How do skinned skeletal muscle fibers relax?

When free calcium is rapidly removed from skinned fibres using the photolab ile Ca2+ chelator diazo-a, they relax without an appreciable change in sarc omere length (less than or equal to 1.7%). This relaxation is faster than t he turnover rate of cross-bridges in steadily contracting muscle, Therefore a fall in the free calcium does not just prevent the recruitment of new cr oss-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 diphosphate (MgADP) slow relaxation, indicating th at during relaxation cross-bridges must undergo both a Pi and an ADP releas e step. In addition, we have observed that muscle relaxes in twee phases, P hase 1, a linear phase, is greatly truncated in diazo-2 relaxation compared to intact fibres, supporting the theory that this phase is governed by cal cium removal from the thin filament regulatory system whilst Phase 2 is a r apid exponential decline. Increased MgADP slows phase 1 consistent with a c o operative model of cross-bridge attachment where strongly-bound crossbrid ges can maintain thin filament activation. The transition between phase 1 a nd 2 is not marked by any significant changes in sarcomere length (less tha n or equal to 0.17%). (C) 1999 Academic Press.