Post-reextension force decay of relaxing cardiac muscle.

Residual active cardiac muscle force during ventricular filling causes deviations of the pressure-volume and pressure-segment length relations from passive left ventricular compliance curves. A possible interaction at the myocardial level between muscle reextension and subsequent active force decay has not yet been investigated. We therefore studied the relation between isolated cat papillary muscle reextension, load during reextension, and isometric force decay after isotonic reextension. Both timing and extent of the isotonic muscle reextension phase were altered while load during reextension was lowered, subsequent residual isometric force was decreased. The extent of reextension or the final muscle length did not alter residual active isometric force after isotonic reextension at an identical load. Moreover, irrespective of the loading history of the shortening phase of the contraction, equal loads during reextension resulted in superimposable subsequent isometric force decay traces. From these results it therefore appears that residual isometric force after isotonic reextension is determined by the load during reextension. Extrapolation of these results to the filling ventricle implies the existence of a dynamic interaction between instantaneous extent of filling, wall stress, and residual force development.