The activation dependence of the rate of force redevelopment is significantly reduced in porcine-permeabilized ventricular myocardium. The steady-state isometric forces and ktr values measured during submaximal Ca²⁺ activation were normalized to their respective maximal values measured in pCa 4.5 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.