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 degrees C; neither k(tr) nor k(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.