Effects of a shortened preovulatory follicular phase on genital blood flow and endometrial hormone receptor concentrations in Holstein-Friesian cows

The goal of this study was to determine the effects of the length of the preovulatory phase on genital blood flow and mRNA expression of endometrial hormone receptors in cattle (Bos Taurus). Ovulation was synchronized in 50 Holstein-Friesian cows using a modified Ovsynch (ovulation synchronization) protocol, in which the second gonadotropin-releasing hormone (GnRH) administration was given 40 h (G40, n = 17) or 60 h (G60, n = 16) after the prostaglandin F2α (PGF2α) administration. The third group (S, n = 17) did not receive a second GnRH administration. Transrectal color Doppler examinations were carried out 24 h before (Day –1) and on Day 7 after ovulation (Day 0). Follicular size (FS) and luteal size (CLS) were quantified by measuring the areas of these structures on cross-sectional B-mode ultrasound images. Follicular blood flow (FB) and luteal blood flow (CLB) were quantified by determining the colored areas of these structures. Uterine blood flow was measured using the time-averaged maximum velocities (TAMVs) and the pulsatility indices (PIs) of both uterine arteries. Endometrial mRNA transcript abundance of estrogen receptors α and β as well as oxytocin and progesterone receptor were determined on Days –1 and 7 in G40 and G60 cows. In all cows, plasma progesterone (P4) values were measured on Day 7. On Day –1, FS and FB values were lower (P ≤ 0.05) in G40 cows compared with those in S cows but did not differ (P > 0.05) between G60 cows on the one hand and G40 cows and S cows, respectively, on the other hand. On Day 7, CLS and P4 did not differ (P > 0.05) between cows of Groups G40, G60, and S; CLB was lower (P ≤ 0.05) in G40 cows than in G60 and S cows, but no difference (P > 0.05) occurred between G60 and S cows. The uterine TAMV and PI values did not differ among the three groups (P > 0.05). Gene expression of hormone receptors did not differ (P > 0.05) between Groups G40 and G60 on Days –1 and 7. Results of this study indicate that a shortened preovulatory follicle phase primarily affects ovarian but not the measured uterine events in cows undergoing synchronization of ovulation.