Roles of troponin isoforms in pH dependence of contraction in rabbit fast and slow skeletal and cardiac muscles

Skinned fibers prepared from rabbit fast and slow skeletal and cardiac muscles showed acidotic depression of the Ca²⁺ sensitivity of force generation, in which the magnitude depends on muscle type in the order of cardiac > fast skeletal > slow skeletal. Using a method that displaces whole troponin-complex in myofibrils with excess troponin T, the roles of Tn subunits in the differential pH dependence of the Ca²⁺ sensitivity of striated muscle were investigated by exchanging endogenous troponin I and troponin C in rabbit skinned cardiac muscle fibres with all possible combinations of the corresponding isoforms expressed in rabbit fast and slow skeletal and cardiac muscles. In fibers exchanged with fast skeletal or cardiac troponin I, cardiac troponin C confers a higher sensitivity to acidic pH on the Ca²⁺ sensitive force generation than fast skeletal troponin C independently of the isoform of troponin I present. On the other hand, fibres exchanged with slow skeletal troponin I exhibit the highest resistance to acidic pH in combination with either isoform of troponin C. These results indicate that troponin C is a determinant of the differential pH sensitivity of fast skeletal and cardiac muscles, while troponin I is a determinant of the pH sensitivity of slow skeletal muscle.