MYOSIN HEAVY-CHAIN TRANSITIONS DURING DEVELOPMENT FUNCTIONAL IMPLICATIONS FOR THE RESPIRATORY MUSCULATURE

The myosin heavy chain (MHC) exists as multiple isoforms that are enco ded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expre ssion during maturation. Developmental expression of MHC isoforms corr elate well with postnatal changes in actomyosin ATPase activity, speci fic force generation (P(0/)CSA), maximum unloaded velocity of shorteni ng (V-0), and fatigue resistance. More specifically, as the expression of MHCneonatal declines and MHC2A, MHC2X, and MHC2B increase, actomyo sin ATPase activity, P-0/CSA, V-0, and muscle fatigability increase. T he increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fat igue resistance declines with development it is apparent that the ener gy 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. (C) 1 998 Elsevier Science Inc. All rights reserved.