Graduate Thesis Or Dissertation
 

Cellular mechanisms of altered bovine luteal function in response to exogenous gonadotropin-releasing hormone

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  • To determine whether membrane-related events may be involved in attenuated luteal function after gonadotropin-releasing hormone (GnRH) administration, corpora lutea (CL) were removed from 10 beef heifers on day 7 of the estrous cycle after i.v. injection of GnRH or saline on day 2 of the cycle. Luteal slices were incubated with saline (control), luteinizing hormone (LH) or 8-bromo-cAMP for 2 h. 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). Results of this experiment suggested that GnRH-induced alteration of bovine luteal function may be due to an effect distal to the point of cAMP accumulation. To explore further the effect of GnRH on luteal cell function, 10 heifers were injected with saline or GnRH and CL removed as above. Dissociated (mixed) and small luteal cells (SC) were cultured overnight, then incubated for 2 h with medium alone (control), LH or cAMP. In vitro treatment with LH and cAMP increased progesterone in the medium relative to controls (p<0.01), however, there was no effect of GnRH injection on progesterone production (p>0.05) nor in the percentage of large cells (LC) present in the mixed cell cultures (p=0.95). It has been previously found that the ratio of LC to SC increases in GnRH-treated animals. Many LC can be ruptured during dissociation of the CL, and it is possible that this procedure altered the number of LC, such that any differences that may have existed between the saline and GnRH-exposed CL were minimized. These data suggest that differences in the LC to SC ratio may indeed account for attenuated luteal function after exposure to GnRH. To examine if early administration of GnRH alters response of the CL to prostaglandin (PG) Fav beef heifers were injected with saline or GnRH on day 2 of the cycle (n=4/group), then injected with PGF[subscript 2α], on day 8 and the CL removed 60 min later. Blood samples were collected for oxytocin (OT) analysis at frequent intervals after PGF[subscript 2α], injection and for progesterone at 0 and 60 min. Induction of the early response gene c-jun or release of OT by PGF[subscript 2α], was not altered by GnRH injection (p>0.05). Injection of PGF[subscript 2α], decreased serum progesterone by 60 min post-injection (p<0.05), but was also unaffected by GnRH (p>0.05). These data support the hypotheses that c-jun expression and OT release are involved in PGF[subscript 2α]-induced luteolysis, but early administration of GnRH did not affect these processes.
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