The effect of ovarian arterial infusion of human recombinant inhibin and bovine follicular fluid on ovarian hormone secretion by ewes with an autotransplanted ovary

1996 ◽  
Vol 149 (3) ◽  
pp. 531-540 ◽  
Author(s):  
B K Campbell ◽  
R J Scaramuzzi
Keyword(s):  
Follicular Fluid ◽  
Luteal Phase ◽  
Ovarian Function ◽  
Hormone Secretion ◽  
Follicular Phase ◽  
Ovarian Hormone ◽  
Inhibitory Effects ◽  
Arterial Infusion ◽  
Lh Surge ◽  
High Doses

Abstract Recombinant human inhibin A (rhInh) or steroid-free bovine follicular fluid (bFF) were infused into the ovarian artery of anoestrous ewes with ovarian autotransplants induced to ovulate with a pulsatile regimen of GnRH applied after a 10-day pretreatment with progestagen sponges. In the period 12–24 h after sponge withdrawal ewes received ovarian arterial infusions of saline (n=6), 0·3 μg rhInh/h (n=5), 1·6 μg rhInh/h (n=5) or 25 μl bFF/h (n=4). Controls had a normal follicular phase with an LH surge 43 ± 3 h after sponge withdrawal which resulted in ovulation (six out of six). Both doses of rhlnh increased ovarian venous inhibin concentrations in a dose-related fashion (P<0·05) but resulted in depressions (P<0·05) in FSH concentrations of similar magnitude. Both doses of rhInh acutely inhibited ovarian oestradiol and androstenedione secretion (P<0·01) but at the end of rhInh infusion oestradiol secretion was quickly re-established without a corresponding increase in FSH. LH surges were detected in five out of five and three out of five ewes infused with low and high doses of rhInh respectively, and progesterone concentrations during the subsequent luteal phase were depressed (P<0·05). Infusion of bFF had no effect on inhibin or FSH concentrations but resulted in acute inhibition (P<0·01) of ovarian oestradiol, androstenedione and inhibin secretion, a delay (P<0·05) in the time to the LH surge and a depression (P<0·05) in luteal-phase progesterone concentrations. In conclusion, while the depression in FSH induced by rhlnh cannot be excluded as a cause for the inhibitory effects of rhInh treatment on ovarian function, such a mechanism cannot fully explain the ovarian responses obtained to rhInh infusion. These results therefore support a direct ovarian role for inhibin in the modulation of ovarian function in addition to its indirect role in controlling FSH. This conclusion is supported by the demonstration that bFF can induce similar inhibitory effects on ovarian function without changing FSH. Journal of Endocrinology (1996) 149, 531–540

The effect of ovarian arterial infusion of transforming growth factor α on ovarian follicle populations and ovarian hormone secretion in ewes with an autotransplanted ovary

1994 ◽  
Vol 143 (1) ◽  
pp. 13-24 ◽  
Author(s):  
B K Campbell ◽  
B M Gordon ◽  
R J Scaramuzzi
Keyword(s):  
Growth Factor ◽  
Luteal Phase ◽  
Transforming Growth Factor ◽  
Hormone Secretion ◽  
Follicular Phase ◽  
Ovarian Hormone ◽  
Arterial Infusion ◽  
Transforming Growth Factor Α ◽  
Progesterone Secretion ◽  
Factor Α

Abstract Transforming growth factor α (TGFα) inhibits hormone production by cultured follicular cells but evidence of an effect of TGFα on ovarian hormone secretion in vivo is still required. Eleven ewes with an autotransplanted ovary received, by ovarian arterial infusion, either 5 μg/h recombinant rat TGFα (n=6) or placebo (n=5) for 12 h on day 10 of the luteal phase. Two hours before the start and 1 hour before the end of the infusion each ewe received a single injection of gonadotrophin-releasing hormone (GnRH; 150 ng i.v.). Two hours after the end of the infusion luteal regression was induced with prostaglandin F2α (PGF2α; 125 μg i.m.). Ovarian and jugular venous blood samples were taken at 10-min, 15-min or 4-h intervals from 2 h before the start of the infusion until 96 h after PGF2α and the rates of secretion of ovarian oestradiol, inhibin, progesterone and androstenedione were determined. Jugular venous concentrations of LH and FSH were also measured and follicle populations monitored by real-time ultrasound scanning. Infusion of TGFα resulted in a significant (P<0.05) depression in the amplitude of the pulsatile response of oestradiol and androstenedione secretion to the GnRH-induced LH pulse at the end of the infusion. Ovarian inhibin secretion was acutely suppressed by TGFα infusion (P<0·001) and remained lower than controls for the period of the experiment. Luteal phase progesterone secretion was also acutely inhibited (P<0·001) by infusion of TGFα and in one treated ewe progesterone secretion was elevated 48–84 h after PGF2α. Jugular venous concentrations of FSH in TGFα-treated ewes were significantly (P<0·001) elevated over controls during the first 48 h of the follicular phase and the LH surge was delayed for about 10 h (P<0·05). Infusion of TGFα caused a marked decline (P<0·05) in the number of large follicles within 12 h of the end of the infusion. Two of the six treated ewes, including the one with high follicular phase progesterone, had unusually large (8·7 and 10 mm) follicles present from 48–96 h after PGF2α. We conclude that direct arterial infusion of TGFα results in acute inhibition of ovarian steroid and inhibin secretion that is associated with induction of atresia in the population of large follicles. The lack of feedback of ovarian hormones results in a rebound increase of FSH which stimulates the growth of more ovarian follicles and the eventual re-establishment of ovarian hormone secretion and normal cyclicity. Journal of Endocrinology (1994) 143, 13–24

Ovine follicular fluid suppresses the ovarian secretion of androgens, oestradiol and inhibin

1990 ◽  
Vol 127 (1) ◽  
pp. 23-32 ◽  
Author(s):  
D. T. Baird ◽  
B. K. Campbell ◽  
A. S. McNeilly
Keyword(s):  
Follicular Fluid ◽  
Venous Blood ◽  
Hormone Secretion ◽  
Experimental Period ◽  
Follicular Phase ◽  
Ovarian Hormone ◽  
Lh Surge ◽  
Fourfold Increase ◽  
Testosterone Secretion ◽  
Venous Plasma

ABSTRACT An experiment was conducted to examine the effect of steroid-free ovine follicular fluid (oFF) on ovarian hormone secretion. Eight Merino × Finnish Landrace ewes in which the left ovary and vascular pedicle had been autotransplanted to a site in the neck were studied during the breeding season. Luteal regression was induced in all animals by injection of cloprostenol (100 μg, i.m.) on day 10 of the luteal phase. Four of the eight animals were treated with steroid-free oFF (3 ml, s.c.) in the early follicular phase, 24 and 36 h after injection of cloprostenol. Samples of both ovarian and jugular venous blood were collected at 4-h intervals from 20 h before until 96 h after injection of cloprostenol. Ovarian and jugular venous blood samples were also collected at 10-min intervals from 48 to 52 h after injection of cloprostenol to investigate the pattern of pulsatile secretion of ovarian hormones. Samples were assayed for oestradiol, androstenedione, testosterone and inhibin and the ovarian secretion rates calculated. Both injections of oFF resulted in a fourfold increase in the concentration of inhibin in jugular venous plasma within 4–8 h of administration (P < 0·01) with concentrations remaining increased (P < 0·05) until 56 h after cloprostenol (32 h after the first oFF injection). Following oFF injection there was a profound (100%; P < 0·001) and prolonged decrease in the peripheral concentration of FSH until 60 h after cloprostenol at which time the concentration of FSH increased five- to sixfold (P < 0·001) to a peak lasting 24 h. In contrast to FSH, the concentration of LH in jugular venous plasma rose immediately following oFF treatment and continued to increase, exhibiting a profile similar to that described for FSH. No preovulatory LH surge was detected in any of the oFF-treated ewes while untreated ewes had an LH surge within 58·0±1·2 (s.e.m.) h. Within 8 h of the first injection of oFF the ovarian secretion rate of oestradiol, androstenedione and inhibin began to decline to reach a nadir of less than 1 ng/min within 32–36 h (56–60 h after cloprostenol; P < 0·01). Testosterone secretion, already barely detectable, did not change significantly following injection of oFF but remained low for 36 h following oFF and did not exhibit the increase observed over this period in controls. After injection of oFF the episodic secretion of oestradiol, androstenedione, testosterone and inhibin was markedly suppressed in spite of numerous pulses of LH. Re-establishment of inhibin, androstenedione and testosterone secretion began from around 36 h after injection of oFF and continued to increase for the remainder of the experimental period (P < 0·001). The re-establishment of oestradiol secretion, however, took until 60 h after oFF treatment (84 h after cloprostenol). This increase in ovarian hormone secretion was temporally related to the decrease in the concentration of FSH and LH in jugular venous plasma that was observed at the end of the experimental period. We conclude that treatment of ewes with steroid-free oFF during the follicular phase of the oestrous cycle results in the immediate inhibition of the ovarian secretion of oestradiol, inhibin, androstenedione and testosterone. This effect can most probably be attributed to the depression in FSH that occurs following oFF injection, although the possibility exists that other factors present in oFF are acting directly on the ovary to inhibit follicular growth. Journal of Endocrinology (1990) 127, 23–32

Evidence that the switch from negative to positive feedback at the level of the pituitary gland is an important timing event for the onset of the preovulatory surge in LH in the ewe

1995 ◽  
Vol 145 (2) ◽  
pp. 271-282 ◽  
Author(s):  
I J Clarke
Keyword(s):  
Pituitary Gland ◽  
Negative Feedback ◽  
Luteal Phase ◽  
Bolus Injection ◽  
Pulse Frequency ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Lh Surge ◽  
Small Rise ◽  
Lh Secretion

Abstract Experiments were performed to test the hypothesis that there is a negative feedback 'clamp' of ovarian hormones on the hypothalamus and pituitary gland during the follicular phase of the oestrous cycle that limits the secretion of GnRH and LH. GnRH secretion was monitored by sampling the hypophysial portal blood of ewes during the luteal phase of the oestrous cycle and either 24 h or 48 h after the induction of luteolysis by the injection of cloprostenol, a prostaglandin analogue. There was an increase in GnRH pulse frequency in the transition from the luteal to the follicular phase of the cycle. A reduction in the amplitude of GnRH pulses did not occur until 48 h after cloprostenol, suggestive of negative feedback at the level of the hypothalamus that is more profound in the latter part of the follicular phase. The responsivity of the pituitary gland to GnRH was monitored in ewes during the luteal phase of the oestrous cycle and 24 h or 48 h after cloprostenol. Injections of 250 ng or 1000 ng GnRH were given (i.v.) to ewes that had been anaesthetised to suppress endogenous secretion of GnRH and LH. Using the lower dose, the responses 48 h after cloprostenol were not significantly different from those in the luteal phase. With the higher dose of GnRH, a significant (P<0·05) increase in mean responsivity was seen 48 h after cloprostenol. There was, however, a marked variation in response, with some ewes showing profound increases in LH secretion in response to GnRH and others showing responses that were similar to those obtained during the luteal phase of the cycle. These data are interpreted to mean that the secretion of LH is 'clamped' during the follicular phase of the oestrous cycle and the 'clamp' is only released near the time of the preovulatory LH surge. To test whether or not a rise in GnRH input to the pituitary gland could over-ride the 'clamp' on the pituitary secretion of LH in the late follicular phase of the cycle, sheep were treated 40 h after cloprostenol with either a bolus injection of 500 ng GnRH or four pulses of 125 ng GnRH given at 10-min intervals. These treatments caused small elevations in LH secretion but did not always cause preovulatory LH surges. In some cases, a small rise in LH secretion was induced by GnRH treatments and levels of LH in plasma returned to baseline with the preovulatory LH surge occurring a few hours later. In one clear case, a bolus injection of GnRH induced an LH surge. The overall data from the GnRH-treated groups, however, indicated a significant delay in the onset of the LH surge which may have been due to perturbation of the subcellular mechanisms in the gonadotrophs. These data were interpreted to mean that the secretion of LH from the pituitary gland is inhibited up to very soon before the onset of the LH surge. The inhibitory factor could be oestrogen but could also be some other pituitary feedback hormone such as gonadotrophin surge-attenuating factor. It is concluded that the increase in the secretion of GnRH at the time of the onset of the LH surge is closely linked to an increase in the responsivity of the gonadotrophs to GnRH. The latter is not caused by the increase in the secretion of GnRH. Journal of Endocrinology (1995) 145, 271–282

Differential effects of progesterone on secretion of gonadotropic hormones in the rhesus monkey

1981 ◽  
Vol 240 (5) ◽  
pp. E489-E492
Author(s):  
J. A. Resko ◽  
W. E. Ellinwood ◽  
E. Knobil
Keyword(s):  
Menstrual Cycle ◽  
Macaca Mulatta ◽  
Luteal Phase ◽  
Rhesus Macaques ◽  
Follicular Phase ◽  
Base Line ◽  
Inhibitory Effects ◽  
New Information ◽  
Lh Secretion ◽  
Gonadotropic Hormones

To obtain new information on the site of the inhibiting actions of progesterone (P) during the follicular phase of the cycle, we administered P continuously to 13 rhesus macaques (Macaca mulatta) from days 5 through 12 of the menstrual cycle. This treatment produced luteal-phase levels (approximately 6 ng/ml of serum) within 4 h. (FSH) concentrations dropped significantly from pretreatment amounts (P less than 0.01). During this time, both immunoreactive luteinizing hormone (LH) (n = 8 animals) and bioactive LH (n = 5) remained stable. The 17 beta-estradiol (E2) concentrations dropped significantly below base line 24 h after treatment (P less than 0.05). The decline in E2 occurred after the decline in FSH. Inhibition of FSH continued for 40 h, after which both FSH and LH rose steadily in a way that resembled a preovulatory surge on days 8 or 9 of the menstrual cycle, and then returned to base line by day 10. When gonadotropins were at their zenith, the ovary did not respond by secreting E2. None of the effects mentioned above were found in control animals treated with blank implants. These data demonstrate that P administered during the follicular phase inhibits FSH but not LH secretion. This initial inhibition probably occurs at a hypothalamic-pituitary site, but after 40 h direct inhibitory effects on the ovary cannot be ruled out. P stimulates the release of gonadotropin in female monkeys despite reduced production of E2 by the ovary.

scholarly journals The early luteal phase administration of estrogen and progesterone does not induce premature luteolysis in normo-ovulatory women

2006 ◽  
Vol 155 (2) ◽  
pp. 355-363 ◽  
Author(s):  
Nicole G M Beckers ◽  
Peter Platteau ◽  
Marinus J Eijkemans ◽  
Nicholas S Macklon ◽  
Frank H de Jong ◽  
...  
Keyword(s):  
Corpus Luteum ◽  
Luteal Phase ◽  
Controlled Trial ◽  
Normal Weight ◽  
High Dose ◽  
Ovarian Hyperstimulation ◽  
Lh Surge ◽  
High Doses ◽  
Phase Length

Objective: The luteal phase after ovarian hyperstimulation for in vitro fertilization (IVF) is insufficient. Therefore, luteal phase supplementation is routinely applied in IVF. It may be postulated that premature luteolysis after ovarian hyperstimulation is due to supraphysiological steroid levels in the early luteal phase. In the present study, high doses of steroids are administered after the LH surge in normo-ovulatory volunteers in order to investigate whether this intervention gives rise to endocrine changes and a shortening of the luteal phase. Design: Randomized controlled trial. Methods: Forty non-smoking, normal weight women, between 18 and 37 years of age, with a regular menstrual cycle (24–35 days), received either high dosages of estradiol (E2), progesterone (P), E2+P or no medication. Blood sampling was performed every other day from the day of the LH surge until LH+14. Duration of the luteal phase and endocrine profiles were the main study outcomes. Results: Early luteal phase steroid concentrations achieved by exogenous administration were comparable with levels observed following ovarian hyperstimulation for IVF. No difference in the luteal phase length was observed comparing all groups. However, a significant decrease in LH levels could be observed 6 days after the mid-cycle LH surge (P<0.001) in women receiving P, resulting in accelerated decrease of inhibin A production by the corpus luteum (P=0.001). Conclusion: The present intervention of high-dose steroid administration shortly after the LH surge failed to induce a premature luteolysis regularly in cyclic women. It seems that the induced transient suppression in LH allowed for a timely recovery of corpus luteum function. Other additional factors may be held responsible for the distinct reduction in luteal phase length observed after ovarian hyperstimulation for IVF.

Mobilization of LH secretory granules in gonadotrophs in relation to gene expression, synthesis and secretion of LH during the preovulatory phase of the sheep oestrous cycle

1995 ◽  
Vol 147 (2) ◽  
pp. 259-270 ◽  
Author(s):  
R J W Currie ◽  
A S McNeilly
Keyword(s):  
Electron Microscopy ◽  
Luteal Phase ◽  
Secretory Granules ◽  
Follicular Phase ◽  
Oestrous Cycle ◽  
Single Type ◽  
Maximum Diameter ◽  
Northern Analysis ◽  
Mrna Levels ◽  
Lh Surge

Abstract Changes in LH secretory granules in pituitary gonadotrophs throughout the sheep oestrous cycle were determined by immunogold localisation of LH at ultrastructural level by electron microscopy. Oestrous cycles in Welsh Mountain ewes were initially synchronised with progestagen sponges and studies carried out in the subsequent cycle. Animals were allocated at random to six groups each of five animals, one killed on day 12 of the luteal phase and the other groups after prostaglandin (PG)-induced luteal regression at PG plus 18 h (early follicular phase), oestrus (PG plus 33·6±1·0 h), oestrus plus 9 h just before the preovulatory LH surge, 1 h after GnRH agonist-induced LH surge at PG plus 48 h (mid-LH surge) and oestrus plus 24 h, after the preovulatory LH surge. Blood samples collected throughout confirmed the pulsatile secretion of LH before and the timing in relation to the preovulatory LH surge. Pituitaries were dissected and processed for transmission electron microscopy and frozen for later extraction of mRNA. Only a single type of LH cell was present in the sheep pituitary. In the luteal phase, LHimmunopositive secretory granules were distributed throughout the cytoplasm in 80% of cells while in 20% of cells granules were polarised to the region of the cell next to a vascular sinusoid. The percentage of polarised cells increased during the follicular phase to 45% at oestrus, 75% at oestrus plus 9 h just before the LH surge and 90% in mid-LH surge. Cell size increased in parallel with polarisation. Gonadotrophs after the LH surge were almost totally devoid of LH granules but prominent LHβ immunoreactivity was observed in the rough endoplasmic reticulum. Analysis of granule diameters revealed a single class of granules with a maximum diameter of 300 nm. Polarised cells had significantly fewer 130–150 nm granules than non-polarised cells, suggesting preferential exocytosis of LH-containing granules of this size from polarised cells. Northern analysis showed that LHβ mRNA levels decreased from luteal through the follicular phase. These results suggest that the preovulatory LH surge in sheep is not related to a change in synthesis of LH but to a progressive recruitment of gonadotrophs into a releasing state, priming, as indicated by polarisation of secretory granules to the region of the cell next to the vascular system. Journal of Endocrinology (1995) 147, 259–270

THE SEQUENCE OF PITUITARY RESPONSES TO SYNTHETIC LUTEINIZING HORMONE RELEASING HORMONE (LH-RH) THROUGHOUT THE NORMAL MENSTRUAL CYCLE

1975 ◽  
Vol 79 (4) ◽  
pp. 625-634 ◽  
Author(s):  
Elwyn M. Grimes ◽  
Irwin E. Thompson ◽  
Melvin L. Taymor
Keyword(s):  
Menstrual Cycle ◽  
Luteal Phase ◽  
Follicular Phase ◽  
Lh Surge ◽  
Luteinizing Hormone Releasing Hormone ◽  
Late Luteal Phase ◽  
Normal Menstrual Cycle ◽  
Early Follicular Phase ◽  
Lh Rh ◽  
Late Follicular Phase

ABSTRACT Thirty-one ovulatory women between 20 and 33 years of age were given 150 μg of synthetic LH-RH during different phases of the menstrual cycle. Five patients were studied during the early follicular phase (days 4–7); 10 patients during the late follicular phase (days 9–12); 6 patients during the "LH Surge"; 5 patients during the early luteal phase (days 14–16); 3 patients during mid-luteal phase (days 17–21); and 2 patients during late luteal phase (days 22–27). Oestrogen, progesterone, FSH and LH levels were determined from 30 min prior to LH-RH administration to 90 min thereafter in all cases. LH response to LH-RH increased progressively during the follicular phase. Enhanced pituitary responsiveness to LH-RH occurred at mid-cycle for both LH and FSH and maximum LH responses occurred during the "LH Surge" and early luteal phase. LH responses during the mid and late luteal phases were similar to late follicular phase responses. There were no significant differences between FSH responses during the early follicular, late follicular, mid-luteal and late luteal phases. Maximum pituitary responsiveness appears to occur in a gonadal steroid milieu of high oestrogen levels in association with rising but low progesterone levels. Progesterone or a crucial oestrogen: progesterone ratio may in fact potentiate pituitary release of LH during the early stages of corpus luteum formation. Pituitary responsiveness to LH-RH correlates positively with basal LH and oestrogen levels during the menstrual cycle and with the oestrogen:progesterone ratio during the luteal phase.

LUTEINIZING HORMONE LEVELS DURING THE MENSTRUAL CYCLE OF THE BABOON (PAPIO HAMADRYAS)

1979 ◽  
Vol 91 (1) ◽  
pp. 49-58 ◽  
Author(s):  
N. Goncharov ◽  
A. V. Antonichev ◽  
V. M. Gorluschkin ◽  
L. Chachundocova ◽  
D. M. Robertson ◽  
...  
Keyword(s):  
Menstrual Cycle ◽  
Luteinizing Hormone ◽  
Plasma Levels ◽  
Luteal Phase ◽  
Follicular Phase ◽  
Papio Hamadryas ◽  
Biologically Active ◽  
Plasma Progesterone ◽  
Lh Surge

ABSTRACT The peripheral plasma levels of luteinizing hormone (LH) as measured by an in vitro bioassay method were determined in daily plasma samples collected throughout one menstrual cycle in 8 normally menstruating baboons (Papio hamadryas). In addition LH was measured in plasma at three hourly intervals throughout the day in the follicular, peri-ovulatory and luteal phases of the cycle in 7, 3 and 6 animals respectively. The plasma levels of progesterone and oestradiol were also determined in the same samples throughout the menstrual cycle and during the period of the midcycle LH surge. The circulating LH profile measured throughout the cycle was characterized by a sharp mid-cycle surge (completed within one day) which was followed by a series of LH surges of varying intensity during the luteal phase of the cycle. The initial surge was considered to be pre-ovulatory as indicated by its relationship to the peak of plasma oestradiol and to the first significant increase in the levels of plasma progesterone above values found earlier in the follicular phase. A circadian rhythm of LH was observed during the luteal phase of the cycle; a 3 fold rise in LH was noted during the hours 15.00 to 24.00. No differences were observed throughout the day in the follicular phase of the cycle. The LH profile in three animals studied during the mid-cycle LH surge showed pronounced circadian changes with a major peak at 24.00 h. Plasma progesterone levels during this period rose sharply to values normally found in the mid-luteal phase of the cycle. A comparison of plasma levels of biologically active LH during the menstrual cycle of the baboon with those found in normally menstruating women reveals that in the baboon the LH peak is of much shorter duration and the levels in the follicular and peri-menstrual phases are significantly lower than in the human.

Corpus luteum life span and pituitary oestrogen and progesterone receptors in cyclic and gonadotrophin-releasing hormone-treated anoestrous ewes

2005 ◽  
Vol 17 (7) ◽  
pp. 721 ◽  
Author(s):  
C. Tasende ◽  
M. Rodríguez-Piñón ◽  
S. Acuña ◽  
E. G. Garófalo ◽  
M. Forsberg
Keyword(s):  
Luteal Phase ◽  
Luteinising Hormone ◽  
Binding Assay ◽  
Hormone Secretion ◽  
Progesterone Receptors ◽  
Oestrous Cycle ◽  
Inhibitory Effects ◽  
Releasing Hormone ◽  
Expected Time ◽  
Gonadotrophin Releasing Hormone

The present study investigated the pituitary oestrogen (ER) and progesterone (PR) receptor concentrations in ewes during the oestrous cycle in the breeding season (n = 19), and in anoestrous ewes treated with gonadotrophin-releasing hormone (GnRH) (n = 11) and anoestrous ewes treated with progesterone + GnRH (n = 11). The pituitary ER and PR concentrations at the expected time of ovulation and in the early and late luteal phases were measured by binding assay. The pattern of pituitary ER and PR concentrations in the progesterone + GnRH-treated ewes resembled the pattern found during the normal oestrous cycle, with ER and PR concentrations decreasing from the time of ovulation to the early luteal phase. In contrast, in ewes treated with GnRH alone, ER and PR concentrations increased in the early luteal phase, which may increase the inhibitory effects of steroid hormones on luteinising hormone secretion, ultimately leading to the development of subnormal luteal phases.

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