Relaxin is a 6000-d polypeptide, structurally related to insulin and the insulin-like
growth factors. Unlike insulin, the structure of which is remarkably well conserved
among the vertebrates, relaxin sequences can vary by more than 50% between different
species. Despite these large sequence variations, relaxins (with few exceptions) have
very similar biologic activities in animal test systems. The reason for this has recently
come to light: the receptor binding region of the B chain, in contrast to the rest
of the molecule, is highly conserved between species. Relaxin is measured by bioassays
employing interpubic ligament formation in mice and guinea pigs, and by inhibition
of uterine motility. A more sensitive and efficient bioassay is urgently needed. In
women, the target organs for relaxin are the uterine cervix, myometrium, endometrium,
and decidua. Other presumptive but unproven targets are the pubic symphysis and sacroiliac
joints, mammary glands, and pituitary gland. Circulating relaxin is secreted by the
corpus luteum. The placenta, decidua, or both also produce relaxin, which does not
enter the circulation but may act in an autocrine or paracrine fashion. hCG is a stimulus
to luteal relaxin secretion. Other regulatory factors are poorly defined. Aluteal
women are hyporelaxinemic, and yet are capable of normal vaginal delivery of their
infants. Local effects of placental or decidual relaxin cannot be discounted in such
subjects. Hyperrelaxinemia may occur in women with multiple gestations and ovarian
stimulation, and may be associated with increased premature births. Serum relaxin
also is elevated in pregnant diabetics, but its role in this condition has not been
defined. Clearly, further investigations are needed to delineate the precise role
of relaxin in human pregnancy.