Deactivation of Contraction by Quick Releases in the Isolated Papillary Muscle of the Cat

In the isolated cat papillary muscle, the time course of activation during isometric contraction was measured using instantaneous velocity of shortening after quick releases at constant contractile element length and controlled load. Activation rose and declined ahead of isometric force. This course of activation could be altered by the mode of the quick release. Undamped quick releases increasingly deactivated contraction with time. Deactivation was reduced by damping the rapid movement of the lever after the release and by increasing the afterloads to which the releases were made, thus lessening the extent of the releases. The curves of activation obtained from releases to different afterloads converged after the first half of the rising phase of the twitch. Deactivation from quick releases was eliminated by tetanizing the muscle. Hence, the instantaneous relation of force and velocity of shortening was not unique relative to time except when the muscle was tetanized. Deactivation was also reduced by lowering the temperature, by adding 10 m M caffeine, or by increasing the Ca 2+ concentration in the bathing medium in the presence of 10 m M caffeine. These results suggest that quick releases deactivate contraction by uncoupling crossbridges which can be reformed if the Ca 2+ available to activate contractile sites is not sequestered elsewhere.