The role of relaxin in human reproduction

Although relaxin was discovered almost 60 years ago as a hormone that relaxed the pubic symphysis of the guinea-pig prior to parturition, it has only very recently been purified and characterized. Found in all species studied, it is a polypeptide hormone with a similar structural identity to insulin and nerve growth factor. Its main source appears to be the corpus luteum of pregnancy but it is also produced by the male prostate gland and is found in many target tissues such as cervix, myometrium, decidua and breast connective tissue. Its main mechanism of action appears to be the facilitation of remodelling of connective tissue in target tissues to allow the necessary changes in organ structure during pregnancy and parturition. A secondary mode of action, in some mammals at least, is to inhibit myometrial contractility until near the end of pregnancy. Rat and porcine relaxins have been highly purified and the latter relaxin is now being used in human trials where it has been shown to be a cervical ripening agent probably working in sequence with prostaglandins and oestradiol. A human relaxin polypeptide has recently been characterized from the identification of a genomic clone for relaxin and there is evidence that there may be more than one gene for relaxin in the human. Sufficient amounts of synthetic human relaxin for clinical trials should soon be available using recombinant DNA techniques and this important biochemical advance should facilitate specific in vitro and in vivo studies of the role of relaxin in the human. Possible roles for relaxin in the human which merit further investigation include the enhancement of sperm motility and penetration, implantation of the blastocyst, uterine stromal remodelling during pregnancy, the inhibition of premature labour, cervical ripening at parturition and, in the non-pregnant state collagen biosynthesis in disorders of collagen metabolism such as scleroderma and arthritis.