[Alterations in the passive and active performance of cat papillary muscle induced by variation of preload (author's transl)].

In the isolated cat papillary muscle a rapid change in length induces a viscoelastic process of relaxation, during which the diastolic tension attains its new equilibrium after a delay. Its time course may be approximated both after stretching and releasing by a total of four exponential functions being marked by highly diverse time constants. During the stretch-induced relaxation phase the isometrically active papillary muscle shows a marked increase in mechanogram amplitudes, which is preceded in the first seconds by a short-term decrease. An opposite behavior is to be recorded after abrupt releases. The mechanograms of the stationary state prove to be exclusively a function of the degree of stretch, while the contractions in the early relaxation phases are dependent on the speed, direction and scale of the preceding change in length. The higher the stretching step chosen, the more clearly reduced are the mechanogram amplitudes of the early relaxation phase in comparison to the stationary state. This applies especially right of the optimum of force-development and contradicts a viscoelastic interpretation of the systolic phenomena in the poststretch phase. The findings after abrupt stretching point to either an initial decrease of amplitude or to a delayed approach of the contractions to their stationary state. Stretch-induced changes in the time course of the action potentials would constitute an adequate basis for the interpretation of these phenomena.