In vivo and in vitro responses of the bovine corpus luteum after exposure to exogenous gonadotropin-releasing hormone and prostaglandin F2α

Abstract

Administration of gonadotropin-releasing hormone (GnRH) early in the estrous cycle has been shown to cause subsequent altered luteal function. To determine whether membrane-related events may be involved in GnRH-attenuated luteal function, corpora lutea (CL) were removed from beef heifers on day seven of the estrous cycle after iv injection of GnRH or saline on day two of the cycle (n=5/group). Luteal slices were incubated with saline (control), luteinizing hormone (LH), or 8-bromo-cAMP for 2h. In vivo administration of GnRH reduced LH and cAMP-stimulated progesterone production by tissue (p<0.01), but basal progesterone production was not affected (p>0.05). Luteal adenylyl cyclase activity did not differ between saline and GnRH-treated animals (p>0.05). Then to examine if early administration of GnRH alters response of the CL to prostaglandin (PG) F(2α), beefheifers were injected with GnRH as described above (n=4/group), and then injected with PGF(2α) on day eight and the CL removed 60 min later. Blood samples were collected for oxytocin (OT) analysis at frequent intervals after PGF(2α) injection and for progesterone at 0 and 60 min. Induction of the early response gene c-jun or release of OT by PGF(2α) was not altered by GnRH injection (p>0.05). Injection of PGF(2α) decreased serum progesterone by 60 min postinjection (p<0.05), but concentrations of this steroid were unaffected by GnRH (p>0.05). Collectively, these data suggest that GnRH-induced alteration of bovine luteal function may be owing to events distal to cAMP synthesis that do not interfere with PGF(2α)-induced expression of c-jun or OT release, cellular phenomena involved in luteolysis.