The influence of Ca2+ on isometric force kinetics was studied in skinn
ed rat ventricular trabeculae by measuring the kinetics of force redev
elopment after a transient decrease in force. Two protocols were emplo
yed 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) f
or 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 Ca2activation. Similar results were found at submaximum Ca2+ levels when
sarcomere length control by laser diffraction was used. The lack of ac
tivation dependence of k(tr) contrasts with results from fast skeletal
fibers, in which k(tr) varies 10-fold from low to high activation lev
els, and suggests that Ca2+ does not modulate the kinetics of cross-br
idge attachment or detachment in mammalian cardiac muscle.