Load and time considerations in the force-length relation of cardiac muscle.

A study of the end-systolic force length relations in isolated mammalian myocardium was undertaken in an effort to clarify controversies concerning the uniqueness of this relationship for a given state of contractility. In studies of cat and rat papillary muscles, differences between isometric and isotonic end-systolic force length relations in twitch contractions were shown to be due to different levels and durations of mechanical activity engendered by the different modes of contraction. This was further supported by the constancy of end-systolic force-sarcomere length relation in small well-oxygenated muscles, and tetanic force-length relations in larger muscles. The dependence of the end-systolic force-length relation on total load was shown not to vary with length or load history during the contraction if the final total load or length was achieved prior to 75% of time to peak tension. Beyond this point, end-systolic force-length relations in isolated muscle become load and time-dependent. The results from isolated muscle studies can be closely correlated qualitatively with results from studies of the intact ventricle, and this analogy may help to further explain observations of ventricular performance in conditions such as heart failure or cardiomyopathy, where significant alterations in the intensity and time course of mechanical activity are known to occur. Additional studies of intact ventricular performance are needed to confirm whether or not the ventricle behaves like its component muscle in a simplified model-independent way.