Thyroidectomized myocardium (0%α-MHC, •, n= 10) and hyperthyroid myocardium (100%α -MHC, ○, n= 10) displayed similar force–pCa relationships. Data points are means ±s.e.m. Forces measured at submaximal free [Ca2+] were expressed relative to the maximal force obtained at pCa 4.5. The smooth lines were fitted using the Hill equation: P/Po=[Ca2+]n/(kn+[Ca2+]n)], where P is the force measured at submaximal free [Ca2+], Po is the force measured at maximal free [Ca2+] (pCa 4.5), nH is the Hill coefficient, and k is the Ca2+ concentration required for half-maximal activation.