Effect of denervation on ATP consumption rate of diaphragm muscle fibers

Denervation (DNV) of rat diaphragm muscle (DIAm) decreases myosin heavy chain (MHC) content in fibers expressing MHC2X isoform but not in fibers expressing MHCslow and MHC2A. Since MHC is the site of ATP hydrolysis during muscle contraction, we hypothesized that ATP consumption rate during maximum isometric activation (ATP(iso)) is reduced following unilateral DIAm DNV and that this effect is most pronounced in fibers expressing MHC2X. In single-type-identified, permeabilized DIAm fibers, ATP(iso) was measured using NADH-linked fluorometry. The maximum velocity of the actomyosin ATPase reaction (V-max ATPase) was determined using quantitative histochemistry. The effect of DNV on maximum unloaded shortening velocity (V-o) and cross-bridge cycling rate [estimated from the rate constant for force redevelopment (k(TR)) following quick release and restretch] was also examined. Two weeks after DNV, ATP(iso) was significantly reduced in fibers expressing MHC2X, but unaffected in fibers expressing MHCslow and MHC2A. This effect of DNV on fibers expressing MHC2X persisted even after normalization for DNV-induced reduction in MHC content. With DNV, V-o and k(TR) were slowed in fibers expressing MHC2X, consistent with the effect on ATP(iso). The difference between V-max ATPase and ATP(iso) reflects reserve capacity for ATP consumption, which was reduced across all fibers following DNV; however, this effect was most pronounced in fibers expressing MHC2X. DNV-induced reductions in ATP(iso) and V-max ATPase of fibers expressing MHC2X reflect the underlying decrease in MHC content, while reduction in ATP(iso) also reflects a slowing of cross-bridge cycling rate.