Effects of physiological beating on the contractility of cat ventricular muscle

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.