Plasticity in Skeletal, Cardiac, and Smooth Muscle: Selected Contribution: Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development

It has beenfound that maximum specific force (F_max; force percross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change inF_max reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridgesin parallel) and/or a greater force per cross bridge in fibersexpressing fast MHC isoforms compared with slow and neonatal MHCisoforms (MHC_slow and MHC_neo, respectively).Single Triton 100-X-permeabilized fibers were activated at a pCa of4.0. 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, fibersexpressing MHC_slow and coexpressing MHC_neoproduced less force than fibers expressing fast MHC isoforms. Weconclude that lower force per cross bridge in fibers expressingMHC_slow and MHC_neo contributes to the lowerF_max seen in early postnatal development.