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How do skinned skeletal muscle fibers relax?

When free calcium is rapidly removed from skinned fibres using the photolabile Ca2+ chelator diazo-2, they relax without an appreciable change in sarcomere length (</=1.7%). This relaxation is faster than the turnover rate of cross-bridges in steadily contracting muscle. Therefore a fall in the free calcium does not just prevent the recruitment of new cross-bridges but must also increase the rate of decay of cross-bridges which are already generating force. Increased levels of free phosphate (Pi) and free magnesium adenosine di-phosphate (MgADP) slow relaxation, indicating that during relaxation cross-bridges must undergo both a Pi and an ADP release step. In addition, we have observed that muscle relaxes in two phases. Phase 1, a linear phase, is greatly truncated in diazo-2 relaxation compared to intact fibres, supporting the theory that this phase is governed by calcium removal from the thin filament regulatory system whilst Phase 2 is a rapid exponential decline. Increased MgADP slows phase 1 consistent with a co-operative model of cross-bridge attachment where strongly-bound cross-bridges can maintain thin filament activation. The transition between phase 1 and 2 is not marked by any significant changes in sarcomere length (</=0.17%).