Contractile activation and recovery in skinned frog muscle stimulated by ionic substitution

Contractile activation of skinned (sarcolemma removed) skeletal muscle fibers stimulated by ionic substitution has been studied. Stimulating solutions contained varying amounts of Cl- and K+ with the [K+] x [Cl-] product kept constant at 368 mM2. Activation is a graded function of the ionic content of the stimulating solution. Mechanical threshold is reached when the [Cl-] is changed from 4 to 6.5 mM. Maximal activation occurs at 20 mM Cl-. After stimulation, fibers do not respond to a second stimulus unless allowed to recover. Contractile height reaches 50% of control levels after 30 s in the standard recovery solution (4 mM Cl). Full recovery is reached after approximately 2-4 min. The degree of contractile recovery after a constant interval (30 s) depends on the ionic composition of the recovery solution. Contractile recovery decreases 50% when the [Cl-] of the recovery solution is raised from 4 to 5.5 mM. The activation and recovery phenomena described for skinned fibers stimulated by ionic substitution are similar to those described for intact cells stimulated by elevated [K+]. This finding is consistent with the hypothesis that depolarization of resealed transverse tubules triggers release of Ca2+ from the sarcoplasmic reticulum of skinned fibers.