Luteal P4 synthesis in early pregnant gilts after induction of estrus with PMSG/hCG
Introduction
The corpus luteum (CL) plays a key role in the regulation of reproductive events associated with pregnancy establishment and maintenance because it is the main source of progesterone (P4). The concentration of P4 in peripheral blood increases with maturation of CLs. P4 is the hormone of pregnancy in all mammals and is required for maternal support of conceptus survival and development (Spencer and Bazer, 2002). Sustained P4 action on the uterus is essential for the attainment of endometrial receptivity for implantation in ruminants and pigs, and determines endometrial expression of several genes encoding proteins participating in embryo-maternal communication (Bazer et al., 2010, Spencer et al., 2004, Ziecik et al., 2011).
In the pig, the most established treatment protocol for the control of ovulation includes using a combination of pregnant mare serum gonadotropin (PMSG) to induce follicular development and human chorionic gonadotropin (hCG) to trigger ovulation (Britt et al., 1989, Estill, 2000). Although the treatment of gilts with exogenous gonadotropins can induce a fast and synchronous ovulatory response (Britt et al., 1989, Knox et al., 2000), administration of hormones may also have a detrimental effect on reproductive functions. Temporal relationships among serum estrogens, prolactin and luteinizing hormone (LH) concentrations were different in prepubertal gilts induced to ovulate compared with mature gilts (Pinkert et al., 1988). The patterns of morphological and biochemical development of ovarian follicles also differ between gilts exhibiting gonadotropin-stimulated vs. natural estrus (Wiesak et al., 1990). Moreover, a greater proportion of gilts with follicular cysts and poorly formed CLs was observed after hormone treatment (Breen and Knox, 2012). Administration of hormones to induce estrus in domestic animals may cause decreased concentrations of circulating P4, which results from impairment of the luteal P4 metabolic pathway (Kineman et al., 1987a) and/or reduced sensitivity of CLs to luteotropic factors (Kineman et al., 1987b, Skarzynski et al., 2009). Moreover, induced CLs are more sensitive to luteolytic prostaglandin (PG) F2α, resulting in pregnancy failure (Puglisi et al., 1978, Rampacek et al., 1976a). Low concentrations of P4 in blood serum of heifers affected the expression of endometrial genes that potentially contribute to histotroph composition and conceptus elongation. This altered endometrial transcriptome was associated with a reduced capacity of the uterus to support conceptus development after embryo transfer (Forde et al., 2011, Forde et al., 2012).
In the pig, lower concentrations of P4 in blood serum, accompanied by decreased expression of homeobox A10, leukemia inhibitory factor and transforming growth factor β1 genes in the endometrium, were observed on day 12 in pregnant gilts after induction of puberty with PMSG/hCG (Blitek et al., 2010a). Moreover, the expression of PG synthesis enzymes in the endometrium and PG content in the uterine lumen and peripheral blood were negatively affected by the administration of gonadotropins (Blitek et al., 2010b). These changes may be attributed to impaired endometrial preparation for implantation resulting from insufficient P4 synthesis and/or secretion by luteal tissue. Therefore, the current study was conducted to examine the effect of estrus induction with PMSG/hCG in prepubertal gilts on the quality of subsequent CLs with respect to P4 synthesis. Using the same gilts as described previously (Blitek et al., 2010a), we analyzed: (1) the profiles of steroidogenic acute regulatory protein (StAR), cytochrome P450 family 11 subfamily A polypeptide 1 (CYP11A1) and 3β-hydroxysteroid dehydrogenase (3βHSD) mRNA and protein expression in luteal tissue; (2) the concentrations of P4 in luteal tissue and blood serum; and (3) the expression of LH receptor (LHR) and estrogen receptors (ESR1 and ESR2) mRNA in the CL. Samples were collected on days 10, 12, and 15 after insemination, because this period is critical for prolonged CL lifespan and pregnancy establishment in response to conceptus signals.
Section snippets
Animals and sample collection
All procedures involving the use of animals were conducted in accordance with the national guidelines for agricultural animal care and were approved by the Animal Ethics Committee, University of Warmia and Mazury in Olsztyn, Poland.
In this study, CLs from the same cross-bred gilts (Polish Landrace × Duroc) as described previously by Blitek et al. (2010a) were used. Prepubertal gilts of similar age (165–175 d old), weight (100–110 kg) and genetic background were assigned randomly into two groups.
Effect of estrus induction on the number of ovulations
The number of ovulations, assessed by the total number of CLs formed on both ovaries, did not differ between control and gonadotropin-treated animals (P = 0.47). The number of ovulations measured on each day of pregnancy studied in the control and PMSG/hCG-stimulated animals, respectively, were as follows: 11.2 ± 0.6 vs. 9.3 ± 1.0 on day 10, 11.6 ± 0.6 vs. 11.8 ± 2.0 on day 12, and 11.6 ± 0.9 vs. 11.2 ± 1.8 on day 15.
Effect of estrus induction on StAR, CYP11A1 and 3βHSD mRNA expression in the CL
StAR mRNA expression was affected by the day (P = 0.004) and by the treatment of gilts with
Discussion
Hormonal induction of estrus has become a standard procedure to advance puberty in gilts. However, administration of PMSG/hCG can impact both ovarian (Bogacki et al., 2014, Wiesak et al., 1990) and endometrial (Blitek et al., 2010a, Blitek et al., 2010b; Kiewisz et al., 2011) development. Previously, we demonstrated that the expression of several genes described as “receptivity markers” was decreased in the endometrium of PMSG/hCG-treated gilts compared with naturally-ovulated animals (Blitek
Conclusions
Estrus induction with PMSG/hCG in premature gilts impaired the luteal P4 synthesis system by decreasing expression of StAR, CYP11A1 and 3βHSD mRNA and/or protein and resulted in lower concentrations of P4 in both luteal tissue and blood serum. On the other hand, greater expression of LHR and ESR1 mRNA in CLs of gonadotropin-treated animals indicates the existence of a local mechanism that may support P4 synthesis by increasing the number of binding sites for luteotropic factors.
Conflict of interest
None of the authors have any conflict of interest to declare.
Acknowledgements
The authors are grateful to K. Gromadzka-Hliwa and J. Klos for technical assistance and M. Blitek for help in care and handling of animals. This research was supported by National Science Centre (N311 04732/2777) and basic grant of Polish Academy of Sciences.
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