Effect of substitution of troponin C in cardiac myofibrils with skeletal troponin C or calmodulinon the Ca²⁺ -and sr²⁺-sensitive ATPase activity

Troponin C was removed almost completely from the porcine cardiac myofibrils by the same extraction procedureusing CDTA as that previously reported for the rabbit skeletal myofibrils (Morimoto, S. & Ohtsuki, I. (1987)J. Biochem. 101, 291-301), and the effects of substitution of troponin C in cardiac myofibrils with rabbit skeletal troponin C or bovine brain calmodulin were examined. While the ATPase activity of intact cardiac myofibrils or cardiac troponin C-reconstituted cardiac myofibrils was activated at only a little higher concentration of Sr²⁺ than Ca²⁺ the skeletal troponin C-substituted cardiac myofibrils, as well as intact rabbit skeletal myofibrils, required more than 10 times higher concentration of Sr²⁺ than Ca²⁺ for activation of the myofibrillar ATPase activity. However, the concentrations of Ca²⁺ and Sr²⁺ required for the activation of the ATPase activity of the skeletal troponin C-substituted cardiac myofibrils were both about 5 times higher than those of intact skeletal myofibrils. The skeletal troponin C-substituted cardiac myofibrils, as well as intact skeletal myofibrils, also showed higher cooperativity in the Ca²⁺ of the ATPase activity than intact or cardiac troponinC-reconstituted cardiac myofibrils. The ATPase activity of calmodulin-substituted cardiac myofibrils was activated at a several times lower concentration of Ca²⁺ or Sr²⁺ than that of calmodulin-substituted skeletal myofibrils, while the ratios of the concentration of Sr²⁺ to Ca²⁺ required for activation were almost the same in both cases. The results indicate that, among the characteristics of divalent cation regulation in the contraction of skeletal and cardiac muscles, the higher sensitivity to Ca²⁺ relative to Sr²⁺ and the higher cooperativity in the divalent cation activation in skeletal muscle than in cardiac muscle are determined solely by the species of troponin C, but thesensitivities to the divalent cations in the contraction of skeletal and cardiac muscle are determined through the interactions between troponin C and other troponin components.