Mechanism underlying double-hyperbolic force-velocity relation in vertebrate skeletal muscle

The force-velocity relation of frog striated muscle exhibits two distinct curvatures located on either side of a breakpoint that occurs near 80% of maximum isometric force (Po) where the shortening velocity is approximately 1/10 of Vmax. The present experiments have been performed to further elucidate the high-force deviation of the force-velocity curve in frog single muscle fibres. The biphasic shape of the force-velocity curve appears at the same relative values of Po and Vmax also after depressing the isometric force to 80% of the control value by dantrolene, a substance known to reduce the release of activator calcium from the sarcoplasmic reticulum. This finding suggests that the breakpoint of the force-velocity curve is not related to the force level per se but rather to the speed of shortening of the contractile system. Thus as the speed of shortening goes below 1/10 of Vmax, the performance of the myofilament system is changed such that less force and less motion are produced than expected from the main part of the force-velocity curve. In a series of experiments active force and fibre stiffness were simultaneously recorded while the fibre shortened at various speeds during tetanus. Stiffness was measured as the change in force that occurred in response to a 4 kHz sinusoidal length oscillation of the fibre. A plotting of stiffness against force recorded under these conditions provides a biphasic relationship with a distinct transition between the two phases near 80% of Po, i.e. at the same relative force at which the breakpoint occurs in the force-velocity curve. Above 0.8 Po stiffness increases more steeply with force than below this point. This means that while more crossbridges than expected attach to the thin filaments when the load is raised above 0.8 Po, the force output and the speed of shortening become lower than predicted from measurements at low and intermediate loads. The results suggest that the kinetics of crossbridge function is changed as the speed of filament sliding is reduced below a critical level, 1/10 of Vmax. Beyond this point a greater portion of myosin crossbridges would seem to accumulate in a state where less force is being produced. Data are also presented to further elucidate the force-velocity relation at negative loads. In these experiments the passive tension at long sarcomere lengths has been utilized to produce a longitudinal compressive force on the sarcomeres during unloaded shortening (force-clamp recording).(ABSTRACT TRUNCATED AT 400 WORDS)