Aim: 

The finding that more than 50% of the half-sarcomere (hs) compliance is in the actin and myosin filaments indicates that the rate of quick force recovery following a length step, the mechanical manifestation of the working stroke in the myosin motors, cannot be used as a direct estimate of the state transition kinetics of force generation by myosin motors (Huxley & Simmons, Nature 233:533, 1971). Here we use BTS, a dose-dependent inhibitor of isometric force (T₀) (Cheung et al. Nat. Cell Biol. 4:83, 2001), to determine the effect of modulating the myofilament strain on the kinetics of force generation by myosin motors.

Methods: 

Force transients were elicited by imposing length steps of different size (from -4 to +2 nm/hs) on tetanized single muscle fibres (Rana temporaria, 4 °C, 2.1 mm sarcomere length) in the control Ringer solution (NR) and in the presence of different [BTS] (range 0.3–1 mM).

Results: 

The analysis of phase 1 of the force transient (the elastic response) indicated that the fractional contribution of myofilament compliance to hs compliance decreases with the BTS induced reduction of T₀ from 0.7 in NR to 0.2 in 1 mM BTS, when the force is 0.14 T_0,NR. The rate of quick force recovery (the working stroke response) increases with BTS: for a 2 nm step release it is 1.7 0.1 ms^-1 in NR and becomes 4 0.4 ms^-1 in 1 mM BTS. The force transient is simulated with an implemented version of Huxley & Simmons model that takes myofilament compliance into account.

Conclusion: 

The simulation shows that the increase in the rate of quick force recovery by BTS is fully accounted for by the reduction in the relative contribution of myofilament to hs strain and provides the estimate of the transition kinetics in the absence of myofilament compliance.

Supported by MiUR, NIH (R01 AR049033).