Time Course and Strain Dependence of ADP Release during Contraction of Permeabilized Skeletal Muscle Fibers

A phosphorylated, single cysteine mutant of nucleoside diphosphate kinase, labeled with N-[2-(iodoacetamido)ethyl]-7-diethylaminocoumarin-3-carboxamide (P∼NDPK-IDCC), was used as a fluorescence probe for time-resolved measurement of changes in [MgADP] during contraction of single permeabilized rabbit psoas fibers. The dephosphorylation of the phosphorylated protein by MgADP occurs within the lattice environment of permeabilized fibers with a second-order rate constant at 12°C of 105 M−1 s−1. This dephosphorylation is accompanied by a change in coumarin fluorescence. We report the time course of P∼NDPK-IDCC dephosphorylation during the period of active isometric force redevelopment after quick release of fiber strain at pCa2+ of 4.5. After a rapid length decrease of 0.5% was applied to the fiber, the extra NDPK-IDCC produced during force recovery, above the value during the approximately steady state of isometric contraction, was 2.7 ± 0.6 μM and 4.7 ± 1.5 μM at 12 and 20°C, respectively. The rates of P∼NDPK-IDCC dephosphorylation during force recovery were 28 and 50 s−1 at 12 and 20°C, respectively. The time courses of isometric force and P∼NDPK-IDCC dephosphorylation were simulated using a seven-state reaction scheme. Relative isometric force was modeled by changes in the occupancy of strongly bound A.M.ADP.Pi and A.M.ADP states. A strain-sensitive A.M.ADP isomerization step was rate-limiting (3–6 s−1) in the cross-bridge turnover during isometric contraction. At 12°C, the A.M.ADP.Pi and the pre- and postisomerization A.M.ADP states comprised 56%, 38%, and 7% of the isometric force-bearing AM states, respectively. At 20°C, the force-bearing A.M.ADP.Pi state was a lower proportion of the total force-bearing states (37%), whereas the proportion of postisomerization A.M.ADP states was higher (19%). The simulations suggested that release of cross-bridge strain caused rapid depopulation of the preisomerization A.M.ADP state and transient accumulation of MgADP in the postisomerization A.M.ADP state. Hence, the strain-sensitive isomerization of A.M.ADP seems to explain the rate of change of P∼NDPK-IDCC dephosphorylation during force recovery. The temperature-dependent isometric distribution of myosin states is consistent with the previous observation of a small decrease in amplitude of the Pi transient during force recovery at 20°C and the current observation of an increase in amplitude of the ADP-sensitive NDPK-IDCC transient.