Background: Atrial fibrillation (AF) is one of the most common causes of cardiac arrhytmia in the heart (1). A relationship between AF and exercise is controversial as exercise seems to protect the heart, in contrast, extremely vigorous long-term exercise could increase cardiac problems (2). AF causes poor performance in horses, and knowledge about the molecular initiation of AF of the pulmonary vein (PV) (3) in the athletic horse is still (4) unknown. Ion channels, responsible for cardiac action potential generation and electrical propagation are target-study of AF causes, in addition to this, the interest in investigating them is because they seem to interact with AF treatments (3). Therefore, this study aims to investigate the molecular ion channel expression pattern in the PV of the athletic horse to get a global gene map expression of this condition.
Methods: Heart samples were obtained from ten horses. Five horses were healthy, and the other five clinically reported suffering AF condition. All ten horses were humanly euthanized due to cardiac affections, and the healthy horses were selected because of musculoskeletal problem- related. Horses belonged to the Hong Kong Jockey Club and were clinically evaluated by veterinarians. Heart tissue biopsies were taken from the left and right atria and ventricle respective sides. Myocardial sleeves from the PV were dissected from the heart. RNA from heart tissue was analysed by RNA sequencing; protein and immunohistochemistry were performed in the samples of healthy and AF horses.
Differential expression of a selection of ion channels and electrophysiological proteins were evaluated. We analysed results by hierarchical clustering and left atrium and PV sequenced samples. PV gene expressions were compared with paired Mann-Whitney-Wilcoxon test. Bioinformatics data and statistics analysis were performed with R software. Results from PV and left atrium samples were clustered separately based on their gene expression. Seven ion channels were significantly (p<0.05) increased while six ion channels were significantly under- expressed (p<0.05) in the PV sleeves of horses. Gene expression of potassium ion channels such as KCNA4, KCNQ1, KCNH2, KCND3, KCNJ2, KCNN1, KCNN2, and KCNN3 were positively expressed. In addition, calcium-handling proteins (NCX1 and PLN), connexins (CX40), and HCN4 channel ion genes were expressed. Genes in the PV sleeves and left atrium coincide with the differences in the electrophysiological characteristics and conduction velocities. Variations in KCNQ1 and NCX1 genes showed the repolarization patterns of the potassium and calcium homeostasis in the PV sleeves and left atrium. The left atrium and the PV sleeves gene expression showed a specific ion channel pattern of the pulmonary sleeves in the horse.
Conclusion: Our findings show that PV sleeves in athletic horses could initiate AF, this was observed by gene and molecular expression patterns. Therefore, the study of the PV in racing horses using bioinformatics tools analyses could help us clarify relevant biological data and provide us with an understanding location gene map. This study focuses on cardiac dysrhythmias as AF in athlete horses will provide knowledge in human athlete’s studies due to similarities in ion channel expression patterns.