Myosin heavy chain transitions during development: Functional implications for the respiratory musculature

The myosin heavy chain (MHC) exists as multiple isoforms that are encoded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expression during maturation. Developmental expression of MHC isoforms correlate well with postnatal changes in actomyosin ATPase activity, specific force generation ( P 0/CSA), maximum unloaded velocity of shortening ( V 0), and fatigue resistance. More specifically, as the expression of MHC neonatal declines and MHC 2A, MHC 2X, and MHC 2B increase, actomyosin ATPase activity, P 0/CSA, V 0, and muscle fatigability increase. The increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fatigue resistance declines with development it is apparent that the energy costs of contraction are not fully matched by an increase in energy production. Developmental transitions in smooth muscle MHC phenotype also occur although their functional importance remains unclear.