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 i n fibers expressing fast myosin heavy chain (MHC) isoforms and 2) the effec t 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 per formed 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 cur ve of known MHC concentrations. After 2 of wk DNV, the maximum specific for ce of fibers expressing MHC2x was reduced by similar to 40% (MHC2x expressi on was absent), whereas the maximum specific force of fibers expressing MHC 2A and MHCslow decreased by only similar to 20%. DNV also reduced the MHC c ontent in fibers expressing MHC2x with no effect on fibers expressing MHC2A and MHCslow. When normalized for MHC content per half sarcomere, force gen erated by DNV fibers expressing MHC2x and MHC2A was decreased compared with control fibers. These results suggest the force per cross bridge is also a ffected by DNV.