Abrupt Shifts in Relaxin and Progesterone Secretion by Aging Luteal Cells: Luteotropic Response in Hysterectomized and Pregnant Pigs*

Abstract

Although a prominent role of conceptuses in the hormonal control of normal parturition in the pig is well founded, mechanisms are entrained that cause a timed relaxin release and an abrupt decrease in progesterone secretion in the complete absence of the uterus. This study focused on the steroidogenic and peptide hormone secretory capacities of aging porcine luteal cells in culture on specific days during a narrow window (days 110-116) when abrupt shifts occur in their function. Dispersed luteal cells (3 x 10(5) cells/well) from hysterectomized and pregnant gilts on days 110, 113, and 116 after estrus were cultured for 24 h with porcine (p) LH, pPRL, prostaglandin E1 (PGE1), PGE2, and (Bu)2cAMP. Progesterone and relaxin concentrations in cells preceding culture and in the spent medium were quantified by RIA. Progesterone in cells before culture was less (P less than 0.05) than that released into the medium, while relaxin levels were greater (P less than 0.05) than those in the medium. Relaxin stored in luteal cells of hysterectomized gilts was consistently greater (P less than 0.01) than that during the same days of late pregnancy; however, progesterone was greater (P less than 0.01) in hysterectomized compared with pregnant gilts only on day 116. Relaxin release was greater (P less than 0.01) in hysterectomized compared with pregnant gilts on days 113 and 116, whereas progesterone was greater (P less than 0.05) on days 110 and 116 in hysterectomized compared with pregnant animals. pLH, pPRL, and (Bu)2cAMP increased (P less than 0.05) relaxin secretion in hysterectomized gilts on days 110, 113, and 116, but only on day 110 in pregnant animals. These hormones consistently increased (P less than 0.05) progesterone release only on day 110 in both groups of gilts. PGE1 increased (P less than 0.01) relaxin release in hysterectomized, but not pregnant, gilts; it was not effective in altering progesterone in either group. PGE2 increased the release of both hormones on day 113 in both groups. The percentage of large cells (i.e. greater than 20 microns in diameter) was consistently greater (P less than 0.01) on days 110-116 in hysterectomized compared with pregnant gilts. Both luteal cell relaxin concentrations and the population of electron-dense granules decreased from days 110-116 in pregnant, but not in hysterectomized, gilts; luteal cells from postpartum animals were nearly depleted of granules, and empty vesicles were prominent. The secretory response of the cells to tropic hormone stimulation is preprogrammed from the previous estrus that occurred more than 110 days previously.(ABSTRACT TRUNCATED AT 400 WORDS)