Release of calcium into the myofibrillar space in response to active shortening of striated muscle

Aim: The study was undertaken to explore whether shortening of striated muscle during activity is associated with release of bound Ca²⁺ into the myofibrillar space as has previously been proposed in order to explain the depressant effect of active shortening. Methods: The experiments were carried out on single muscle fibres isolated from the anterior tibialis muscle of Rana temporaria. The fibres were loaded with the calcium sensitive indicator Fluo-3. The fibres, stimulated to produce a partially fused isometric tetanus, were subjected to a shortening ramp or, alternatively, to a stretch ramp during activity while force, fibre length, sarcomere length and the Fluo-3 signal were recorded. Results: A shortening ramp performed during a partially fused tetanus caused an increase in the myofibrillar free calcium concentration and produced, simultaneously, a decrease in active force. The isometric force recovered gradually after the shortening ramp, while the intracellular Ca²⁺ concentration stayed above the control level during the remainder of the stimulation period. A stretch ramp applied during a partially fused tetanus caused a considerably smaller change in the myofibrillar Ca²⁺ concentration. Conclusion: The results provide evidence that the myosin cross-bridges interact with the calcium binding sites on the thin filaments during active shortening, causing sustained release of calcium and reduced contractile strength.