Serum Relaxin Quantity and Timing

Relaxin is a peptide hormone. The main source of circulating relaxin is the corpus luteum of the ovary. Humans are unusual among mammals, but not unique among primates, in that relaxin is detectable in the luteal phase of non-conception cycles. Unlike rodents, guinea pigs and pigs, there is no near-term increase or surge in serum relaxin in human or non-human primates.

In the non-conception ovarian cycle of anthropoid primates, serum relaxin concentration peaks about 10 – 12 days after ovulation. Data are available for the following species: human, chimpanzee, baboon, rhesus macaque, cynomolgus macaque (where peak luteal-phase level is reported at 16 days post-ovulation), and common marmoset (Hayes, 2004).

If fertilization occurs, relaxin rises to a peak in the first third of pregnancy, then declines slightly and stabilizes for the remainder of pregnancy in humans, chimpanzees, baboons and macaques. Chimpanzees may have absolutely higher concentrations of circulating relaxin in pregnancy as well as during the luteal phase of the cycle (Steinetz et al., 1992). In marmosets the rise in serum relaxin during pregnancy does not peak until mid-gestation (Einspanier et al., 1999). Relaxin was originally isolated from pigs. In this species as well as guinea pigs and rodents, relaxin secretion peaks at the end of gestation and the physiological role of relaxin appears related to softening of the cervix and relaxation of pelvic ligaments both of which facilitate passage of the fetuses. Exogenously administered relaxin has a similar affect on women, however, since there is no endogenous rise in circulating relaxin near-term the physiological role of relaxin in human and non-human primate parturition is uncertain.

There are three genes (H1, H2, H3) that encode relaxin in humans (Hayes, 2004). Circulating relaxin, produced by the ovary, is primarily the result of transcription of H2. In the great apes (chimps, gorillas, and orangutans) there are two genes, equivalent to H1 and H2. H1 and its homologue in the great apes probable arose as a duplication event of H2. In baboons, macaques, and marmosets there is one gene that encodes relaxin, it is equivalent to H2. An H3 equivalent has not been identified in non-human primates but is known from the mouse (M3). H3 is expressed only in the brain.