Endogenous troponin complex replacement in single skeletal and cardiac myofibrils

The replacement of native troponin complex (Tn) with exogenous Tn in single myofibrils is a powerful tool for the understanding of the regulation mechanism of striated muscle contraction (Piroddi & al., J Physiol. 2003, 552, 917-931). In rabbit psoas myofibrils we have studied the mechanical sequelae of the replacement of native fast skeletal Tn with cardiac Tn or with chimeric Tn complexes containing different combinations of the C, T, and I subunit isoforms (fast skeletal, cardiac, and slow skeletal). The results show that Tn replacement in myofibrils by Tn of homogeneous origin strongly affects calcium sensitivity and cooperativity of force generation while maximal isometric force and apparent rates of force generation are unchanged. These results suggest that cross bridge kinetics and mechanics are basically set by the myosin isoform and are unaffected by Tn isoforms. We are now extending these replacement studies to cardiac myofibrils. The relative ease of Tn exchange even in human cardiac myofibrils means that replacement can be made in myofibrils using Tn forms associated with cardiac diseases to further investigate the role of regulatory proteins on cross-bridge properties in human cardiomyopathies. Supported by EU grant HPRN-CT- 2000-0091.