Abstract
In order to clarify the mechanisms of cardiac muscle contraction, sarcomere dynamics in isolated cardiac muscle was analyzed by the laser diffraction method and the servo-control system. In muscle isometric contraction, a considerable internal shortening of sarcomere occurred during contraction. In sarcomere isometric contraction, tension developed very rapidly to the plateau level and then declined exponentially. Sarcomere force-velocity relation for shortening was a hyperbolic curve. The force generated by a cardiac sarcomere during lengthening increased towards a constant value when the lengthening velocity became larger than 2μm/s. The sarcomere became less able to shorten and more resistant to lengthening with the increase of the time after stimulation pulse. Isotonic velocity transients were observed only when the sarcomeres were held isometrically before the isotonic release. Deactivation by active shortening was observed and the extent of deactivation increased with increase of shortening velocity and the time after stimulation pulse.
A kinematic cross-bridge model, which relates the formation and degradation of the cross-bridge with the force generated by the stiffness of the cross-bridge, could explain the major characteristics of cardiac sarcomere dynamics. The cross-bridge model with parameters estimated from force-velocity relation predicted that the stiffness within cross-bridge was linear and that its size was 5nm in length.
