Follicular Growth and Estradiol Influence on Luteal Function in Mares

Abstract

Follicular growth, circulating estradiol concentrations and endometrial prostaglandin F2 alpha (PGF2 alpha) production were measured to determine whether there is an interrelationship among these factors associated with luteolysis. Follicular growth was monitored by rectal palpation every other day during diestrus in 16 mares. Plasma estradiol was determined for daily samples during all estrous cycles. Endometrial tissue was removed for PGF2 alpha analysis by radioimmunoassay on d 10, 12, 14 or 16 during several normal cycles and after d 30 during spontaneously prolonged cycles. Circulating estradiol concentrations were highly correlated with follicular diameter. Follicular growth was initiated before d 14 in normal cycles, thus estradiol was significantly elevated during the portion of diestrus prior to luteolysis. This was not the case during spontaneously prolonged cycles in which a) there was no correlation between follicular size and circulating estradiol, b) follicular growth was initiated an average of 19 d postovulation, and c) estradiol concentrations did not increase. Diestrous estradiol concentrations, during normal cycles, were significantly elevated immediately (72 to 24 h) prior to the time at which peak endometrial PGF2 alpha production occurred (d 12 and 14). During spontaneously prolonged cycles, estradiol concentrations did not rise at the time of expected luteolysis, and endometrial PGF2 alpha production was minimal in d 30 biopsies. Results suggest that diestrus follicular growth and its consequent estradiol production may play a role in initiating luteolysis through endometrial PGF2 alpha production and that delay or failure of this process may result in spontaneously prolonged corpora luteal function.

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

“…The divergence in E2 concentrations between the groups beginning on Day 16 represented increasing concentration in the IOI group but not in the PCL group, consistent with a report that E2 did not increase during the late diestrus in mares with PCL [26]. These findings for E2 apply to mares in which PCL has already been established and thereby represent the consequences of persistent P4 concentration rather than for a role of E2 as an etiologic factor.…”

“…In the present study, divergence in LH between the groups began within a day after the reported diameter of the largest follicle at deviation (22-25 mm; [47,55]). In a previous study, follicle emergence was delayed in mares that developed spontaneous PCL [26]. In contrast, a negative effect on follicles in the present study was indicated by a slower growth of the largest follicle but not by a delay in day of emergence.…”

“…The main rationale for the hypothesis was the report that urinary estrogens are higher from Days 5 to 10 in mares that develop PCL [25]. A report on the associations among follicles, luteal function, and plasma E2 found no difference between the early (Days 2-5) and late (1-3 days before luteolysis) luteal phases in mares that developed PCL [26]. However, although apparently not analyzed or commented on, tabulated data indicate that E2 from Days 2 to 5 may have been greater in a group that developed PCL (24.7 AE 1.8 pg/mL) than that in a group that did not develop PCL (20.9 AE 0.7 pg/mL) [26].…”

“…In another study, mean urinary estrogen concentration was greater during Days 5 to 10 in mares that developed PCL than that in mares that did not [25]. Concentration of E2 was also reported to be similar between the early luteal phase and the expected time of luteolysis in mares with PCL, contrasting with an increase in E2 between the two luteal phases in IOIs [26]. Prolactin increases during luteolysis during the IOI and during the expected time of luteolysis in mares with PCL [11].…”

“…In one mare with PCL, a small follicle was detected by transrectal palpation at the end of diestrus but then regressed and no other follicles were detected [16]. Another report indicated that follicular growth was initiated before Day 14 during an IOI and at mean Day 19 during spontaneously prolonged cycles [26]. Individual profiles for P4, LH, and estradiol (E2) for PCL have been described in two mares [16].…”