Effects of inotropic interventions on end systolic length-force curve of cat ventricular muscle

A computer controlled servosystem was used to generate physiological contractions in which cat papillary muscles went through a cycle of force and length changes similar to that undergone by muscle fibres in the heart during systole and diastole, net external work being done during each cardiac cycle. End systolic length-force curves were obtained by varying the setpoint force against which the muscle shortened. The end systolic length-force curve was very similar to the isometric length-force curve obtained under comparable experimental conditions. It was altered in the same way by inotropic interventions (increase in stimulus frequency, increasing the bathing calcium concentration, and exposure to isoprenaline), each of which increased the slope of the curve and decreased its (extrapolated) intercept on the length axis. As these results were obtained from studies of muscles producing physiological contractions at 37 degrees C, they provide evidence for length dependence of activation in the working myocardium.