AJP - Heart and Circulatory Physiology, Vol 248, Issue 6 894-H900, Copyright © 1985 by American Physiological Society
Effects of physiological beating on the contractility of cat ventricular muscle
D. A. Hanck and B. R. Jewell
Force and length changes corresponding to those undergone by muscle fibers
in the ventricular wall of the heart during systole and diastole were
generated in isolated cat ventricular muscles contracting at 30 degrees C.
The effect on contractility of such cycles of force and length changes
("physiological" contractions) was assessed by measuring force production
in isometric test beats. Contractility rose over the 1st min of
physiological beating but then fell over the next 7-10 min. These changes
in contractility were compared with those occurring during a series of
isotonic contractions or modified isotonic contractions in which muscles
were held at the short length during relaxation (reversed relaxation
contractions). In all cases contractility increased to the same extent over
the 1st min, but over the next 10 min it fell significantly more when the
contractions were physiological than when they were isotonic (normal or
reversed relaxation). This difference could be accounted for experimentally
by the diastolic length changes that occur in physiological contractions.
This study demonstrates 1) that contractility depends on length changes
occurring in the muscle during both systole and diastole, and 2) that
length-dependent changes in contractility contain both positive and
negative components.
