Ultracentrifugation study on interactions among calmodulin, cardiac troponin and tropomyosin-actin complex

In an effort to clarify the regulation of contractions in cardiac muscle, we performed ultracentrifugation studies on the interactions between cardiac troponin and tropomyosin-actin complex in the presence of Ca²⁺ or Sr²⁺. When troponin C and troponin I were centrifuged with tropomyosin-actin complex, troponin I was not removed from tropomyosin-actin complex in the presence of bivalent-cation. Troponin C was observed to bind very weakly to troponin T-tropomyosin-actin complex in either the presence or absence of bivalent-cation. When troponin C was replaced by calmodulin, troponin I was not removed from tropomyosin-actin complex in the presence of bivalent-cation. Calmodulin bound to the troponin I-troponin T-tropomyosin-actin complex only in the presence of bivalent-cation. These results suggest that the inhibitory action of troponin I is neutralized by troponin C or calmodulin upon binding of bivalent-cation while troponin I binds to tropomyosin-actin complex in cardiac muscle. Therefore cardiac muscle seems to differ from skeletal muscle in regard to regulation of its contraction.