Excitation-contraction coupling--cardiac muscle events in the myofilament.

The interaction of myosin and actin is by intracellular Ca2+ concentration, which in turn is controlled by the sarcoplasmic reticulum. In muscle--including cardiac muscle--of vertebrates, and some invertebrates, the site of Ca2+ control is in the thin, actin-containing filaments. These filaments contain tropomyosin and troponin; the latter is a complex of three subunits. When Ca2+ combines with troponin C, the Ca-binding subunit, a shift occurs in the position of tropomyosin that makes it possible for the myosin heads to bind to actin. This process is inhibited by a conformational change in troponin C, resulting in the release of the troponin complex from one of the binding sites on the thin filament. This process exhibits cooperative aspects which have been analyzed in terms of the Ca-binding process and the effect of Ca2+ on actomyosin ATPase activity.