Mechanisms for human sinoatrial node (SAN) dysfunction are poorly understood and whether human SAN excitability requires voltage-gated sodium channels (Nav) remains controversial. Here, we report that neuronal (n)Nav blockade and selective nNav1.6 blockade during high-resolution optical mapping in explanted human hearts depress intranodal SAN conduction, which worsens during autonomic stimulation and overdrive suppression to conduction failure. Partial cardiac (c)Nav blockade further impairs automaticity and intranodal conduction, leading to beat-to-beat variability and reentry. Multiple nNav transcripts are higher in SAN vs atria; heterogeneous alterations of several isoforms, specifically nNav1.6, are associated with heart failure and chronic alcohol consumption. In silico simulations of Nav distributions suggest that I Na is essential for SAN conduction, especially in fibrotic failing hearts. Our results reveal that not only cNav but nNav are also integral for preventing disease-induced failure in human SAN intranodal conduction. Disease-impaired nNav may underlie patient-specific SAN dysfunctions and should be considered to treat arrhythmias.

Background Atrial fibrillation (AF) driver mechanisms are obscured to clinical multielectrode mapping approaches that provide partial, surface‐only visualization of unstable 3‐dimensional atrial conduction. We hypothesized that transient modulation of refractoriness by pharmacologic challenge during multielectrode mapping improves visualization of hidden paths of reentrant AF drivers for targeted ablation. Methods and Results Pharmacologic challenge with adenosine was tested in ex vivo human hearts with a history of AF and cardiac diseases by multielectrode and high‐resolution subsurface near‐infrared optical mapping, integrated with 3‐dimensional structural imaging and heart‐specific computational simulations. Adenosine challenge was also studied on acutely terminated AF drivers in 10 patients with persistent AF. Ex vivo, adenosine stabilized reentrant driver paths within arrhythmogenic fibrotic hubs and improved visualization of reentrant paths, previously seen as focal or unstable breakthrough activation pattern, for targeted AF ablation. Computational simulations suggested that shortening of atrial refractoriness by adenosine may (1) improve driver stability by annihilating spatially unstable functional blocks and tightening reentrant circuits around fibrotic substrates, thus unmasking the common reentrant path; and (2) destabilize already stable reentrant drivers along fibrotic substrates by accelerating competing fibrillatory wavelets or secondary drivers. In patients with persistent AF, adenosine challenge unmasked hidden common reentry paths (9/15 AF drivers, 41±26% to 68±25% visualization), but worsened visualization of previously visible reentry paths (6/15, 74±14% to 34±12%). AF driver ablation led to acute termination of AF. Conclusions Our ex vivo to in vivo human translational study suggests that transiently altering atrial refractoriness can stabilize reentrant paths and unmask arrhythmogenic hubs to guide targeted AF driver ablation treatment.

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Background: Up to fifty percent of the adult human sinoatrial node (SAN), is composed of dense connective tissue. Cardiac diseases including heart failure (HF) may further increase fibrosis within the SAN pacemaker complex, leading to impaired automaticity and conduction of electrical activity to the atria. However, unlike the role of cardiac fibroblasts in pathological fibrotic remodeling and tissue repair, nothing is known about fibroblasts that maintain the inherently fibrotic SAN environment. Methods: Intact SAN pacemaker complex was dissected from cardioplegically arrested explanted non-failing (non-HF, n=22; 48.7±3.1y.o,) and HF human hearts (n=16; 54.9±2.6y.o.). Connective tissue content was quantified from Masson's trichrome stained head-center and center-tail SAN sections. Expression of extracellular matrix (ECM) proteins, including Collagens 1, 3A1, cartilage intermediate layer protein 1 (CILP1) and periostin, fibroblast and myofibroblast numbers were quantified by in situ and in vitro immunolabeling. Fibroblasts from the central intramural SAN pacemaker compartment (~10x5x2 mm 3 ) and right atria (RA) were isolated, cultured, passaged once, and treated ±transforming growth factor beta-1 (TGFβ1) and subjected to comprehensive high-throughput next-generation sequencing of whole transcriptome, microRNA and proteomic analyses. Results: Intranodal fibrotic content was significantly higher in SAN pacemaker complex from HF vs non-HF hearts (57.7±2.6% vs 44.0±1.2% p <0.0001). Proliferating phosphorylated histone3 + /vimentin + /CD31 - fibroblasts were higher in HF SAN. Vimentin + /alpha smooth muscle actin + /CD31 - myofibroblasts along with increased interstitial periostin expression were found only in HF SAN. RNA sequencing and proteomic analyses identified unique differences in mRNA, long non-coding RNA, microRNA and proteomic profiles between non-HF and HF SAN and RA fibroblasts, and TGFβ1-induced myofibroblasts. Specifically, proteins and signaling pathways associated with ECM flexibility, stiffness, focal adhesion and metabolism were altered in HF SAN fibroblasts compared to non-HF SAN. Conclusions: This study revealed increased SAN-specific fibrosis with presence of myofibroblasts, CILP1 and periostin-positive interstitial fibrosis only in HF vs non-HF human hearts. Comprehensive proteo-transcriptomic profiles of SAN fibroblasts identified upregulation of genes and proteins promoting stiffer SAN ECM in HF hearts. Fibroblast-specific profiles generated by our proteo-transcriptomic analyses of the human SAN, provide a comprehensive framework for future studies to investigate the role of SAN-specific fibrosis in cardiac rhythm regulation and arrhythmias.

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