Follicular fluid and plasma concentrations of progesterone following hCG administration before ultrasound-guided ablation of dominant follicles ≥25mm in mares

Ultrasound-guided transvaginal follicle ablation of dominant follicles ≥25mm during the follicular phase results in luteinization and an increase in circulating concentrations of progesterone (Ignácio et al., Theriogenology. 2021; 160:128-133). Although LH receptors are present in early antral follicles of 5mm diameter (Goudet et al., Biol Reprod. 1999; 60:1120-7) and increase after deviation (Bastos et al., Theriogenology. 2014; 82:1131-6), the ablation-induced luteinization rate may be less for smaller-sized dominant follicles <30mm and progesterone may not achieve concentrations >2ng/mL in a timely manner (Mozzaquatro et al., Reprod Domest Anim. 2012; 47:288-92). The objective of the present study was to determine if hCG administered 24h before ablation of dominant follicles <30 or ≥30mm increases the luteinization rate and results in progesterone concentrations >2ng/mL to support pregnancy. Cycling mares were randomly assigned to four ablation groups: 1) ≥25 (ablation of follicles 25-29mm, n=9), 2) hCG+≥25 (2500 IU hCG, i.v. 24h before ablation of follicles 25-29mm, n=11), 3) ≥30 (ablation of follicles ≥30mm, n=13), and 4) hCG+≥30mm (2500 IU hCG, i.v. 24h before ablation of follicles ≥30mm, n=10). Subsequent to PGF2α – induced luteolysis in all mares and administration of hCG to respective groups, D0 marked the day whenthe dominant follicle reached 25-29mm or ≥30mm and follicular fluid was collected. Blood samples were collected daily until D15, fresh embryos were transferred when plasma plasma progesterone (PP4) was >2ng/mL on D7. Follicular fluid progesterone (FFP4) and PP4 were measured by a commercial RIA kit. Wilcoxon test was used to evaluate FFP4. Repeated measures models and Tuckey test were used to evaluate PP4. Regardless of follicle size on D0, mean FFP4 but not PP4 was higher (P<0.05) in mares that received hCG (115.0ng/mL) vs those that did not (53.0ng/mL). Based on PP4 >2ng/mL, luteinization rate was 43 vs 75% in the ≥25 vs hCG+≥25 group (P<0.05) and 77 vs 67% in the ≥30 vs hCG+≥30 group (P>0.05). From D0, daily PP4 gradually increased and on D7 mean concentrations did not differ among the ≥25, hCG+≥25, ≥30, and hCG+≥30 groups, respectively. Regardless of follicle size, embryo transfer resulted in pregnancy rates of 43% (3/7) for the follicle ablation alone and 43% (6/14) for the hCG + ablation groups. In conclusion, administration of hCG before ablation of follicles 25-29mm but not ≥30mm enhanced the luteinization rate with PP4 >2ng/mL at D7 among groups. This was sufficient to support pregnancy after embryo transfer.