Cardiac Excitation-Contraction Coupling

Contraction of the heart, required to pump the blood out of the left ventricle, is a complex process that involves electrical excitation of the cardiac muscle, finely tuned changes in the intracellular concentration of the ubiquitous second-messenger calcium (Ca), and activation of the contractile apparatus of cardiac myocytes. This process is called 'excitation-contraction (E-C) coupling'. The electrical impulse generated by the cardiac pacemaker cells propagates to the ventricle where it triggers cardiac myocyte depolarization. Depolarization opens voltage-dependent Ca channels, allowing Ca to enter cardiac myocytes and trigger Ca release from the sarcoplasmic reticulum. The combination of Ca entry and Ca release raises the intracellular Ca concentration, allowing Ca to bind to the myofilaments and activate contraction. Relaxation occurs when Ca is extruded from the cytosol. Defective E-C coupling leads to heart failure (HF), a syndrome that develops when the amount of blood pumped from the heart is inadequate to meet the metabolic demands of the body. HF is characterized by both contractile dysfunction and increased propensity for fatal arrhythmias. In this article, we review the E-C coupling process in healthy hearts and the main alterations that lead to reduced contractions and arrhythmias in HF.