Effects of various inotropic interventions on the dynamic properties of the contractile elements in heart muscle of t he cat

Force-velocity-length (FVL) relations were obtained by determining die phase-plane tracings of velocity of shortening vs. length during isotonic con-tractions. These measurements were then replotted in a three-dimensional graph after correction for the series elastic extension during the isometric phase of the contractions as derived from a quick release contraction. On examining the in-fluence of temperature (29 ° and 37°), preload, frequency of stimulation (12/min and 24-30/min), paired stimulation (PS), calcium (2.5 B M and 7.5 HIM), and iso-proterenol (10~5M), it was shown that the surface created by the three-dimen-sional FVL relations of the contractile element (CE) is unique for a given state of contractility. Furthermore, the course of velocity vs. length is determined only by the instantaneous CE length, regardless of the contractile state of die CE, and is independent of the time after stimulation over a large portion of the shortening. From the intersection of the linear load-shortening relation observed for the CE with the load axis the maximum force (Po) development of the C