Experimental protocol for determining steady-state tension and the kinetics of force redevelopment in mammalian skinned myocardium. (A and B) Representative steady-state isometric tension data were obtained from permeabilized myocardial preparations isolated from the porcine left atrium (A) and porcine left ventricle (B) during maximal Ca²⁺ activation in a solution of pCa 4.5 (upper trace). Maximal Ca²⁺-activated tension (P_o) was determined by subtracting Ca²⁺-independent tension (P_rest) measured in a solution of pCa 9.0 (lower trace) from the total tension measured in the solution of pCa 4.5. Mechanical properties for porcine myocardium were as follows: atria (cross-sectional area = 2.2 × 10⁻⁸ m²; P_o = 17.1 mN mm⁻²; P_rest = 0.5 mN mm⁻²; ktr = 14.8 s⁻¹) and ventricle (cross-sectional area = 3.5 × 10⁻⁸ m²; P_o = 22.5 mN mm⁻²; P_rest = 0.8 mN mm⁻²; ktr = 2.4 s⁻¹).