Force–pCa relationships in mammalian skinned myocardium. Force–pCa relationships were determined in permeabilized myocardial preparations. Smooth lines were generated by fitting the mean data with the Hill equation: $P/Po=[Ca2+]n/(kn+[Ca2+]n)$⁠, where n is the Hill coefficient and k is the [Ca²⁺] required for half-maximal activation (i.e., pCa₅₀). Ca²⁺-activated force–pCa relationships were determined in murine ventricular myocardium (filled circle: pCa₅₀ = 5.78 ± 0.01, n_H = 4.0 ± 0.2), porcine atrial myocardium (triangle: pCa₅₀ = 5.69 ± 0.02, n_H = 3.2 ± 0.2), and porcine ventricular myocardium (empty circle: pCa₅₀ = 5.70 ± 0.01, n_H = 4.6 ± 0.1).