Relaxin is secreted during pregnancy, but it has no verified effects i
n humans. The objective of the present study was to identify the cells
containing specific relaxin-binding sites in the uterine cervix, vagi
na, uterus, mammary glands, mammary nipples, and term placenta in the
human. The uterine cervix, vagina, and uterus were obtained from hyste
rectomy specimens. Mammary glands and nipples were obtained after modi
fied radical mastectomy. Placenta was obtained after normal delivery.
Tissue samples were cut into slices (0.5-3 cm(3)), frozen in liquid ni
trogen, and cryosectioned (8 mu m). Cells that bind relaxin were ident
ified by sequential application of biotinylated porcine relaxin probe,
antibiotin immunoglobulin G conjugated to 1 nm colloidal gold, and si
lver enhancement for signal amplification. Relaxin bound with specific
ity to epithelial cells, smooth muscle cells, and blood vessels in the
cervix, vagina, uterus, and mammary nipples; to epithelial cells and
blood vessels in the mammary glands; and to skin of the mammary nipple
s. In addition, relaxin bound to individual cell types within the term
placenta (amnion epithelium, syncytiotrophoblasts, blood vessels), an
d to sebaceous glands within the nipples, We conclude that the specifi
c relaxin-binding cells probably contain relaxin receptors. Identifica
tion of putative relaxin receptors may provide insight into physiologi
cal and/or therapeutic roles of relaxin in the human.