Active Length-Tension Relations Compared in Isometric, Afterloaded and Isotonic Contractions of Cat Papillary Muscle

Isometric, afterloaded and isotonic contractions of cat right ventricular papillary muscle were studied in vitro at 30°C or 37°C. The length to which muscle contracted under a given total load varied directly with initial muscle length. The discrepancies between contracted muscle lengths depended upon load and the inotropic state of the muscle. In control contractions at 12/min, the maximum discrepancy occurred at a load of 0.5 g or 1 g and averaged 0.47 ± 0.04 (SD) mm while after acetylstrophanthidin (0.75 μg/ml) the maximum discrepancy occurred between 3- and 5-g load and averaged 0.62 ± 0.07 mm (P < 0.001). At low loads acetylstrophanthidin decreased the discrepancy at 0.5-g load from 0.41 ± 0.08 mm to 0.22 ± 0.05 mm (P < 0.001). Acetylstrophanthidin increased developed tension at optimum muscle length by 71 ± 23% (P < 0.001). Increasing frequency of contraction from 12/min to 60/min induced greater decrease in time to peak tension than occurred with acetylstrophanthidin (30% vs. 15%) but produced little change in either isotonic or isometric contracted length-tension relations. Since the differences between contracted length-tension relations were not influenced by abbreviation of active state induced by frequency changes, it is unlikely that the lesser abbreviation with acetylstrophanthidin was responsible for the changes it produced. Rather, they may be related to the intensity and duration of active state reflecting increased energy release in the contractile process.