Decline In Plasma Progesterone Concentrations Research Articles

Canine parturition: what is known about the hormonal setting?

Parturition is a challenging physiological process with perfect timing dictated by the events leading to the end of pregnancy in the female, and by the maturation of the fetus(es). The process of parturition remains an intricate interaction of hormones in a fine-tuned timing that remains to be better elucidated in the dog. In the dog pregnancy is maintained by the progesterone production by the corpora lutea, in which some hormones play a luteotrophic action. At term of pregnancy, the pre-parturient luteolytic cascade is the most apparent event, characterized by a rapid decline in plasma progesterone concentrations and useful in predicting the onset of spontaneous parturition. In contrast, cortisol plasma concentrations measurement showed high variability and suggested to be related to the stress condition instead of the onset of parturition. Both prostaglandin F2α and E2 concur in the process of parturition in the dog. The measurement of oxytocin plasma concentrations, also very variable, is implicated in uterine contractions. The measurement of plasma oxytocin concentrations showed to be useful to distinguish between normal and disturbed parturition, allowing the prompt recognition of dystocia and the immediate obstetrical intervention. In contrast to other species, no significant roles of estrogens for the initiating of parturition were demonstrated. Relaxin, the main pregnancy hormonal marker in the dog, beside an endocrine action, is also supposed to play a paracrine/autocrine role at the utero-placental unit and to support the maintenance of high plasma progesterone concentrations through a luteotrophic action, concurring to the main action of prolactin. Although important information has been provided, some aspects in the understanding the hormonal interactions and action timing implications in the process of parturition in the dog remains to be better investigated and represent intriguing topic for basic knowledge and applied research.

A compartmental model describing changes in progesterone concentrations during the oestrous cycle

The objective of this study was to develop a mathematical model that accurately describes the rise and decline in plasma progesterone concentrations, and is able to define parameters describing progesterone appearance and disappearance during the bovine oestrous cycle. Daily plasma progesterone data from 27 cows were used to develop a compartmental model consisting of an appearance function and an appearance modulating function. Model outputs included an apparent appearance or secretion duration, appearance rate and an average disappearance rate (expressed as arbitrary units per day; units/d). Shape-based clustering identified three common shape-based groups (or clusters) of progesterone profiles defined as either 'peaked' profile, with the profile reaching a distinguishable peak, 'structured', with the profile exhibiting a wave-like pattern, or 'flat top', with the profile reaching a plateau. Differences in the model parameters for the three different shapes of progesterone profiles were examined: peaked (n=13), flat top (n=7) and structured (n=7). The mean duration of apparent appearance was 11.49 (SD 0.17 d) for all 27 profiles. The model estimates for total appearance of progesterone (area under the curve; ng/ml per cycle), mean appearance rate and maximum appearance rate were 69.04 ng/ml per cycle (SD 15.2 ng/ml per cycle), 3.19 ng/ml per cycle (sd 0.7 ng/ml per d) and 6.70 ng/ml (SD 1.31 ng/ml), respectively. The average disappearance rate was 1.0 units/d (SD 0.04 units/d). The apparent appearance duration was greatest (P<0.01) in the flat top profiles (12.54, SD 0.41 d) followed by the structured (11.77, SD 0.66 d) and the peaked (10.80, SD 0.30 d) profiles. Total and mean progesterone appearance, maximum progesterone appearance rate, and the progesterone disappearance rates were not different between the profiles. The model successfully simulated all components of the progesterone profile and was able to define specific parameters of different shaped progesterone profiles. A simple model able to estimate parameters describing progesterone appearance and disappearance can be used to explore the relationships between profile shapes and reproductive outcomes.

The hormonal profile during the estrous cycle and gestation in Damascus goats

Plasma progesterone, estradiol, LH, FSH and prolactin levels were studied before and during an estrous cycle in 11 Damascus does, exposed to different suckling regimens (Group A: does suckled for 60 days; Group B: does suckled for 120 days; Group C: non-lactating does). In addition, plasma progesterone levels were determined during pregnancy in 10 pregnant does. The mean plasma progesterone levels in the lactating does before the onset of the first postpartum estrus, which included the follicular phase, were significantly lower (0.2 ± 0.1 and 0.1 ± 0.02 ng/ml, respectively), compared to the luteal phase (5.4 ± 0.9 and 2.6 ± 0.3, respectively). The mean plasma progesterone levels in non-lactating does prior to and after the onset of the first seasonal estrus were 0.5 ± 0.1 and 3.3 ± 0.4, respectively. A decline in plasma progesterone concentration was observed 24–72 h before the second postpartum (Groups A and B) and seasonal (Group C) estrus, indicating CL regression occurring in all does. The mean plasma progesterone concentrations in pregnant does reached a value of 4.6 ± 2.8 ng/ml at week 2 of gestation, and increased to a level of 24.5 ± 3.1 ng/ml 12 weeks later. During the luteal phase estradiol levels in the does of Groups A–C were lower (45.7 ± 3.1, 30.8 ± 1.4 and 61.04 ± 6.5 pg/ml, respectively) than the first peak, whether at the onset of postpartum (Groups A and B) seasonal estrus (Group C), or during the estrous period (46.1 ± 5.9, 41.6 ± 5.2 and 93.1 ± 39.5 pg/ml, respectively). The difference in the profiles of plasma estradiol levels at the first and the second postpartum (Groups A and B) seasonal (Group C) estrus may be due to the negative effect of progesterone levels. An LH surge was detected in only two lactating does in Group A (59.3 and 33.6 mIU/ml) 24 and 32 h after the onset of the second postpartum estrus following an increase in plasma estradiol concentration. Plasma FSH levels fluctuated in all groups with no distinct trend during the estrous cycle. Prolactin levels at the onset of the postpartum (Groups A and B)/seasonal (Group C) estrus were higher than levels detected before and during estrous period. The Damascus does manifested great variation in hormonal profiles of the individual animals. All does showed normal cycles, not affected by the different suckling regimens or lactation.

Increasing feed intake in late gestation does not affect plasma progesterone concentration in the sow

Rate of decline in plasma progesterone concentration may influence the success of lactogenesis in the sow. The aim of this experiment was to investigate whether progesterone concentration and rate of decline of progesterone in the periparturient sow could be manipulated by changing her feeding level. Forty-two sows received either 1.15 or 2 times maintenance energy daily from day 100 of gestation up until and including the day of farrowing. Blood samples were taken on days 98 (pre-treatment baseline) and 109 of gestation, during farrowing, 6 h after farrowing and at 09:00 h for the 3 days following farrowing. Plasma progesterone concentration was determined and progesterone half-life was calculated for each sow. High intake feeding had no effect on plasma progesterone concentration at any time of sampling. Progesterone half-life averaged 41.2±3.81 h and did not differ between treatments. There was no relationship between progesterone concentration, or half-life, and litter weight gain, although there was a weak correlation between decline in progesterone in the first 6 h after birth and piglet growth rate from birth to 6 days of age ( R 2=0.109, P<0.05). It was concluded that increasing feed intake in late gestation cannot be used to increase progesterone clearance rate and hasten the onset of lactogenesis in sows.

Pulsatile output of prostaglandin F(2alpha) does not increase around the time of luteolysis in the pregnant goat.

Prostaglandin (PG) F(2alpha) secreted from the uterus is the luteolysin of the estrous cycle and is also believed to be responsible for luteolysis in the pregnant doe at term. We have reported that basal progesterone concentrations decrease before basal PGF(2alpha) concentrations increase, which is inconsistent with this view. In this study we investigated whether luteolysis is associated with increased frequency or amplitude of pulsatile PGF(2alpha) secretion in does over the last 2 wk of gestation. Progesterone concentrations decreased approximately 1 wk before parturition. There was no accompanying increase in PGF(2alpha) concentrations or pulse frequency, and those pulses that were observed were of lesser amplitude and duration than those that have been associated with luteolysis in cycling ewes. A small increase in PGF(2alpha) pulse frequency was identified during the 3 days before parturition, but this was not associated with any change in progesterone concentrations. The biological significance of these small changes in PGF(2alpha) pulse frequency is obscure, although the high concentration of this eicosanoid at labor may have been related to the final, precipitous decline in plasma progesterone concentrations. These findings do not support the notion that PGF(2alpha) is the principal luteolysin in the pregnant doe at term.

Ovarian responses to progesterone and oestradiol benzoate administered intravaginally during dioestrus in cattle.

This study examined the effects of administering progesterone and oestradiol benzoate (ODB) during mid-dioestrus, on ovarian follicular dynamics in cattle. Twelve cycling cows were used in a 4 x 4 latin square design, with the 4 treatments being initiated on Day 13 of the cycle (oestrus = Day 0) and comprising intravaginal insertion for 5 days of: (i) a progesterone releasing device (CIDR; 'P4'); (ii) a CIDR device with a gelatin capsule containing 10 mg ODB and 1 g lactose (CIDIROL; 'P4/ODB') attached; (iii) a placebo CIDR device with the 10 mg ODB capsule (ODB); and, (iv) a placebo CIDR device alone (CTRL). The ovaries of each cow were examined daily by transrectal ultrasonography from Day 7 of the cycle until subsequent ovulation. Blood samples were collected daily from Day 11, and at intervals of 2-4 h during the 24 h period either side of treatment initiation. The second dominant follicle (DF2) emerged on Day 10.7 +/- 0.2 (mean +/- SEM), and was 8.5 +/- 0.2 mm in diameter by Day 13. The DF2 developed through to ovulation (2-wave cycles) in half of the animals in the CTRL group; while in the other half of cases, the ovulatory follicle originated from the third follicle wave that emerged on Day 17.2 +/- 0.4. Administration of a CIDR device alone (P4 group) did not alter the 1:1 ratio of 2 and 3-wave cycles, but the third dominant follicle (DF3) in those cows with 3-wave cycles emerged earlier on Day 15.6 +/- 0.2. In contrast, the DF2 of every animal in the ODB and P4/ODB groups became atretic and was replaced by a DF3 which emerged 4.0 +/- 0.3 days later. The effects of ODB on luteal function were limited to an earlier decline in plasma progesterone concentrations from 2 to 4 days after device insertion and a reduction in diameter of the corpus luteum when administered concurrently with progesterone. Intravaginal administration of 10 mg ODB on Day 13 of the oestrous cycle, with or without progesterone, was effective in promoting follicle wave turnover. In the absence of ODB, progesterone administration alone (P4 group) did not alter the ratio of animals with 2 or 3-wave cycles from that observed in animals in the CTRL group, but did advance the timing of subsequent follicle wave emergence in those animals with 3-wave cycles.

Changes in the gene expression of progesterone receptor and prolactin receptor in sow mammary gland between late gestation and lactation

Lactogenesis is triggered by a rapid decline in plasma progesterone concentration combined with a peak in plasma prolactin concentration; in mice, there is a concurrent loss of mammary progesterone receptors (Haslam and Shyamala, 1980). The aims of this experiment were to determine the pattern of change of progesterone receptor and prolactin receptor mRNA during late gestation and early lactation and to determine whether abundance of mRNA for the two receptors are related to each other, to plasma concentrations of progesterone and prolactin or to piglet performance.

Changes in the gene expression of progesterone receptor and prolactin receptor in sow mammary gland between late gestation and lactation

Lactogenesis is triggered by a rapid decline in plasma progesterone concentration combined with a peak in plasma prolactin concentration; in mice, there is a concurrent loss of mammary progesterone receptors (Haslam and Shyamala, 1980). The aims of this experiment were to determine the pattern of change of progesterone receptor and prolactin receptor mRNA during late gestation and early lactation and to determine whether abundance of mRNA for the two receptors are related to each other, to plasma concentrations of progesterone and prolactin or to piglet performance.

Luteal support of pregnancy in red deer (Cervus elaphus): effect of cloprostenol, ovariectomy and lutectomy on the viability of the post-implantation embryo

Abstract In two experiments with red deer hinds we investigated (1) the sensitivity of the corpus luteum of pregnancy and the post-implantation embryo to a single injection of a prostaglandin analogue, and (2) the effects of removal of ovarian and luteal tissues on the maintenance of pregnancy. In the first experiment, groups of hinds (n = 6 per group) were given a single intramuscular injection of cloprostenol on either Day 30, 40 or 60 of pregnancy, with fetal viability assessed by repeated rectal ultrasonography and luteal status assessed by measurement of plasma progesterone concentrations. Control hinds (n = 5 or 6) received saline injections at the same stages of pregnancy. Pregnancy was maintained to term in all control hinds. However, 9 18 (50%) of cloprostenol treated hinds aborted 48–120 h after injection, at a frequency of 3 6 on Day 30, 5 6 on Day 40 and 1 6 on Day 60. All hinds receiving cloprostenol exhibited a marked decrease in plasma progesterone concentrations within 12 h of injection, although those that maintained pregnancy appeared to undergo luteal recovery 56–72 h later. In the second experiment, groups of hinds (n = 3 or 4 were subjected to either ovariectomy, lutectomy or sham surgery on Day 33 or 75 of pregnancy. Contemporary controls were included in the trial. Fetal viability was assessed by rectal ultrasonography and recovery of reproductive tracts at hind slaughter 30 days after surgery. Plasma progesterone concentrations were measured during this period. Ovariectomy and lutectomy resulted in fetal loss at both stages of pregnancy. Although one hind lutectomised on Day 75 retained pregnancy until slaughter (30 days after surgery), the fetus appeared to be developmentally retarded relative to those retrieved from control and sham-operated hinds. Both ovariectomy and lutectomy resulted in a marked decline in plasma progesterone concentrations. These studies have indicated that the presence of luteal tissue is important for pregnancy maintenance in the first 75 days of gestation in red deer. However, the corpus luteum of pregnancy exhibits a degree of refractoriness to the luteolytic (and hence, abortificient) effects of cloprostenol.

Concentrations of cytochrome P450 cholesterol side-chain cleavage enzyme and 3 beta-hydroxysteroid dehydrogenase during prostaglandin F2 alpha-induced luteal regression in cattle.

Prostaglandin F2 alpha (PGF2 alpha)-induced regression of the corpus luteum causes both plasma progesterone concentrations and luteal concentrations of mRNA encoding the steroidogenic enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) to fall in parallel. To investigate the hypothesis that a decline in the concentrations of mRNA encoding steroidogenic enzymes causes plasma progesterone to fall, the luteal concentrations of the enzymes 3 beta-HSD and cytochrome P450 cholesterol side-chain cleavage were measured during induced luteolysis. Holstein heifers were treated with PGF2 alpha (25 mg Lutalyse) on Day 6 or Day 7 of the oestrous cycle and corpora lutea were collected 0 h, 2 h, 12 h, and 24 h later (n = 6, 4, 4, and 4 respectively). Analyses of the steroidogenic enzymes were carried out by Western immunoblotting. The luteal concentrations of both steroidogenic enzymes did not decrease over the 24-h period. It is concluded that, although the concentrations of mRNA encoding steroidogenic enzymes may decline in response to PGF2 alpha, this does not lead to a sufficiently rapid reduction in the concentrations of the enzymes to precede, and thus cause, the decline in plasma progesterone concentrations. Thus, the mechanism for the initial decline in plasma progesterone concentrations during luteolysis is still not known.

Plasma progesterone concentrations in heifers and cows treated with a new intravaginal device

A new form of intravaginal device (CIDR), manufactured by high temperature injection moulding and containing 1.9 g of progesterone in a silicon elastomer, was used in five trials with 101 cycling heifers, 11 ovariectomized (ovx) heifers and six cycling cows. The average plasma progesterone concentrations (PPC) during the 12 days of treatment with a CIDR device in ovx heifers was 5.6 (± 0.1) ng ml −1 and ranged from 8.7 (± 0.3) ng ml −1 6 h after inserting the device to 2.5 (± 0.2) ng ml −1 at removal. The average increase in PPCs 24 h after CIDR insertion (Treatment Day 1 equals TD 1) was 6.7 (± 0.4) ng ml −1 in ovx heifers compared to 4.9 (± 0.2) ng ml −1 in other cycling heifers ( P < 0.01). This varied from 3.9 ng ml −1 for devices inserted in pro-oestrus to 5.8 ng ml −1 for mid-cycle insertions ( P < 0.01). PPCs in ovx heifers were maintained at levels equivalent to those found in dioestrous heifers for the first three days after device insertion only. If a CIDR device was inserted during metoestrus (Cycle Days 2 or 3; oestrus equals Cycle Day 0) the average PPCs did not change significantly from 5.0 ng ml −1 on TD 1 with values of 5.3 ng ml −1 and 5.9 ng ml −1 recorded on TD 7 and TD 12, respectively ( P < 0.10). The concurrent treatment of some of these heifers with 10 mg oestradiol benzoate (OB), either in a gelatin capsule placed in the grooved surface of a CIDR device before insertion, or by i.m. injection significantly reduced PPCs on TD 7 (4.8 vs. 6.2 ng ml −1; P < 0.01) and TD 12 (5.0 vs. 7.9 ng ml −1; P < 0.01). Device insertion during metoestrus for 12 days significantly reduced oestrous cycle length compared to contemporary control animals (18.2 vs. 21.0 days; P < 0.001). Dioestrous heifers which had CIDRs inserted on Cycle Days 9 to 12 for four days had PPCs increased by 5.8 ng ml −1 on TD 1 (8.7 to 14.5 ng ml −1). The average decline in PPCs by 24 h after device removal was 4.5 ng ml −1, but this decline was significantly greater in those heifers which had CIDRs removed on Cycle Day 16 (7.7 ng ml −1; P < 0.01). CIDR-treated dioestrous heifers which were injected with a luteolytic dose of prostaglandin F 2α either 18 h before, or two days after CIDR insertion had similar average PPCs to ovx heifers at TDs 7 and 12 (3.2 and 2.7 ng ml −1). The average PPC in three cows treated with three CIDR devices for 15 days was three times higher than that in three cows treated with one device (8.4 vs. 2.8 ng ml −1; P < 0.01). In summary, these studies showed that changes in PPCs following insertion or removal of a CIDR device were influenced by the stage of treatment, the type of animal (ovx vs. cycling), the stage of cycle when treatment was initiated or terminated, the number of devices inserted, and concurrent treatment with oestrogen or prostaglandin F 2α.

Prolactin and progesterone concentrations during early pregnancy and relationship to pregnancy loss prior to day 45

Summary Eleven multiparous Quarter Horse and Thoroughbred mares were used to determine the plasma concentrations of progesterone and prolactin during early pregnancy and to examine the relationship of plasma progesterone and prolactin to pregnancy loss prior to d 45 of gestation. Plasma samples were collected at two day intervals beginning on d 14 of pregnancy (d 0 = ovulation) and countinued to d 80. Ovulation and pregnancy status were determined by ultrasonography. Four mares experienced pregnancy loss between d 28 and 44 and plasma samples were collected for 10 days beyond the detected loss. Seven mares had successful pregnancies (Term group). Plasma progesterone concentrations peaked by d 28 in the Term group, with individual peak values ranging from 14.9 to 31.9 ng/ml. Values then declined until d 36, followed by a rise until d 80. Prior to d 45 of gestation 5 of the 7 mares had a peak in excess of 15 ng/ml and 3 of these had brief periods, ranging fron 1 to 8 days when progesterone dropped to less than 2 ng/ml. The other 2 mares had peak plasma progesterone concentrations of less than 7 ng/ml, but maintained concentrations in excess of 2 ng/ml during this period. Within the mares experiencing pregnancy loss, 2 mares had a decline in plasma progesterone concentrations prior to pregnancy loss, while the other 2 had typical progesterone patterns beyond the detected loss. Differences in individual patterns of plasma progesterone concentration were observed in both groups (P Plasma prolactin concentrations ranged from less than .32 to 4.58 ng/ml in the Term group and from .31 to 1.9 ng/ml in the mares experiencing pregnancy loss. Differences in the individual patterns of prolactin secretion were observed in both groups (P A correlation between progesterone and prolactin was observed in the Term group between day 14 and 37 (P 2 =.88).

Effect of hysterectomy on the short life-cycle corpus luteum produced after GnRH-induced ovulation in the anoestrous ewe

Anoestrous Romney Marsh ewes were treated with small-dose (250 ng) multiple injections of GnRH. Ewes in Groups 1 and 3 were hysterectomized 2 weeks before treatment, while those in Groups 2 and 4 were intact controls. Groups 1 and 2 were primed with progesterone (+P) and treated with 2 h injections of GnRH (250 ng) for 36 h, while Groups 3 and 4 were not pretreated (-P) but were given 2 h injections of GnRH (250 ng) for 18 h. Both treatment regimens were terminated with a bolus injection of GnRH (125 micrograms), given to synchronize the timing of the LH surge and subsequent luteal progesterone production. The plasma progesterone profiles of 5/5 animals in Group 2 (+P controls) and 2/5 animals in Group 4 (-P controls) were indicative of normal luteal function, while the remaining 3/5 animals in Group 4 produced plasma progesterone profiles typical of abnormal luteal function. However, in all the hysterectomized animals (Groups 1 and 3) peripheral plasma progesterone concentrations rose to reach a mean peak value of 1.3 ng/ml plasma on Day 8 which was maintained in all animals irrespective of progesterone pretreatment. The absence of a fall in progesterone concentrations precluded the identification of any animal in Group 4 showing abnormal luteal function. It was also noted that, after hysterectomy, although the corpus luteum was maintained, it was with reduced secretory capacity. The prevention of the expected proportion (70%) of -P animals from displaying a decline in plasma progesterone concentration after hysterectomy provides firm evidence that the uterus is involved in the premature regression of the short-cycle corpus luteum.

Prostaglandin E 2 opunteracts the effects of PGF 2α in indomethacin treated cycling gilts

Twenty crossbred gilts with at least 2 consecutive estrous cycles of 18 to 21 days in length were used to study the effects of prostaglandins E 2 and F 2α (PGE 2 and PGF 2α) on luteal function in indomethacin (INDO) treated cycling gilts. Intrauterine and jugular vein catheters were surgically palced before day 7 of the treatment estrous cycle and gilts were randomly assigned to 1 of 5 treatment groups (4/groups). With exception of the controls (Group I) all gilts received 3.3 mg/kg INDO every 8 h, Groups III, IV and V received 2.5 mg PGF 2; 2.5 mg PGF 2α + 400 μg PGE 2 every 4 hr, or 400μg PGE 2 every 4 h, respectively. All treatments were initiated on day 7 and continued until estrus or day 23. Jugular blood for progesterone analysis was collected twice daily from day 7 to 30. Estradiol-17β (E 2-17β) concentrations were dtermined in samples collected twice daily, from 2 d before until 2 d following the day of estrus onset. When compared to pretreatment values, estrous cycle length was unaffected (P>0.05) in Group I, prolonged (P<0.05) in Groups II, IV and V; and shortened (P<0.05) in Group III. The decline in plasma progesterone concentration that normally occurs around day 15 was unaffected (P>.05) in Group I; delayed (P<0.05) in Groups II, IV and V; and occurred early (P<0.05) in Group III. Mean E 2-17β remained high (31.2 ± 4.9 to 49.3 ± 3.1 pg/ml) in Groups III and IV, while the mean concentrations in Groups III and V varied considerably (17.0 ± 2.0 to 52.2 ± 3.5 pg/ml). The results of this study have shown that PGE 2 will counteract the effects of PGF 2α in INDO treated cycling gilts. The inclusion of PGF 2α appeared to either stimulate E 2-17β secretion or maintain it at a higher level than other treatments.

Concentrations of PGF-2 alpha and PGE-2 in the uterine venous blood of rabbits during pseudopregnancy and pregnancy.

Peripheral plasma progesterone concentrations in intact and hysterectomized pseudopregnant rabbits decreased from Day 14. The rate of decrease in intact, but not in hysterectomized, rabbits accelerated on Day 17 and was associated with an increase in PGF-2 alpha levels in the uterine venous plasma. There was no change in uterine PGE-2 output during pseudopregnancy. In pregnant rabbits, peripheral plasma progesterone concentrations remained high up to Day 30, and there was no increase in uterine venous plasma levels of PGF-2 alpha on Day 17, PGF-2 alpha levels were elevated in uterine venous plasma on Day 25 of pregnancy, but there was no concurrent decline in plasma progesterone concentrations. Uterine venous plasma levels in PGE-2 increased markedly after Day 11 of pregnancy and reached very high concentrations in some rabbits.

The luteolytic and abortifacient potential of an estrogen-bromergocryptine regimen in the baboon

Administration of 1 mg of estradiol benzoate (E2B) daily to normally cycling baboons on days 6, 7 and 8 after ovulation significantly shortens the luteal phase to 13.8 +/- 0.3 days (n = 5) from 16.0 +/- 0.6 days (n = 8) in vehicle-treated controls, and causes an early decline in plasma progesterone concentration. When 1 mg/kg bromergocryptine (CB-154) was administered twice daily in combination with the E2B treatment, there was an earlier depression of plasma progesterone and a further shortening of the luteal phase to 12.3 +/- 0.5 days (n = 4). In five pregnant baboons, the E2B/CB-154 treatment initiated on postconception day 20 produced a substantial depression of plasma progesterone, followed by a posttreatment rebound to control values, without apparent effect on the pregnancy. In both cycling and pregnant baboons, this dose of CB-154 produced a significant and sustained reduction in plasma prolactin levels.

Peripheral plasma progesterone and utero-ovarian prostaglandin F concentrations in the cow around parturition

The concentration of prostaglandin F in utero-ovarian venous plasma and progesterone in jugular venous plasma were determined by radioimmunoassay in 3 cows over the last 2–3 weeks of gestation. Utero-ovarian prostaglandin F concentrations did not show any consistent pattern in two of three cows until 48–72 h before term when the levels rose sharply from 1 ng/ml to a maximum 4–9 ng/ml during labour. The concentration of progesterone in jugular venous plasma tended to fall gradually over the last 20 days of gestation with a further fall occurring 48-36 h before delivery. In two other cows at around 240 days of gestation the concentration of plasma progesterone in ovarian venous plasma was 50 to 150 times the concentration of progesterone in uterine or jugular venous plasma. It is concluded from these results that the ovaries are the major source of progesterone in cows during late pregnancy. The findings also suggest that prostaglandin F may be the luteolytic factor responsible for the sharp decline in plasma progesterone concentrations over the last 48-36 h preceding parturition.

Efficacy of a prostaglandin analogue in reproduction in the anestrous mare

A prostaglandin F analogue was studied in anestrous mares: a dose-response study; a study in mares presumed pregnant; and a field evaluation of effective doses in breeding establishments. A dose of 2.0mg given by single subcutaneous injection to mares with initial plasma progesterone levels greater than 1.0ng/ml, caused luteolysis on the basis of decline in plasma progesterone concentrations. Follicle maturation leading to ovulation, accompanied by estrus, was observed, and fertility at mating either by natural service or artificial insemination was satisfactory. A dose of 1.0mg was generally effective for luteolysis, but pregnancy rates were lower than after 2.0mg. A proportion of mares which had less than 1.0mg of plasma progesterone at the time of injection ovulated and became pregnant.

Multiple progesterone injections and the duration of estrus in ovariectomized guinea pigs

The duration of sexual receptivity in guinea pigs primed with 3.3 ug estradiol benzoate and given 0.1 mg progesterone 36 hr later in oil or propylene glycol vehicle was about 6–7 hr. Maintenance of elevated plasma progesterone levels (as measured by radioimmunoassay)throughout the period of heat by multiple 0.1 mg progesterone injections significantly prolonged (by about 2.7 hr) heat duration, but heat terminated despite the presence of high plasma progesterone levels. The experiment suggests that a decline in plasma progesterone concentration is not necessary for the termination of heat. Methods by which termination of heat could occur are discussed.

Functional Luteolysis in the Rhesus Monkey: The Role of Estrogen12

Silastic capsules containing estradiol– 17β were implanted into normally cycling rhesus monkeys 2–3 days following the preovulatory LH surge. The resultant plasma estrogen concentrations, which were maintained throughout the luteal phase of the cycle, were approximately one—half those observed on the day of the LH peak but exceeded normal, luteal phase levels by about 150 pg/ml. This increment in circulating estrogen levels was associated with a premature decline in plasma progesterone concentrations which became undetectable 6.4 days before those in control animals, a 43% reduction in the functional life—span of the corpus luteum. These findings demonstrate that small increments in circulating estradiol can induce functional luteolysis in the rhesus monkey and support the hypothesis that estrogens play a role in the physiological regression of the corpus luteum during the normal menstrual cycle of primates. This luteolytic action of estradiol could not always be associated with measurable decrements in p...