Dependence of the work done by ATP-induced actin-myosin sliding on the initial baseline force: its implications for kinetic properties of myosin heads in muscle contraction.

The properties of the ATP-dependent actin-myosin sliding responsible for muscle contraction was studied using an in vitro force-movement assay system, in which a myosin-coated glass microneedle was made to slide on actin filament arrays (actin cables) in the giant algal cell with iontophoretic application of ATP. With a constant amount of ATP application, the amount of work done by the actin-myosin sliding increased with increasing baseline force from zero to 0.4-0.6 Po, and then decreased with further increasing baseline force, thus giving a bell-shaped work versus baseline force relation. The result that the maximum actin-myosin sliding velocity did not change appreciably with increasing baseline force up to 0.4-0.6 Po implies, together with the limited number of myosin heads involved, that (1) the rate of power output of actin-myosin sliding is determined primarily by the amount of external load rather than the velocity of actin-myosin sliding, and (2) the bell shaped work versus baseline force relation (and also the hyperbolic force-velocity relation) results from the kinetic properties of individual myosin head rather than the change in the number of myosin heads involved.