Concentration of progesterone during the development of the ovulatory follicle: II. Ovarian and uterine responses.

Two experiments evaluated the influence of altering the concentrations of progesterone during the development of the ovulatory follicle on the composition of the follicular fluid, circulating LH and PGF(2α) metabolite (PGFM), and expression of endometrial progesterone receptor and estrogen receptor-α. In both experiments, the estrous cycles were presynchronized (GnRH and progesterone insert followed by insert removal and PGF(2α) 7 d later, and GnRH after 48 h) and cows were then enrolled in 1 of 2 treatments 7 d later (study d -16): high progesterone (HP) or low progesterone (LP). In experiment 1 (n=19), cows had their estrous cycle synchronized starting on study d -9 (GnRH and progesterone insert on d -9, and insert removal and PGF(2α) on d -2). In experiment 2 (n=25), cows were submitted to the same synchronization protocol as in experiment 1, but had ovulation induced with GnRH on study d 0. In experiment 1, plasma was sampled on d -4 and analyzed for concentrations of LH; the dominant follicle was aspirated on d 0 and the fluid analyzed for concentrations of progesterone, estradiol, and free and total IGF-1. In experiment 2, follicular development and concentrations of progesterone and estradiol in plasma were evaluated until study d 16. Uterine biopsies were collected on d 12 and 16 for progesterone receptor and estrogen receptor-α protein abundance. An estradiol/oxytocin challenge for PGFM measurements in plasma was performed on d 16. In experiments 1 and 2, LP cows had lower plasma concentrations of progesterone and greater concentrations of estradiol, and had larger ovulatory follicle diameter (20.4 vs. 17.2mm) at the end of the synchronization protocol than HP cows. Concentration of LH tended to be greater for LP than HP cows (0.98 vs. 0.84 ng/mL). The dominant follicle of LP cows had greater concentration of estradiol (387.5 vs. 330.9 ng/mL) and a lower concentration of total IGF-1 (40.9 vs. 51.7 ng/mL) than that of HP cows. In experiment 2, estradiol and progesterone concentrations did not differ between treatments from d 0 to 16; however, the proportion of cows with a short luteal phase tended to increase in LP than HP (25 vs. 0%). Concentrations of PGFM were greater for LP than HP. Uterine biopsies had a greater abundance of progesterone receptor, and tended to have less estrogen receptor-α abundance on d 12 compared with d 16. An interaction between treatment and day of collection was detected for estrogen receptor-α because of an earlier increase in protein abundance on d 12. Reduced concentrations of progesterone during the development of the ovulatory follicle altered follicular dynamics and follicular fluid composition, increased basal LH concentrations, and prematurely increased estrogen receptor-α abundance and exacerbated PGF(2α) release in the subsequent estrous cycle.