Orthovanadate and orthophosphate inhibit muscle force via two different pathways of the myosin ATPase cycle

Measurements of the half-sarcomere stiffness during activation of skinned fibers from rabbit psoas (sarcomere length 2.5 μm, temperature 12°C) indicate that addition of 0.1 mM orthovanadate (Vi) to the solution produces a drop to ∼1/2 in number of force-generating myosin motors, proportional to the drop in steady isometric force (T₀), an effect similar to that produced by the addition of 10 mM phosphate (Pi). However, in contrast to Pi, Vi does not change the rate of isometric force development. The depression of T₀ in a series of activations in presence of Vi is consistent with an apparent second-order rate constant of ∼1 × 10³ M⁻¹ s⁻¹. The rate constant of T₀ recovery in a series of activations after removal of Vi is 3.5 × 10⁻² s⁻¹. These results, together with the finding in the literature that the ATPase rate is reduced by Vi in proportion to isometric force, are reproduced with a kinetic model of the acto-myosin cross-bridge cycle where binding of Vi to the force-generating actomyosin-ADP state induces detachment from actin to form a stable myosin-ADP-Vi complex that is not able to complete the hydrolysis cycle and reenters the cycle only via reattachment to actin upon activation in Vi-free solution.