels and comparison to a standard curve of known MHC concentrations. After 2 of wk DNV, the maximum specific force of fibers expressing MHC2X was reduced by approximately 40% (MHC(2B) expression was absent), whereas the maximum specific force of fibers expressing MHC2A and MHC(slow) decreased by only approximately 20%. DNV also reduced the MHC content in fibers expressing MHC2X, with no effect on fibers expressing MHC2A and MHC(slow). When normalized for MHC content per half sarcomere, force generated by DNV fibers expressing MHC2X and MHC2A was decreased compared with control fibers. These results suggest the force per cross bridge is also affected by DNV.