Abstract: | Postnatal transitions in myosin heavy chain (MHC) isoformexpression were found to be associated with changes in both isometric and isotonic contractile properties of rat diaphragm muscle(Diam). Expression of MHCneo predominated inneonatal Diam fibers but was usually coexpressed withMHCslow or MHC2A isoforms. Expression ofMHCneo disappeared by day 28. Expression ofMHC2X and MHC2B emerged at day 14 andincreased thereafter. Associated with these MHC transitions in theDiam, maximum isometric tetanic force (Po), maximum shortening velocity, and maximum power output progressively increased during early postnatal development. Maximum power output ofthe Diam occurred at ~40% Po at days0 and 7 and at ~30% Po in older animals.Susceptibility to isometric and isotonic fatigue, defined as a declinein force and power output during repetitive activation, respectively,increased with maturation. Isotonic endurance time, defined as the timefor maximum power output to decline to zero, progressively decreasedwith maturation. In contrast, isometric endurance time, defined as thetime for force to decline to 30-40% Po, remained>300 s until after day 28. We speculate that with thepostnatal transition to MHC2X and MHC2Bexpression energy requirements for contraction increase, especiallyduring isotonic shortening, leading to a greater imbalance betweenenergy supply and demand. |