Stiffness and tension during and after sudden length changes of glycerinated single insect fibrillar muscle fibres

Abstract

Rapid length changes were applied (within 0.2 ms or 0.4 ms) to single isometrically contracted glycerol extracted muscle fibres of the dorsal longitudinal muscle ofLethocerus maximus suspended in an Ca²⁺ and ATP containing solution at 20–23‡ C. Force transients and the fibre stiffness were measured during and after rapid length changes.

At length changesbelow 0.5% of the initial fibre length (∼ 2.4 Μm sarcomere length) the mechanical transients were characterized as follows: (1) After stretch and after release the force regains at least partly the value of tension before the length change within a quick phase of tension recovery. The quick phase induced by stretch was nearly completed within 1–2 ms. (2) A pulse in length of 1.5 ms duration, i.e., a stretch followed by a release to the initial length or a release followed by a stretch to the initial length, was applied to the fibre. The force transient induced by this procedure regains after the second length change the value of the isometric tension before the procedure. (3) The stiffness was constant during each length change of the “pulse” and was equal during the first and the second length changes.

These findings are predicted by the muscle contraction model of Huxley and Simmons (1971):

The identical force before and after a length pulse may indicate that the rotation of cross bridges after the first length change is followed by a rotation into the original position after the second length change. The constancy of the stiffness during the length changes may indicate a Hookean elastic element of the cross bridge. The similarity of the stiffness during the first and the second length changes, i.e., before and after the quick phase, gives evidence that the quick phases after stretch and after release are not accompanied by a change in the net number of attached cross bridges.

If stretches ofmore than 0.5% of the initial length were applied, the mechanical transient of the muscle fibre changed as follows: (1) An ultra fast tension decay phase (duration < 0.4 ms) was observed in addition to the slower decay phase induced by the smaller stretches. (2) If the initial stretch was followed by a release to the initial length, no fast recovery phase was observed, which returns the force to the value before the stretch. The reduced tension value persists for a longer period in time than 10 ms. (3) If the muscle was stretched and released repetitively an ultra fast quick phase was induced only by the first stretch. (4) The stiffness increased during stretch, but was found to be the same in the isometrically contracting muscle and after the quick tension decay phase following a large stretch.

These findings indicate that the contraction model of Huxley and Simmons has to be extended by a further process additional to cross bridge rotation in case of large stretches (> 0.5%L ini). The findings are taken to indicate a rapid detachment and reattachment of overstrained cross bridges, i.e., a cross bridge slippage induced by large stretches.