Physiological role of atrial myocytesThe heart undergoes a cycle of events that cause blood to be propelled to the lungs and the body. These events can be divided into two stages: diastole and systole. During the diastolic stage, the atrial and ventricular myocytes are relaxed. Systole refers to the period of contraction and consequent ejection of blood from the ventricles to the pulmonary artery or aorta. The cardiac cycle is initiated by the sinoatrial nodea group of specialised non-contractile cardiac myocytes positioned in the wall of the right atrium. All of the cells in the heart have an intrinsic ability generate action potentials (electrical impulses). However, the sinoatrial node acts as the primary pacemaker because its cells naturally discharge at a high rate, and so override the other potential pacemaking sites. Hormonal modulation of the sinoatrial node is one of the principal mechanisms for altering the frequency of the heartbeat. From the sinoatrial node, the action potential spreads over the atria causing them to contract and push blood into the ventricles. As the wave of depolarisation sweeps across the heart, it reaches the atrioventricular node, which filters and relays the signal to the ventricles via specialised conduction tissue including the Purkinje fibres. The atrial chambers contract and relax before the ventricular systole, and their activation is evident as separate electrical activity in an electrocardiogram. The time course of contraction is marginally shorter in atria compared to ventricles. Both cell types reach peak contraction within a few tens of milliseconds (Luss et al., 1999).The ventricles contract more forcefully than the atrial chambers and are predominantly responsible for forcing blood out of the heart. However, atria can play a significant role in altering the amount of blood that loads into the ventricles before to systole. When a person is at rest, the contribution of atria to the filling of ventricles with blood is relatively low. The majority (~90%) of ventricular refilling occurs as a consequence of the venous pressure of blood returning to the heart. However, if the heart rate increases, such as during exercise or stress, then atrial contraction can account for ~20-30% of the volume of blood in the ventricles. This contribution to ventricular refilling is known as 'atrial kick' (Lo et al., 1999). Under some pathological conditions (such as mitral valve stenosis) and during ageing, the dependence on atrial kick can be critical (Nicod et al., 1986).A common form of cardiac dysrhythmia known as 'atrial fibrillation' occurs when electrical impulses do not solely arise from the sinoatrial node, but instead spontaneously occur with high frequency from sites around the atria (~350 discharges per minute, compared with the normal sinoatrial rhythm of 60-80 beats per minute). The rapid and irregular electrical discharges during atrial fibrillation cause the atria to quiver and thereby