The Cardiac Troponin C Isoform and the Length Dependence of Ca2+Sensitivity of Tension in Myocardium

The Ca2+sensitivity of tension in cardiac muscle is length dependent, such that the sensitivity is diminished with decreasing sarcomere length below 2.4μm. This length dependence of Ca2+sensitivity of tension also forms the basis for the Frank–Starling mechanism in the heart. The fast-twitch skeletal muscle has a much lower length dependence of Ca2+sensitivity. In a recent study of skinned cardiotrabeculae, we indicated that the exchange of endogenous cardiac troponin C (TnC) for skeletal troponin C also resulted in a major reduction in the length dependence to the level of skeletal muscle. These findings suggested that cardiac troponin C has a key role in the length-sensing mechanism. The present investigation supports this conclusion and delineates the specific domain in cardiac TnC responsible for the length effect. Chimeras splicing either 41, 61, or 96 N-terminal cardiac amino acids with the remaining skeletal residues have indicated that while Ca2+binding in all three constructs is similar to that in wild type cardiac TnC, the functional responsiveness of the 96-cardiac residue construct is improved over the other two. This 96-cardiac residue construct yielded a tension response indistinguishable from that of wild-type cardiac TnC. A tryptophan variant of the chimera indicated fluorescence characteristics indistinguishable from cardiac troponin C. The findings provide further support for the idea that cardiac troponin Cin situis modified in response to sarcomere length change and thereby participates in the Frank–Starling mechanism. Moreover, the study indicates that the tropinin C length-sensing attribute originates within the N-terminal domain constituted by these 96 residues.