Molecular control of early luteolysis in Nelore (Bos indicus) cows supplemented with injectable long-acting progesterone during the early luteal phase

Long-acting injectable (i) progesterone (P4) supplementation at the onset of diestrus increases conception rate in beef cattle. However, earlier onset of functional and structural regression of the corpus luteum (CL) has been observed, resulting in pregnancy loss in 30 to 50% of animals subjected to this treatment. Therefore, we tested the hypothesis that iP4-induced premature luteolysis is associated with earlier decrease in the abundance of endometrial P4 receptor (PGR) and an earlier rise in the abundance of oxytocin receptors (OXTR). Cycling, multiparous, nonlactating Nelore cows (n = 24) were selected after detection a CL in at least one of two consecutive exams, conducted 12 days apart. Cows received a P4-releasing device (Sincrogest®, OuroFino Saude Animal), 2 mg of estradiol benzoate i.m. (Sincrodiol®, Ouro Fino Saude Animal) and PGF2α treatment (500 µg sodium cloprostenol; Sincrocio®, Ouro Fino Saude Animal). After 8 days an additional PGF2α treatment was performed. The day of ovulation was defined as Day 0. According to pre-ovulatory follicle size, cows that ovulated were assigned to receive no treatment (Control group; n = 12) or 300 mg of iP4 on Day 3 post-ovulation (Sincrogest®, Ouro Fino Saude Animal, Brazil; iP4 group; n = 12). Endometrial tissue was collected transcervically from every cow using a cytological brush on days 3, 5, 7, 9, 11, 13 and 16, immediately submerged in 1 mL Trizol reagent and stored at - 80°C. Color Doppler ovarian ultrasonography was conducted every 12 h from device removal to verify ovulation and daily from Day 3 to 25 for evaluation of development and regression of the CL. The day of structural luteolysis was defined as the day when CL area (cm2 ) decreased by 25% of the average CL area measured on Days 7 and 8. Data were examined for normality using Shapiro-Wilk test. Data that were not normally distributed were transformed to natural logarithms, square or ranks. The SAS PROC MIXED procedure (version 9.2; SAS Institute, Cary, NC, USA) was used for analysis. Fisher`s exact test distribution was used for comparisons of frequency data. Data are presented as the mean ± SEM, unless otherwise indicated. Structural luteolysis occurred earlier (P = 0.03) in the iP4 group (16.4 ± 0.9 d) compared to the control group (18.8 ± 0.7 d). The frequency of cows that began functional luteolysis before Day 17 was greater (P = 0.03) in the iP4 group (50.0%) than in the control group (8.3%). There was a group by time interaction (P < 0.05) for abundance of the OXTR gene. Cows in the iP4 group that presented earlier onset of luteolysis had a greater abundance of OXTR on Day 16 then the previous days while cows of P4 group that did not have early luteolysis showed similar abundance of OXTR across the experiment. There was a group by time interaction (P < 0.10) for abundance of the PGR transcript. Cows in the iP4 group that presented earlier onset of luteolysis had greater abundance of PGR on Day 16 compared to Day 13, while cows of iP4 group that did not have early luteolysis showed greatest abundance of PGR on Day 5 that decreased on Day 7 and then remained low across time. In conclusion, iP4-induced early luteolysis is associated with an early rise in the abundance of endometrial OXTR and PGR. We speculate that this molecular event is associated with earlier release of endometrial PGF2α pulses, involved in CL regression.