Effects of Various Inotropic Interventions on the Dynamic Properties of the Contractile Elements in Heart Muscle of the Cat

Force-velocity-length (FVL) relations were obtained by determining the phase-plane tracings of velocity of shortening vs. length during isotonic contractions. 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 influence of temperature (29° and 37°), preload, frequency of stimulation (12/min and 24-30/min), paired stimulation (PS), calcium (2.5 mM and 7.5 mM), and iso-proterenol (10 -5 M), it was shown that the surface created by the three-dimensional 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 the 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 (P o ) development of the CE was derived. The highest values of this corrected P o (1.80 ± 0.13 kg/cm 2 ) were seen following inotropic interventions such as PS, calcium, or isoproterenol. Using the corrected P o , truly hyperbolic force-velocity curves were constructed from which V max and the Hill equation constants a and b were derived for the various inotropic interventions.