Relaxin production and release after hysterectomy in the pig

Relaxin secretion by luteal tissue and into peripheral blood was examined in relation to changes in the fine structure of aging porcine corpora lutea. Sequential bleedings every sixth day reveal a similar increase in serum relaxin concentrations from days 6-114 in hysterectomized and pregnant gifts. The span of the estrous cycle is about 21 days, and the length of pregnancy is about 115 days in pigs. Estrone (E1) and 17 beta-estradiol (17 beta-E2) serum levels increase with placental development; after hysterectomy, they are low from days 12-168. In luteal tissue, peak quantities of relaxin and electron-dense granules occur on day 100 in hysterectomized and pregnant gilts. By day 112, luteal tissue relaxin levels and granule populations decrease by about half, while relaxin secretion into peripheral blood approaches peak concentrations about day 114 in hysterectomized as well as pregnant gilts. Between days 114 and 120, serum relaxin decreases by half even though the corpora lutea persist to day 150 in hysterectomized animals. Thereafter, cytoplasmic granules and blood and luteal tissue levels of relaxin decrease gradually to day 150 in hysterectomized gilts; in contrast, they disappear abruptly after parturition. In hysterectomized gilts, relaxin release coincides with diminished populations of granules in corpora lutea, and the initiation of these events seems to be precisely timed. E1 and 17 beta-E2 secretion is unrelated to relaxin secretion in pregnant and hysterectomized gilts. High peripheral blood levels of E1 and 17 beta-E2, primarily of placental origin, or even the presence of the uterus are not required in the release of relaxin at day 114. By using steroidogenic and protein synthetic capabilities of luteal cells as criteria essential to the evolution of reproduction in this species, the results presented here suggest that 114 days rather than 21 days characterize the reproductive cycle, even in the absence of the uterus. These results indicate that the production and, particularly, the release of relaxin into peripheral blood on about day 114 in hysterectomized gilts as well as pregnant animals may be genetically controlled.