Localization of specific relaxin-binding cells in the ovary and testis of pigs

It is not known whether relaxin has physiological roles in the gonads in mammalian species. Limited evidence indicates that relaxin may act locally to regulate ovarian function in pigs. The possibility of a role for relaxin in testicular function in pigs has not been investigated. A major initial step toward the establishment of direct effects of relaxin on the ovary and/or testis is to demonstrate that relaxin binds with specificity to the gonads. Accordingly, the first objective of this study was to employ an immunohistochemical localization technique to determine whether relaxin-binding cells are present in the ovaries and/or testis of pigs. Once they were found to be present, the second objective was to determine whether relaxin-binding sites noticeably change either within the ovary at different stages of the estrous cycle and pregnancy, or within the testis at sexual maturity. Ovaries were collected from four stages of the estrous cycle (midfollicular, late follicular, early luteal, and midluteal) and three stages of the pregnancy (Day 40, Day 80, and Day 110). Two gilts were used for each of the stages of the estrous cycle and pregnancy. Testes were collected from a 5-mo-old immature boar and a 36-mo-old mature boar. Tissues were cut into cubes (3-4 cm3), frozen in liquid nitrogen, and cryosectioned (8 microm). Specific cell types that bind relaxin were identified by sequential application of a biotinylated relaxin probe, antibiotin immunoglobulin G conjugated to 1 nm colloidal gold, and silver for signal amplification. In the ovary, specific relaxin-binding sites were localized in both the theca and granulosa cells of developing follicles, luteal cells, and blood vessels. In the testis, specific relaxin-binding sites were localized in the Leydig cells. There were no apparent differences in relaxin-binding distribution within the ovary at different stages of the estrous cycle and pregnancy in gilts, or within the testis at sexual maturity in boars. We conclude that the specific relaxin-binding cells within the ovary and testis of the pig may contain relaxin receptors. Therefore, relaxin may have effects in the ovary and testis of pigs.