The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1 (RLN1), 2 (RLN2) and 3 (RLN3), and the insulin-like (INSL) peptides, INSL3, INSL4, INSL5 and INSL6. The functions of relaxin-3, INSL4, INSL5, INSL6 remain uncharacterised.
In the male, it is produced in the prostate and is present in human semen.
- See also: Insulin/IGF/Relaxin family
Relaxin is produced from its prohormone, "prorelaxin", by splitting off one additional peptide chain reaction.
In females, relaxin is produced mainly by the corpus luteum, in both pregnant  and nonpregnant females; it rises to a peak within approximately 14 days of ovulation, and then declines in the absence of pregnancy, resulting in menstruation Template:Citation needed). During the first trimester of pregnancy, levels rise and additional relaxin is produced by the decidua. Relaxin's peak is reached during the 14 weeks of the first trimester and at delivery. It is known to mediate the hemodynamic changes that occur during pregnancy, such as increased cardiac output, increased renal blood flow, and increased arterial compliance. It also relaxes other pelvic ligaments. It is believed to soften the pubic symphysis.
In males, relaxin enhances motility of sperm in semen.
In other animals
In animals, relaxin widens the pubic bone and facilitates labor; it also softens the cervix (cervical ripening), and relaxes the uterine musculature. Thus, for a long time, relaxin was looked at as a pregnancy hormone. However, its significance may reach much further. Relaxin affects collagen metabolism, inhibiting collagen synthesis and enhancing its breakdown by increasing matrix metalloproteinases. It also enhances angiogenesis and is a potent renal vasodilator.
In the European Rabbit, (Oryctolagus cuniculus), relaxin is associated with squamous differentiation and is expressed in tracheobronchial epithelial cells as opposed to being involved with reproduction.
Relaxin interacts with the relaxin receptor LGR7 (RXFP1) and LGR8 (RXFP2), which belong to the G protein-coupled receptor superfamily. They contain a heptahelical transmembrane domain and a large glycosylated ectodomain, distantly related to the receptors for the glycoproteohormones, such as the LH-receptor or FSH-receptor.
A recombinant form of human relaxin-2 has been developed as investigational drug RLX030 (serelaxin).
Relaxin 1 and Relaxin 2 arose from the duplication of a proto-RLN gene between 44.2 and 29.6 million years ago in the last common ancestor of catarrhine primates. The duplication that led to RLN1 and RLN2 is thought to have been a result of positive selection and convergent evolution at the nucleotide level between the relaxin gene in New World monkeys and the RLN1 gene in apes . As a result, Old World monkeys, a group that includes the subfamilies colobines and cercopithecines, have lost the RLN1 paralog, but apes have retained both the RLN1 and the RLN2 genes ; Lawrence and Cords, 2012).
- Relaxin family peptide hormones
- Insulin/IGF/Relaxin' family
- Relaxin'/insulin-like family peptide receptor 1
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- MeSH Relaxin'
- "Relaxin". Human Protein Reference Database. Johns Hopkins University and the Institute of Bioinformatics. Retrieved 2009-05-20.