Actomyosin bond properties during the quick force recovery following a sudden sarcomere length change in single frog muscle fibres.

Force generation at molecular level in muscle contractionoccurs in two steps: a relatively slow attachment of myosin heads to actin without force, followed by a fast process, the myosin “power stroke”, which stretches the elastic component of the half-sarcomere and develops force. We characterized the mechanical properties of the actomyosin bond by applying to intact activated frog muscle fibres, fast stretches (~25 nmhs-1 amplitude and ~350µs duration) which forcibly detached the crossbridges. Stretches were applied before and after a conditioning stretch or release length change (~4 nm hs-1 amplitude), during the quick force recovery which it though to represent the synchronized power stroke of the crossbridges. The rupture force of the crossbridge ensemble (critical force, Pc) and the sarcomere elongation at Pc (critical length, Lc) were measured. Experiments were performed at 5°C; force and sarcomere length were recorded with a fast force transducer (~40kHz natural frequency) and a striation follower device. In contrast with the data obtained previously on the tetanus rise (Bagni et al., 2005), Pc was almost independent of the tension developed by the fibre at the time of the stretch and Lc was not constant but changed with a characteristic time course. In agreement with the theory we show that the crossbridge state at the end of the quick recovery is characterized by a lower potential energy respect to isometric state. The individual crossbridge rupture force does not change significantly. The forced rupture of the actomyosin bond produced by fast ramp stretches occurs through the reversal of the process leading to force generation.