Possible cooperativity in crossbridge detachment in muscle fibers having magnesium pyrophosphate at the active site

When rabbit psoas muscle fibers bathed in solutions containing the ATP analogue magnesium pyrophosphate (MgPPi) are first stretched rapidly and then held isometric, a force is generated during the stretch which decays during the subsequent isometric period (Schoenberg, M., and E. Eisenberg. 1985. Biophys. J. 48:863-871). Previously we showed that the force decay is due to crossbridge heads detaching and reattaching in positions of lesser strain, the rate of decay of force reflecting the crossbridge detachment rate constants. Since the crossbridge detachment rate constants with MgPPi bound to the active site are so much faster than without analogue bound, at subsaturating concentrations of analogue, if the heads act independently and nucleotide association and dissociation is rapid, the rate of force decay should simply be proportional to the number of heads with bound analogue. That is, the analogue concentration dependence of the rate of force decay should have the same form as the Michaelis-Menten equation. Here we report that the concentration dependence of the rate of force decay is not described by the Michaelis equation, but is instead sigmoidal. This suggests possible cooperativity in the detachment of the crossbridge heads, the amount of cooperativity being described by an interaction coefficient of approximately 2. One idea put forward to explain the data is that both of the heads of a crossbridge may need to bind analogue before the crossbridge can relax a substantial fraction of the tension it supports.