Relaxin' is a protein hormone of about 6000 Da first described in 1926 by Frederick Hisaw.
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 female, it is produced by the corpus luteum of the ovary, the breast and, during pregnancy, also by the placenta, chorion, and decidua.
In the male, it is produced in the prostate and is present in human semen.
Structurally, relaxin' is a heterodimer of two peptide chains of 24 and 29 amino acids linked by disulfide bridges, and it appears related to insulin.
Relaxin' is produced from its prohormone, "prorelaxin'", by splitting off one additional peptide chain.
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). 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 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.
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.
Relaxin' receptors have been found in the heart, smooth muscle, the connective tissue, and central and autonomous nervous system.
Specific disorders related to relaxin' have not been described, yet a link to scleroderma and fibromyalgia has been suggested.
A recombinant form of human relaxin'-2 has been developed as investigational drug RLX030 (serelaxin').