125 CHANGES IN INTRAFOLLICULAR CONCENTRATIONS OF FREE INSULIN-LIKE GROWTH FACTOR-1, ACTIVIN-A, INHIBIN-A, VASCULAR ENDOTHELIAL GROWTH FACTOR, ESTRADIOL, AND PRL BEFORE OVULATION IN MARES

Abstract

Intrafollicular factors and hormones during the process of follicle deviation have been studied extensively in mares. However, knowledge is still lacking concerning the dominance and preovulatory (postdeviation) phases of the oestrous cycle. Furthermore, information about intrafollicular growth factors and hormones during postdeviation growth is critical for understanding the ovulatory and post-ovulatory physiological mechanisms that have important in vivo and in vitro applications, such as oocyte maturation, ovulation, and embryo development and survival. Changes in intrafollicular growth factors and hormones were evaluated in vivo in postdeviation and impending ovulation follicles. Hormone and growth factor concentrations were determined by ELISA or RIA. Mares (n = 30) were randomly assigned to 5 experimental groups based on target diameters of 25, 30, 35, 40 mm, and impending signs of ovulation. Data were analysed by one-way ANOVA, paired-test, and Pearson correlation. Data belonging to 2 or more proximal diameter groups that were not different were combined and regrouped for each factor separately. When the data were regrouped (all results shown below), follicular fluid-free insulin-like growth factor-1 was higher (P < 0.003) in 35-mm follicles than in the 25- to 30-mm and 40-mm impending ovulation follicle groups. The 25- to 30-mm follicle group had the lowest insulin-like growth factor-1 concentration. However, concentrations of insulin-like growth factor binding protein-2 in follicular fluid did not differ (P > 0.05) among groups. Additionally, follicular fluid activin-A tended (P < 0.06) to be higher in impending ovulation follicles when compared to the 25- to 40-mm follicle group. Concentrations of intrafollicular oestradiol were higher (P < 0.0001) in 40-mm impending ovulation follicles than in the other follicle groups. Follicular fluid concentrations of inhibin-A and vascular endothelial growth factor were lower (P < 0.02–0.002) in the 40-mm impending ovulation follicle group when compared to the 25- to 35-mm follicle group. Systemic and intrafollicular PRL were lower (P < 0.05) in the impending ovulation group when compared to the 25- to 40-mm follicle group. PRL concentrations were higher (P < 0.05) in the follicular fluid than in the plasma. The novel findings of this study, a decrease in intrafollicular free insulin-like growth factor-1, inhibin-A, vascular endothelial growth factor, and PRL during the final stages of follicular growth, document for the first time the occurrence of dynamic changes among intrafollicular factors and hormones during the stages of follicle dominance and as ovulation approaches. Overall, the elucidation of the role of follicular fluid factors and hormones and their interactions need to be studied further to allow a better understanding of the ovulatory process in mares. This knowledge will potentially assist in obtaining better outcomes during the use of assisted reproductive technologies and will also help to understand mechanisms of ovulatory dysfunctions in mares and perhaps in other species.