Load-dependence of physiologically relaxing cardiac muscle.

Abstract

In order to better mimic the relaxation sequence of the average ventricular wall muscle fibre, isotonic relaxation of cat papillary muscle was studied after reversal of the isotonic-isometric relaxation sequence into an isometric-isotonic relaxation sequence. During this physiological relaxation, mammalian cardiac muscle remains very sensitive to load. In addition, the reversal of the relaxation sequence allows us to evaluate separately the influence of either external load or muscle length on isotonic relaxation. At any given external load during lengthening a relationship between peak lengthening velocity and muscle length at peak shortening is obtained, i.e. peak relaxation velocity increases with decreasing muscle length at peak shortening. When the external load during lengthening is lowered, the curve shifts towards the right and below, indicating that isotonic relaxation is very sensitive to the external load. The influence of muscle length cannot be accounted for by an internal restoring force since the observed velocity-length relation is obtained for muscle lengths between slack length and the muscle length at which active force development is maximal (Lmax). Instead, the major driving force during lengthening appears to be the difference between the external load, which promotes lengthening, and the parallel elastic force, which opposes lengthening and increases exponentially with muscle length according to the passive length-tension relation of the muscle.