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 (MHC_Neo) to adult fast MHC isoform expression in Dia_m 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 (∼55-60%) across age groups, although at day 28 and in adults the reserve capacity for ATP consumption was slightly higher for fibers expressing MHC_Slow compared with fast MHC isoforms. At day 28 and in adults, both V_max ATPase and ATP_iso were lower in fibers expressing MHC_Slow followed in rank order by fibers expressing MHC_2A, MHC_2X, and MHC_2B. For fibers expressing MHC_Neo, V_max ATPase, and ATP_iso were comparable to values for adult fibers expressing MHC_Slow but significantly lower than values for fibers expressing fast MHC isoforms. We conclude that postnatal transitions from MHC_Neo to adult fast MHC isoform expression in Dia_m fibers are associated with corresponding but disproportionate changes in V_max ATPase and ATP_iso.