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

in actomyosin ATP consumption rate in rat diaphragm mus-cle fibers during postnatal development. J Appl Physiol 94: 1896–1902, 2003. First published January 31, 2003; 10.1152/ japplphysiol.00617.2002.—Early postnatal development of rat diaphragm muscle (Diam) is marked by dramatic transi-tions 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 max-imum velocity of the actomyosin ATPase reaction (Vmax ATPase) and the ATP consumption rate during maximum isometric activation (ATPiso). Rat Diam fibers were evaluated at postnatal days 0, 14, and 28 and in adults (day 84). Across all ages, Vmax ATPase of fibers was significantly higher than ATPiso. The reserve capacity for ATP consumption [1 (rati