Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development

It has been found that maximum specific force (F-max; force per cross-secti onal area) of rat diaphragm muscle doubles from birth to 84 days (adult). W e 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 bridg e in fibers expressing fast MHC isoforms compared with slow and neonatal MH C isoforms (MHCslow and MHCneo, respectively). Single Triton 100-X-permeabi lized fibers were activated at a pCa of 4.0. MHC isoform expression was det ermined by SDS-PAGE. MHC content per half-sarcomere was determined by densi tometric analysis and comparison to a standard curve of known MHC concentra tions. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibers expressing MHCslow and coexpressing MHCneo produced less force than fibers expressing fast MHC isoforms. We conclude that lower force per cross bridge in fibers expressing MHCslow and MHCneo contributes to the lower F- max seen in early postnatal development.