Isotonic Segment Dynamics of Isolated Cardiac Muscle

Summary

To determine dynamic properties of contracting cardiac muscle, we studied the isotonic segment length changes of isolated rat right ventricular papillary muscles in response to step tension reductions during Ba²⁺ contracture and the chemically skinned Ca²⁺ -activated state, using a newly developed segment control system. The segment length responses comprised four different phases, which were consistent with those properties of heart muscle in Ba²⁺ contracture previously characterized using step and sinusoidal length changes [4–6, 12]. The first phase is a rapid and minute shortening during the tension reduction. The second phase is a slow further shortening, and the third a slow lengthening. The fourth is an extremely slow and mild shortening. Increasing temperature slightly decreased the amplitude and markedly shortened the duration of segment length response in the second and third phases. Unlike that of the activated muscle, the segment length responses of the resting and the rigor muscle changed in a nearly stepwise fashion. From these results, we conclude that the transient segment length response following a rapid tension reduction is mostly determined by the kinetics of the attachment and detachment of cross-bridges between actin and myosin filaments. Lastly, we discuss the length- and activation-dependence of segment length response, particularly of the velocity of segment shortening, in terms of the cross-bridge kinetics.