Altered performance of rat cardiac muscle follows changes in mechanical stress during relaxation.

We compared two sets of loading conditions for their effect on the mechanical performance of isolated rat trabecular muscle: (1) loading which approximated that of the intact ventricle; (2) nonphysiological afterloaded isotonic contractions. Improved performance, manifested by increased shortening and rate of shortening at constant load, was seen when lengthening occurred at light loads, as in the intact heart. In contrast, lengthening occurring at the same load against with shortening had taken place was followed by diminished performance. Viscous elements in series with the muscle could not account for this phenomenon. The results suggest the presence of an autoregulatory response of myocardium to mechanical stress during relaxation.