Relaxin: peptide hormone in reproduction, tissue remodeling and cardiovascular function

Structure and receptors

Processed relaxin peptides resemble insulin in overall fold but have distinct sequence features. Their cellular actions are mainly mediated by G protein–coupled receptors of the RXFP family. RXFP1 is the principal receptor for the canonical relaxin peptide in many tissues. Other RXFP receptors (including RXFP2 and RXFP3) interact with related family peptides and support diverse signalling outcomes through intracellular pathways such as cyclic AMP, nitric oxide and kinase cascades.

Physiological roles

Relaxin contributes to connective tissue remodelling by modulating matrix metalloproteinases and collagen turnover; this action is important in the reproductive tract around parturition in many species. It also affects the cardiovascular and renal systems, promoting vasodilation, increasing renal blood flow and influencing extracellular matrix composition. Certain relaxin-family peptides act in the central nervous system with roles in appetite, stress responses and behaviour.

Clinical research and applications

Because of antifibrotic and vasodilatory effects, relaxin has been investigated as a potential therapy for conditions such as organ fibrosis and acute heart failure. Recombinant human relaxin showed promising haemodynamic and symptom effects in early studies, but larger trials yielded mixed outcomes and clinical use remains limited. Research continues into receptor pharmacology and therapeutics that target specific pathways.

Evolution, species differences and practical notes

Relaxin biology varies by species: sources include the corpus luteum and decidua in humans, and placental or ovarian tissues in other mammals. Gene families (often named RLN1, RLN2, RLN3 and related insulin-like peptides) illustrate specialization within the family. These differences complicate direct translation of animal findings to humans and are an important consideration in research design.

• Key mechanisms: connective tissue remodelling, modulation of nitric oxide and matrix metalloproteinases.
• Primary receptor family: RXFP (notably RXFP1).
• Clinical interest: antifibrotic therapies, cardiovascular modulation; outcomes remain under investigation.