ISOTONIC CONTRACTILE AND FATIGUE PROPERTIES OF DEVELOPING RAT DIAPHRAGM MUSCLE

Postnatal transitions in myosin heavy chain (MHC) isoform expression w ere found to be associated with changes in both isometric and isotonic contractile properties of rat diaphragm muscle (Dia(m)). Expression o f MHCneo predominated in neonatal Dia, fibers but was usually coexpres sed with MHCslow or MHC2A isoforms. Expression of MHCneo disappeared b y day 28. Expression of MHC2X and MHC2B emerged at day 14 and increase d thereafter. Associated with these MHC transitions in the Dia(m), max imum isometric tetanic force (P-o), maximum shortening velocity, and m aximum power output progressively increased during early postnatal dev elopment. Maximum power output of the Dia(m) occurred at similar to 40 % P-o at days 0 and 7 and at similar to 30% P-o in older animals. Susc eptibility to isometric and isotonic fatigue, defined as a decline in force and power output during repetitive activation, respectively, inc reased with maturation. Isotonic endurance time, defined as the time f or maximum power output to decline to zero, progressively decreased wi th maturation. In contrast, isometric endurance time, defined as the t ime for force to decline to 30-40% P-o, remained >300 s until after da y 28. We speculate that with the postnatal transition to MHC2X and MHC 2B expression energy requirements for contraction increase, especially during isotonic shortening, leading to a greater imbalance between en ergy supply and demand.