Half-Sarcomere Mechanics and Energetics Indicate that Myosin Motors Slip Between Two Consecutive Actin Monomers during their Working Stroke

Abstract

The coupling between chemical and mechanical steps of actomyosin ATPase cycle was studied in situ by using fast mechanical protocols in Ca2+-activated demembranated fibres from rabbit psoas under sarcomere length control (sarcomere length 2.4 μm, temperature 12°C). We determined the effects of the concentration of inorganic phosphate (Pi) on the force-velocity relation (T-V), on the stiffness-velocity relation (e-V) and on the isotonic velocity transient following a stepwise drop in force from the isometric plateau force (T0) (Piazzesi et al. J Physiol 545:145, 2002). With respect to control (no added Pi), the increase of [Pi] to 10 mM, i) reduced T0 by 50-60%, decreased the curvature of the T-V relation by 30% and increased the unloaded shortening velocity (V0) by 19%; ii) decreased the relative half-sarcomere stiffness at each shortening velocity by an extent that indicates that Pi has little effect on the force per attached myosin motor; iii) did not change the rate of early rapid shortening (phase 2) following the stepwise drop in force, while reduced its size and made the subsequent pause of shortening (phase 3) briefer. Steady state and transient mechanical responses and the known related energetics (Potma and Stienen J Physiol 496:1, 1996) are simulated with a kinetic-mechanical model of the actomyosin ATPase cycle that incorporates Huxley and Simmons mechanism of force generation. Muscle power and efficiency during isotonic shortening at high and intermediate loads can be predicted only if myosin motors at an intermediate stage of both the working stroke and product release can slip to the next Z-ward actin monomer. Supported by MIUR, Ministero della Salute and Ente Cassa di Risparmio di Firenze (Italy).