Genetic Complexity of Sinoatrial Node Dysfunction

The pacemaker cells of the cardiac sinoatrial node (SAN) are essential for normal cardiac automaticity. Dysfunction in cardiac pacemaking results in human sinoatrial node dysfunction (SND). SND more generally occurs in the elderly population and is associated with impaired pacemaker function causing abnormal heart rhythm. Individuals with SND have a variety of symptoms including sinus bradycardia, sinus arrest, SAN block, bradycardia/tachycardia syndrome, and syncope. Importantly, individuals with SND report chronotropic incompetence in response to stress and/or exercise. SND may be genetic or secondary to systemic or cardiovascular conditions. Current management of patients with SND is limited to the relief of arrhythmia symptoms and pacemaker implantation if indicated. Lack of effective therapeutic measures that target the underlying causes of SND renders management of these patients challenging due to its progressive nature and has highlighted a critical need to improve our understanding of its underlying mechanistic basis of SND. This review focuses on current information on the genetics underlying SND, followed by future implications of this knowledge in the management of individuals with SND.

Concomitant genetic ablation of L-type Cav1.3 (α1D) and T-type Cav3.1 (α1G) Ca2+ channels disrupts heart automaticity

Cardiac automaticity is set by pacemaker activity of the sinus node (SAN). In addition to the ubiquitously expressed cardiac voltage-gated L-type Cav1.2 Ca2+ channel isoform, pacemaker cells within the SAN and the atrioventricular node co-express voltage-gated L-type Cav1.3 and T-type Cav3.1 Ca2+ channels (SAN-VGCCs). The role of SAN-VGCCs in automaticity is incompletely understood. We used knockout mice carrying individual genetic ablation of Cav1.3 (Cav1.3−/−) or Cav3.1 (Cav3.1−/−) channels and double mutant Cav1.3−/−/Cav3.1−/− mice expressing only Cav1.2 channels. We show that concomitant loss of SAN-VGCCs prevents physiological SAN automaticity, blocks impulse conduction and compromises ventricular rhythmicity. Coexpression of SAN-VGCCs is necessary for impulse formation in the central SAN. In mice lacking SAN-VGCCs, residual pacemaker activity is predominantly generated in peripheral nodal and extranodal sites by f-channels and TTX-sensitive Na+ channels. In beating SAN cells, ablation of SAN-VGCCs disrupted late diastolic local intracellular Ca2+ release, which demonstrates an important role for these channels in supporting the sarcoplasmic reticulum based “Ca2+clock” mechanism during normal pacemaking. These data implicate an underappreciated role for co-expression of SAN-VGCCs in heart automaticity and define an integral role for these channels in mechanisms that control the heartbeat.

Abstract 247: Dose-dependent Role of Inwardly Rectifying Potassium Current on Cardiac Automaticity of Human Embryonic Stem Cell-derived Cardiomyocytes

The inwardly rectifying potassium current (IK1), encode by Kir2 family, is responsible for maintaining the negative resting potential, and contributes to phase 3 repolarization of the cardiac action potential. IK1 was generally thought to suppress cardiac automaticity, while the suppression of IK1 in adult ventricular cardiomyocytes (CMs) could engineer bio-artificial pacemaker-like cells to spontaneously fire action potential. Our studies also showed that overexpressed the gene of Kir2.1 could facilitate the electrophysiological maturing of mouse and human embryonic stem cell-differentiated CMs (ESC-CMs), which have the high degree of automaticity with nearly 50% of cells that can spontaneously fire action potential. In this study, we extensively analyzed the electrophysiology of mouse and human ESC-CMs, and found that the maximum diastolic potential in spontaneously firing ESC-CMs, -72.1±1.3 mV in atrial cells and -75.0±2.1 mV in ventricular cells, were significantly more hyperpolarized than that in quiescent ESC-CMs (-64.4±2.1 mV in atrial cells and -67.1±3.2 mV in ventricular cells). Applying a small amount of IK1 to hyperpolarize the membrane potential could enable those quiescent ESC-CMs to spontaneously fire action potential, indicating the enhancement of cardiac automaticity, while a large amount of IK1 could quiet those spontaneously firing cells down. By combining computational and experimental analyses, we confirmed that the synergistic interaction of IK1 and pacemaker current (If) could efficiently regulate cardiac automaticity during the differentiation. Our studies disclosed a dose-dependent role of IK1 on cardiac automaticity that a small amount of IK1 enhances and a large amount of IK1 suppresses cardiac automaticity in ESC-CMs during differentiation.

Spontaneous electrical activity of guinea-pig sinoatrial cells under modulation of two different pacemaker mechanisms

The main cellular determinants of cardiac automaticity are the hyperpolarization-activated cationic current If, and the electrogenic Na+/Ca2+ exchanger which generates an inward current after each action potential (AP). Our goal was to evaluate their relative role in pacemaking, by means of application of Ivabradine (IVA) (specific If blocker) and Ryanodine (RYA) (known to abolish calcium transient) on enzimatically isolated guinea-pig pacemaker cells. Spontaneous APs were recorded in patch-clamp whole cell configuration at 36°C from 7 cells perfused with the following sequence of solutions: physiological normal tyrode (NT), IVA 3 mM, NT and RYA 3 mM. Cycle length (CL, ms) and diastolic depolarization rate (DDR, V/s) were also calculated. Both blockers displayed similar effects, increasing CL (by 27 and 30%, respectively), and decreasing DDR (by 34 and 42%) with respect to NT exposure. These results suggest that both mechanisms are involved into pacemaking mechanism at a similar degree.