Oscillatory work and the step that generates force in single myofibrils from rabbit psoas

Abstract

The elementary molecular step that generates force by cross-bridges (CBs) in an active muscle has been under intense investigation in the field of muscle biophysics. The question asked here is whether this step is before phosphate (Pi) release or after its release. It is known that an increase in the concentration of Pi ([Pi]) diminishes isometric force in Ca 2+ activated fibers, indicating a tight coupling between the force-generating step and the Pi release step. We investigated the effect of Pi on oscillatory work production in single myofibrils and found that Pi-attached state(s) to CBs is essential for its production. Oscillatory work is the mechanism that allows an insect to fly by beating its wings, and it also has been observed in skeletal and cardiac muscle fibers, implying that it is an essential feature of all skeletal muscle types. With our studies, oscillatory work disappears in the absence of Pi in experiments using myofibrils. This suggests that force is generated during a transition between steps of oscillatory work production and that the states involved in its production must have Pi attached. With sinusoidal analysis, we obtained the kinetic constants around the Pi release steps, established a CB scheme, and evaluated force generated (and supported) by each CB state. Our results demonstrate that force is generated before Pi is released, and the same force is maintained after Pi is released. Stretch activation and/or delayed tension can also be explained with this CB scheme and forms the basis of force generation and oscillatory work production.