Muscle cross-bridge kinetics in rigor and in the presence of ATP analogues.

Recently we reported preliminary mechanical experiments on freshly skinned rabbit psoas fibers that suggested that while almost all of the cross-bridges are attached to actin in the presence of 4 mM adenyl-5'-yl-imidodiphosphate (AMP-PNP) (ionic strength, 0.13 M), there is an equilibrium between the attached and detached states, so that, in the presence of 4 mM AMP-PNP, fibers should not be able to maintain tension (Schoenberg, et al., 1984, in Contractile Mechanisms in Muscle, Pollack and Sugi, editors., Plenum Publishing Corp., NY). Since this suggestion was at variance with published results of Clarke and Tregear (1982, FEBS [Fed. Eur. Biochem. Soc.] Lett, 143:217), we reinvestigated the ability of rabbit psoas fibers to support tension following a 2-nm stretch in rigor and in the presence of the nucleotide analogues, PPi and AMP-PNP, for analogue concentrations ranging from 0.25 to 4 mM. We find that, whereas in rigor there is very little tension decay following a stretch, in 4 mM nucleotide analogue solution, the force generated by stretch quickly decays to zero. The force decay is not exponential; rather, it can be described by rate constants that range from approximately 0.1 to 100 s-1 in 4 mM PPi, and 0.01 to 10 s-1 in 4 mM AMP-PNP. This large range of decay rate constants may be partially related to the dependence of either analogue binding or cross-bridge dissociation upon strain, since we find that the rate constants for force decay decrease with decreasing size of stretch or with decrease of analogue concentration below the maximum studied (4 mM). In general the results are consistent with an equilibrium model for cross-bridge binding to actin, where the rate constants for cross-bridge detachment determine the rate of tension decay.