Oxytocin does not contribute to the effects of cervical dilation on progesterone secretion and embryonic development in mares

The aim of the present study was, to investigate the effects of oxytocin administration on Day 7 post-ovulation on progesterone secretion, pregnancy rate and embryonic growth in mares. Endogenous stimulation of oxytocin release was compared to the administration of native oxytocin or the long-acting oxytocin analogue carbetocin. At Day 7 after ovulation, mares had to undergo four treatments in a crossover design: (a) control, (b) oxytocin (10 IU i.v.), (c) carbetocin (280 microg i.m.) and (d) cervical dilation. On Day 13, all mares (8 of 8 mares) were pregnant on groups control, oxytocin and carbetocin and only 6 of 8 mares on group dilation. In one mare uterine fluid accumulation and uterine edema from Day 6 to 13 and early embryonic death by Day 11 occurred during dilation treatment. Another mare, which did not become pregnant during dilation treatment, developed uterine fluid accumulation and uterine edema from Day 10 to 14. Mean growth rates of the conceptuses did not differ among treatment groups and individual growth rates varied in a wide range from -0.1 to 0.8 cm per day. At Day 13, mean diameters of conceptuses yielded 1.4+/-0.1 cm in control group, 1.5+/-0.1 in oxytocin and carbetocin group and 1.3+/-0.2 cm in dilation group. Secretion of progesterone was not affected by treatments. Administration of oxytocin and carbetocin caused similar maximum plasma concentrations of oxytocin, but onset and duration of peaks differed. Maximum concentrations after intramuscular application of carbetocin were obtained almost 20 min later when compared to intravenous administration of oxytocin. Duration of peaks after injection of the long-acting oxytocin analogue was more than three-fold longer than after administration of native oxytocin. In conclusion, the present study showed that single administration of oxytocin or its long-acting analogue carbetocin at Day 7 after ovulation did not affect progesterone secretion, pregnancy rate and embryonic growth. Two possible scenarios concerning the effects of cervical dilation were observed: In the majority of mares, dilation of the caudal half to two-third of the cervical lumen up to a diameter of 4.5 cm had no negative consequences on progesterone secretion and pregnancy outcome. However, cervical dilation caused uterine inflammation and subsequent luteolysis in two mares and early embryonic death in one of them. Thus, manipulation of the cervix itself seems not to have negative impact on success rates of transcervical transfer of embryos in the mare.     

Controlling interrelationships of progesterone/LH and estradiol/LH in mares

The interrelationships of progesterone, estradiol, and LH were studied in mares (n=9), beginning at the first ovulation (Day 0) of an interovulatory interval. An increase in mean progesterone concentrations began on Day 0 and reached maximum on Day 6, with luteolysis beginning on Day 14. A common progesterone threshold concentration of about 2 ng/ml for a negative effect on LH occurred at the beginning and end of the luteal phase. Progesterone and LH concentrations decreased at a similar rate from Day 6 until the onset of luteolysis on Day 14, consistent with a decreasing positive effect of LH on progesterone. Concentrations of LH during the increase in the ovulatory surge consisted of two linear regression segments involving a rate of 0.4 ng/ml/day for Days 14-22 and 1.8 ng/ml/day for Day 22 to 1 day after the second ovulation. The end of the first segment and beginning of the second segment was 2 days before ovulation and was the day the ovulatory estradiol surge was at a peak.     

Urinary and plasma estrogen conjugates, estradiol and estrone concentrations in nonpregnant and early pregnant mares

A direct radioimmunoassay for estrogen conjugates (EC) was applied to paired blood and urine samples collected from 20 mares and compared against estrone (E(1)) and estradiol-17beta (E(2)) to monitor changes in estrogen production during ovulatory cycles and early pregnancy. Blood samples were taken daily from five mares through two consecutive ovulations and from six mares at 6-h intervals starting 48 hours prior to ovulation and continuing after ovulation had occurred. Blood samples were also collected daily or three times per week from conception until Day 60 of pregnancy in nine pregnant mares. The mean urinary EC, plasma EC and plasma E(2) dynamics were parallel in nonpregnant mares, with a 3-fold increase in mean urinary EC concentrations from baseline to the ovulatory peak, a 1.8-fold increase in mean plasma EC concentrations and a 1.4-fold increase in mean plasma E(2) concentrations. In early pregnancy, a two-fold increase in mean plasma E(1) and EC concentrations occurred in concert with a five-fold rise in mean urinary EC concentrations, whereas plasma E(2) did not change. Following hydrolysis and chromatographic separation, E(1) and E(2) were identified as the hydrolytic products in the urine of nonpregnant and pregnant mares; however, an unidentified estrogen was the major hydrolytic product in nonpregnant mares and pregnant mares prior to Day 38 of pregnancy. The increased resolution of the EC profiles compared with the profiles of other estrogen components indicates that the determination of EC in urine or plasma provides a useful alternative method for monitoring reproductive events in mares.     

Deslorelin on Day 8 or 12 postovulation does not luteinize follicles during an artificially maintained diestrous phase in the mare

Practical estrus synchronization schemes are needed for mares. The Ovsynch synchronization protocol for cattle involves the administration of gonadotropin-releasing hormone (GnRH) to induce ovulation or luteinization of dominant follicles during the luteal phase and prostaglandin 7 days later to cause regression of any luteal tissue and development of a preovulatory follicle. An Ovsynch-type synchronization program potentially could be developed for horses if luteinization or ovulation of diestrous follicles occurred in response to GnRH treatment. The objective of this study was to determine if administration of the GnRH agonist, deslorelin acetate, on Day 8 or 12 postovulation would induce luteinization or ovulation of diestrous follicles in the mare. The model used was cycling mares maintained in an artificial luteal phase by administration of a synthetic progestin following prostaglandin-induced luteal regression. On the day of ovulation, 21 light horse mares were randomly assigned to one of three groups: (1) no GnRH, altrenogest from Days 5 to 15 postovulation with prostaglandin on Day 15; (2) GnRH on Day 8, altrenogest from Days 5 to 15 with prostaglandin given on Day 6 to induce luteolysis of the primary corpus luteum, an implant containing 2.1mg of deslorelin acetate inserted on Day 8 and removed on Day 10, with a second prostaglandin treatment on Day 15; (3) GnRH on Day 12, altrenogest from Days 9 to 19, prostaglandin on Day 10, a deslorelin acetate implant injected on Day 12 (subsequently removed on Day 14), and a second dose of prostaglandin administered on Day 19. Follicular development was monitored every other day from Day 5 until a 30-mm sized follicle was observed, and then daily to detection of ovulation. Serum progesterone concentrations were determined daily for 12 consecutive days. Progesterone concentrations in Group 1 remained elevated until approximately Day 12 postovulation. Prostaglandin administration on Day 15 resulted in complete luteolysis in all seven mares. In Group 2, progesterone concentrations in six of seven mares declined to baseline after prostaglandin treatment. No increase in serum progesterone was noted in any of the six mares that were given GnRH on Day 8, including three mares that had diestrous follicles > or =30mm in diameter at the time of treatment. Similarly, progesterone concentrations in six of seven mares in Group 3 declined to baseline after prostaglandin and there was no increase in progesterone after administration of GnRH on Day 12. No ultrasound evidence of luteinization or ovulation of diestrous follicles were noted after GnRH administration in any mares of Group 2 or 3. In conclusion, administration of the GnRH agonist deslorelin acetate to mares failed to induce luteinization or ovulation of diestrous follicles. Consequently, the Ovsynch program (as used in cattle) has little efficacy for synchronization of estrus in mares.     

Progestagen synchronisation in the absence of a corpus luteum results in the ovulation of a persistent follicle in cyclic ewe lambs

Progestagens are widely used to synchronise oestrous in sheep but the effects on follicular dynamics are not clear. We tested the hypothesis that when luteolysis occurs early during progestagen synchronisation prolonged growth of the ovulatory follicle will occur. Cyclic ewe lambs (40.0+/-0.3 kg) were divided into three groups: eight ewes (Long group) received a progestagen sponge (60 mg medroxyprogesterone acetate) from Days 5 to 19 after oestrous and eight ewes (Short group) received a progestagen sponge on Day 5 which was replaced on Day 10 and again on Day 15, and removed on Day 19 after oestrous. On Days 6 and 7, ewes in both groups received prostaglandin. A third group (n=5, Control) did not receive any treatment. The growth and development of follicles > or =2 mm in diameter were characterised using daily transrectal ultrasonography. On Day 18, blood samples were collected every 12 min for 8 h from five ewes in the Long and Short groups. Data were analysed by ANOVA. The maximum diameter and age (emergence to ovulation) of the ovulatory follicle was greater (P<0.01) in ewes in the Long group (7. 4+/-0.2 mm and 12.1+/-0.6 days) than in ewes in the Short group (6. 3+/-0.2 mm and 5.1+/-0.5 days) and Control group (6.3+/-0.4 mm and 6. 8+/-0.6 days). On Day 18 of the cycle, LH pulse frequency and oestradiol concentrations were greater (P<0.05) in ewes in the Long group (3.2+/-1.1 pulse per 8 h and 1.15+/-0.09 pg ml(-1)) than the Short group (0.8+/-0.4 pulses per 8 h and 0.54+/-0.08 pg ml(-1)).We suggest that the negative feedback efficacy of a long-term progestagen sponge decreased with time and led to an increase in LH pulse frequency and prolonged growth of the ovulatory follicle. We conclude that, in the absence of luteal progesterone, synchronisation with a single progestagen sponge for 14 days resulted in higher LH pulse frequency and ovulation of a persistent follicle with a larger maximum diameter, compared with controls.     

Effects of oxytocin on cloprostenol-induced luteolysis, follicular growth, ovulation and corpus luteum function in heifers

Twenty-five normally cyclic Holstein heifers were used to examine the effects of oxytocin on cloprostenol-induced luteolysis, subsequent ovulation, and early luteal and follicular development. The heifers were randomly assigned to 1 of 4 treatments: Group SC-SC (n=6), Group SC-OT (n=6), Group OT-SC (n=6) and Group OT-OT (n=7). The SC-SC and SC-OT groups received continuous saline infusion, while Groups OT-SC and OT-OT received continuous oxytocin infusion (1:9 mg/d) on Days 14 to 26 after estrus. All animals received 500 microg, i.m. cloprostenol 2 d after initiation of infusion (Day 16) to induce luteolysis. Groups SC-OT and OT-OT received oxytocin twice daily (12 h apart) (0.33 USP units/kg body weight, s.c.) on Days 3 to 6 of the estrous cycle following cloprostenol-induced luteolysis, while Groups SC-SC and OT-SC received an equivalent volume of saline. Daily plasma progesterone (P4) concentrations prior to cloprostenol-induced luteolysis and rates of decline in P4 following the induced luteolysis did not differ between oxytocin-infused (OT-OT and OT-SC) and saline-infused (SC-SC and SC-OT) groups (P >0.1). Duration of the estrous cycle was shortened in saline-infused heifers receiving oxytocin daily during the first week of the estrous cycle. In contrast, oxytocin injections did not result in premature inhibition of luteal function and return to estrus in heifers that received oxytocin infusion (OT-OT). Day of ovulation, size of ovulating follicle and time of peak LH after cloprostenol administration for oxytocin and saline-treated control heifers did not differ (P >0.1). During the first 3 d of the estrous cycle following luteal regression, fewer (P <0.01) follicles of all classes were observed in the oxytocin-infused animals. Day of emergence of the first follicular wave in heifers treated with oxytocin was delayed (P <0.05). The results show that continuous infusion of oxytocin during the mid-luteal stage of the estrous cycle has no effect on cloprostenol-induced luteal regression, timing of preovulatory LH peak or ovulation. Further, the finding support that an episodic rather than continuous administration of oxytocin during the first week of the estrous cycle results in premature loss of luteal function. The data suggest minor inhibitory effects of oxytocin on follicular growth during the first 3 d of the estrous cycle following cloprostenol-induced luteolysis.     

Persistence of the luteal phase following ovulation during altrenogest treatment in mares

Two experiments were conducted to test the efficacy of altrenogest treatment in mares. The response to 15-d altrenogest treatment (Experiment 1) was characterized in 20 mares that were given 22 mg daily of altrenogest in oil (n = 10) or in gel (n = 10) from Day 10 to 25 after ovulation. In 17 mares, luteolysis occurred during altrenogest treatment (Day 17.7 +/- 0.5), while 2 mares retained their corpus luteum (CL), and 1 mare had a diestrous ovulation on Day 16, resulting in a prolonged luteal phase. Ten of the 17 mares in which the CL had spontaneously regressed returned to estrus after the end of treatment, and ovulated 5.7 +/- 0.8 d after the end of altrenogest treatment. Two of these 17 mares ovulated 2 and 3 d after the end of altrenogest treatment but ovulation was not accompanied by estrous behavior, and 5 mares ovulated during altrenogest treatment resulting in an interovulatory interval of 22.4 +/- 1.1 d (range: 20 to 25d). Five mares which ovulated during altrenogest treatment and 2 mares which ovulated during silent estrus after the end of altrenogest treatment failed to regress the CL around 14 d post ovulation, and had a prolonged luteal phase. In Experiment 2, the effect of altrenogest administered from luteolysis to ovulation on duration of the subsequent luteal period was analyzed. In 6 mares altrenogest was begun on Day 14 post ovulation and continued until the hCG-induced ovulation. The interval from ovulation during altrenogest treatment to spontaneous luteolysis was 45.6 +/- 2.4 d (range: 40 to 54d) in altrenogest-treated mares and was significantly longer than in 10 untreated control mares (14.5 +/- 0.3 d, range: 13 to 16d). The results suggest that the oil and gel altrenogest preparations are equally effective in modulating estrous behavior and time to estrus and ovulation. Altrenogest treatment started late in diestrus appears to result in a high incidence of ovulation during treatment and when luteolysis and ovulation occur during treatment; the subsequent luteal phase is frequently prolonged due to failure of regression of the CL.     

Acute effects of prostaglandin F(2alpha) on systemic oxytocin and progesterone concentrations during the mid- or late-luteal phase in mares

The acute effects of prostaglandin F(2alpha) (PGF) on circulating oxytocin and progesterone concentrations were characterized in mares during the mid- or late-luteal phase. Pony mares were randomly assigned to the following experimental groups based on treatment with PGF (2.5mg) or saline on Day 8 or Day 13 (Day 0=ovulation): PGF-8, PGF-13, saline-8, or saline-13 (n=7/group). Mares were fitted with indwelling, jugular vein catheters and two blood samples (-5 and 0 min) were collected prior to treatment. Treatments were administered into the jugular vein (0 min) and blood collection continued thereafter at 1 min intervals until 5 min and then at 5 min intervals until 60 min. Based on the combined data of -5 and 0 min samples, mares on Day 8 had greater (P<0.05) oxytocin concentrations than mares on Day 13. On Day 8, PGF treatment resulted in a biphasic pattern of oxytocin release. Oxytocin concentrations increased (P<0.05) 1 min after PGF treatment, decreased (P<0.05) from 1 to 10 min, and increased (P<0.05) from 10 to 30 min. Oxytocin concentrations were greater (P<0.05) from 1 to 3 min in PGF-treated than saline-treated mares and at most sample times from 15 to 60 min. On Day 13, oxytocin concentrations were greater (P<0.05) in PGF-treated than in saline-treated mares for most sample times. Mares treated with PGF on Day 8 had greater (P<0.05) oxytocin concentrations at 25, 30, and 40 min than mares on Day 13. Progesterone concentrations on Day 8 also increased by 1 min after PGF, decreased toward basal concentrations by 2-3 min, and then increased to a maximum 10 min after treatment. Subsequently, circulating progesterone decreased (P<0.05) below pretreatment concentrations by 40-50 min after PGF. In conclusion, treatment with PGF resulted in an immediate and biphasic increase in progesterone concentrations prior to the expected decrease. Treatment of mares with PGF on Day 8 resulted in an overall greater increase in systemic oxytocin concentrations compared to treatment on Day 13, and the increase on Day 8 was biphasic.     

Oxytocin administration prolongs luteal function in cyclic mares

Recent evidence indicates that, in mares, as in the domestic ruminants, oxytocin and its endometrial receptor play important roles in stimulating the pulsatile releases of prostaglandin F2 alpha from the endometrium that effect luteolysis. In the present experiment, continuous administration of oxytocin by subcutaneous minipump to five mares during days 8-20 after ovulation abolished luteolysis in four of them, while all four of the control mares infused similarly with saline underwent luteolysis at the expected time. When oxytocin administration began on day 10, instead of on day 8, after ovulation luteolysis occurred rapidly in two of the five treated mares, indicating that the development of oxytocin responsiveness begins on or about day 10 of dioestrus in cyclic mares.     

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.     

Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles

The aim was to define precisely the FSH secretion pattern in mares during the two ovulatory cycles before, and for 24 days after, the last ovulation of the season and to compare this with the profiles of other reproductive hormones and follicular growth to identify changes which may lead to the termination of follicular cycles. Jugular blood was collected every 6 h from ten light horse mares for 6 weeks in autumn. Samples were assayed for FSH, LH, prolactin, inhibin, oestrone conjugates and progesterone. Luteolysis occurred earlier and periovulatory oestrone, but not inhibin, concentrations were significantly lower in the last than in the second to last cycles. In ovulatory and anovulatory cycles, daily mean FSH concentrations were low at the expected time of ovulation and high between days 9 and 11 (day 0 = ovulation), which were usually after luteolysis. However, the periovulatory FSH nadir was prolonged in the last compared with the second to last cycles, and the difference between peak and trough values was not significant in anovulatory cycles. Between day 5 and day 8, the FSH interpulse interval was approximately 2 days, and did not vary in successive cycles. The LH profile also showed progressive changes as mares entered acyclicity; the surge terminated sooner in the last than in the second to last cycles, and failed to occur when expected in acyclicity. Sporadic prolactin pulses occurred at luteolysis in a similar proportion of ovulatory and anovulatory cycles. These results indicate that inadequate gonadotrophin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, and eventually acyclicity. Moreover, the time relationships amongst changes in pituitary and ovarian hormones and follicular growth become increasingly disrupted during the autumn transition, which may contribute to the cessation of cyclicity.     

Effect of dose and day of treatment on uterine response to oxytocin in mares

To determine the effect of dose and day of oxytocin treatment on intrauterine pressure, 6 normal mares were treated with 10 or 25 IU oxytocin 2 days before ovulation, on the day of ovulation and 2 days after ovulation. Intrauterine pressure (IUP) was measured using micro-tip-catheters (one placed intrauterine, a second and third serving as reference sensors in the vagina and external to the mare) and transmitted by telemetry for 30 min to establish a baseline before saline was administered, iv, and for an additional 30 min after saline administration. Oxytocin was then given, iv, and IUP was recorded for 60 min. No change in IUP was observed after saline injection. The administration of both 10 (n=16) and 25 (n=10) IU oxytocin induced a response (P<0.01). The intensity of response depended on the day of administration (P<0.01) and the dose of oxytocin (P<0.001). The variation of response was significantly greater after 10 IU oxytocin (CV 15.78%) compared with 25 IU oxytocin (CV 6.42%). The uterine response was greatest on Day 2 prior to ovulation and lowest on Day 2 after ovulation. The response was negatively correlated to increasing plasma progesterone (10 IU oxytocin: r = -0.435, 25 IU oxytocin: r = -0.265). There was no correlation between the uterine response and plasma estradiol-17beta concentration (P<0.01). In conclusion the results of this study show that oxytocin administration to mares before ovulation provides a greater response than after ovulation. A decline in the intensity of response after ovulation can be compensated for with a higher dose of oxytocin. Furthermore, the use of the multiple catheter technique is an effective method for assessing changes in uterine pressure.     

Control of endometrial oxytocin receptor and uterine response to oxytocin by progesterone and oestradiol in the ewe

The effects of administration of progesterone and oestradiol on ovine endometrial oxytocin receptor concentrations and plasma concentrations of 13,14-dihydro-15-keto prostaglandin F-2 alpha (PGFM) after oxytocin treatment were determined in ovariectomized ewes. Ewes received progestagen pre-treatment, progesterone and/or oestradiol in 11 different treatment schedules. Progestagen pre-treatment decreased oxytocin receptor concentrations in endometrium from ewes treated subsequently with either progesterone for 5 days or progesterone for 5 days plus oestradiol on Days 4 and 5 of progesterone treatment. Oestradiol increased endometrial oxytocin receptor concentrations when administered on Days 4 and 5 of 5 days progesterone treatment. Progestagen pre-treatment followed by progesterone treatment for 12 days caused a large increase in oxytocin receptors and no further increase occurred when ewes were given oestradiol on Days 11 and 12, or when progesterone was withdrawn on Days 11 and 12, or these two treatments were combined. Oxytocin administration caused an increase in plasma PGFM concentrations in ewes which did not receive progestagen pre-treatment, and subsequently received progesterone treatment for 5 days and oestradiol treatment on Days 4 and 5 of progesterone treatment. Similarly treated ewes which received progestagen pre-treatment did not respond to oxytocin. Oxytocin administration also increased plasma PGFM concentrations in ewes which received progestagen pre-treatment followed by progesterone treatment for 12 days, progesterone treatment for 12 days plus oestradiol on Day 11 and 12 of progesterone treatment, progesterone withdrawal on Day 11 and 12, or progesterone withdrawal and oestradiol treatment combined. The results indicate that (1) progesterone pre-treatment affects oxytocin receptor concentrations in the endometrium and uterine responsiveness to oxytocin and (2) progesterone treatment alone for 12 days after a treatment which mimics a previous luteal phase and oestrus is sufficient to induce oxytocin receptors and increase oxytocin-induced PGF release. These results emphasize the importance of progesterone and provide information which can be used to form an hypothesis for control of luteolysis and oestrous cycle length in the ewe.     

Interrelationship between plasma estradiol concentration and oxytocin-induced PGF2alpha release in heifers

The objective of this study was to determine if the increase in responsiveness to oxytocin toward the time of luteolysis was correlated with an increase in plasma estradiol in the cow. Six heifers each had a cannula placed in the jugular vein on Day 14 of the estrous cycle. Then, beginning on Day 15, growth of the largest follicles was determined by ultrasonography, and a blood sample was taken via the cannula for the measurement of progesterone and estradiol by radioimmunoassay (RIA). After the first blood sample, 3 more samples were taken at 10-min intervals, 100 IU oxytocin were injected into the vein, and a further 3 blood samples were taken at 15, 30 and 60 min after injection. The concentration of 13,14-dihydro-15-keto prostaglandin F2alpha (PGFM) was measured in these frequent samplings and was used to determine the ability of oxytocin to stimulate PGF2alpha release from the uterus. This procedure was repeated daily for at least 7 d. The results showed that the response to oxytocin increased before luteolysis and that there was a significant increase in the response to oxytocin (P<0.05) before any changes in plasma estradiol or progesterone were detected. These data show that an increase in estradiol secretion from the ovulatory follicle does not appear to initiate luteolysis.     

Hormonal changes during luteal regression in farmed fallow deer, Dama dama

Concentrations of progesterone, oxytocin and PGFM (pulmonary metabolite of PGF-2 alpha) were measured in plasma from peripheral blood samples collected from 5 fallow does every hour or 2 h for 12-h periods on Days 15-20 inclusive of the oestrous cycle (i.e. luteolysis). For 3 does that exhibited oestrus on Day 21, plasma progesterone concentrations fluctuated between 3 and 10 ng/ml on Days 15-18 inclusive. Thereafter, values declined progressively to attain minimum concentrations of less than 0.05 ng/ml on Day 20. Basal concentrations of plasma oxytocin and PGFM fluctuated between 5 and 20 pg/ml and 10 and 100 pg/ml respectively. Episodic pulses of plasma oxytocin (greater than 300 pg/ml) occurred on Days 15 and 16, whereas pulses of plasma PGFM (greater than 400 pg/ml) occurred on Days 19 and 20. There was little apparent correlation between episodic pulses of the two hormones. For 2 does that exhibited oestrus on Day 22, plasma progesterone concentrations declined to minimum values of 1.0-1.5 ng/ml by Day 20. One of these does showed very high levels of oxytocin secretion throughout the sampling period while the other showed an apparent paucity of oxytocin secretory periods. Two does hysterectomized on Day 13 of their second oestrous cycle failed to exhibit further oestrous cycles. Continual elevation of plasma progesterone concentrations (2-6 ng/ml) for an 8-month period indicated persistence of the corpus luteum after hysterectomy. It is concluded that luteolysis in fallow deer involves episodic secretion of both oxytocin and PGF-2 alpha.     

Seasonal effects on attempts to synchronize estrus and ovulation by intravaginal application of progesterone-releasing device (PRID) in mares

To investigate seasonal effects on the efficacy of estrus synchronization in mares, we administered a progesterone-releasing device (PRID) intravaginally to eight Haflinger mares for 11 days. In January 3 of 8 mares responded to the treatment with estrus and ovulation, in March 7 with estrus and 6 of 7 mares with ovulation, in June 6 of 7 and in October 7 of 8 mares with estrus and ovulation. Follicle distribution patterns at PRID insertion were different between January/October, March/June and June/October (P<0.05). Number of follicles decreased during PRID treatment in January, March and June (difference of number of follicles at Day 12 minus number of follicles at Day 1: -4.2+/-2.7, -0.9+/-0.9 and -4.9+/-1.5 follicles), while it increased in October (3.9+/-1.2 follicles; P<0.05). Mean progesterone concentrations were lowest in January (0.3+/-0.1 ng mL(-1)) when compared with March (3.5+/-1.8 ng mL(-1); P=0.063), June (4.4+/-1.4 ng mL(-1); P<0.05) and October (2.2+/-0.9 ng mL(-1); P<0.05). At Day 2 of PRID treatment, mean progesterone concentrations significantly increased in all mares. Except from January, mean LH concentrations decreased within one day after PRID insertion and remained at low levels during treatments in January and March. Total secretion of LH during PRID-treatment was significantly lower in January and March when compared with June and October. In the 5 of 7 mares that ovulated during PRID treatment a distinct increase of plasma LH concentrations after ovulation was detected. Administration of the progesterone releasing intravaginal device PRID combined with the PGF2alpha analogue cloprostenol was able to induce estrus and ovulation in mares at different times of the year. However, efficacy of the treatment was not satisfactory concerning effectiveness in relation to season and synchrony of intervals from removal of PRID to ovulation in mares.     

Continuous administration of low-dose GnRH in mares II. Pituitary and ovarian responses to uninterrupted treatment beginning near the autumnal equinox and continuing throughout the anovulatory season

We tested the hypothesis that continuous subcutaneous treatment with low-dose GnRH, administered to mares from late September/early October through March, would prevent the development of seasonal anovulation. Quarter Horse mares (n=20) were stratified by age and body condition score and assigned randomly to either a saline control (n=9) or a GnRH (n=11) treatment group. Gonadotropin-releasing hormone was delivered continuously via osmotic minipumps, with sham pumps placed in control mares. Initial pumps were inserted on Day 3 following ovulation or during the follicular phase if the next anticipated ovulation did not occur by 9 October. Delivery rate of GnRH was 2.5 microg/h (60 microg/day) for the first 60 days, followed by 5.0 microg/h (120 microg/day) thereafter. Pumps were replaced every 30 days. Eighty and 100% of all mares had become anovulatory by 1 November and 1 December, respectively, and remained anovulatory through the end of February. Neither serum concentrations of LH throughout the study nor total releasable pools of LH in March differed between groups. Although control mares that exhibited ovulatory cycles after study onset had greater (P<0.05) mean concentrations of LH during the follicular phase and metestrus compared to GnRH-treated mares, neither size of ovulatory follicles nor interovulatory intervals differed between groups. Serum concentrations of FSH were not affected by treatment, but were lowest (P<0.05) from November through January. Continuous infusion of low-dose GnRH, beginning soon after autumnal equinox and continuing until just after vernal equinox, failed to prevent the occurrence of or to hasten transition from seasonal anovulation.     

Growth and regression of ovarian follicles during the follicular phase of the oestrous cycle in heifers undergoing spontaneous and PGF-2 alpha-induced luteolysis

Ultrasonography was used to monitor the growth, ovulation and regression of individual ovarian follicles greater than or equal to 5 mm during the late luteal and follicular phases of the oestrous cycle in heifers treated with injections of PGF-2 alpha to induce luteolysis and in heifers undergoing spontaneous luteolysis. Six heifers were given a single injection of PGF-2 alpha between Day 12 and 15 of the oestrous cycle and their ovaries were examined daily by transrectal ultrasonography until ovulation occurred. Another group of 5 heifers was examined daily by ultrasound from Day 14 or 15 of the cycle through spontaneous luteolysis and ovulation. Blood samples were taken twice daily from this group and analysed for progesterone to determine when luteolysis occurred. All heifers were checked for oestrous behaviour twice daily. Mean diameters of ovulatory follicles on each of the 3 days before oestrus were not different between PGF-2 alpha-treated and untreated heifers. In both groups there was large variation among heifers in the sizes and growth rates of the ovulatory follicles. At 3 days before oestrus the diameters of ovulatory follicles were between 7.5 and 11 mm in PGF-2 alpha-treated heifers and between 6 and 11.5 mm in untreated heifers. Non-ovulatory follicles decreased in size during the 3 days before oestrus and the number of non-ovulatory follicles within the size ranges of ovulatory follicles decreased. The ovulatory follicle was not consistently the largest follicle on the ovaries until the day of oestrus but was always one of the 2 largest follicles during the 3 days before oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of human chorionic gonadotrophin and exogenous progesterone on luteal function during early pregnancy in pony mares

Previous reports have indicated that repeated administrations of human chorionic gonadotrophin (hCG) to pony mares before day 38 of gestation caused pregnancy failure, but that this could be prevented by the prior or simultaneous administration of progesterone. The present paper describes the circulating progestagen concentrations during these experiments, and shows that pregnancy failure resulted from luteolysis after hCG administration. Most of the mares which received progesterone maintained their pregnancies after hCG injection, but progestagen concentrations fell, suggesting that partial lysis of the corpus luteum occurred. After day 38 of gestation repeated doses of hCG had no effect on plasma progestagen concentrations.     

Effect of PGF2alpha and 13,14-dihydro-15-keto-PGF2alpha (PGFM) on corpora luteal function in nonpregnant mares

The objective of this study was to determine if the primary circulating metabolite of PGF2alpha, 13,14-dihydro-15-keto-PGF2alpha (PGFM), is biologically active and would induce luteolysis in nonpregnant mares. On Day 9 after ovulation, mares (n = 7/group) were randomly assigned to receive: 1) saline control, 2) 10 mg PGF2alpha or 3) 10 mg PGFM in 5 mL 0.9% sterile saline i.m. On Days 0 through 16, blood was collected for progesterone analysis. In addition, blood was collected immediately prior to treatment, hourly for 6 h, and then at 12 and 24 h after treatment for progesterone and PGFM analysis; PGFM was measured to verify that equivalent amounts of hormone were administered to PGF2alpha- and PGFM-treated mares. Mares were considered to have undergone luteolysis if progesterone decreased to < or = 1.0 ng/mL within 24 h following treatment. Luteolysis was induced in 0/7 control, 7/7 PGF2alpha-treated, and 0/7 PGFM-treated mares. There was no difference (P>0.1) in the occurrence of luteolysis in control and PGFM-treated mares. More (P<0.001) PGF2alpha-treated mares underwent luteolysis than control or PGFM-treated mares. There was no difference (P>0.1) in progesterone concentrations between control and PGFM-treated mares on Days 10 through 16. Progesterone concentrations were lower (P<0.01) on Days 10 through 14 in PGF2alpha-treated compared with control and PGFM-treated mares. There was no difference (P>0.05) in PGFM concentrations between PGF2alpha- and PGFM-treated mares; PGFM concentrations in both groups were higher (P<0.001) than in control mares. These results do not support the hypothesis that PGFM is biologically active in the mare, since there was no difference in corpora luteal function between PGFM-treated and control mares.