End-systolic force-length relationship of nonexcised canine papillary muscle

End-systolic force-length (F-L) relationships were studied in blood-perfused papillary muscles remining in the canine right ventricle. The muscles contracted spontaneously at a regular rate of 114 +/- 4 (SE, N = 31) beats/min at 37 degrees C. In each muscle, end-systolic F-L curves were obtained in different modes of contraction at different diastolic lengths between the unstressed length (9.4 +/- 0.7 mm) and 143 +/- 4% of it. At the latter length, muscles developed a peak isometric force of 46 +/- 4 g (5.1 +/- 0.5 g/mm2). The isotonic F-L curve was lower than the isometric F-L curves to various extents. An auxotonic F-L curve fell between the two curves. At identical end-systolic lengths peak isometric forces were greater than isotonic forces by 14.4 +/- 1.4% in the first transient beats after switching the contraction mode from isometric to isotonic. The difference increased to 28.5 +/- 3.5% in the steady-state beats. The reduced force in the first isotonic beat was suspected to be due to the uncoupling effect of shortening. The further reduction of isotonic force in the steady-state beats was caused by viscous properties (creep and stress relaxation) and by a gradual decrease in contractility.