MEASUREMENTS OF ACTIVE MYOCARDIAL TENSION UNDER A WIDE-RANGE OF PHYSIOLOGICAL LOADING CONDITIONS

Active tension developed while cardiac muscle shortens has been studie d extensively under afterloaded isotonic or isovelocity conditions. Ho wever, these are not true in vivo loading conditions. To obtain more p hysiological loading, we controlled sarcomere length to follow the tim e courses that we observed previously in a beating canine left ventric le. Sarcomere length was measured by laser diffraction in 12 rat cardi ac trabeculae, superfused with Krebs-Henseleit solution (25 degrees C; [Ca] = 1.5 mM). Force was measured by a silicon strain gauge. Sarcome re length time courses were scaled slightly in time to account for tem perature and species differences. We examined the relationships betwee n active tension and sarcomere length under loading observed over a wi de range of left ventricular preloads and afterloads, and at two sites . Under all loading conditions, active tension was not isotonic but de clined steadily throughout the ejection period. While there were major differences in peak tension dependent on loading conditions and the i ncidence of 'pre-ejection' sarcomere shortening, these factors did not influence the relationship between sarcomere length and peak active t ension. This study provides excellent illustrations of the potential d ifferences in stress (1) within a ventricular wall, and (2) under diff erent operating conditions. Moreover, it provides data for developing models of fiber contraction to be synthesized into a whole heart for p redicting potential differences in stress at all sites and under all l oading conditions. Copyright (C) 1996 Elsevier Science Ltd.