Relaxin

Relaxin is a peptide hormone that was first described in 1926 by Frederick Hisaw.

Different forms of relaxin have been described: relaxin 1, 2, and 3.

Production
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, relaxin is produced in the testes.

Structure
Structurally, relaxin is a heterodimer of two peptide chains of 24 and 29 amino acids that are linked by disulfide bridges and it appears related to insulin.

Relaxin is produced from its prohormone, “pro-relaxin”, by splitting off one additional peptide chain.

Function
The function of relaxin in humans is not clear, although it has several functions in animals.

In humans
In women relaxin levels rise after ovulation as a result of its production by the corpus luteum. In the absence of pregnancy its level declines at menstruation. During the first trimester of pregnancy levels rise and additional relaxin is produced by the decidua.

Relaxin's role or necessity in human pregnancy remains under investigation, as in humans its peak is reached during the first trimester, not toward the end of pregnancy.

In 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.

Receptors
Relaxin interacts with the relaxin receptor LGR7 and LGR8  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.

Disorders
Specific disorders related to relaxin have not been described, yet it has been suggested that it could be linked to scleroderma and to fibromyalgia.