Phosphorylation of troponin I and the inotropic effect of adrenaline in the perfused rabbit heart

THE regulation of contractile activity in cardiac muscle by changes in the intracellular Ca 2+ concentration involves the regulatory protein system, consisting of tropomyosin and the troponin complex that is located in the I filament. Although the regulatory protein system of cardiac muscle is essentially similar in function to that of skeletal muscle, the components differ in the two tissues. Cardiac tropomyosin consists principally of α subunits, whereas the skeletal protein is composed of α and β subunits 1 . Troponin I, troponin C and troponin T are all specific for cardiac muscle in that they have different chemical and immunochemical properties from the corresponding skeletal muscle proteins 2–4 . In addition, cardiac troponin I and cardiac troponin C have been shown to have different amino acid sequences from their skeletal muscle counterparts 3,5 .