Seasonal variation in oestrus, the secretion of oestrogen and progesterone and LH levels before ovulation in the ewe

Ewes with cervical ovarian autotransplants were studied after PGF-2alpha-induced luteal regression in November and February (mid- and late-breeding season) and June (anoestrum). Progesteron and oestradiol-17beta secretion rates and LH concentrations were determined in serial ovarian venous blood samples and the ewes were frequently tested for oestrus. All 4 ewes in November and 3 of the 4 ewes in February exhibited oestrus and endocrine changes indicative of ovulation. The remaining ewe in February and the three ewes in June failed to show elevated oestradiol-17beta secretion rates after luteal regression, indicating the absence of follicles in the final stages of maturation. The preovulatory rise in the oestradiol-17beta secretion rate, the LH surge and the display of oestrus all occurred earlier, with respect to the PGF-2alpha infusion, in November than in February, suggesting a greater stimulation of folliculogenesis, and therefore a greater availability of maturing follicles, in November than in February.     

Duration of oestrus, ovulation rate, time of ovulation and plasma LH, total oestrogen and progesterone in Galway adult ewes and ewe lambs

The mean duration of oestrus, ovulation rate, duration of the preovulatory LH discharge, time interval between sponge removal and beginning of the LH discharge, total LH discharged, maximum LH value observed and the concentration of progesterone in the peripheral plasma during the luteal phase of the oestrous cycle was similar in Galway adult ewes and 8-month-old ewe lambs after treatment with intravaginal sponges containing 30 mg cronolone for 12 days and injection of 500 i.u. PMSG. The interval between sponge removal and the onset of oestrus was shorter for adults than for ewe lambs; the interval between the onset of oestrus and the beginning of the LH discharge was longer in adults. During the period 12-36 h after sponge removal the mean plasma total oestrogen concentration was significantly higher in lambs than in adults. In a separate study of the time of ovulation in Galway ewe lambs given the same progestagen-PMSG treatment, ovulation did not occur in any lamb before 17 h after the onset of oestrus and the majority ovulated close to the end of oestrus.     

Effect of mouse epidermal growth factor on plasma concentrations of FSH, LH and progesterone and on oestrus, ovulation and ovulation rate in merino ewes

The 24 h i.v. infusion of Merino ewes with 60 or 100 microgram mouse epidermal growth factor (EGF)/kg body weight on Days 4, 9 or 14 of the oestrous cycle decreased the strength of wool attachment and caused marked changes in subsequent reproductive performance. In ovaries removed 2 days after EGF treatment all follicles greater than or equal to 0.6 mm diameter were atretic. After 7 days either a normal pattern of atresia or no atresia was evident while after 12 days the pattern of follicular atresia was similar to that in controls. Irrespective of stage of cycle EGF caused dose-dependent increases in plasma FSH concentrations that persisted for up to 14 days. Changes in plasma LH concentrations were generally similar after infusion on Days 4 and 14, but were smaller and shorter-lived after infusion on Day 9. Irrespective of dose, the infusion of EGF on Days 4 and 14 caused immediate luteolysis then the formation of a luteinized follicle in many ewes. Most ewes treated on Day 4 returned to oestrus between Days 17 and 21 with the same ovulation rate (1.3) as the controls. Of those infused on Day 14 oestrus occurred about a cycle length later than expected and their ovulation rate then (1.9) was also similar to that of the controls (1.7). Luteal function was not affected in ewes infused on Day 9, and most returned to oestrus between Days 17 and 20 with an ovulation rate of 3.2. Fertile rams were not placed with the ewes until after the differences in ovulation rate had been observed. Mating occurred generally 2-4 weeks after treatment, and there were no differences between EGF-treated and control ewes in fertility or fecundity. The results are interpreted as indicating that mouse EGF induces ovarian follicular atresia but has differential effects on luteal function according to the stage of the oestrous cycle at which it is given. As a consequence of these two effects, which lead to differential changes in gonadotrophin secretion, ovarian function may be temporarily impaired, little affected or improved.     

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.     

Effect of passive immunization against inhibin on FSH secretion, folliculogenesis and ovulation rate during the follicular phase of the estrous cycle in mares

Physiological roles of inhibin in mares were investigated by means of passive immunization using an antiserum to inhibin that had been raised in a castrated goat. Eight mares were given an intravenous injection of either 100 mL (n = 4) or 200 mL (n = 4) of inhibin antiserum 4 d after a single intramuscular injection of PGF2 alpha on Day 8 after ovulation, 4 control mares were treated with 100 mL castrated goat serum in the same manner. Jugular vein blood samples were collected after treatment with the serum until 192 h post treatment. Follicular growth and ovulations were monitored by ultrasound examination at 24-h intervals. The ability of the inhibin antiserum to neutralize the bioactivity of equine inhibin was examined in vitro using a rat pituitary cell culture system. Suppression of secretion of FSH from cultured rat pituitary cells by equine follicular fluid was reversed by the addition of increasing doses of the inhibin antiserum, thereby indicating its bioactivity. Plasma levels of FSH and estradiol-17 beta were higher in mares treated with the inhibin antiserum. The ovulation rate was significantly higher in mares treated with antiserum (100 mL = 3.75 +/- 0.63; 200 mL = 4.50 +/- 0.65) than in control mares (1.25 +/- 0.25). These results demonstrate that inhibin is important in regulating FSH secretion and folliculogenesis in mares. They also show that neutralization of the bioactivity of inhibin may become a new method for the control of folliculogenesis and ovulation rate in mares.     

The effect of active immunization against progesterone on plasma concentrations of total and free progesterone, estradiol-17beta and LH in the cyclic ewe

Nine mature cyclic ewes were actively immunized against progesterone which was rendered immunogenic by conjugation to bovine serum albumin (BSA). Seven control ewes were immunized with BSA. In ewes immunized against progesterone, the concentration of total plasma progesterone increased to 24.3 ng/ml vs 2.8 ng/ml in control animals (P<0.001). However, immunization did not affect the plasma levels of free, unbound progesterone. The correlation coefficient between total plasma progesterone concentrations on Days 4 to 11 of the estrous cycle and antibody titer was r=0.983. Estradiol-17beta concentrations in immunized ewes were higher than in controls on Days 6 to 15 of the estrous cycle (P approximately 0.05). Frequent sampling for LH over a 6-h period on Days 2, 5, 8, 11 and 14 of the cycle revealed no significant differences in the frequency and amplitude of LH pulses between immunized and control ewes. The immunized ewes had estrous cycles of normal length and maintained normal pregnancies. It is suggested that the immunized cyclic ewe is capable of maintaining adequate levels of free progesterone by greatly increasing progesterone synthesis, thus neutralizing the effect of the antibodies.     

Effect of active immunization against oxytocin on gonadotrophin secretion and the establishment of pregnancy in the ewe

A group of 14 ewes was actively immunized against oxytocin coupled to carrier protein, and comparisons of the reproductive status of these animals were made against ewes immunized against carrier protein only (N = 5) and untreated controls (N = 6). The last two groups were indistinguishable and were therefore combined as a single control group for analysis of the results. Oestrous cycle lengths were significantly extended in oxytocin-immunized ewes (P less than 0.005) with 42% of cycles lasting greater than 18 days. Cloprostenol treatment in the mid-luteal phase resulted in apparently normal luteal regression and re-ovulation, but luteal phase FSH concentrations and follicular phase LH concentrations were elevated in the immunized ewes, although surge levels of both hormones were unaffected. Measurements of free oxytocin concentrations in the blood suggested that these were significantly raised in treated animals. Progesterone concentrations in peripheral plasma were not altered by treatment. Mating resulted in a conception rate of 91% in control ewes compared with only 28% in oxytocin-immunized animals (P less than 0.01). There was no evidence of any conceptus material in the uteri of non-pregnant immunized ewes 25 days after service. Some had re-ovulated, whereas the ovaries of others contained mature corpora lutea which had been maintained. Ovarian histology appeared normal. We conclude that active immunization against oxytocin influences gonadotrophin secretion and reduces fertility. The site(s) of action for both of these effects needs to be determined.     

Effect of progesterone on basal LH and episodic LH and FSH secretion in heifers

Heifers between Days 6 and 10 of the cycle were allocated at random to groups of 8 and treated with (i) a 4% progesterone-releasing intravaginal device (PRID) + oestrogen capsule for 12 days; (ii) 4% PRID for 12 days; (iii) 20% PRID for 12 days; (iv) 4% for 14 days; or (v) 20% PRID for 14 days. Blood was obtained daily during treatment and at 2- or 4-h intervals for 72 h after removal of PRIDs. Some animals were sampled every 20 min for 4.676 h on the 3rd day after PRID insertion, and 1 day before and 36 h after removal of the PRID insertion, and 1 day before and 36 h after removal of the PRID. During progesterone treatment there was: (i) no correlation between concentrations of progesterone and LH within days; (ii) a significant negative correlation between progesterone and days (P less than 0.01) and also between progesterone and LH over days (P less than 0.01); (iii) the overall correlation co-efficient between LH and days was positive (P less than 0.05). The amplitude of LH or FSH episodes was not affected as progesterone concentrations declined during PRID treatment, but the number of LH (but not FSH) episodes was increased (p less than 0.01). After PRID removal, the amplitude of both LH and FSH episodes increased (P less than 0.01). We suggest that progesterone is part of a negative feedback complex on LH secretion in cattle and that this effect is apparently mediated through frequency of episodic LH release.     

Opioid modulation of LH secretion in the ewe

Administration of opioid agonists and antagonists and measurement of resulting hormone changes were used to study the possible effects of opioids on reproductive function in the ewe. Intravenous administration of the long-acting methionine-enkephalin analogue FK33-824 (250 micrograms/h for 12 h) to 3 ewes during the follicular phase of the oestrous cycle depressed episodic LH secretion. This effect was reversed by administration of the opiate antagonist naloxone (25 mg/h) in combination with the FK33-824 treatment; in fact LH secretion was enhanced by the combined regimen. Naloxone (25 mg/h for 12 h) administered alone to 3 ewes in the follicular phase also enhanced LH secretion. In 3 animals treated with FK33-824 during the follicular phase, progesterone remained basal for 14 days after treatment, suggesting that ovulation was blocked. Jugular venous infusion of naloxone (25, 50 or 100 mg/h for 8h) into 5 ewes during the early and mid-luteal phase of the cycle resulted overall in a significant increase in mean plasma LH concentrations and LH episode frequency. To investigate whether endogenous opioids suppress LH release in seasonally anoestrous sheep, naloxone was infused intravenously into mature (25, 50 or 100 mg/h for 8 h) and yearling ewes (12 . 5, 25 or 50 mg/h for 8 h) during early, mid- and late anoestrus and plasma LH concentrations were measured. In the mature ewes, there was a trend for naloxone to increase LH values during the early anoestrous period but naloxone was without effect during mid- and late anoestrus. In the yearlings, naloxone infusion consistently increased plasma LH concentrations as a result of a significant increase in LH episode frequency. These experiments indicate that endogenous opioid peptides probably modulate gonadotrophin secretion during both the follicular and luteal phases of the oestrous cycle. However, the follicular phase of the sheep cycle is of short duration, and there may be residual effects of luteal-phase progesterone during this period. Secondly, there may be an age-dependent effect of naloxone on LH secretion during seasonal anoestrus in the ewe, with opioids playing a part in the suppression of LH in young but not in mature animals.     

Influence of perioestrous suprabasal progesterone levels on cycle length,oestrous behaviour and ovulation in heifers

In order to induce suprabasal plasma concentrations of progesterone after luteolysis and to determine their effect on oestrous behaviour and ovulation, heifers subcutaneously received silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3) or 10 (n = 4) g of progesterone, or an empty implant (controls, n = 5) between days 8 and 25 of the cycle (ovulation designated Day 0). Growth of dominant follicles and time of ovulation were determined by ultrasound, and signs of oestrus were recorded and scored. Blood was collected at 2–4 h intervals from Days 15 to 27 and assayed for progesterone concentration. In all heifers, plasma concentrations of progesterone sharply decreased during Days 16–18. Control heifers had their lowest progesterone levels on Days 20.5 and 21, standing oestrus on Day 19.5 ± 0.4 (mean ± SEM), and ovulated on Day 20.7 ± 0.4. A similar pattern was observed in heifers treated with 2.5 and 5 g progesterone. Heifers treated with 6, 7.5 and 10 g of progesterone showed an extended (P < 0.05) interovulatory interval. Onset of prooestrus and time of maximum expression of signs of oestrus were not significantly different from those in controls. However, there was an absence of standing oestrus in most of the cases, signs of oestrus lasted longer (P < 0.05) and were weaker in intensity when doses increased. In these groups, the lowest progesterone concentrations were attained shortly after implant removal. Some heifers treated with 6 and 7.5 g of progesterone had standing oestrus and post oestrous bleeding as seen in the controls but ovulation occurred from Days 24.5 to 27. When plasma progesterone concentrations were over 1 nmol 1⁻¹, disturbed oestrus and delayed ovulation occurred. The extended period of prooestrus and oestrus and delayed ovulation were similar to that described in cases of repeat breeding. It is suggested that suprabasal plasma concentrations of progesterone, after luteolysis, may lead to asynchrony between onset of oestrus and ovulation and consequently be a cause of repeat breeding in cattle.     

Influence of nutrition on ovulation rate in the ewe

The effect of nutrition on ovulation rate in the ewe is reviewed with particular reference to the role of protein and energy and the time of effect during the cycle. Ovulation rate is increased by both protein and energy. In the case of protein this was shown to be accompanied by increased plasma levels of FSH and androstenedione at about the time of luteolysis, while levels of LH were unaffected. Increased hepatic oxidative enzyme activity is proposed as a mechanism by which nutrient intake may influence ovulation rate.     

Influence of oxytocin infusion during oestrus and the early luteal phase on progesterone secretion and the establishment of pregnancy in ewes

In Experiment 1, an osmotic minipump containing oxytocin was implanted s.c. in ewes for 12 days beginning on Day 10 of the oestrous cycle, producing approximately 100 pg oxytocin/ml in the plasma. Two days after the start of infusion, all ewes were injected with 100 micrograms cloprostenol and placed with a fertile ram. At slaughter 22 days later, 9 (75%) of the 12 control (saline-infused) ewes were pregnant compared with 1 (11%) of the 9 ewes infused with oxytocin. In the control group, midcycle plasma concentrations of oxytocin were significantly higher in nonpregnant than in pregnant ewes. In Experiment 2, an infertile ram was used throughout to avoid any possible effects of pregnancy and oxytocin infusions were given at different stages of the oestrous cycle. Otherwise the protocol was similar to that in Exp. 1. Oxytocin infusion during luteolysis and the early follicular phase had no effect on the subsequent progesterone secretion pattern, but infusions beginning the day before cloprostenol-induced luteolysis and lasting for 7 or 12 days and infusions beginning on the day of oestrus for 4 days all delayed the subsequent rise in plasma progesterone by approximately 3-4 days. In these animals, the cycle tended to be longer. It was concluded that an appropriate oxytocin secretion pattern may be necessary for the establishment of pregnancy in ewes and that a high circulating oxytocin concentration during the early luteal phase delays the development of the young corpus luteum.     

Ability of ram introduction to induce LH secretion, estrus and ovulation in fall-born ewe lambs during anestrus.

The ability of ram introduction (RI) and progesterone pre-treatment to induce increases in LH secretion and ovulation, and the ability of progesterone pre-treatment with or without estrogen to induce estrus and ovulation in fall-born ewe lambs during seasonal anestrus was investigated. In early July, lambs of mixed breeds (41.8+/-0.6 kg and 250.7+/-1.3 days of age) were assigned to receive no treatment (C, n=7), to be introduced to rams (7:1 ewe:ram ratio; R, n=7), to be treated with progesterone (a used CIDR device) for 5 days (P, n=5), to be treated with progesterone and introduced to rams at CIDR removal (PR, n=11), or to receive the latter treatment plus an injection of estradiol benzoate (25 microg, E2beta i.m.) 24 h after CIDR withdrawal/RI (PER, n=11). Blood samples were collected from all lambs every 4h for 60 h beginning at RI/CIDR withdrawal (0 h), to characterize the LH surge profile and in groups R and C every 15 min for 8 h between 12 and 20 h for determination of LH pulse frequencies. Ultrasonographic examinations of the ovaries were conducted at 0, 36 and 60 h. In ram-exposed groups lambs were also observed for raddle marks every 4h from 0 to 60 h. The LH pulse frequency (pulses/8 h) was higher in group R (P<0.01; 7.7+/- 0.5) than group C lambs (2.7+/- 0.8). More lambs in groups exposed to rams than in the C or P groups showed an LH surge (P<0.05; 0, 100, 0, 72.7 and 100%, for C, R, P, PR and PER groups, respectively). Time from RI/CIDR removal to initiation of the LH surge was greater in lambs in the PR (43.5+/- 3.8h) than in the R (32.6+/- 4.6h; P=0.08) or PER (33+/- 1.2h; P<0.01). Diameter of the largest follicle at 0 h (3.2+/- 0.2mm) was not different among groups. Growth rate of the largest follicle between 0 and 36 h was greater (P<0.05) in RI than in C or P groups. Diameter of the largest follicle at 36 h was larger (P<0.05) in lambs in R (5.6+/- 0.2mm) and PR (5.1+/- 0.5mm) than C (4.0+/- 0.6mm) or P (3.8+/- 0.4mm) groups, and in R than PER (4.3+/- 0.4mm) treatment groups. Only lambs in the RI groups ovulated. Among RI groups the percentage of lambs ovulating was greater in the R (P<0.05; 85.7%) than PR (33.3%) groups with an intermediate response observed in lambs in treatment group PER (71.4%). The estrous response in progesterone pre-treated groups was greater (P<0.05) in lambs also treated with estrogen (PER; 81.8%), than in lambs introduced to rams alone (PR; 45.5%). In conclusion, ram introduction by itself, but not progesterone treatment alone, induces increases in LH pulse frequency, follicular development, and ovulation in fall-born ewe lambs during seasonal anestrus, further, P4 pre-treatment and RI when combined with E2 results in a high estrous response.     

Effect of high progesterone concentrations during the early luteal phase on the length of the ovulatory cycle of goats

The effect of exogenous progesterone exposure early in the oestrous cycle on the duration of the interovulatory interval was studied in dairy goats. A controlled intravaginal drug release (CIDR-G) device was inserted for 5 days starting at day 0 (D0 group, n=6) or day 3 (D3 group, n=5) postovulation. A third group was composed of untreated control goats (control group, n=7). Daily transrectal ultrasound was carried out during the interovulatory interval to assess the ovarian dynamics. Oestrous behaviour was checked twice a day and serum progesterone levels were assayed in daily jugular blood samples. Treated goats showed two different responses. In three D0 goats and one D3 goat, progesterone concentrations fell immediately after CIDR withdrawal and this was followed by oestrus and ovulation between days 8 and 11 (short cycles). In the other three D0 goats and in four D3 goats the treatment significantly reduced the interovulatory interval (18.3+/-0.3 and 18.5+/-0.3 days, respectively) (shortened cycles) compared with the control group (20.0+/-0.2 days; P<0.05), but the intervals with progesterone concentrations over 1 ng/ml were not different (15.7+/-0.3, 15.8+/-0.7 and 16.0+/-0.5 days for D0, D3 and control goats, respectively). In all D0 goats with a short cycle response, the ovulatory follicle arose from the first follicular wave but in the D3 goat with a short cycle it arose from the second follicular wave. These results showed that premature progesterone exposure early in the ovulatory cycle of the goat affected its length inducing short or shortened cycles. The effect of progesterone could either affect luteotropic support of the corpus luteum (CL) and/or stimulate a premature release of the luteolysin.     

The effect of the infusion of insulin during the luteal phase of the estrous cycle on the ovulation rate and on plasma concentrations of LH, FSH and glucose in ewes

The role of insulin in mediating pituitary responses to nutrition was investigated in 30 mature Border Leicester X Merino ewes. The ewes were infused with saline (n = 15) or bovine insulin at 0.4 IU/kg/d (n = 15) for 72 h during the luteal phase of the estrous cycle The ewes were housed in individual pens and were fed, ad libitum, a diet of low quality straw. Their estrous cycles were synchronized with prostaglandin (PG), with infusions given over Days 9 to 11 of the estrous cycle. A further injection of PG was given at the end of the infusion, and the subsequent ovulation rate was determined by endoscopy 12 d later. Blood samples were collected every 4 h from Day 8 until 52 h after the final PG injection for the determination of plasma FSH, insulin and glucose concentrations. On Day 11 blood samples were also taken every 20 min for 24 h for the determination of LH pulse characteristics. During the infusion of insulin, its concentration rose 4-fold and remained elevated until the end of infusion, when it fell to pretreatment concentrations. Glucose concentrations were significantly reduced during the insulin infusion and rose to pretreatment concentrations after infusion. In control ewes glucose and insulin concentrations did not change. Ovulation rate of treated ewes was not affected by the insulin (1.9 +/- 0.07) compared with that of control ewes (2.0 +/- 0.10). Neither were FSH concentrations affected by treatment with insulin, although a significant interaction of treatment with time was observed in the 36 h after infusion. The pre-ovulatory decline in FSH concentrations was delayed by about 8 h in the insulin treated ewes. The mean (+/- SEM) LH pulse frequency (4.3 +/- 0.4 vs 1.8 +/- 0.3 pulses per 24 h) and the mean (+/- SEM) concentration of LH (0.48 +/- 0.04 vs 0.32 +/- 0.03 ng/ml) were both significantly reduced by insulin. These results indicate that insulin-induced hypoglycaemia inhibits LH secretion in cyclic ewes and implicates insulin as a mediator of normal hypothalamo-pituitary function.     

Effect of stress due to laparoscopy on plasma cortisol levels, the preovulatory surge of LH, and ovulation in the ewe

The ovulation which is induced in seasonally anovular ewes by the introduction of rams was used as a model to examine the effects of the stress of repeated laparoscopy on the preovulatory surge of LH and ovulation. The number of ewes which experienced LH surges and ovulation was reduced (10 23 vs 1 20 ) when laparoscopy was repeated every four hours. Plasma cortisol levels were used as an indicator of stress, and were found to increase after laparoscopy: a single laparoscopy produced a transient rise, while repeated laparoscopy induced and maintained levels between 70 and 100 ng/ml.     

Relationship of oestrus synchronization method, circulating hormones, luteinizing hormone and prostaglandin F-2 alpha receptors and luteal progesterone concentration to premature luteal regression in superovulated sheep

Ewes were treated with exogenous follicle-stimulating hormone (FSH) and oestrus was synchronized using either a dual prostaglandin F-2 alpha (PGF-2 alpha) injection regimen or pessaries impregnated with medroxy progesterone acetate (MAP). Natural cycling ewes served as controls. After oestrus or AI (Day 0), corpora lutea (CL) were enucleated surgically from the left and right ovaries on Days 3 and 6, respectively. The incidence of premature luteolysis was related (P less than 0.05) to PGF-2 alpha treatment and occurred in 7 of 8 ewes compared with 0 of 4 controls and 1 of 8 MAP-exposed females. Sheep with regressing CL had lower circulating and intraluteal progesterone concentrations and fewer total and small dissociated luteal cells on Day 3 than gonadotrophin-treated counterparts with normal CL. Progesterone concentration in the serum and luteal tissue was higher (P less than 0.05) in gonadotrophin-treated ewes with normal CL than in the controls; but luteinizing hormone (LH) receptors/cell were not different on Days 3 and 6. There were no apparent differences in the temporal patterns of circulating oestradiol-17 beta, FSH and LH. High progesterone in gonadotrophin-treated ewes with normal CL coincided with an increase in total luteal mass and numbers of cells, which were primarily reflected in more small luteal cells than in control ewes. Gonadotrophin-treated ewes with regressing CL on Day 3 tended (P less than 0.10) to have fewer small luteal cells and fewer (P less than 0.05) low-affinity PGF-2 alpha binding sites than sheep with normal CL. By Day 6, luteal integrity and cell viability was absent in ewes with prematurely regressed CL. These data demonstrate that (i) the incidence of premature luteal regression is highly correlated with the use of PGF-2 alpha; (ii) this abnormal luteal tissue is functionally competent for 2-3 days after ovulation, but deteriorates rapidly thereafter and (iii) luteal-dysfunctioning ewes experience a reduction in numbers of small luteal cells without a significant change in luteal mass by Day 3 and, overall, have fewer low-affinity PGF-2 alpha binding sites.     

Pattern of ovarian progesterone secretion during the luteal phase of the ovine estrous cycle

Pulsatile secretion of progesterone has been observed during the late luteal phase of the menstrual cycle in the rhesus monkey and human. As the luteal phase progresses in each of these species, there is a pattern of decreased frequency and increased amplitude of progesterone pulses. The present study was designed to determine the pattern of progesterone secretion during the late luteal phase (Days 10-16) of the normal ovine estrous cycle. Five unanesthetized ewes, each bearing an indwelling cannula in the utero-ovarian vein, were bled every 15 min from 0800 h on Day 10 through 0800 h on Day 16 of the estrous cycle. With the computer program PULSAR, it was determined that progesterone secretion was episodic, with pulsations observed on all days. Analysis of variance was used to determine differences in frequency, amplitude, and interpeak interval (IPI) of progesterone pulses among ewes and days. The ewes averaged 8.0 +/- 0.63 pulses of progesterone per 24 h. Mean frequency of pulses was not different between days but showed differences between ewes. Mean amplitude of progesterone pulses was 7.0 +/- 0.27 ng/ml, with no differences observed either between days or between ewes. Mean IPI was 197 +/- 7.1 min, and, like frequency, the IPI was not different between days, but varied between ewes. No consistent temporal relationship was found between progesterone pulses and luteinizing hormone (LH), as determined by bioassay and radioimmunoassay, on Day 14 of the cycle in one ewe. The results indicate that progesterone secretion is episodic during the luteal phase of the ovine estrous cycle and is independent of LH pulses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of chronic treatment with a GnRH agonist on oestrous behaviour and on the secretion of LH and progesterone in the ewe

A study was carried out to investigate a novel approach to oestrus synchronization in the ewe by treatment with a gonadotrophin releasing hormone (GnRH) agonist. Groups of ewes were initially treated on Day 2, 10 or 14 of the oestrous cycle with 10 mug GnRH analogue (D-Ser(Bu(t)) 6 des Gly GnRH ethylamide) per ewe per day for 14 days. Behavioural oestrus was inhibited during GnRH agonist treatment and recurred from 8 to 38 days after the treatment in an unsynchronized manner. Luteal activity during treatment was not impaired but reduced progesterone concentrations occurred in cycles after the treatment. The rhythm of ovarian function, generally characterized by prolonged follicular development, was impaired. During the treatment and subsequent recovery period, integrity of pituitary function was examined by measuring luteinizing hormone (LH) after GnRH agonist was injected, and after stimulation test doses of 150 ng natural GnRH were administered. During treatment there was, with time, a decline in pituitary response to the agonist which suggested that pituitary release of LH was exhausted. After the 14-day treatment the stimulation test with GnRH revealed a gradual return to normal responsiveness although this was not complete three weeks after the treatment when compared to control ewes. This lowered pituitary activity could cause the impaired ovarian function.