Myofibrillar determinants of rate of relaxation in skinned skeletal muscle fibers.

Muscle relaxation occurs when Ca2+, sequestrated by the sarcoplasmic reticulum (SR) Ca2+ pump, dissociates from troponin (Tn) to deactivate the thin filaments leading to cross-bridge detachment and force decay. It is well established that the rate of Ca2+ sequestration by the SR can control relaxation kinetics.1 The aim of the present investigation is to determine the relative contribution of Ca2+ dissociation from TnC and cross-bridge detachment to relaxation rate induced by rapid sequestration of Ca2+. Three possibilities can be envisioned. First, Ca2+ dissociation from TnC may be much faster than cross-bridge detachment. In this case, only cross-bridge detachment kinetics would affect relaxation rate. Second, Ca2+ dissociation from TnC may be similar to cross-bridge detachment. If this relationship were true, both cross-bridge and TnC kinetics would affect relaxation rate. Third, Ca2+ dissociation from TnC may be slower than cross-bridge detachment. If this possibility were true, then only the kinetics of Ca2+ dissociation from TnC would affect relaxation rate.