The absence of corpus luteum formation alters the endocrine profile and affects follicular development during the first follicular wave in cattle

We previously established a bovine experimental model showing that the corpus luteum (CL) does not appear following aspiration of the preovulatory follicle before the onset of LH surge. Using this model, the present study aimed to determine the profile of follicular development and the endocrinological environment in the absence of CL with variable nadir circulating progesterone (P₄) concentrations during the oestrous cycle in cattle. Luteolysis was induced in beifers and cows and they were assigned either to have the dominant follicle aspirated (CL-absent) or ovulation induced (CL-present). Ultrasound scanning to observe the diameter of each follicle and blood collection was performed from the day of follicular aspiration or ovulation and continued for 6 days. The CL-absent cattle maintained nadir circulating P₄ throughout the experimental period and showed a similar diameter between the largest and second largest follicle, resulting in co-dominant follicles. Oestradiol (E₂) concentrations were greater in the CL-absent cows than in the CL-present cows at day - 1, day 1 and day 2 from follicular deviation. The CL-absent cows had a higher basal concentration, area under the curve (AUC), pulse amplitude and pulse frequency of LH than the CL-present cows. After follicular deviation, the CL-absent cows showed a greater basal concentration, AUC and pulse amplitude of growth hormone (GH) than the CL-present cows. These results suggest that the absence of CL accompanying nadir circulating P₄ induces an enhancement of LH pulses, which involves the growth of the co-dominant follicles. Our results also suggest that circulating levels Of P₄ and E₂ affect pulsatile GH secretion in cattle.