Crossbridge properties during the quick force recovery in single frog muscle fibers.

Fast stretches (~25 nmhs-1 amplitude and ~400 µs duration) which induced the forced rupture of the crossbridge ensemble were applied to intact muscle fibers to investigate the actomyosin bond properties during the force recovery following a step length change (release or stretch of 2 or 4 nm amplitude). Force and sarcomere length were measured with a fast force transducer (~50 kHz natural frequency) and a striation follower device. To reduce fiber damaging by the stretches and to reduce the influence of myofilament compliance on the measurements, experiments were made on the tetanus rise at tension of about 0.5 the maximum plateau tension. Fast stretches were applied before and at progressively increasing times (up to 20 ms) after the step length change. The rupture force of the crossbridge ensemble (Pc) and the sarcomere elongation at Pc ( Lc) were measured. In contrast with the data obtained previously on the tetanus rise (Bagni et al. J. Physiol., 2005; 565). The results showed that: (1) Pc was almost independent of the tension developed by the fiber and (2), Lc was not constant but increased immediately after the release and decreased after the stretch. These changes were still present 2 ms later when the quick recovery was almost complete and disappeared completely within 15-20 ms. Data analysis suggests that: 1) crossbridge number remains almost constant during the quick force recovery; 2) crossbridge detachment by the fast stretch is preceded by the reversal of the myosin head power stroke and, 3) the extent of the power stroke can be measured by the changes in Lc occurring during the quick recovery.