Maximum lengthening velocity during isotonic relaxation at preload in canine papillary muscle

Abstract
The characteristics of the lengthening velocity during isotonic relaxation at preload in the blood perfused canine papillary muscle were investigated. The force imposed on the papillary muscle is shifted from total load to preload at the instant the muscle reaches the end-systolic fiber length in the conventional afterloaded contraction. The isotonic bar is kept at the end-systolic position until the contractile force of the papillary muscle falls to the predetermined preload. Then the isotonic lengthening at preload follows. The lengthening continues until the muscle length reaches its initial fiber length. Maximum lengthening velocity of the cardiac muscle, -dL/dtmax, is linearly related to the extent of muscle shortening (.DELTA.L). -dL/dtmax divided by .DELTA.L is independent of preload, total load and contractility under conditions of constant temperature of 37.degree. C. ln[(-dL/dtmax)/.DELTA.L] decreases linearly with the reciprocal of absolute temperature over a range of 25.3-38.5.degree. C under conditions of constant preload and total load. Frequency of contraction ranging from 45-180 min-1 has almost no effect on (-dL/dtmax)/.DELTA.L at 37.degree. C.