H2-Helical Region of Cardiac Troponin T Contributes to Length-Dependent Regulation of Cardiac Contractile Activation

An alpha-helical region of cardiac troponin T, cH2(T), is centrally positioned in the core domain of the troponin complex (Takeda et al., Nature 2003 vol.424(6944):p35-41). cH2(T)-troponin I and cH2(T)-troponin C interactions have been illustrated through biochemical studies and visualized in the crystal structure of the core domain of human cardiac troponin, suggesting an important regulatory role for cH2(T). However, little is known about how the cardiac-specific structure of cH2(T) relates to cardiac-specific contractile function. To better understand the functional significance of cH2(T), we created a chimeric rat cardiac troponin T (TnT) in which cH2(T) was replaced by the corresponding helical region of rat slow skeletal TnT and studied how replacement of native TnT with chimeric TnT affected contractile function of rat cardiac muscle. We measured isometric force, ATPase activity, and length-dependent contractile dynamics in detergent skinned papillary muscle bundles reconstituted with either wild-type or chimeric TnT, held at either sarcomere length (SL) 2.0 μm or 2.2 μm. Preliminary studies suggest that the SL dependence of Ca2+-activated maximal force production and tension cost was depressed in bundles containing chimeric TnT. For example, bundles containing the chimeric TnT showed an 8.4% decrease in force production when SL was decreased from 2.2 μm to 2.0 μm, whereas bundles containing the wild-type TnT showed a 43.9% decrease in force production. In addition, length-dependent contractile dynamics were significantly altered in bundles containing the chimeric TnT. For example, the length-dependent rate constant of crossbridge recruitment was slower, and the rate constant of crossbridge detachment was faster in bundles containing the chimeric TnT. Thus, our data suggest that cH2(T) plays a role in cardiac-specific length-mediated myofilament activation, an important mechanism underlying the Frank-Starling relationship.