Influence of Ca²⁺ on force redevelopment kinetics in skinned rat myocardium

The influence of Ca²⁺ on isometric force kinetics was studied in skinned rat ventricular trabeculae by measuring the kinetics of force redevelopment after a transient decrease in force. Two protocols were employed to rapidly detach cycling myosin cross-bridges: a large-amplitude muscle length ramp followed by a restretch back to the original length or a 4% segment length step. During the recovery of force, the length of the central region of the muscle was controlled by using a segment marker technique and software feedback control. Tension redevelopment was fit by a rising exponential governed by the rate constant k_tr for the ramp/restretch protocol and k_step for the step protocol. k_tr and k_step averaged 7.06 s^-1 and 15.7 s^-1, respectively, at 15°C; neither k_tr nor fc_step increased with the level of Ca²⁺ activation. Similar results were found at submaximum Ca²⁺ levels when sarcomere length control by laser diffraction was used. The lack of activation dependence of k_tr contrasts with results from fast skeletal fibers, in which k_tr varies 10-fold from low to high activation levels, and suggests that Ca²⁺ does not modulate the kinetics of cross-bridge attachment or detachment in mammalian cardiac muscle.