The Ca ²⁺ and activation dependence of the rate of force redevelopment in mammalian skinned myocardium. The rate constant of force redevelopment (k_tr) following a rapid release/restretch maneuver was measured in permeabilized myocardial preparations isolated from murine ventricular myocardium (filled circle), porcine atrial myocardium (triangle), and porcine ventricular myocardium (empty circle). Data points represent means and the error bars are the SEM. (A) The Ca²⁺ dependence of the rate constant of force redevelopment. (B) The rate constant of force redevelopment was recorded as a function of relative steady-state force (P/P_o) measured at each pCa.