Paper B Analysis of the Controlled Auxotonic Twitch in Isolated Cardiac Muscle

In the intact heart, there are simultaneous changes in stress and length throughout the cardiac cycle. In a recently developed digital control system for experiments on isolated papillary muscles from rabbit right ventricle, we included a controlled auxotonic twitch. This twitch allowed proportional changes in stress and length, as if the muscle was acting against an ideal spring. The auxotonic twitches had no discontinuities in length, velocity, stress, or derivative of stress and represented a smooth transition from isotonic to isometric loading. We wanted to use such twitches to analyze the dependence on auxotonic load and initial length of contraction and relaxation. Timing of contraction was independent of load when initial length was lmax. Timing of relaxation was highly correlated with peak stress. Twitch prolongation at increased initial length was more pronounced at lower loads. Similar results were obtained when post extra-systolic potentiation was included as an inotropic intervention. The relations between the load dependence of auxotonic contraction and relaxation, presented in this paper, may have implications for the in vivo left ventricular function analysis.