Evidence of Superior and Inferior Sinoatrial Nodes in the Mammalian Heart

The initiation of rhythmic heartbeat is the earliest manifestation of new life after conception. A normal heartbeat originates as an action potential in a group of pacemaker cells of the sinoatrial node (SAN) 1 . Pacemaker cells are evident since early embryogenesis when the entire sinus venosus exhibits electric automaticity and possesses specific gene expression profile distinct from that of working myocardium 2,3 . During the subsequent looping and ballooning stages of cardiac development, pacemaker cells eventually localize into SAN near the superior vena cava (SVC) when the two horns of the sinus venosus coil in formation of the atria 2 . The heart rate and anatomical site of origin of pacemaker activity dynamically change in response to various physiological input such as autonomic stimuli and pharmacological interventions 4 . However, the mechanisms of dominant pacemaker shift are not well understood. Here, we present functional and molecular evidence of two competing right atrial pacemakers localized near the SVC and the inferior vena cava (IVC), which we call the superior and inferior SANs: sSAN and iSAN. Using ex vivo optical mapping techniques and RNA sequencing of rat and human hearts, we demonstrate that sSAN and iSAN preferentially control the fast and slow heart rates during sympathetic or parasympathetic stimulation, respectively. RNAseq confirmed unique transcriptional profiles of sSAN and iSAN, which differs from both atria and ventricles. We speculate that the anatomical locations of sSAN and iSAN are a result of the way the two horns of the sinus venosus twist into a mature chambered heart. We anticipate these findings would clarify previously observed migration of dominant pacemaker and corresponding changes in P-wave morphology in many species. Furthermore, we expect these findings will shed light onto pathogenesis of aberrant pacemakers responsible for life-threatening arrhythmias near orifices of other major vessels: SVC and IVC, coronary sinus, pulmonary veins, aorta and the right ventricular outflow tract.