UNLOADED SHORTENING OF SKINNED MAMMALIAN SKELETAL-MUSCLE FIBERS - EFFECTS OF THE EXPERIMENTAL APPROACH AND PASSIVE FORCE

In rabbit, rat and human skinned skeletal muscle fibres the length-tim e relationship of isotonic releases was determined after maximal Ca2activation. Slack test experiments provided information about unloaded conditions. Force clamp experiments of different load were extrapolat ed for zero load and compared with the slack test data. The course len gth-time relationship for unloaded conditions was similar using both a pproaches. However, slack test data showed a triphasic shape which cou ld be fitted by three straight lines (phase I, II, III), whereas the d ata of force clamp experiments exhibited a steady curved shape. Conseq uently, the instantaneous slopes differed in the two relationships, bu t the distance which was shortened during the time interval of phase I I was similar in both approaches. The ratio between these unloaded sho rtening velocities resulting from force clamp and slack test experimen ts was 1.01 +/- 0.05 (SD) (n = 25). The effects of passive force on th e velocity of fibre shortening was investigated in skinned rabbit musc le fibres using slack test experiments. A significant increase in the unloaded shortening velocity was observed when the sarcomere length of the fibres was increased to values which exhibited considerable amoun ts of passive force. The high reproducibility of the isotonic releases required in this study was achieved by improving some methodological details. Using these improved techniques an identity between the relat ive fibre and sarcomere shortening was observed during the isotonic re leases.