Effects of Length Changes on Force Produced by Ca2+ and ADP-Activated Myofibrils along the Ascending Limb of the Force-Length Relation

Abstract

Isometric forces produced by skeletal muscles are higher after stretch and smaller after shortening. A few studies investigating these length-dependent changes in force were conducted on the ascending limb of the force-length (FL) relation, showing conflicting results with elusive mechanisms. The purposes of this study were: (i) to evaluate the effects of muscle stretching and shortening on forces along the ascending limb of the FL relation, (ii) to evaluate if sarcomere length dispersion changes after the imposed length changes, and (iii) to assess if cross-bridges play a role in the length-induced force changes. Rabbit psoas myofibrils were attached between two pre-calibrated micro-needles, and their images were projected into a photodiode array for measurements of individual sarcomere length (SL). Myofibrils were activated by Ca2+ or ADP - the later induces cross-bridge attachment to actin independently of Ca2+. After activation myofibrils were subjected to three stretches or shortenings (∼4%SL), with isometric periods allowed between length changes so that force would achieve a steady-state. Forces of ADP-activated myofibrils were greater (7-8%) than those of Ca2+-activated myofibrils at corresponding SLs (range: 2.2-2.4μm) after shortening, but forces were similar after stretch. Forces were greater (26% with ADP and 15% with Ca2+, SL: 2.2μm) after stretch than after shortening. Sarcomere dispersion was similar after stretch or shortening in Ca2+ and ADP-activated myofibrils. The results suggest that stretching and shortening affects isometric forces on the ascending limb of the FL relation through different mechanisms, and are not associated with SL dispersion. While cross-bridges seem to be involved in force depression, they are likely not involved in force enhancement.