Isotonic contractile and fatigue properties of developing rat diaphragm muscle

Postnatal transitions in myosin heavy chain (MHC) isoform expression were found to be associated with changes in both isometric and isotonic contractile properties of rat diaphragm muscle (Dia(m)). Expression of MHC(neo) predominated in neonatal Dia(m) fibers but was usually coexpressed with MHC(slow) or MHC(2A) isoforms. Expression of MHC(neo) disappeared by day 28. Expression of MHC(2X) and MHC(2B) emerged at day 14 and increased thereafter. Associated with these MHC transitions in the Dia(m), maximum isometric tetanic force (P(o)), maximum shortening velocity, and maximum power output progressively increased during early postnatal development. Maximum power output of the Dia(m) occurred at ~40% P(o) at days 0 and 7 and at ~30% P(o) in older animals. Susceptibility to isometric and isotonic fatigue, defined as a decline in force and power output during repetitive activation, respectively, increased with maturation. Isotonic endurance time, defined as the time for maximum power output to decline to zero, progressively decreased with maturation. In contrast, isometric endurance time, defined as the time for force to decline to 30-40% P(o), remained >300 s until after day 28. We speculate that with the postnatal transition to MHC(2X) and MHC(2B) expression energy requirements for contraction increase, especially during isotonic shortening, leading to a greater imbalance between energy supply and demand.