Female-biased renin release in response to relaxin in isolated rat renal cortex: potential implications for RAAS-mediated congestion with relaxin analogues

Background Relaxin, a pleiotropic vasoactive peptide and agonist of relaxin family peptide receptor 1 (RXFP1), has been evaluated in heart failure (HF) and pulmonary hypertension due to left heart disease (PH-LHD; Group 2). Recent clinical findings with the long-acting relaxin analogue volenrelaxin have raised signals consistent with worsening congestion/fluid retention. RXFP1 agonism has also been linked to increases in circulating renin, typically ascribed to reflex sympathetic/neurohumoral activation during vasodilation; however, a kidney-autonomous mechanism cannot be excluded. Purpose To test whether relaxin stimulates renin release in the absence of systemic sympathetic input. Methods Kidney cortex from 10–12-week-old male and female Sprague-Dawley (SD) rats were studied ex vivo. Tissue was incubated in DMEM with human relaxin-2 (100 or 300 nmol/L) or vehicle (n=4/sex, total n=8 per group). Renin concentration in the medium was quantified at 0, 3, and 6 h (ELISA). Rxfp1 mRNA expression was measured by RT-qPCR in renal cortex (n=6/sex). Data were analysed using an unpaired Student's t-test (Rxfp1 mRNA) and a repeated-measures mixed-effects model evaluating the interaction of sex, relaxin concentration, and time. Procedures complied with the Principles of Laboratory Animal Care (NIH Publication No. 85-23, 1985) Results The analysis revealed a significant main effect of time, with basal renin accumulation in the medium increasing by 6 h (p=0.017). A significant dose-by-time interaction was observed: female-derived renal cortex treated with the highest relaxin dose (300 nmol/L) exhibited a significant surge in renin secretion into the medium at 6 h (p<0.001). However, a significant three-way interaction (sex × dose [300 nmol/L] × time [6 h]) demonstrated a significant sexual dimorphism in this secretory response; the marked relaxin-induced renin surge observed in female tissue was absent in male-derived renal cortex (Fig. 1). Renal cortical Rxfp1 mRNA was significantly higher in females than males (p<0.05, Fig. 2). Conclusions To our knowledge, this is the first study to demonstrate that relaxin directly stimulates renin release from renal tissue, supporting a kidney-autonomous RXFP1-mediated mechanism. The pronounced female-predominant renin response aligns with higher baseline cortical Rxfp1 expression. These findings highlight the potential for RAAS activation and subsequent fluid congestion during therapeutic RXFP1 agonism, underscoring the critical need for sex-stratified mechanistic and clinical evaluations. Figure Legend Fig 1. Renin concentration in incubation medium, normalized to tissue mass (pg/mg tissue), measured at 0, 3, and 6 h. Data are mean ± SEM (n=4/sex, total n=8 per group). Female renal cortex exhibited significantly greater renin response to relaxin, most pronounced at 300 nmol/L at 6 h. Fig 2. Female renal cortex showed higher mRNA levels of Rxfp1 (arbitrary units, means ± SEM, n=6/sex).Figure 1Figure 2