Role of troponin C in determining the Ca²⁺-sensitivity and cooperativity of the tension development in rabbit skeletal and cardiac muscles¹

The role of troponin C in determining the characteristic differences in contraction between fast-twitch skeletal and cardiac muscles was investigated with rabbit skinned psoas fibers and left ventricular trabeculae by exchanging their troponins C using a technique involving extraction and replacement. In both muscle types, skeletal troponin C conferred higher cooperativity on the Ca²⁺-activated tension development than cardiac troponin C, indicating that troponin C plays a crucial role in determining the cooperativity in Ca₂₊ activation of vertebrate striated muscle. Furthermore, the foreign type of troponin C was found to confer lower Ca²⁺sensitivity on the tension development in both fast-twitch skeletal and cardiac muscles than the native type of troponin C. This indicates that the Ca²⁺sensitivity of each muscle type is determined by troponin C interactions with other troponin subunits and not by troponin C alone.