History-dependent mechanical properties of permeabilized rat soleus muscle fibers

Permeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 /₀, ± 0.1 /₀ s^-1 imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca²⁺ activation. Actively contracting fibers exhibited a characteristic tension response to stretch: Tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca²⁺ activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca²⁺ activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca²⁺ concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca²⁺ concentration and cross-bridge strain.