63 EQUINE FOLLICLES MODULATE CORTISOL LEVELS AND CAPABILITY OF OOCYTES TO ADAPT TO STRESS SITUATIONS

Abstract

Glucocorticoids are mediators of the systemic stress response. Acute or chronic stress characterised by high cortisol concentrations in the periphery impairs reproductive function in a variety of species and therefore may affect fertility. The ovary has been shown to be a target tissue for glucocorticoids in many species, including the mare. This study hypothesised that the equine ovary possesses internal mechanisms to modulate cortisol activity and that supraphysiologic levels of glucocorticoids do not affect oocyte IVM rates. Light horse mares (n = 9) were used in this study. Growing follicles from an induced follicular wave were divided into the following groups: G1: 5–9 mm, G2: 10–14 mm, G3: 15–19 mm, G4: 20–24 mm, and G5: ≥25 mm. Follicular fluid (FF) and compact cumulus‐oocyte complexes (COCs) were obtained by ultrasound-guided transvaginal aspiration. Blood samples were collected at the beginning and the end of every aspiration session. Cortisol (DE1887, Demeditec, Kiel-Wellsee, Germany), progesterone (ADI-901–011, Enzo Life Sciences, Farmingdale, NY, USA), and corticosteroid binding globulin (CBG, MBS047353, MyBioSource, San Diego, CA, USA) concentrations were determined by ELISA. COCs (n = 80) were randomly distributed to either the control group (DMEM-F12⁺ medium) or the following hydrocortisone treatment groups: 0.1 µg mL^–1, 1 µg mL^–1, 5 µg mL^–1, 10 µg mL^–1. Maturation rate was assessed 30 h after incubation. Statistical analysis was performed with the SPSS Statistics 22 software. Data were analysed using one-way ANOVA, Pearson correlation, and chi-squared test. Cortisol (115.4 ± 13.3 ng mL^–1) and progesterone (22.1 ± 3.1 ng mL^–1) FF concentrations were higher (P < 0.05) in G5 follicles than in all other groups, and were positively correlated (r = 0.8; P < 0.001). Plasma concentrations of cortisol (118.6 ± 7.8 v. 120.3 ± 12.2 ng mL^–1), progesterone (2.4 ± 0.5 v. 2.5 ± 0.4 ng mL^–1), and CBG (11.1 ± 5.1 v. 9.9 ± 3.2 µg mL^–1) did not differ before and after follicle aspiration. However, plasma CBG and progesterone were negatively correlated (r = –0.56; P < 0.01). Maturation rates did not differ among groups, regardless of the hydrocortisone concentration added to the culture medium. Our results demonstrated higher cortisol concentrations in preovulatory follicles in vivo, suggesting its importance for oocyte maturation. The greater unbound cortisol available in the FF of preovulatory follicles can be indicative of the displacement of cortisol from CBG in favour of progesterone. Furthermore, equine oocytes were capable of surviving cortisol concentrations 100 times higher than those physiologically present in preovulatory follicles. This finding suggests the ability of equine oocytes to modulate cortisol levels and adapt to stress situations.