Calcium sensitivity is modified by contraction.

This paper summarizes three lines of experimental evidence showing that crossbridge interaction affects calcium sensitivity and probably also affects calcium binding. Evidence is presented that this is a true hysteresis, not just a slow approach to equilibrium. In barnacle single muscle fibers injected with aequorin to monitor intracellular Ca, a long duration stimulus under voltage clamp conditions can produce a long duration calcium transient and force record which both approach steady levels. However, if the stimulus is briefly elevated to transiently produce a higher force early in the contraction, the same steady state Ca level can eventually maintain a higher steady force. Thus Ca sensitivity is modified. In "skinned" barnacle muscle activated by Ca in the presence of buffered Ca, MgATP, and pH, force was measured in split, detergent treated fiber while it was transferred consecutively between solutions which were relaxing, submaximal contracting, maximal contracting, the same submaximal contracting, and finally relaxing. The submaximal contracting solution produced more force when stepping down in Ca concentration (as in relaxation) than when stepping up. Extending the time in the initial submaximal contracting solution did not result in more force. Thus, the force-pCa relationship shows marked hysteresis. The same phenomenon was seen in frog and mammalian muscle fibers. These experiments confirm the findings that contraction modifies Ca sensitivity. In the barnacle single muscle fiber preparation (under both voltage clamp and controlled length conditions), phasic (400 msec) depolarization leads to a calcium transient and a twitch contraction. Releasing the muscle to allow it to shorten rapidly during the declining phase of the calcium transient causes the force to fall and leads to extra Ca in the sarcoplasm. Rapidly stretching the muscle produces the opposite effect. The extra Ca probably comes from a myofilament Ca activating site. Thus, a length change (force change) affects Ca binding.