Time course and duration of the depressant effect of active shortening in cardiac muscle.

Shortening deactivation is studied in isolated preparations of rat cardiac muscle by comparing tension redevelopment in partially fused isometric contractions with quick releases of different extent ("control" and "test" displacements). The active shortening takes place during the ascending phase of the 1st contraction of the couple and the comparison of redeveloped tension is carried out at the peaks of both 1st and 2nd contractions. The depressant effect of active shortening can be demonstrated at the 1st peak and also at the 2nd one, after the restimulation of the muscle. This effect slowly decreases with time and disappears within approximately one second after active shortening took place. By keeping constant the control shortening and changing the test shortening, a clear dependence of depressant effect on the amount of displacement can be shown in the 1st and in the 2nd contraction.