ATP consumption rate per cross bridge depends on myosin heavy chain isoform

In the present study, we tested the hypothesis that intrinsic differences in ATP consumption rate per cross bridge exist across rat diaphragm muscle (Dia_m) fibers expressing different myosin heavy chain (MHC) isoforms. During maximum Ca²⁺ activation (pCa 4.0) of single, Triton X-permeabilized Dia_m fibers, isometric ATP consumption rate was determined by using an NADH-linked fluorometric technique. The MHC concentration in single Diam fibers was determined by densitometric analysis of SDS-PAGE gels and comparison to a standard curve of known MHC concentrations. Isometric ATP consumption rate varied across Dia_m fibers expressing different MHC isoforms, being highest in fibers expressing MHC_2X (1.14 ± 0.08 nmol·mm^-3·s^-1) and/or MHC_2B (1.33± 0.08 nmol·mm^-3·s^-1), followed by fibers expressing MHC2A (0.77 ± 0.11 nmol·mm^-3·s^-1 and MHC_Slow (0.46 ± 0.03 nmol·mm^-3·s^-1). These differences in ATP consumption rate also persisted when it was normalized for MHC concentration in single Diam fibers. Normalized ATP consumption rate for MHC concentration varied across Diam fibers expressing different MHC isoforms, being highest in fibers expressing MHC_2X (2.02 ± 0.19 s^-1) and/or MHC_2B (2.64 ± 0.15 s^-1), followed by fibers expressing MHC_2A (1.57 ± 0.16 s^-1) and MHC_Slow (0.77 ± 0.05 s^-1). On the basis of these results, we conclude that there are intrinsic differences in ATP consumption rate per cross bridge in Dia_m fibers expressing MHC isoforms.