The cardiac conduction system undergoes electrophysiological remodelling in response to endurance exercise, resulting in rhythm ‘disturbances’ such as sinus bradycardia i.e. a low resting heart rate. For example, elite athletes can present with heart rates as low as 30 beats per minute. In the long term this can become maladaptive, underscoring a range of bradyarrhythmias including sinus pauses and heart block and necessitating electronic pacemaker implantation, known to be more frequent in veteran athletes compared to the general population.
Our recent work has demonstrated that sinus bradycardia in rodent models of endurance exercise is a result of diffuse ion channel remodeling in the sinus node, the pacemaker of the heart. Notably, we reported a downregulation of the key pacemaker channel HCN4 and a corresponding reduction in the pacemaker current If. Experiments with the If selective blocker Ivabradine suggest analogous HCN4 remodeling in human athletes and hence our current focus is on deciphering the underlying transcriptional and post-translational mechanisms with specific emphasis on microRNAs (miR) and transcription factors. Using a combination of techniques including next generation sequencing, luciferase reporter gene assays, tissue and single cell electrophysiology in swim-trained mice, we identified that miR-423-5p exerts post translational control over HCN4. Targeting miR-423-5p with antisense oligonucleotides reversed sinus bradycardia, HCN4 and If remodeling, thus holding therapeutic potential for the treatment of bradarrythmias in the veteran athlete.
Previous work has also demonstrated an increased incidence of bradyarrhythmias during sleep in athletes, a phenomenon currently attributed to heightened vagal tone. However, in Langendorff-perfused mouse hearts and in ex vivo sinus node preparations we found an intrinsic sinus bradycardia at night- explained by a circadian variation in the expression of HCN4 and current density of If. Using bioluminescent reporters and knockout mouse models we discovered a functioning circadian clock in the sinus node characterised by endogenous rhythms in canonical clock transcription factors such as PERIOD, CLOCK and BMAL1. Further investigations in BMAL1 knockout mice and chromatin immunoprecipitation experiments indicated that HCN4 is a transcriptional target of BMAL1, demonstrating involvement of a peripheral clock in heart rate regulation.
Overall, our findings provide new insight into the molecular mechanisms underlying the most common rhythm disturbance in athletes and may have implications for other conditions in which If is dysregulated, e.g. heart failure. At a time when public participation in ultra-endurance events is on the rise, our work is the first step towards small molecule therapies for arrhythmias in athletes.