Force response to rapid and slow small amplitude length releases in isometrically contracting normal width and radially compressed trabeculae from rat heart.

The early transient force response to small amplitude ramp length release, applied at the plateau of calcium activated force development, was studied in normal width and radially compressed trabeculae from rat right ventricle. Force recovery immediately following "rapid" (5 ms ramp duration) release was less in compressed than in normal width muscle. Force recovery immediately following "slow" (ramp duration greater than 160 ms) release was virtually absent in both compressed and normal width muscle. The difference in force levels between the beginning and end of slow releases was small in normal width muscle; in compressed muscle a noticeable difference in force levels was seen. The rate of force recovery immediately following "rapid" release, as given by the rate constant obtained by fitting a single exponential equation to the force recovery vs time data, was not altered by compression. These results suggest that radial compression divides crossbridges into two distinct populations: one which cycles normally and another which cycles very slowly, if at all.