Temporal relationships of a pulse of prolactin (PRL) to a pulse of a metabolite of PGF2α in mares

Concentrations of progesterone and a PGF2α metabolite during the interovulatory interval compared to the corresponding days of pregnancy in mares

The concentrations of progesterone (P4) and a metabolite of PGF2α (PGFM) in mares were compared between the interovulatory interval (IOI; n = 8) and the corresponding days of pregnancy (n = 9). In daily blood samples, P4 increased between the day of ovulation (Day 0) and ∼Day 6 and then gradually decreased until the beginning of luteolysis in the IOI group. Before the beginning of luteolysis, there were no significant differences in P4 concentrations between the IOI and early pregnancy. In the IOI, PGFM concentration on the day before the beginning of luteolysis began to increase (P < 0.04) and reached a maximum mean (42.9 ± 11.6 pg/mL) on Day 14. In pregnancy, a novel increase in PGFM occurred from Day 12 to a maximum mean on Day 15 (16.7 ± 3.1 pg/mL). Daily PGFM concentrations were not different between the two groups until the increase just before luteolysis in the IOI. During 8-h sessions of hourly blood sampling, the mean and maximum PGFM concentrations were significantly greater in IOI than in pregnancy for each 8-h session on Days 13, 14, and 15. The minimum was not different between groups on any day. Pulses of PGFM were identified by coefficient of variation during the hourly 8-h sessions on day-sets of Days 4-7, 9-11, and 13-16. Despite the PGFM increase in daily samples between Days 12 and 15 of pregnancy, the amplitude and peaks of CV-identified pulses did not differ in the pregnant mares among the three day-sets. The pulses were similarly small for day-sets 4-7 and 9-11 in the IOI and for all day-sets in pregnancy (eg, amplitude on Days 13-16: 43.4 ± 15.6 pg/mL vs 5.4 ± 1.1 pg/mL for IOI vs pregnancy). Hypothesis 1 was not supported that daily PGFM concentrations in an IOI increase at the intersection between the end of the rapid P4 increase and the gradual P4 decrease. Hypothesis 2 was supported that pregnant mares have low amplitude PGFM pulses during the days of the high amplitude pulses at luteolysis in the IOI.

Endogenous and exogenous effects of PGF2α during luteolysis in mares

An inhibitor of PGF2α biosynthesis (flunixin meglumine, FM) was used to study the role of endogenous PGF2α on the luteolytic effect of exogenous PGF2α in mares. A 2-h infusion of PGF2α at a constant rate (total dose, 0.1 mg) on Day 10 (ovulation = Day 0) was used to mimic the maximal concentrations of a spontaneous pulse of a PGF2α metabolite (PGFM). Treatment with FM (1.7 mg/kg) was done 1 h before and 5 h after the start of PGF2α infusion. In hourly blood samples beginning 1 h before the start of PGF2α infusion, progesterone decreased (P < 0.05) similarly by 5 h in each of the PGF2α and PGF2α+FM groups but not in the controls (n = 5). In a study of spontaneous luteolysis, the same FM dose was given every 6 h from Day 13 until Day 17 or earlier if CL regression was indicated by an 80% decrease in luteal blood-flow signals. Blood was sampled for progesterone assay each day and 8 h of hourly blood sampling was done each day to characterize PGFM concentrations and pulses. Progesterone (P4) was lower (P < 0.05) in controls than in an FM group (n = 7) by Day 15. Luteolysis (P4 < 1 ng/mL) ended on Days 14-19 in individual controls. In contrast, luteolysis did not end until after Day 20 in 4 of 7 FM-treated mares. In the three mares with completion of luteolysis before Day 20 in the FM group, the interval from beginning to end of luteolysis was longer (P < 0.02) (4.5 ± 0.6 days) than in the controls (3.0 ± 0.4 days). During 8-h sessions of hourly blood sampling on Day 14, concentration of PGFM was significantly lower in the FM group for the minimal, mean, and maximal per session. Pulses of PGFM were identified by a CV methodology on each day in 7 of 7 and 3 of 7 mares in the controls and FM group, respectively. The four FM-treated mares without a CV-identified pulse were the four mares in which luteolysis did not occur before Day 20. In mares with detected pulses, PGFM was lower at each nadir and at the peak (86% lower) in the FM group than in controls, but the interval between nadirs or base of a pulse was not different between groups. Hypothesis 1 that endogenous PGF plays a role in the luteolytic effect of exogenous PGF2α was not supported. Hypothesis 2 that an inhibitor of PGF2α biosynthesis prevented or minimized the prominence of PGFM pulses and increased the frequency of persistent CL was supported.

Hemodynamics of the corpus luteum in mares during experimentally impaired luteogenesis and partial luteolysis

The aim of the current project was to characterize the luteal vascularity and the plasma concentrations of progesterone (P4), prolactin (PRL) and 13,14-dihydro-15-keto-PGF_2α (PGFM) in mares with luteal disturbances during early and mid-diestrus. In Experiment 1, twenty-one mares were treated with 2 mL of 0.9% NaCl, or 1 mg Dinoprost, or 10 mg Dinoprost on day two after ovulation (Control-D2, 1/10PGF-D2 and PGF-D2 groups, respectively; n = 7 mares/group). In Experiment 2, similar treatments were performed eight days post-ovulation using a different cohort of 21 mares (Control-D8, 1/10PGF-D8 and PGF-D8 groups, respectively; n = 7 mares/group). Blood samples were collected hourly and power-Doppler examinations of the corpus luteum (CL) were performed every 6 h from H0 (moment immediately before treatment) to H48. Data collection was also done once a day from D0 (day of ovulation) to D20. In Experiment 1, the PGF-D2 and 1/10PGF-D2 groups had lower increase of plasma concentration of P4 until H48 and reduced maximum P4 concentrations on D8-D11 than mares from the Control-D2 group. However, no differences among groups were detected for luteal vascularity during early and mid-diestrus. In Experiment 2, complete and partial luteolysis were detected in mares from the PGF-D8 and 1/10PGF-D8 groups, respectively. Luteal vascularity and plasma P4 concentrations differed among Control-D8, PGF-D8 and 1/10PGF-D8 groups on H48. Partially regressed CLs (1/10PGF-D8 group) generated more Doppler signals than completed regressed CLs (PGF-D8 group) between D10 and D13. In both experiments, a transient increase in PRL activity was observed in parallel to the PGFM pulse in mares receiving 1 or 10 mg Dinoprost. The use of prostaglandin on D2 at conventional or 1/10 of the dose impaired the luteal development in mares. Moreover, the low dose of prostaglandin lead to partial regression of mature CLs. The blood supply was reduced in partially regressed CLs, but not in CLs undergoing impaired luteogenesis.

Concentrations of progesterone, a metabolite of PGF2α, prolactin, and luteinizing hormone during development of idiopathic persistent corpus luteum in mares

In experiment 1, daily blood samples were available from Days 0 to 20 (Day 0 = ovulation) in mares with an interovulatory interval (IOI, n = 5) and in mares that developed idiopathic persistent corpus luteum (PCL, n = 5). The PCL was confirmed by maintenance of progesterone (P4) concentration until end of the experiment (Day 20). Significant interactions of group and day revealed the novel findings that luteinizing hormone (LH) was lower (P < 0.05) in the PCL group than that in the IOI group on Days 0 to 4, and prolactin was lower (P < 0.05) on Days 1, 4, 6, and 7. In experiment 2, treatment with a gonadotropin-releasing hormone antagonist (n = 6) significantly reduced LH on Days 1 to 6 compared with the controls (n = 6) but did not support the hypothesis that low LH during the postovulatory period increases the frequency of PCL. In experiment 3, P4, PGFM (a PGF2α metabolite), and prolactin concentrations on Days 12 to 20 from 2 reported experiments were combined to increase the number of mares with an IOI (n = 11) or a PCL (n = 11). An abrupt and complete decrease in P4 (luteolysis) began on Day 13 in the IOI group compared with a gradual and partial P4 decline after Day 12 in the PCL group. Concentrations of PGFM and prolactin were lower (P < 0.05) in the PCL group than those in the IOI group on the day at the end of the most pronounced decrease in P4. The PCL mares were subgrouped into those with an abrupt but incomplete P4 decrease (partial luteolysis; n = 5) at the expected time and those without partial luteolysis (n = 6). There were no significant differences between the 2 subgroups in concentrations of PGFM and prolactin, but on a tentative basis (P < 0.10), the concentration of PGFM seemed more focused on the day of the most pronounced decrease in P4 in the subgroup with partial luteolysis. Results for PCL compared with IOI indicated (1) postovulatory LH and prolactin were lower, (2) treatment to reduce postovulatory LH did not increase the incidence, and (3) both PGFM and prolactin were lower on the day of the most pronounced decrease in P4.

Changes in intrafollicular concentrations of free IGF-1, activin A, inhibin A, VEGF, estradiol, and prolactin before ovulation in mares

Changes in intrafollicular growth factors and hormones were evaluated in vivo in postdeviation and impending ovulation follicles. Mares (n = 30) were randomly assigned to five experimental groups based on target diameters of 25, 30, 35, 40 mm, and impending signs of ovulation. Furthermore, data belonging to two or more proximal diameter groups that were not different were combined and regrouped for each factor separately. Follicular fluid-free insulin-like growth factor 1 was highest (P < 0.003) in 35-mm follicles, followed by the 40-mm and impending ovulation follicle group, and the 25- to 30-mm follicle group. However, concentrations of insulin-like growth factor binding protein 2 in follicular fluid did not differ (P > 0.05) among groups. Additionally, follicular fluid activin A tended (P < 0.06) to be higher in impending ovulation follicles when compared with the 25- to 40-mm follicle group. Concentrations of intrafollicular estradiol were higher (P < 0.0001) in 40-mm and impending ovulation follicles than in the other follicle groups. Follicular fluid concentrations of inhibin A and vascular endothelial growth factor were lower (P < 0.05) in the 40-mm and the impending ovulation follicle group when compared with the 25- to 35-mm follicle group. Systemic and intrafollicular prolactin levels were lower (P < 0.05) in the impending ovulation group when compared with the 25- to 40-mm follicle group. Prolactin concentrations were higher (P < 0.05) in the follicular fluid than in the plasma. The novel findings of this study, a decrease in intrafollicular-free insulin-like growth factor 1, inhibin A, vascular endothelial growth factor, and prolactin during the final stages of follicular growth, document for the first time the occurrence of dynamic changes among intrafollicular factors and hormones during the stages of follicle dominance and as ovulation approaches.

Hormonal, luteal, and follicular changes during initiation of persistent corpus luteum in mares

Mares with persistent CL (PCL) with no known etiology (idiopathic) were matched with mares with an interovulatory interval (IOI) of apparent physiological length, so that ovulation at the beginning of each PCL and IOI occurred during the same month (n = 6/group). Blood samples were collected daily from Days 12 to 22 (Day 0 = ovulation). Mean progesterone (P4) decreased in both groups on Days 14 and 15 and then diverged with a continued decrease in the IOI group and the beginning of constant and greater (P < 0.05) P4 concentration on each day in the PCL group. Before P4 divergence between groups, P4 in the PCL group decreased either abruptly (apparent incomplete luteolysis) or gradually. Concentration of PGFM (a metabolite of PGF2α) was not different between groups and reached maximum on mean Day 15 in each group. After the divergence in P4 between groups, LH and estradiol (E2) remained low in the PCL group. There was no indication that an increase in a luteotropic effect of LH in the PCL group accounted for the divergence in P4. Differences in prolactin between the groups were inconclusive. The hypothesis that secretion of PGF2α at the time of expected luteolysis is defective in mares with idiopathic PCL was not supported. The hypothesis that E2 concentration before expected luteolysis is greater in mares with PCL than those without PCL was not supported; however, a difference on Day 12 approached significance (P < 0.06) and tentatively indicated greater E2 in the PCL group before the beginning of luteolysis.

Inhibition of prostaglandin biosynthesis during postluteolysis and effects on CL regression, prolactin, and ovulation in heifers

The beginning of postluteolysis (progesterone, <1 ng mL(-1)) in heifers was targeted by using 8 h after ultrasonic detection of a 25% decrease in CL area (cm2) and was designated Hour 0. Flunixin meglumine (FM; n=10) to inhibit PGF2α secretion or vehicle (n=9) were given intramuscularly at Hours 0, 4, 8, 16, 24, 32, and 40. The dose of FM was 2.5 mg/kg at each treatment. Blood sampling and measurement of the CL and dominant follicle were done every 8 h beginning 14 days postovulation in each group. Blood samples for detection of pulses of PRL and pulses of a metabolite of PGF2α (PGFM) were obtained every hour for 24 h beginning at Hour 0. Pulse concentrations of both PGFM and PRL were lower in the FM group than in the vehicle group. Concentration of PRL was greatest at the peak of a PGFM pulse. Neither CL area (cm2) nor progesterone concentration differed between groups during Hours 0 to 48 (postluteolysis). Ovulation occurred in nine of nine heifers in the vehicle group and in three of 10 heifers in the FM group. The anovulatory follicles in the FM group grew to 36.2±2.9 mm, and the wall became thickened from apparent luteinization. The hypothesis that PGF2α was involved in the continued P4 decrease and structural CL regression during postluteolysis was not supported. However, the hypotheses that pulses of PGFM and PRL were temporally related and that systemic FM treatment induced an anovulatory follicle were supported.