Dose-dependent exercise promotes the transition from atrial protection to arrhythmogenic vulnerability: the intermediary role of calcium handling instability in substrate remodeling

Background Endurance athletes carry an elevated risk of atrial fibrillation (AF), yet the exercise dose threshold at which physiological cardiac adaptation gives way to maladaptive remodeling, and the potential involvement of calcium (Ca 2+ ) handling in this transition, remain poorly delineated. Methods and results C57BL/6 mice were stratified into sedentary control, low-, moderate-, and high-dose forced treadmill groups for 10 weeks. A modified eight-parameter behavioral panel confirmed progressively compromised exercise tolerance in the high-dose group. Surface ECG revealed significant P-wave morphological alterations and PR interval shortening, while HRV analysis demonstrated sympathovagal co-activation confined to the high-dose group. Echocardiography identified adverse atrial geometric remodeling, and histopathology demonstrated extensive fibrosis with inflammatory infiltration. Programmed electrical stimulation uncovered a striking dose-response gradient: AF inducibility reached 62.5% in the high-dose group versus 0% in the moderate-dose group. RNA sequencing identified 758 - 2,237 DEGs across the three comparisons, of which 13.6 - 17.2% were annotated to Ca 2+ -regulatory processes. KEGG and cluster analyses revealed enrichment of Ca 2+ signaling genes in both up- and down-regulated gene modules, while protein-protein interaction (PPI) network analysis suggested that inter-group transcriptional differences were largely related to calcium handling, cardiac contraction, myocardial remodeling, adrenergic signaling, autonomic regulation, and ECM remodeling. Optical mapping delineated dose-dependent remodeling of atrial Ca 2+ transient dynamics, accompanied by concordant action potential duration alterations. Conclusion High-dose exercise is associated with progressive maladaptive atrial remodeling encompassing structural, electrical, and Ca 2+ handling alterations. Impaired Ca 2+ release-reuptake balance and transcriptional alterations in Ca 2+ signaling pathways are potentially linked to a pro-arrhythmic substrate, suggesting that Ca 2+ handling instability may serve as a plausible intermediary between high-dose exercise and increased arrhythmogenic vulnerability.