Changes in actomyosin ATP consumption rate in rat diaphragm muscle fibers during postnatal development

Early postnatal development of rat diaphragm muscle (Dia(m)) is marked by dramatic transitions in myosin heavy chain (MHC) isoform expression. We hypothesized that the transition from the neonatal isoform of MHC (MHCNeo) to adult fast MHC isoform expression in Diam fibers is accompanied by an increase in both the maximum velocity of the actomyosin ATPase reaction (V-max ATPase) and the ATP consumption rate during maximum isometric activation (ATP(iso)). Rat Dia(m) fibers were evaluated at postnatal days 0, 14, and 28 and in adults ( day 84). Across all ages, V-max ATPase of fibers was significantly higher than ATP(iso). The reserve capacity for ATP consumption [1 - ( ratio of ATP(iso) to V-max ATP(ase))] was remarkably constant (similar to55-60%) across age groups, although at day 28 and in adults the reserve capacity for ATP consumption was slightly higher for fibers expressing MHCSlow compared with fast MHC isoforms. At day 28 and in adults, both V-max ATPase and ATP(iso) were lower in fibers expressing MHCSlow followed in rank order by fibers expressing MHC2A, MHC2X, and MHC2B. For fibers expressing MHCNeo, V-max ATPase, and ATP(iso) were comparable to values for adult fibers expressing MHCSlow but significantly lower than values for fibers expressing fast MHC isoforms. We conclude that postnatal transitions from MHCNeo to adult fast MHC isoform expression in Dia(m) fibers are associated with corresponding but disproportionate changes in V-max ATPase and ATP(iso).