Effect of unilateral denervation on maximum specific force in rat diaphragm muscle fibers

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 MHC_2X was reduced by ∼40% (MHC_2B expression was absent), whereas the maximum specific force of fibers expressing MHC_2A and MHC_slow decreased by only ∼20%. DNV also reduced the MHC content in fibers expressing MHC_2X, with no effect on fibers expressing MHC_2A and MHC_slow. When normalized for MHC content per half sarcomere, force generated by DNV fibers expressing MHC_2X and MHC_2A was decreased compared with control fibers. These results suggest the force per cross bridge is also affected by DNV.