AJP - Heart and Circulatory Physiology, Vol 247, Issue 4 548-H555, Copyright © 1984 by American Physiological Society
Isotonic length transient of cat heart muscle in Ba2+-induced contracture
Y. Saeki, T. Shibata and K. Yanagisawa
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.
