Reduction in Circulating Estradiol by Ablation of Follicles Delays Luteolysis in Heifers.

Abstract

Circulating estradiol (E2) and progesterone (P4) may regulate timing of prostaglandin F2α (PGF2α) release from the uterus. We hypothesized that removal of circulating E2 by ablating follicles >4 mm would therefore delay luteolysis. On Day 9 (Day 0 = ovulation), Holstein heifers were randomized by replicate into three groups. Follicle ablations (all follicles ≥ 4 mm ablated) were done either once on Day 9 (FA9; n=6), on Days 9, 11, 13 and 15 (FA9-15; n=6), or on Days 9, 11, 13, 15, 17, 19 and 21 (FA9-21; n=7). A duplex B-mode and pulsed-wave color-Doppler ultrasound scanner equipped with a linear array 7.5 MHz transrectal transducer was used to estimate corpus luteum (CL) blood flow, based on percentage of the CL with color-Doppler signals during real time scanning. Blood samples were collected daily and plasma assayed by RIA for P4 and E2 concentrations. The CL blood-flow and hormone data were analyzed by SAS MIXED procedure with a REPEATED statement for main effects of group and day and their interaction. Both main effects and the interaction were significant (P<0.01) for P4, E2, and CL blood flow. Mean P4 concentrations first began to decrease (P<0.02) on Day 14 in FA9 and were lower (P<0.05) than in the other two groups on Days 15 and 16. In FA9-15, P4 first began to decline on Day 16 (P<0.01) and was lower (P<0.05) than in FA9-21 on Days 17, 18 and 19. In FA9-21, the first P4 decrease (P<0.01) occurred on Day 19, and there was no significant difference among groups after Day 19. For CL blood flow, the first decrease (P<0.05) began in FA9 on Day 15 and the values were lower (P<0.05) than in FA9-15 on Day 16. In FA9-15, the first decrease (P<0.05) in blood flow occurred on Day 17 and the values were lower (P<0.05) from Day 17 to Day 21 than in FA9-21. The first decrease (P<0.05) in CL blood flow for FA9-21 occurred on Day 19. In FA9, the E2 concentration began to increase (P<0.01) on Day 13 and the values were higher (P<0.05) from Day 14 to Day 17 than in FA9-15 and FA9-21. In FA9-15, the first increase (P<0.01) in E2 occurred on Day 16, whereas in FA9-21 the first increase (P<0.04) occurred on Day 22. Follicular ablation delayed the rise in circulating E2 with earlier (P<0.002) mean peak E2 concentrations in FA9 (17.6±0.7 days after ovulation) than in FA9-15 (20.3±0.3 d) and FA9-21 (24.9±0.3 d). In FA9 and FA9-15 luteolysis did not begin until after the last ablation and 1 day after the first continuous increase in E2 plasma concentration, but in FA9-21 luteolysis began 3 days before the last ablation and first continuous increase in E2 concentration. The first significant decreases in blood flow occurred one day after the first significant decrease in P4 (FA9) or both occurred on the same day (FA9-15, FA9-21). Sequential ablations of all follicles (≥4 mm) delayed both functional (decreased P4) and structural (decreased CL blood flow) indicators of luteolysis. Thus, normal follicle growth was essential for normal timing of luteolysis in heifers. It seems likely that the delay in luteolysis caused by follicular ablation was due to reduced circulating E2, but luteolysis occurred before a detectable increase in plasma E2 concentration when the CL was maintained for long period as in the FA9-21 group.