Automaticity of the heart: New insight

Automaticity is the property of cardiac cells to undergo spontaneous slow diastolic depolarization and initiate an electrical impulse in the absence of external electrical stimulation. Automaticity occurs in the sinoatrial node (the pacemaker tissue of the heart) and in cardiac tissues including some parts of atria, pulmonary vein myocardium, the atriaoventricular node and the His-Purkinje system under physiological conditions (normal automaticity) and in working atrial and ventricular cells when their resting membrane potential is depolarized under pathological conditions (depolarization-induced abnormal automaticity). The slow diastolic depolarization of the membrane potential is the result of the development of a net inward current during the phase 4 after action potential repolarization, which is attributed to interactions of several types of sarcolemmal ion channels and transporters (membrane potential clock). In addition, intracellular Ca2+ cycling mediated by sarcoplasmic reticulum Ca2+ release and uptake (Ca2+ clock) is also known to be involved in the genesis of the slow diastolic depolarization. In this symposium, molecular and electrophysiological mechanisms and regulation of normal and abnormal automaticity in the cardiac pacemaker tissues and its roles in arrhythmogenic activities under pathophysiological conditions will be discussed.