Molecular and Functional Remodeling of Superior and Inferior SAN in a Rat Model of HCM.

BACKGROUND: Recently, our laboratory presented functional and molecular evidence for the presence of 2 competing sinoatrial node (SAN) pacemakers in healthy human and rat hearts. Anatomically localized near the superior vena cava and inferior vena cava, the superior and inferior SANs (sSAN and iSAN, respectively) preferentially control fast and slow normal heart rates. However, only 1 dominant pacemaker, primarily the sSAN, was functional in the failing rat heart with hypertrophic cardiomyopathy. OBJECTIVES: This study aimed to determine the transcriptional basis of functional silencing of 1 of 2 dominant pacemakers in failing rat hearts. METHODS: Ascending aortic constriction was performed on 1-week-old male Sprague-Dawley rat pups to induce left ventricular hypertrophy and heart failure. The dominant pacemaker was anatomically mapped in adult (10-12 weeks old) healthy and failing rat hearts using optical mapping in isolated right atrial tissue preparations. RNA sequencing was used to identify regional sSAN/iSAN gene expression differences between healthy and failing rat hearts. RESULTS: In all failing rat hearts optically mapped in this study (n = 4), only the sSAN pacemaker was functional, while the iSAN was silent. Compared to healthy rat hearts, a total of 3,640 genes were downregulated, and 4,518 genes were upregulated in failing rat hearts. The functional quiescence of the iSAN in these failing rat hearts may be explained by their downregulation of sodium, potassium, and calcium ion channels as well as their downregulation of specific structural genes, including ankyrin, titin, and myosin heavy chain. Moreover, the iSAN showed predominant downregulation of several key transcription factors such as Tbx5, Tbx3, Shox2, and Smad9. CONCLUSIONS: Pressure-overload-induced heart failure resulted in significant downregulation of critical transcription factors, ion channels, and structural transcripts of the iSAN, which could explain the functional silencing of the iSAN in failing rat hearts.

Electrophysiology and Arrhythmogenesis in the Human Right Ventricular Outflow Tract.

BACKGROUND: Right ventricular outflow tract (RVOT) is a common source of ventricular tachycardia, which often requires ablation. However, the mechanisms underlying the RVOT's unique arrhythmia susceptibility remain poorly understood due to lack of detailed electrophysiological and molecular studies of the human RVOT. METHODS: We conducted optical mapping studies in 16 nondiseased donor human RVOT preparations subjected to pharmacologically induced adrenergic and cholinergic stimulation to evaluate susceptibility to arrhythmias and characterize arrhythmia dynamics. RESULTS: We found that under control conditions, RVOT has shorter action potential duration at 80% repolarization relative to the right ventricular apical region. Treatment with isoproterenol (100 nM) shortened action potential duration at 80% repolarization and increased incidence of premature ventricular contractions (P=0.003), whereas acetylcholine (100 µM) stimulation alone had no effect on action potential duration at 80% repolarization or premature ventricular contractions. However, acetylcholine treatment after isoproterenol stimulation reduced the incidence of premature ventricular contractions (P=0.034) and partially reversed action potential duration at 80% repolarization shortening (P=0.029). Immunolabeling of RVOT (n=4) confirmed the presence of cholinergic marker VAChT (vesicular acetylcholine transporter) in the region. Rapid pacing revealed RVOT susceptibility to both concordant and discordant alternans. Investigation into transmural arrhythmia dynamics showed that arrhythmia wave fronts and phase singularities (rotors) were relatively more organized in the endocardium than in the epicardium (P=0.006). Moreover, there was a weak but positive spatiotemporal autocorrelation between epicardial and endocardial arrhythmic wave fronts and rotors. Transcriptome analysis (n=10 hearts) suggests a trend that MAPK (mitogen-activated protein kinase) signaling, calcium signaling, and cGMP-PKG (protein kinase G) signaling are among the pathways that may be enriched in the male RVOT, whereas pathways of neurodegeneration may be enriched in the female RVOT. CONCLUSIONS: Human RVOT electrophysiology is characterized by shorter action potential duration relative to the right ventricular apical region. Cholinergic right ventricular stimulation attenuates the arrhythmogenic effects of adrenergic stimulation, including increase in frequency of premature ventricular contractions and shortening of wavelength. Right ventricular arrhythmia is characterized by positive spatial-temporal autocorrelation between epicardial-endocardial arrhythmic wave fronts and rotors that are relatively more organized in the endocardium.