Estradiol-induced blockade of ovulation in the cow: effects on luteinizing hormone release and follicular fluid steroids

Administration of 10 mg estradiol valerate (EV) to nonlactating Holstein cows on Days 16 of the estrous cycle prevented ovulation in 7 of 8 cows for 14 days post-injection. In these 7 cows, the timing of luteolysis and the luteinizing hormone (LH) surge was variable but within the normal range. At 14 days post-treatment, each of these cows had a large (greater than 10 mm) follicle, with 558 +/- 98 ng/ml estradiol-17 beta, 120 +/- 31 ng/ml testosterone, and 31 +/- 2 ng/ml progesterone in follicular fluid (means +/- SE). A second group of animals was then either treated with EV as before (n = 22), or not injected (control, n = 17) and ovariectomized on either Day 17, Day 18.5, Day 20, or Day 21.5 (24, 60, 96, or 132 h post-EV). Treatment with EV did not influence the timing of luteolysis, but surges of LH occurred earlier (59 +/- 8 h post-EV vs. 100 +/- 11 h in controls). The interval from luteolysis to LH peak was reduced from 44 +/- 6 h (controls) to 6.9 +/- 1.5 h (treated). Histologically, the largest follicle in controls tended to be atretic before luteolysis, but nonatretic afterwards, whereas the largest follicle in treated animals always tended to be atretic. Nonatretic follicles contained high concentrations of estradiol (408 +/- 59 ng/ml) and moderate amounts of testosterone (107 +/- 33 ng/ml) and progesterone (101 +/- 21 ng/ml), whereas atretic follicles contained low concentrations of estradiol (8 +/- 4 ng/ml) and testosterone (12 +/- 4 ng/ml), and either low (56 +/- 24 ng/ml) or very high (602 +/- 344 ng/ml) concentrations of progesterone. This study suggests that EV prevents ovulation by inducing atresia of the potential preovulatory follicle, which is replaced by a healthy large follicle by 14 days post-treatment.     

Temporal interrelationships among luteolysis, FSH and LH concentrations and follicle deviation in mares

The effect of altered LH concentrations on the deviation in growth rates between the 2 largest follicles was studied in pony mares. The progestational phase was shortened by administration of PGF2alpha on Day 10 (Day 0=ovulation; n=9) or lengthened by daily administration of 100 mg of progesterone on Days 10 to 30 (n=11; controls, n=10). All follicles > or = 5 mm were ablated on Day 10 in all groups to initiate a new follicular wave. The interovulatory interval was not altered by the PGF2alpha treatment despite a 4-day earlier decrease in progesterone concentrations. Time required for growth of the follicles of the new wave apparently delayed the interval to ovulation after luteolysis. The FSH concentrations of the first post-ablation FSH surge were not different among groups. A second FSH surge with an associated follicular wave began by Day 22 in 7 of 11 mares in the progesterone group and in 0 of 19 mares in the other groups, indicating reduced functional competence of the largest follicle. A prolonged elevation in LH concentrations began on the mean day of wave emergence (Day 11) in the prostaglandin group (19.2 +/- 2.2 vs 9.0 +/- 0.7 ng/mL in controls; P<0.05), an average of 4 d before an increase in the controls. Concentrations of LH in the progesterone group initially increased until Day 14 and then decreased so that by Day 18 the concentrations were lower (P<0.05) than in the control group (12.9 +/- 1.6 vs 20.2 +/- 2.6 ng/mL). Neither the early and prolonged increase nor the early decrease in LH concentrations altered the growth profile of the second-largest follicle, suggesting that LH was not involved in the initiation of deviation. However, the early decrease in LH concentrations in the progesterone group was followed by a smaller (P<0.05) diameter of the largest follicle by Day 20 (26.9 +/- 1.7 mm) than the controls (30.3 +/- 1.7 mm), suggesting that LH was necessary for continued growth of the largest follicle after deviation.     

Role of LH in luteolysis and growth of the ovulatory follicle and estradiol regulation of LH secretion in heifers

The role of LH in luteolysis and development of the ovulatory follicle and the involvement of GnRH receptors in estradiol (E2) stimulation of LH secretion were studied in heifers. A pulse of PGF_2α, as indicated by a metabolite, was induced by E2 treatment on Day 15 (Day 0 = ovulation) and LH concentration was reduced with a GnRH-receptor antagonist (acyline) on Days 15, 16, and 17. Blood samples were collected every 6 h on Days 14-17 and hourly for 10 h beginning at the Day-15 treatments. Four groups were used (n = 6): control, acyline, E2, and E2/acyline. The number of LH pulses/heifer during the 10 h posttreatment was greater (P < 0.0002) in the E2 group (2.3 ± 0.4, mean ± SEM) than in the acyline group (0.2 ± 0.2) and was intermediate in the E2/acyline group (1.4 ± 0.2). Concentrations of progesterone in samples collected every 6 h on Day 15 showed a group-by-hour interaction (P < 0.02); concentrations decreased in the acyline group but not in the control group. The 12 heifers in the combined acyline and E2/acyline groups had three follicular waves compared to two waves in 10 of 12 heifers in the combined control and E2 groups. Results (1) supported the hypothesis that LH delays the progesterone decrease associated with luteolysis, (2) supported the hypothesis that LH has a positive effect on the continued development and growth of the selected ovulatory follicle, and (3) indicated that E2 stimulates LH production through an intracellular pathway that involves GnRH receptors on the gonadotropes and a pathway that does not involve the receptors.     

Decreased pulsatile LH secretion in heifers superovulated with eCG or FSH

This study was designed to test the hypothesis that treatment with super-ovulatory drugs suppresses endogenous pulsatile LH secretion. Heifers (n=5/group) were superovulated with eCG (2500 IU) or FSH (equivalent to 400 mg NIH-FSH-P1), starting on Day 10 of the estrous cycle, and were injected with prostaglandin F(2alpha) on Day 12 to induce luteolysis. Control cows were injected only with prostaglandin. Frequent blood samples were taken during luteolysis (6 to 14 h after PG administration) for assay of plasma LH, estradiol, progesterone, testosterone and androstenedione. The LH pulse frequency in eCG-treated cows was significantly lower than that in control cows (2.4 +/- 0.4 & 6.4 +/- 0.4 pulses/8 h, respectively; P<0.05), and plasma progesterone (3.4 +/- 0.4 vs 1.8 +/- 0.1 ng/ml, for treated and control heifers, respectively; P<0.05) and estradiol concentrations (25.9 +/- 4.3 & 4.3 +/- 0.4 pg/ml, for treated and control heifers, respectively; P<0.05) were higher compared with those of the controls. No LH pulses were detected in FSH-treated cows, and mean LH concentrations were significantly lower than those in the controls (0.3 +/- 0.1 & 0.8 +/- 0.1, respectively; P<0.05). This suppression of LH was associated with an increase in estradiol (9.5 +/- 1.4 pg/ml; P<0.05 compared with controls) but not in progesterone concentrations (2.1 +/- 0.2 ng/ml; P>0.05 compared to controls). Both superovulatory protocols increased the ovulation rate (21.6 +/- 3.9 and 23.0 +/- 4.2, for eCG and FSH groups, respectively; P>0.05). These data demonstrate that super-ovulatory treatments decrease LH pulse frequency during the follicular phase of the treatment cycle. This could be explained by increased steroid secretion in the eCG-trated heifers but not in FSH-treated animals.     

Effect of induced suprabasal progesterone levels around estrus on plasma concentrations of progesterone, estradiol-17beta and LH in heifers

A controlled study was carried out to investigate the effects of suprabasal plasma progesterone concentrations on blood plasma patterns of progesterone, LH and estradiol-17beta around estrus. Heifers were assigned to receive subcutaneous silicone implants containing 2.5 g (n=4), 5 g (n=4), 6 g (n=3), 7.5 g (n=3) or 10 g (n=4) of progesterone, or implants without hormone (controls, n=5). The implants were inserted on Day 8 of the cycle (Day 0=ovulation) and left in place for 17 d. The time of ovulation was determined by ultrasound scanning. Blood was collected daily from Days 0 to 14 and at 2 to 4-h intervals from Days 15 to 27. Control heifers had the lowest progesterone concentrations on Days 20.5 to 21 (0.5 +/- 0.1 nmol L(-1)); a similar pattern was observed in heifers treated with 2.5 and 5 g of progesterone. In the same period, mean progesterone concentrations in the heifers treated with 6, 7.5 and 10 g were larger (P < 0.05) than in the controls, remaining between 1 and 2.4 nmol L(-1) until implant removal. A preovulatory estradiol increase started on Days 16.4 to 18.4 in all the animals. In the controls and in heifers treated with 2.5 and 5 g of progesterone, estradiol peaked and was followed by the onset of an LH surge. In the remaining treatments, estradiol release was prolonged and increased (P < 0.05), while the LH peak was delayed (P < 0.05) until the end of the increase in estradiol concentration. The estrous cycle was consequently extended (P < 0.05). In all heifers, onset of the LH surge occurred when progesterone reached 0.4 to 1.2 nmol L(-1). The induction of suprabasal levels of progesterone after spontaneous luteolysis caused endocrine asynchronies similar to those observed in cases of repeat breeding. It is suggested that suprabasal concentrations of progesterone around estrus may be a cause of disturbances oestrus/ovulation.     

Contraceptive potential of an antiprogestin ZK 98.734: reversal of ZK 98.734--induced blockade of folliculogenesis with FSH and LH and their differential effects in bonnet monkeys.

Studies were undertaken in adult bonnet monkeys to investigate whether treatment with an antiprogestin ZK 98.734 at weekly intervals, starting from day one of menstrual cycle, could arrest ovulation and also to determine if ZK 98.734 induced blockade of ovulation could be reversed with gonadotropins. Adult animals have ovulatory menstrual cycles of normal duration were treated at weekly intervals with ZK 98.734 (25 mg/dose, sc, oil base) for 10 consecutive weeks and its effects on serum levels of estradiol, bioactive LH and progesterone, and endometrial histology were investigated. Following treatment with the antiprogestin they were treated with hMG or hFSH alone. Ovulation was blocked during treatment period in all the animals (n = 14). Typical follicular phase rise in estradiol levels was inhibited, mid cycle surge in the levels of bioactive LH was abolished and serum progesterone levels remained below 1 ng/ml throughout the treatment period. However, prolonged treatment had no significant effect on the basal levels of estradiol which were around 50 pg/ml. ZK 98.734 also had no significant effect on cortisol levels. In animals (n = 4) followed for recovery after the last dose, the treatment cycle length was increased to 117.8 + 6.8 days. In three animals the treatment cycles were anovulatory, whereas in one delayed ovulation with luteal insufficiency was observed. The endometrium had become atrophic. Treatment with hMG (Pergonal: 35 I.U. hLH and 35 I.U. hFSH) or hFSH (Metrodin, 35 I.U.) for 7 consecutive days initiated folliculogenesis and the animals ovulated either spontaneously or after a single im injection of hCG (100 I.U.) on day 8 in ZK 98.734 treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

LH surge in Nelore cows (Bos indicus), after induced estrus or after ovarian superestimulation

Considering that there is limited information about the preovulatory LH surge in Zebu cattle (Bos indicus), the purpose of the present work was to assess the LH surge in Nelore cows during the estrous cycle and after ovarian superestimulation of ovarian follicular development with FSH. This information is particularly important to improve superovulatory protocols associated with fixed-time artificial insemination. Nelore cows (n=12) had their estrus synchronized with an intravaginal device containing progesterone (CIDR-B) associated with estradiol benzoate administration (EB, 2.5 mg, i.m., Day 0). Eight days later all animals were treated with PGF2alpha (Day 8) in the morning (8:00 h) and at night, when CIDR devices were removed (20:00 h). Starting 38h after the first PGF2alpha injection, blood sampling and ovarian ultrasonography took place every 4h, during 37 consecutive hours. Frequent handling may have resulted in a stress-induced suppression of LH secretion resulting in only 3 of 12 cows having ovulations at 46.7+/-4.9 and 72.3+/-3.8 h, respectively, after removal of CIDR-B. Thirty days later, the same animals received the described hormonal treatment associated with FSH (Folltropin), total dose=200 mg) administered twice a day, during 4 consecutive days, starting on Day 5. Thirty-six hours after the first injection of PGF2alpha, to minimize stress, only seven blood samples were collected at 4h interval each, and ultrasonography was performed every 12 h until ovulation. In 11 of 12 cows (92%) the LH surge and ovulation were observed 34.6+/-1.6 and 59.5+/-1.9 h, respectively, after removal of progesterone source. The maximum values for LH in those animals were 19.0+/-2.6 ng/ml (mean+/-S.E.M.). It is concluded that, in Nelore cows submitted to a ovarian superstimulation protocol, the LH surge occurs approximately 35 h after removal of intravaginal device containing progesterone, and approximately 12h before the LH surge observed after an induced estrus without ovarian superstimulation.     

Ovarian capillary blood flow in seasonally anoestrous ewes induced to ovulate by treatment with GnRH

The microsphere technique was used to obtain estimates of ovarian capillary blood flow near ovulation, in 8 seasonally anoestrous ewes, which were induced to ovulate by GnRH therapy. Plasma progesterone concentrations were monitored in jugular blood sampled between Days 4 and 7 after the onset of the preovulatory LH surge. The ewes were then slaughtered. Three of the ewes were treated with a single injection of 20 mg progesterone before GnRH therapy. In these ewes and 1 other, plasma progesterone values increased after ovulation and reached 1.0 ng/ml on Day 7 following the preovulatory LH surge (normal, functional CL), whilst in the other 4 ewes progesterone concentrations increased initially then declined to 0.5 ng/ml by Day 7 (abnormal CL). In the ewes exhibiting normal luteal function, the mean ovarian capillary blood flow was significantly greater (P less than 0.01) than that for ewes having abnormal luteal function. Irrespective of the type of CL produced, capillary blood flow was significantly greater (P less than 0.05) in ovulatory ovaries than in non-ovulatory ovaries. These findings indicate that the rate of capillary blood flow in ovaries near ovulation may be a critical factor in normal development and maturation of preovulatory follicles and function of subsequently formed CL.     

Comparison of the rate of decline in plasma progesterone concentrations at a natural and progesterone-synchronized oestrus and its effect on tonic LH secretion in the ewe

The pattern of change in plasma progesterone and LH concentrations was monitored in Clun Forest ewes at a natural oestrus and compared to that observed after removal of progesterone implants. The rate of decline in plasma progesterone concentrations after implant withdrawal (1.8 +/- 0.2 ng/ml h-1) was significantly greater (P less than 0.001) than that observed at natural luteolysis (0.2 +/- 0.1 ng/ml h-1), and this resulted in an abnormal pattern of change in tonic LH secretion up to the time of the preovulatory LH surge. This more rapid rate of progesterone removal was also associated with a shortening of the intervals from the time that progesterone concentrations attained basal values to the onset of oestrus (P less than 0.05) and the onset of the preovulatory LH surge (P less than 0.01). However, there were no significant differences in the duration of the LH peak, preovulatory peak LH concentration, ovulation rate or the pattern of progesterone concentrations in the subsequent cycle. It is suggested that the abnormal patterns of change in progesterone and tonic LH concentrations may be one factor involved in the impairment of sperm transport and abnormal patterns of oestradiol secretion known to occur at a synchronized oestrus.     

Follicle deviation and diurnal variation in circulating hormone concentrations in mares

The temporal relationships between follicle deviation and systemic hormone concentrations were studied in mares. Blood samples were obtained at 01:00, 07:00, 13:00, and 19:00 h from nine mares throughout an interovulatory interval. Diurnal variation in progesterone occurred on Days 4-12 and in LH on Days 4 and 5; the lowest concentration for both hormones was at 13:00 h. Ultrasonically observed deviation in the ovulatory follicular wave began on Day 15.7+/-0.5 (ovulation=Day 0). An increase (P<0.002) in LH began on Day 14 before the beginning of deviation, and an increase (P<0.05) in estradiol began at the beginning of deviation. Testosterone concentrations began to increase (P<0.05) 2 days after the beginning of deviation and reached maximum 1 day before the next ovulation. The beginning of deviation was encompassed by a decline (P<0.003) in cortisol concentrations, and the concentrations remained low during the preovulatory period.     

Ability of progesterone to reverse anti-androgen (hydroxyflutamide)-induced interference with the preovulatory LH surge and ovulation in PMSG-primed immature rats

In Exp. 1, PMSG was injected to 26-day-old prepubertal rats to induce ovulations. On Day 2 (2 days later, the equivalent of the day of pro-oestrus) they received at 08:00 h 5 mg hydroxyflutamide or vehicle and at 12:00 h 2 mg progesterone or testosterone or vehicle. Animals were killed at 18:00 h on Day 2 or at 09:00 h on Day 3. Progesterone but not testosterone restored the preovulatory LH surge and ovulation in hydroxyflutamide-treated rats. In Exp. 2, 2 mg progesterone or testosterone were injected between 10:30 and 11:00 h on Day 2, to advance the pro-oestrous LH surge and ovulation in PMSG-primed prepubertal rats. Injection of hydroxyflutamide abolished the ability of progesterone to advance the LH surge or ovulation. Testosterone did not induce the advancement of LH surge or ovulation. In Exp. 3, ovariectomized prepubertal rats implanted with oestradiol-17 beta showed significantly (P less than 0.01) elevated serum LH concentrations at 18:00 h over those observed at 10:00 h. Progesterone injection to these animals further elevated the serum LH concentrations at 18:00 h, in a dose-dependent manner, with maximal values resulting from 1 mg progesterone. Hydroxyflutamide treatment significantly (P less than 0.003) reduced the serum LH values in rats receiving 0-1 mg progesterone but 2 mg progesterone were able to overcome this inhibition. It is concluded that progesterone but not testosterone can reverse the effects of hydroxyflutamide on the preovulatory LH surge and ovulation. It appears that hydroxyflutamide may interfere with progesterone action in induction of the LH surge, suggesting a hitherto undescribed anti-progestagenic action of hydroxyflutamide.     

An alteration in the hypothalamic action of estradiol due to lack of progesterone exposure can cause follicular cysts in cattle

Many mammals, including cattle, can develop ovarian follicular cysts, but the physiological mechanisms leading to this condition remain undefined. We hypothesized that follicular cysts can develop because estradiol will induce a GnRH/LH surge on one occasion but progesterone exposure is required before another GnRH/LH surge can be induced by estradiol. In experiment 1, 14 cows were synchronized with an intravaginal progesterone insert (IPI) for 7 days, and prostaglandin F(2alpha) was given on the day of IPI removal. Estradiol benzoate (EB; 5 mg i.m.) was given 3 days before IPI removal to induce atresia of follicles. Cows were given a second EB treatment 1 day after IPI removal to induce a GnRH/LH surge in the absence of an ovulatory follicle. All cows had an LH surge following the second EB treatment, and 10 of 14 cows developed a large-follicle anovulatory condition (LFAC) that resembled follicular cysts. These LFAC cows were given a third EB treatment 15 days later, and none of the cows had an LH surge or ovulation. Cows were then either not treated (control, n = 5) or treated for 7 days with an IPI (n = 5) starting 7 days after the third EB injection. Cows were treated for a fourth time with 5 mg of EB 12 h after IPI removal. All IPI-treated, but no control, cows had an LH surge and ovulated in response to the estradiol challenge. In experiment 2, cows were induced to LFAC as in experiment 1 and were then randomly assigned to one of four treatments 1) IPI + EB, 2) IPI + GnRH (100 microg), 3) control + EB, and 4) control + GnRH. Control and IPI-treated cows had a similar LH surge and ovulation when treated with GnRH. In contrast, only IPI-treated cows had an LH surge following EB treatment. Thus, an initial GnRH/LH surge can be induced with high estradiol, but estradiol induction of a subsequent GnRH/LH surge requires exposure to progesterone. This effect is mediated by the hypothalamus, as evidenced by similar LH release in response to exogenous GnRH. This may represent the physiological condition that underlies ovarian follicular cysts.     

Failure of exogenous LH to prevent PGF2α-induced luteolysis in beef cows

Henderson and McNatty (Prostaglandins 9:779, 1975) proposed that LH from the preovulatory LH surge attached to receptors on luteal cells and that this attachment might protect the early corpus luteum from PGF_2α induced luteolysis. To test this hypothesis, experiments were performed on heifers at day 10–12 of the cycle. Both jugular veins were catheterized and infusions of either saline (0.64 ml/min) or LH-NIH-B9 (10 μg/min; 0.64 ml/min) were given. Saline infusions were from 0–12 h; LH infusions were for 10 h and were preceded by a 2 h saline infusion. All animals were given 25 mg PGF_2α im at 6 h (6 h into the saline infusion and 4 h into the LH infusion). Blood samples were taken at 0.5 h, 1 h and 4 h intervals from 0–12, 13–18 h and 22–24 h respectively. Serum was assayed for LH and progesterone by radioimmunoassay methods. Two animals received saline and two received LH in each experiment. Eact treatment was replicated 6 times. LH infusion resulted in a mean serum LH of 57 ng/ml compared to 0.90 ng/ml in saline infused animals. This elevation of LH did not alter PGF_2α induced luteolysis as indicated by decline in serum progesterone. This experiment does not support the hypothesis that the newly formed corpus luteum is resistant to PGF_2α because of protection afforded by the protestrus LH surge.     

Failure of exogenous LH to prevent PGF2alpha-induced luteolysis in beef cows

Henderson and McNatty (Prostaglandins 9:779, 1975) proposed that LH from the preovulatory LH surge attached to receptors on luteal cells and that this attachment might protect the early corpus luteum from PGF2alpha induced luteolysis. To test this hypothesis, experiments were performed on heifers at day 10-12 of the cycle. Both jugular veins were catheterized and infusions of either saline (0.64 ml/min) or LH-NIH-B9 (10 microgram/min; 0.64 ml/min) were given. Saline infusions were from 0-12 h; LH infusions were for 10 h and were preceded by a 2 h saline infusion. All animals were given 25 mg PGF2alpha im at 6 h (6 h into the saline infusion and 4 h into the LH infusion). Blood samples were taken at 0.5 h, 1 h and 4 h intervals from 0-12h, 13-18 h and 12-42 h respectively. Serum was assayed for LH and progesterone by radioimmunoassay methods. Two animals received saline and two received LH in each experiment. Each treatment was replicated 6 times. LH infusion resulted in a mean serum LH of 75 ng/ml compared to 0.90 ng/ml in saline infused animals. This elevation of LH did not alter PGF2alpha induced luteolysis as indicated by decline in serum progesterone. This experiment does not support the hypothesis that the newly formed corpus luteum is resistant to PGF2alpha because of protection afforded by the proestrus LH surge.     

Stimulation of a pulse of LH and reduction in PRL concentration by a physiologic dose of GnRH before, during, and after luteolysis in heifers

A single physiologic dose (5.0 μg) of GnRH was given to 9 heifers each day (Hour 0) beginning on Day 15 postovulation until regression of the corpus luteum. Blood samples were taken each day for Hours -3, -2, -1, 0, 0.25, 0.5, 0.75, 1, 1.25, 1.50, 1.75, 2, 3, 4, and 5. Based on daily progesterone concentrations, data were grouped into phases of before (n=4), during (n=8), and after (n=7) luteolysis. The number of LH pulses with a peak at pretreatment Hours -2 or -1 (0.35 ± 0.12 pulses/sampling session) was less (P<0.0001) than for a pulse peak at posttreatment Hours 1 or 2 (1.0 ± 0.0 pulses/session). The characteristics and effects of LH pulses on progesterone and estradiol were similar between natural (pretreatment) and primarily induced (posttreatment) LH pulses. The same dose of GnRH stimulated an LH pulse with greater (P<0.05) amplitude after luteolysis than during luteolysis. Concentrations of PRL and number and prominence of PRL pulses decreased (P<0.05) between Hours 0 and 2 within each of the phases of before, during, and after luteolysis. The hypothesis that a physiologic dose of GnRH increases the concentration of PRL was not supported; instead, GnRH reduced the concentration of PRL. Results supported the hypotheses that an appropriate dose of GnRH stimulates an LH pulse during luteolysis that is similar to a natural pulse in characteristics and in the effects on progesterone and estradiol.     

Different effects of subnormal levels of progesterone on the pulsatile and surge mode secretion of luteinizing hormone in ovariectomized goats

This study tested the hypothesis that endocrinological threshold levels of progesterone that induce negative feedback effects on the pulsatile and surge modes of LH secretion are different. Our approach was to examine the effects of subnormal progesterone concentrations on LH secretion. Long-term ovariectomized Shiba goats that had received implants of silastic capsules containing estradiol were divided into three groups. The high progesterone (high P) group received a subcutaneous implant of a silastic packet (50 x 70 mm) containing progesterone, and the low progesterone (low P) group received a similar implant of a small packet (25 x 40 mm) containing progesterone. The control (non-P) group received no treatment with exogenous progesterone. Blood samples were collected daily throughout the experiment for the analysis of gonadal steroid hormone levels and at 10-min intervals for 8 h on Days 0, 3, and 7 (Day 0: just before progesterone treatment) for analysis of the pulsatile frequency of LH secretion. Then estradiol was infused into the jugular vein of all animals at a rate of 3 microg/h for 16 h on Day 8 to determine whether an LH surge was induced. Blood samples were collected every 2 h from 4 h before the start of the estradiol infusion until 48 h after the start of the infusion. In each group, the mean +/- SEM concentration after progesterone implant treatment was 3.3 +/- 0.1 ng/ml for the high P group, 1.1 +/- 0.1 ng/ml for the low P group, and <0.1 ng/ml for the non-P group, concentrations similar to the luteal levels, subluteal levels, and follicular phase levels of the normal estrous cycle, respectively. The estradiol concentration ranged from 4 to 8 pg/ml after estradiol capsule implants in all groups. The LH pulse frequency was significantly (P < 0.05) suppressed on Day 3 (6.2 +/- 0.5 pulses/8 h) and on Day 7 (2.6 +/- 0.9 pulses/8 h) relative to Day 0 (9.0 +/- 0.5 pulses/8 h) in the high P group. In both the low P and non-P groups, however, the changes of pulsatile frequency of LH were not significantly different, and high pulses (7-9 pulses/8 h) were maintained on each of the 3 days they were tested. An LH surge (peak concentration, 100.3 +/- 11.0 ng/ml) occurred in all goats in the non-P group, whereas there was no surge mode secretion of LH in either the high P or the low P group. The results of this study support our hypothesis that the threshold levels of progesterone that regulate negative feedback action on the LH pulse and the LH surge are different. Low levels of progesterone, around 1 ng/ml, completely suppressed the LH surge but did not affect the pulsatile frequency of LH secretion.     

Interrelationships of estradiol, inhibin, and gonadotropins during follicle deviation in pony mares

The changes in circulating concentrations of FSH, LH, estradiol, and total inhibin associated with the beginning of follicle diameter deviation were compared among the last anovulatory follicular wave of the year and the first and second ovulatory waves in pony mares ( n=7 ). Follicle diameters and circulating hormone concentrations for each wave were normalized to the observed beginning of deviation (Day 0). Follicle deviation was demonstrated during the anovulatory wave as well as during the ovulatory waves, and the diameter of the future dominant follicle at the beginning of deviation was similar for the three waves (overall mean: 23.7+/-0.6 mm). Circulating estradiol concentrations did not increase during the last anovulatory wave but increased similarly for the two ovulatory waves, beginning near the onset of deviation. There were no differences among waves in concentrations of inhibin encompassing deviation. The FSH concentrations for the wave-stimulating FSH surge did not differ significantly among the three waves; combined for the three waves, concentrations decreased between Days -3 and 7. Circulating LH did not increase during the last anovulatory wave but increased during the first and second ovulatory waves beginning on Days 6 and -2, respectively. Results indicated that the increase in circulating estradiol at the beginning of deviation was not required for suppression of the wave-stimulating FSH surge and the initiation of deviation, based on an estradiol increase in association with deviation during the ovulatory waves but not during the anovulatory wave. Concentrations of inhibin were similar among waves and, therefore on a temporal basis, the similar suppression of FSH was attributable to inhibin. The later increase in LH before the first ovulation was not attributable to estradiol, based on the similarity between the two ovulatory waves in the increasing estradiol concentrations.     

Effect of endogenous and exogenous progesterone on the oestradiol-induced LH surge in dairy cows

Four cows released an LH surge after 1.0 mg oestradiol benzoate administered i.m. during the post-partum anoestrous period with continuing low plasma progesterone. A similar response occurred in the early follicular phase when plasma progesterone concentration at the time of injection was less than 0.5 ng/ml. Cows treated with a progesterone-releasing intravaginal device (PRID) for 8 days were injected with cloprostenol on the 5th day to remove any endogenous source of progesterone. Oestradiol was injected on the 7th day when the plasma progesterone concentration from the PRID was between 0.7 and 1.5 ng/ml. No LH surge occurred. Similarly, oestradiol benzoate injected in the luteal phase of 3 cows (0.9-2.1 ng progesterone/ml plasma) did not provoke an LH surge. An oestradiol challenge given to 3 cows 6 days after ovariectomy induced a normal LH surge in each cow. However, when oestradiol treatment was repeated on the 7th day of PRID treatment, none released LH. It is concluded that ovaries are not necessary for progesterone to inhibit the release of LH, and cows with plasma progesterone concentrations greater than 0.5 ng/ml, whether endogenous or exogenous, did not release LH in response to oestradiol.     

Control of follicular development and luteal function in the mare: effects of a GnRH antagonist

Control of the equine estrous cycle was studied by suppressing gonadotropin secretion by administration of a GnRH antagonist to cyclic pony mares. Four mares received vehicle (control cycle) or a GnRH antagonist, Antarelix (100 microg/kg) on Day 8 of diestrus, and blood samples were collected at 15-min intervals from 0 to 16 h, 24 to 36 h, and daily until the next ovulation. Ovarian activity was monitored by transrectal ultrasonography, and measurement of plasma concentrations of progesterone and estradiol. Antagonist treatment eliminated large diestrous pulses of LH. Progesterone concentrations had fallen significantly in all mares by the day after treatment and, in three of the four mares, remained low until luteolysis. However timing of luteolysis (ie., progesterone concentrations <1 ng/mL) was not affected by antagonist treatment. The preovulatory surges of estradiol and LH were significantly delayed in the treatment cycle, as was the appearance of a preovulatory follicle >30 mm. Cycle length was significantly longer during the treatment than the control cycle. These results show that treatment of diestrous mares with a GnRH antagonist attenuated progesterone secretion, indicating a role for LH in control of CL function in the mare, and delayed ovulation presumably because of lack of gonadotropic support.     

Diurnal variation in LH and temporal relationships between oscillations in LH and progesterone during the luteal phase in heifers

Diurnal variation in progesterone and LH during the luteal phase and the temporal relationships between oscillations of the two hormones were studied in 10 heifers by collection of blood samples at 0100, 0700, 1300, and 1900 h each day, beginning on Day 1 (Day 0 = ovulation). Concentration of LH on Days 5-9, but not on Days 10-14, was lower (P < 0.05) at 0700 h (0.25 ± 0.02 ng/mL) than at each of the other three hours (combined, 0.32 ± 0.02 ng/mL). An oscillation was defined as an uninterrupted increase and decrease in concentrations. The number of LH oscillations/heifer with the peak at 1900 h (6.1 ± 0.7) throughout the luteal phase was greater (P < 0.01) than for each of the other hours (combined, 4.0 ± 0.2). Diurnal variation in progesterone was not detected. Only statistically defined LH oscillations were used to determine the temporal association between the peak of an LH oscillation and various components of a progesterone oscillation. On Days 5-14, the frequency of the peak of an LH oscillation occurring at the same hour as the peak of a progesterone oscillation (26/48, 54%) was greater (P < 0.0001) than at the progesterone nadir (3/48, 6%). The frequency of the LH peak occurring during increasing (11/34, 32%) and decreasing (8/25, 32%) progesterone concentrations was intermediate (P < 0.05). Results indicated the following: 1) diurnal variation occurred in LH as determined by concentration and by the hour of the peak of an oscillation; and 2) LH oscillations were temporally and positively related to progesterone oscillations.