Depression of mechanical performance by active shortening during twitch and tetanus of vertebrate muscle fibres.

Shortening during activity of frog single muscle fibres caused a graded depression of the contractile force that persisted for 800-900 ms during a partially fused or completely fused tetanus. The depression of force was not associated with a change of the shortening velocity at zero load. Passive shortening performed just before stimulation had no effect on the subsequent course of contraction. The decrease in isometric force produced by shortening was not significantly affected by a stretch applied immediately before or after the shortening was not significantly affected by a stretch applied immediately before or after the shortening phase. For a given amount of shortening the depressant effect during a fused tetanus was 8-28% of that produced during a twitch. The effect was substantially reduced, both during twitch and tetanus, in the presence of 0.5 mM caffeine. The length dependence of the movement effect was studied between 1.7 and 2.9 micrometer sarcomere spacing. Maximum depression of force (in per cent of the control at each length) was obtained at 2.1-2.2 micrometer sarcomere length, the effect being steadily reduced at shorter and more extended lengths. The Q10 of the depressant effect was 0.95 +/- 0.16 (S.D.). The features of the movement effect are consistent with a true deactivation of the contractile system as would occur if shortening reduced the binding of activator calcium to the regulatory proteins of the myofilaments.