Vasodilation of the female reproductive vasculature is essential to supply the increasing demands associated with reproductive processes; including maintenance of a healthy functional luteal structure, and the establishment and continuance of pregnancy. Due to its vasodilatory effects, prostaglandin E₂ has been identified as a potential candidate sustaining such vascular effects. Here, the relationship between systemic concentrations of prostaglandin E₂ metabolite (PGEM) and progesterone (P₄), and uterine arterial vasodilatory changes were evaluated during the oestrous cycle and early pregnancy. Synchronised Angus cows (n = 19) were artificially inseminated (AI) and examined for pregnancy 45 d post-AI. Jugular blood samples were collected on Days 0, 3, 6, 10, 16, 20, 25, and 32 (Day 0 = AI) for serum PGEM and P₄ evaluation. Vasodilatory changes of the uterine arteries were also characterised using the B-mode ultrasound luminal circumference (UAC). Retrospective comparisons were carried out between pregnant (n = 10) and nonpregnant (n = 9) animals. The GLM Procedure of SAS (SAS Institute Inc., Cary, NC, USA) was used to evaluate the effects of pregnancy status and sampling day over PGEM, UAC, and P₄. Pearson correlations between PGEM, UAC, and P₄ were obtained by the CORR Procedure of SAS. All variables were measured in triplicate and thresholds for significance and tendency were fixed at P ≤ 0.05 and 0.06 to 0.10, respectively. From Day 0 to Day 20, the overall PGEM concentration tended to be higher in nonpregnant cows compared to pregnant ones (P = 0.08; 90.97 ± 4.38 v. 78.28 ± 3.01 pg mL^–1, respectively), while a shift in favor of pregnant cows was observed at Day 32 (P = 0.29; 89.88 ± 14.23 v. 106.3 ± 7.17 pg mL^–1, respectively). Both uterine arteries showed similar UAC (P ≥ 0.05); therefore, they were combined in one average value per animal each sampling day. There was an overall increase of the UAC over time, from 10.49 ± 0.01 mm at Day 0 to 12.17 ± 0.02 mm at Day 20 (P ≤ 0.0001), but no significant differences were found between pregnant and nonpregnant cows during this period (P ≥ 0.05). An overall sharp increase in P₄ concentrations was observed from Day 3 to Day 16 (0.60 ± 0.12 to 9.66 ± 0.45 ng mL^–1, P ≤ 0.0001); while from Day 16 to Day 32, pregnant cows maintained steady and higher P₄ concentrations than nonpregnant ones (9.87 ± 0.31 v. 4.40 ± 1.09 ng mL^–1, respectively; P = 0.002). As expected, nonpregnant cows showed a sharp decrease in P₄ concentrations from Day 16 to Day 25, followed by a second increase at Day 32 (9.27 ± 0.20, 0.85 ± 0.33, and 6.04 ± 1.67 ng mL^–1, respectively; P = 0.0007). The UAC and PGEM concentrations were positively correlated in pregnant (r = 0.48; P = 0.002), but not in nonpregnant animals (r = 0.29; P = 0.21). Similarly, UAC and P₄ concentrations were also positively associated in pregnant (r = 0.47; P = 0.002), but not in nonpregnant cows (r = 0.01; P = 0.96). Our findings support the role of PGE₂ as a potential mediator of the vascular function in the female reproductive tract and, subsequently, as an essential factor to ensure adequate P₄ production able to sustain early pregnancy in the bovine.

USDA-ARS Grant#58-6402-3-0120.