The Regulation of Cardiac-Muscle Contraction by Troponin

Abstract

The purpose of this review is to summarize the work we have done over the past several years regarding the regulation of cardiac muscle contraction by troponin (Tn). Studies have been carried out on the Ca2+-binding properties of cardiac Tn (the 1:1:1 molar-ratio complex of TnC, the Ca2+-binding subunit; TnI, the inhibitory subunit; and TnT, the tropomyosin-binding subunit) and TnC, and the effects of Mg2+ and phosphorylation on these parameters have been measured. The relationship between these parameters and the activation of myofibrillar ATPase in cardiac and skeletal muscle will be discussed and related to the known physiological properties of cardiac muscle in different contractile states (e.g., ³-adrenergic stimulation). Since the binding of Ca2+ to Tn is only the first step in the activation of muscle contraction, studies on Ca2+-induced alterations in the interactions of Tn subunits, tropomyosin (Tm), and actin required to bring about muscle contraction or relaxation will also be discussed.