Auto-paracrine action of prostaglandins E2 and F2α in equine corpus luteum

Abstract

The corpus luteum (CL) is a transient endocrine organ that mainly secretes P4, a determinant hormone for implantation. In the mare, gestation depends not only on P4 action, but also on other luteotrophic factors. Prostaglandins (PG) play a critical role in several reproductive events, i.e. ovulation, luteolysis, fertilization and implantation. Prostaglandin E2 is known as a luteotrophic factor, since it stimulates in vitro P4 production by luteal steroidogenic cells in different species. Conversely, PGF2a is known as the main luteolytic factor, which is primarily secreted by the endometrium. Oxytocin (OXT) is suggested as amain inductor of pulsatile PGF2a output from the uterus during luteolysis in the mare. However, the mechanisms regulating PGs secretion and their action in the equine CL are still unknown. Thus, the aim of this study was to determine: (i) the concentration of PGs and mRNA transcription of PG synthesis enzymes in the equine CL, throughout the luteal phase; and (ii) the putative actions of PG on CL secretory function in the mid-luteal phase. In experiment 1, CLs were obtained after slaughter from early, mid-, and late CL luteal phases (n1⁄46/stage). Luteal structures were classified based on plasma P4 levels, follicle size, and the morphological appearance of the CL. Relative quantification (qPCR) of mRNA transcription from prostaglandin-endoperoxide synthase 2 (PTGS-2), synthase prostaglandin E (PGES) and F2a (PGFS) were performed. Additionally, PGE2 and PGF2a were extracted from luteal tissue and its concentration determined by ELISA method. In experiment 2, mid-CL explants (n1⁄45) were cultured for 24h with PGE2 and PGF2a (both 10-7M). Concentration of P4, PGF2a, and OXT in culture media was determined by ELISA and qPCR for 3b-hydroxysteroid dehydrogenase (3bHSD), steroidogenic acute regulatory protein (StAR), PTGS-2, PGES, PGFS, peptidyl-glycine-alpha-amidase (PGA), and OXT receptor (OXTR) were performed. In experiment 1, both PGs presented the lowest concentration level in the mid-CL, with respect to early and late CL (P<0.05), being the highest PGs concentration seen in the early CL (P<0.05). RegardingmRNA transcription, PTGS-2 and PGES presented the highest level in the early CL (P<0.01) and decreased in the subsequent stages of the luteal phase. The PGFS showed the highest expression in the mid-CL (P<0.05). In experiment 2, both PGE2 and PGF2a stimulated P4 and OXT secretion, as well as the mRNA transcription of PTGS-2, PGES, OXTR, 3bHSD (P<0.05), but not StAR and PGFS. In addition, PGE2 inhibit PGF2a secretion and increased mRNA level of PGA (P<0.05). The present results confirm that PGs are actively synthesized in the equine CL and may be involved in its auto-, paracrine regulation. Generally, PGE2 acted as a luteotrophic factor, increasing in vitro P4 secretion and also modulating OXT output in mid-CL in the mare. Further studies are needed to better understand PGs actions in CL.