Boyett, M. R. | Search bioRxiv

bioRxiv2026

Resident cardiac macrophages are not required for normal atrioventricular node conduction

Resident cardiac macrophages are understood to facilitate atrioventricular (AV) node conduction because they purportedly couple to AV node myocytes via connexin43 (Cx43) containing gap junctions. We tested this mechanism using biophysical modelling, high-resolution imaging of mouse and human AV conduction tissue, and pharmacological macrophage depletion. In silico, coupling macrophage membrane phenotypes to HCN4+ AV node myocytes imposed an electrotonic load that suppressed pacemaking and promoted conduction slowing, including stable 2:1 block in strand simulations. Anatomically, HCN4-defined components of the mouse AV conduction axis were essentially devoid of Cx43 and overlap of CD68+ macrophages and Cx43 was negligible in both mouse AV node and human penetrating bundle. Finally, near-complete macrophage depletion with CSF1R inhibition (PLX5622) did not alter AV electrical activity in vivo or ex vivo. Together, these data argue against a physiologically relevant role for Cx43-mediated macrophage-myocyte electrical coupling in normal AV node function. HIGHLIGHTSO_LIModelling predicts that AV node automaticity and conduction would be suppressed if macrophages coupled to AV node myocytes C_LIO_LIThe mouse AV conduction axis is essentially devoid of Cx43, currently considered responsible for macrophage-AV node myocyte coupling C_LIO_LIOverlap of macrophages and Cx43 expression is not discernible in the Cx43-expressing human distal AV node C_LIO_LIMacrophage depletion by CSF1R inhibition does not impact AV electrical activity in vivo or ex vivo C_LI

physiology↗

bioRxiv2025

Endurance exercise remodels pulmonary vein sleeve myocytes and promotes a proarrhythmic atrial substrate

BACKGROUNDAtrial fibrillation (AF) susceptibility is heightened in endurance athletes but the underlying mechanisms are incompletely understood. Because pulmonary vein (PV) myocyte triggers are critical determinants of AF, we investigated PV electrophysiological remodelling in animal models of the athletes heart. METHODSThe following experiments were performed in canines and mice after 16 or 6 weeks, respectively, of daily exercise training (ExT), and compared to sedentary (Sed) controls: ECG recording, echocardiography, pharmacological autonomic block, extrastimulus pacing, multielectrode array mapping, monophasic and intracellular action potential (AP) recording with custom-designed pattern recognition analysis, histology, RNAseq and spatial in situ transcriptomics. RESULTSAF propensity was significantly increased in ExT animals. Mapping studies identified heightened rotational activity in the PV-left atrial (LA) junction of ExT vs. Sed canines in vivo, and enhanced automaticity, triggered activity and AP duration variability ex vivo in ExT canines and mice. Intracellular recordings in mouse PV cardiomyocytes determined at least six AP subtypes with increased frequency of pacemaker-like APs in ExT PV, concomitant with increased expression of pacemaking HCN4, Cav1.3 and Cav3.1 channels. PV spontaneous excitability was also significantly enhanced. Subcellular resolution spatial transcriptomics in mouse PV-LA identified diffuse ion channel remodelling and activation of established AF-promoting pro-inflammatory and pro-fibrotic cytokines and chemokines in ExT PV cardiomyocytes. Conduction slowing in the ExT PV-LA junction was attributable to: gap junction remodelling, reduced Na+ channel expression and increased extracellular matrix deposition with enhanced myofibroblast number and proximity to PV cardiomyocytes. CONCLUSIONSEndurance exercise elicits proarrhythmic electro-anatomical remodelling of the PV-LA junction with enhanced pacemaking ion channel expression and immune-inflammatory pathway activation in PV myocytes as prominent contributors. CLINICAL PERSPECTIVEO_ST_ABSWhat is new?C_ST_ABSO_LIThis work is the first demonstration that endurance training results in proarrhythmic electrophysiological remodelling of PV sleeve myocytes and extracellular matrix deposition in the PV-LA junction. C_LIO_LIWe register electrical and molecular heterogeneity of the PV-LA junction at single cell and subcellular resolution, and for the first time identify the molecular events that underlie increased proarrhythmic activity of the trained PV. These include enhanced pacemaking ion channel expression (e.g., HCN4, Cav1.3, and Cav3.1), pro-inflammatory cytokine activation (e.g., TNF, IL-6), increased myofibroblasts and extracellular matrix deposition. C_LI What are the clinical implications?O_LIWe identify the molecular determinants of PV proarrhythmic activity in the trained heart and present new therapeutic targets for AF prevention in athletes. C_LIO_LIOur findings provide mechanistic rationale for the efficacy of pulmonary vein isolation for AF in athletes. C_LI

physiology↗