Isotonic length transient of cat heart muscle in Ba2+-induced contracture

To characterize mechanical properties of activated heart muscle, the length response to step (4 ms) decrease in tension in Ba2+-induced contracture in kitten papillary muscle was analyzed. The amplitudes of tension steps were varied at the maximum contracture level (Tc), different initial muscle lengths [0.90 Lmax to Lmax], and different temperatures (20-35 degrees C). When the tension decrease was less than 0.7 Tc, the length response comprised four different phases. The amplitude of shortening in the second phase, after the initial rapid shortening in the first phase, increased up to approximately 3.5% of the initial muscle length quite linearly with increasing amplitude of tension reduction. The amount of lengthening in the third phase, however, increased to a maximum (up to approximately 1.4% initial length) at the tension reduction of approximately 0.4 Tc. The third phase of lengthening was larger at shorter initial muscle length. Increasing temperature markedly decreased the amplitude and shortened the duration of the length response in the second and third phase independently of initial muscle length. Unlike that of the muscle in contracture, the length response of the resting and of the rigor muscle changed in a nearly stepwise fashion. One explanation of these phenomena is that the transient length responses following a rapid tension reduction are mostly determined by the kinetics of the attachment and detachment of cross bridges between actin and myosin filaments.