258 ROLE OF PROGESTERONE AND ITS RECEPTORS ON DEVELOPMENTAL COMPETENCE OF OOCYTES IN CATTLE

Abstract

Several studies have demonstrated the importance of progesterone (P4) through its receptors (PR) in the regulation of the ovulatory cycle, but its participation in oocyte maturation in mammals has not yet been clarified. Previous results in our group showed changes in the protein expression of genomic (nPR-A/B) and nongenomic (mPRα/β) PR in bovine cumulus–oocyte complexes (COC) during in vitro maturation (IVM), indicating a possible function for these receptors on bovine oocyte maturation. Therefore, we aimed to study the role of P4 and PR in oocyte developmental competence. Good-quality immature COC were placed in maturation medium [TCM-199 supplemented with 10% (vol/vol) FCS and 10 ng mL^–1 of epidermal growth factor] and cultured at 39°C for 22 h in a humidified atmosphere containing 5% CO₂, in the presence or absence of trilostane (0.001, 0.1, and 10 μM), P4 (50 and 100 ng mL^–1), promegestone (50 and 100 ng mL^–1), RU 486 (0.1, 1, and 10 μM), or antibodies against mPRα or mPRβ. Matured COC were washed and placed in wells containing 250 μL of fertilization medium (25 mM bicarbonate, 22 mM Na-lactate, 1 mM Na-pyruvate, 6 mg mL^–1 fatty acid-free BSA, and 10 mg mL^–1 heparin). In vitro fertilization was performed with 250 μL of frozen–thawed semen at a final concentration of 1 × 10⁶ spermatozoa mL^–1 at 39°C under 5% CO₂ during 20 h. Presumptive zygotes were denuded, washed, and transferred to 25-μL culture droplets (SOF + 5% FCS) at 39°C under 5% CO₂, 90% of N₂, and 5% O₂ atmosphere with maximum humidity. Number of embryos was recorded at day 2, 3, 7, and 8 (Day 0 = day of IVF). Data were analysed using ANOVA analysis using SPSS v. 15.0 software package. Inhibition of P4 production by 10 μM of trilostane during IVM reduced progesterone production in the media, cumulus cells expansion, but had no effect on meiotic resumption in Day 2 and Day 3 embryos; however, there was a significant decrease in the percentage of blastocysts at Day 7 (12 ± 3.5) and Day 8 (14 ± 3.3) compared with the control (30 ± 4.5 and 42 ± 2.5). This reduction on embryo development was overcome by the addition of 50 or 100 ng mL^–1 of P4 at Day 8 (33 ± 5.7 and 32 ± 4.1). The same results were obtained with nonmetabolizable P4, where the reduction on blastocysts with trilostane at Day 8 (20 ± 2.0) was completely overcome by 50 or 100 ng mL^–1 of promegestone (41 ± 5.1 and 40 ± 6.7). Specific inhibition nPR-A/B with 10 μM of RU 486 produced a significant reduction on blastocysts at Day 7 (24 ± 4.3) and Day 8 (29 ± 5.5) compared with the control (44 ± 2.6 and 47 ± 3.0). Inhibition of mPRα reduced cleavage rate and Day 3 embryos, whereas the inhibition of mPRβ had no effect on meiotic resumption or embryo development. In conclusion, the intracellular signaling of P4 on developmental competence of oocytes in cattle seems to be mainly mediated by nPR-A/B receptors and could be associated with cytoplasmic maturation.