Relaxin suppresses atrial fibrillation, reverses fibrosis and reduces inflammation in aged hearts.

Healthy aging results in cardiac structural and electrical remodeling that increase susceptibility to cardiovascular diseases. Relaxin has shown broad cardioprotective effects including anti-fibrotic, anti-arrhythmic and anti-inflammatory outcomes in multiple models. This paper focuses on the cardioprotective effects of Relaxin in a rat model of aging. Sustained atrial or ventricular fibrillation are readily induced in the hearts of aged but not young control animals. Treatment with Relaxin suppressed this arrhythmogenic response by increasing conduction velocity, decreasing fibrosis and promoting substantial cardiac remodeling. Relaxin treatment resulted in a significant increase in the levels of: Nav1.5, Cx43, ßcatenin and Wnt1 in rat hearts. In isolated cardiomyocytes, Relaxin increased Nav1.5 expression. These effects were mimicked by CHIR 99021, a pharmacological activator of canonical Wnt signaling, but blocked by the canonical Wnt inhibitor Dickkopf1. Relaxin prevented TGF-ß-dependent differentiation of cardiac fibroblasts into myofibroblasts while increasing the expression of Wnt1; the effects of Relaxin on cardiac fibroblast differentiation were blocked by Dickkopf1. RNASeq studies demonstrated reduced expression of pro-inflammatory cytokines and an increase in the expression of α- and ß-globin in Relaxin-treated aged males. Relaxin reduces arrhythmogenicity in the hearts of aged rats by reduction of fibrosis and increased conduction velocity. These changes are accompanied by substantial remodeling of the cardiac tissue and appear to be mediated by increased canonical Wnt signaling. Relaxin also exerts significant anti-inflammatory and anti-oxidant effects in the hearts of aged rodents. The mechanisms by which Relaxin increases the expression of Wnt ligands, promotes Wnt signaling and reprograms gene expression remain to be determined.

Periodic injections of Relaxin 2, its pharmacokinetics and remodeling of rat hearts.

Relaxin-2 (RLX), a critical hormone in pregnancy, has been investigated as a therapy for heart failure. In most studies, the peptide was delivered continuously, subcutaneously for 2 weeks in animals or intravenously for 2-days in human subjects, for stable circulating [RLX]. However, pulsatile hormone levels may better uncover the normal physiology. This premise was tested by subcutaneously injecting Sprague Dawley rats (250 g, N = 2 males, 2 females/group) with human RLX (0, 30, 100, or 500 µg/kg), every 12 h for 1 day, then measuring changes in Nav1.5, connexin43, and ß-catenin, 24 h later. Pulsatile RLX was measured by taking serial blood draws, post-injection. After an injection, RLX reached a peak in âˆ¼ 60 min, fell to 50 % in 5-6 h; injections of 0, 30, 100 or 500 µg/kg yielded peak levels of 0, 11.26 ± 3.52, 58.33 ± 16.10, and 209.42 ± 29.04 ng/ml and residual levels after 24-hrs of 0, 4.9, 45.1 and 156 pg/ml, respectively. The 30 µg/kg injections had no effect and 100 µg/kg injections increased Nav1.5 (25 %), Cx43 (30 %) and ß-catenin (90 %). The 500 µg/kg injections also increased Nav1.5 and Cx43 but were less effective at upregulating ß-catenin (up by 25 % vs. 90 %). Periodic injections of 100 µg/kg were highly effective at increasing the expression of Nav1.5 and Cx43 which are key determinants of conduction velocity in the heart and the suppression of arrhythmias. Periodic RLX is effective at eliciting changes in cardiac protein expression and may be a better strategy for its longer-term delivery in the clinical setting.