Role of follicular estradiol-17beta in timing of luteolysis in heifers

Abstract

The hypothesis was tested that estradiol (E2) from the ovarian follicles controls time of luteolysis. Time of luteolysis was evaluated by multiple measures of corpus luteum (CL) structure (area, volume) and function (progesterone [P4], luteal blood flow). The hypothesis for experiment 1 was that repeated ablation of follicles would reduce circulating E2 and delay luteolysis. Heifers were randomly assigned on Day 9 (Day 0 = ovulation) to three groups. All follicles >or=4 mm were ablated on Day 9 (group FA9; n = 6); Days 9-15 (group FA15; n = 6); or Days 9-21 (group FA21; n = 7). As expected, follicular ablation delayed (P < 0.001) the rise in circulating E2 and peak E2 concentrations (FA9, Day 17.6 +/- 0.7; FA15, Day 20.3 +/- 0.3; FA21, Day 24.9 +/- 0.3). Luteolysis (based on each measure) was delayed (P < 0.005) by repeated ablation of follicles, with earlier luteolysis (based on P4 decrease) in FA9 (Day 15.2 +/- 0.8) than FA15 (Day 16.5 +/- 0.4), and a further delay in FA21 (Day 18.3 +/- 0.5). The hypothesis of experiment 2 was that exogenous treatment with E2 would stimulate prostaglandin F(2alpha) (PGF) secretion and prevent the delay in luteolysis associated with follicular ablations. Follicles >or=4 mm were ablated from Day 9 to Day 17 (n = 15). Heifers were treated on Days 13 and 15 with 1.0 mg of estradiol benzoate (FAE2; n = 7) or vehicle (FAV; n = 8). Treatment with E2 induced PGF secretion (detected by PGF metabolite) and induced earlier (P < 0.02) luteolysis in FAE2 than in FAV, whether determined by circulating P4 or by area, volume, or blood flow of CL. In summary, ablation of follicles (>or=4 mm) delayed and treatment with E2 hastened luteolysis in heifers with ablated follicles. Thus, these results are consistent with an essential role for follicle E2 in timing of luteolysis.

FIG. 1.

Experimental procedures used in experiment 1 (upper diagram) and in experiment 2 (lower diagram). FA9, FA15, and FA21 (experiment 1), and FAE2 and FAV (experiment 2) represent the groups in which heifers were allocated to different ablation sessions. Filled circles represent a follicular ablation session, and open circles represent a simulation of the procedure.

FIG. 2.

Mean (±SEM) plasma P4 concentrations (A) and changes in CL blood flow (B) during luteolysis in heifers in which all follicles ≥4 mm were ablated once on Day 9 (FA9; n = 6); ablated every 2 days from Day 9 to Day 15 (FA15; n = 6); or ablated every 2 days from Day 9 to Day 21 (FA21; n = 7). Main effects of group (G), day (D), and their interaction (GD) are shown. Different lowercase letters (a and b) indicate differences (P < 0.05) among groups within a time period. An asterisk (*) indicates the first significant decrease between 2 days.

FIG. 3.

Mean (±SEM) plasma E2 concentrations (A) and plasma FSH concentrations (B) during luteolysis in heifers in which all follicles ≥4 mm were ablated once on Day 9 (FA9; n = 6); ablated every 2 days from Day 9 to Day 15 (FA15; n = 6); or ablated every 2 days from Day 9 to Day 21 (FA21; n = 7). Main effects of group (G), day (D), and their interaction (GD) are shown. Different lowercase letters (a and b) indicate differences (P < 0.05) among groups within a time period. An asterisk (*) indicates the first significant increase in plasma E2 concentration between 2 days.

FIG. 4.

Mean (±SEM) concentrations of plasma E2 (A), P4 (B), and FSH (C) during luteolysis in heifers in which all follicles ≥4 mm were ablated every day from Day 9 to Day 17. Heifers were assigned to receive two i.m. injections of 1.0 mg of estradiol benzoate diluted in sesame oil on Day 13 and Day 15 (FAE2) or the equivalent volume of vehicle (sesame oil; FAV). Main effects of group (G), day (D), and their interaction (GD) are shown.

FIG. 5.

Mean (±SEM) P4 concentrations during luteolysis in heifers in which all follicles ≥4 mm were ablated every day from Day 9 to Day 17. Data were normalized to the first hour that P4 decreased to <1 ng/ml. Heifers were assigned to receive two i.m. injections of 1.0 mg of estradiol benzoate diluted in sesame oil on Day 13 and Day 15 (FAE2) or the equivalent volume of vehicle (sesame oil; FAV). Main effects of group (G), hour (H), and their interaction (GH) are shown.

FIG. 6.

Mean (±SEM) concentrations of PGFM during luteolysis in heifers in which all follicles ≥4 mm were ablated every day from Day 9 to Day 17. Heifers were assigned to receive two i.m. injections of 1.0 mg of estradiol benzoate diluted in sesame oil on Day 13 and Day 15 (FAE2) or the equivalent volume of vehicle (sesame oil; FAV). Main effects of group (G), hour (H), and their interaction (GH) are shown. The arrows indicate the time of E2 treatment.

FIG. 7.

Mean (±SEM) changes in CL blood flow (percent of CL area) as determined by color Doppler ultrasonography during luteolysis in heifers in which all follicles ≥4 mm were ablated every day from Day 9 to Day 17. Heifers were assigned to receive two i.m. injections of 1.0 mg of estradiol benzoate diluted in sesame oil on Day 13 and Day 15 (FAE2) or the equivalent volume of vehicle (sesame oil; FAV). Main effects of group (G), day (D), and their interaction (GD) are shown. An asterisk (*) indicates differences among means within a group.

FIG. 8.

Simplified model to explain the physiology regulating the timing of luteolysis based on the results of the present study and other relevant studies (see Discussion).