Effects of chemical myosin II cross-bridge inhibitors on eccentric force production in skinned muscle fibres.

Eccentric muscle contractions are essential for locomotion and daily activities. Force during these contractions depends on both cross-bridge (XB) and non-cross-bridge (non-XB) elements, including titin, with contributions varying by fibre type and contraction velocity. Despite their importance, mechanisms of force generation during prolonged stretches remain unclear. Chemical XB inhibitors are commonly used to separate XB and non-XB contributions, but their effects have not been systematically compared under identical conditions. We measured eccentric force in single permeabilised fibres from fast-twitch rat extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. Fibres were treated with blebbistatin (Bleb), 2,3-butanedione monoxime (BDM), or N-benzyl-p-toluene sulphonamide (BTS) and stretched from 0.85 to 1.3 times optimal length at three velocities (0.01, 0.1 and 1.0 maximal contraction velocity). XB inhibition produced fibre-type-dependent effects on early- and late-stretch force (slope1, slope2) and on end-of-stretch force (se). Stretch velocity and the type of XB inhibitor influenced se in both SOL and EDL fibres. However, in EDL fibres, se did not differ significantly between XB inhibitors and showed velocity-dependent effects under Bleb and BTS only, but not under BDM. Stretch-induced forces were less reduced than isometric forces, indicating substantial stretch-dependent non-XB contributions. These results show that eccentric force arises from coordinated XB and non-XB mechanisms in a fibre-type-dependent manner and highlight Bleb as an effective tool for probing these contributions.