Relaxin Suppresses Atrial Fibrillation by Reversing Fibrosis and Myocyte Hypertrophy and Increasing Conduction Velocity and Sodium Current in Spontaneously Hypertensive Rat Hearts

<underline>Rationale:</underline> Atrial fibrillation (AF) contributes significantly to morbidity and mortality in elderly and hypertensive patients and has been correlated to enhanced atrial fibrosis. Despite a lack of direct evidence that fibrosis causes AF, reversal of fibrosis is considered a plausible therapy. <underline>Objective:</underline> To evaluate the efficacy of the antifibrotic hormone relaxin (RLX) in suppressing AF in spontaneously hypertensive rats (SHR). <underline>Methods and Results:</underline> Normotensive Wistar-Kyoto (WKY) and SHR were treated for 2 weeks with vehicle (WKY+V and SHR+V) or RLX (0.4 mg/kg per day, SHR+RLX) using implantable mini-pumps. Hearts were perfused, mapped optically to analyze action potential durations, intracellular Ca2+ transients, and restitution kinetics, and tested for AF vulnerability. SHR hearts had slower conduction velocity (CV; P<0.01 versus WKY), steeper CV restitution kinetics, greater collagen deposition, higher levels of transcripts for transforming growth factor-, metalloproteinase-2, metalloproteinase-9, collagen I/III, and reduced connexin 43 phosphorylation (P<0.05 versus WKY). Programmed stimulation triggered sustained AF in SHR (n=5/5) and SHR+V (n=4/4), but not in WKY (n=0/5) and SHR+RLX (n=1/8; P<0.01). RLX treatment reversed the transcripts for fibrosis, flattened CV restitution kinetics, reduced action potential duration at 90% recovery to baseline, increased CV (P<0.01), and reversed atrial hypertrophy (P<0.05). Independent of antifibrotic actions, RLX (0.1 mu mol/L) increased Na+ current density, I-Na (approximate to 2-fold in 48 hours) in human cardiomyocytes derived from inducible pluripotent stem cells (n=18/18; P<0.01). <underline>Conclusions:</underline> RLX treatment suppressed AF in SHR hearts by increasing CV from a combination of reversal of fibrosis and hypertrophy and by increasing I-Na. The study provides compelling evidence that RLX may provide a novel therapy to manage AF in humans by reversing fibrosis and hypertrophy and by modulating cardiac ionic currents.