Uterine oxytocin receptors in cyclic and pregnant cows.

Binding of [3H]oxytocin to uterine subcellular preparations ('oxytocin receptor concentrations') was measured in uterine tissue of heifers and multiparous dairy cows at various stages of the oestrous cycle and during early pregnancy. A method for the assay of ovine uterine oxytocin receptors was optimized for use on bovine tissue. Oxytocin receptor concentrations were increased in cyclic animals around the period of luteolysis and oestrus, rising on Day 15 in endometrium and on Day 17 in myometrium while pregnant animals showed no comparable rise. Receptor concentrations then declined on Day 3 after oestrus in myometrium and on Day 5 in endometrium. Some cyclic animals did not show the expected rise in receptors in the late luteal phase; these animals had abnormally high progesterone concentrations for this stage of the cycle. In animals slaughtered on Day 18 after oestrus and/or insemination which had low oxytocin receptor levels, plasma progesterone concentrations were consistently high; while all animals showing the late luteal phase elevation in receptor values had low progesterone concentrations. Oxytocin receptor and progesterone concentrations were negatively correlated (P less than 0.05). These data support the hypothesis that oxytocin receptor level is a key factor in the process of luteolysis in cattle and that in pregnancy there is suppression of uterine oxytocin receptor at the expected time of luteolysis. We suggest that uterine oxytocin receptor levels are partly controlled by circulating steroid hormones and are suppressed during early pregnancy.     

Peripheral plasma prolactin and progesterone levels in pseudopregnant goats during bromocryptine treatment

Persistence of luteal function and accumulation of fluid within the uterus (hydrometra) are characteristics of pseudopregnancy in goats. To study the luteotrophic role of prolactin in this condition, seven seudopregnant goats were treated with bromocryptine (1 mg subcutaneously, twice daily) for 6 to 10 d. Plasma progesterone (P4) and prolactin (PRL) were measured by radioimmunoassay (RIA) in samples taken twice daily by venipuncture. Ultrasound scanning took place at regular intervals to visualize the presence of fluid within the uterus. Bromocryptine treatment effectively reduced the plasma PRL concentration in six goats. In all seven goats, a gradual decrease of the plasma P4 concentration to levels < 1.8 ng/ml occured during treatment. After bromocryptine treatment, P4 concentrations reached basal levels (<0.1 ng/ml) in two animals. In four goats, P4 concentrations remained close to 1.0 ng/ml, or even temporarily rose above the 2.0 ng/ml level. Spontaneous discharge of uterine fluid took place during (two goats) or within 4 d after bromocryptine treatment (three goats). These results indicate that prolactin plays an important luteotrophic role during pseudopregnancy in goats.     

Effect of progesterone administration on the release of uterine prostaglandin F2α and ovarian oxytocin in the goat

The effects of intramuscular progesterone administration (20 mg·day⁻¹) on plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F_2α (PGFM-pulmonary metabolite of prostaglandin F_2α) and oxytocin were examined in seventeen goats after either bilateral ovariectomy, hysterectomy or during days 12–16 of the estrous cycle. Daily mean values of PGFM in animals treated with progesterone after ovariectomy were significantly greater (P<0.001) than in their corresponding controls on the last two treatment days (10 and 11); concentrations of oxytocin, however, remained at or near the limits of assay sensitivity. In hysterectomized goats PGFM concentrations remained extremely low and oxytocin release appeared steady rather than pulsatile. In the intact animals, undergoing luteolysis, daily mean concentrations of both PGFM and oxytocin were significantly greater (P<0.01) in progesterone-treated goats than in their oil-treated controls; furthermore, in the progesterone-treated goats, increases in PGFM concentrations, observed after the peaks of progesterone, were either coincident with or prior to pulses of oxytocin. These results demonstrate that uterine PGF_2α stimulates the pulsatile release of oxytocin from the ovary during luteolysis in the goat.     

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.     

Effects of clomiphene citrate on early pregnancy in guinea-pigs

Clomiphene citrate (2 mg/kg body wt) given on the day of mating can block or interrupt pregnancy in guinea-pigs. Corpus luteum function, uterine histology, implantation and embryo development were studied in clomiphene-treated and control animals on Days 5, 9 and 20 of pregnancy. Following treatment, only 25% of the females were regularly pregnant, presenting large and healthy foetuses. The other females examined showed either pregnancy with embryos undergoing resorption or no sign of pregnancy. In these females, corpus luteum size was reduced, progesterone concentrations were very low and the endometrial glands and the epithelium were often altered. It is concluded that clomiphene causes a reduction in fertility by altering the uterus and, by directly or indirectly inducing luteolysis, causes later pregnancy loss.     

A combined radioimmunoassay and immunocytochemical study of ovarian oxytocin production during the periovulatory period in the ewe

Corpora lutea and follicles were taken from the ovaries of 12 ewes at intervals from the start of luteolysis until 3 days after ovulation. RIA analysis of the tissue oxytocin content showed that luteal oxytocin concentrations declined during luteolysis to reach basal values at about the time of the next ovulation. Oxytocin was first measurable in the walls of 3 out of 6 preovulatory follicles during the LH surge, with a small increase in concentration to 26.1 +/- 6.6 pg/mg before ovulation, and a further increase in the young corpus luteum to concentrations exceeding 1 ng/mg 2-3 days later. After the LH surge, oxytocin was also found in the follicular fluid at a concentration of 3.4 +/- 0.3 ng/ml. Using immunocytochemical techniques, oxytocin and neurophysin were first detected in the follicle wall immediately before ovulation, and were localized in the granulosa cells. After ovulation the stained cells initially formed strands which appeared to break down to clusters and then to individual cells as the corpus luteum matured. The immunocytochemical picture also suggested that neurophysin immunoreactivity increased within a few hours of ovulation but that processing to oxytocin may be delayed. Measurements of circulating oxytocin concentrations revealed a pulsatile release pattern throughout the follicular phase with the height of the pulses decreasing from 25 +/- 5 pg/ml during luteolysis to a minimum of 11 +/- 2 pg/ml during the LH surge.     

Steroid hormone modulation of prostaglandin secretion in the ruminant endometrium during the estrous cycle

Prostaglandins, produced from membrane phospholipids by the action of phospholipase A2, cyclooxygenase, and specific prostaglandin synthases, are important regulators of ovulation, luteolysis, implantation, and parturition in reproductive tissues. Destruction of the corpus luteum at the end of the estrous cycle in nonpregnant animals is brought about by the pulsatile secretion of prostaglandin F(2alpha) (PGF(2alpha)) from the endometrium. It has been known for many years that progesterone, estradiol, and oxytocin are the hormones responsible for luteolysis. To achieve luteolysis, two independent processes have to be coordinated; the first is an increase in the prostaglandin synthetic capability of the endometrium and the second is an increase in oxytocin receptor number. Although progesterone and estradiol can modulate the expression of the enzymes involved in prostaglandin synthesis, the primary reason for the initiation of luteolysis is the increase in oxytocin receptor on the endometrial epithelial cells. Results of many in vivo studies have shown that progesterone and estradiol are required for luteolysis, but it is still not fully understood exactly how these steroid hormones act. The purpose of this article is to review the recent data related to how progesterone and estradiol could regulate (initiate and then turn off) the uterine pulsatile secretion of PGF(2alpha) observed at luteolysis.     

Effect of passive immunization against progesterone on its distribution in circulation and tissues of mice

Anti-progesterone monoclonal antibody, injected into mice 32 h after mating at a dose that blocks the establishment of pregnancy, produced a significant reduction in the concentration of progesterone in the ovary and uterus within 6 h after treatment. Uterine concentrations remained lower in treated compared with control animals for at least 24 h after injection. There was an associated transient increase in plasma LH and FSH concentrations, but there was no change in plasma prolactin values. The percentage of total progesterone in the circulation that was unbound was reduced after treatment, but the concentration of unbound progesterone was increased. Studies of antibody binding of steroid in the presence of uterine progesterone receptor protein showed that there was a stoichiometric relationship in the distribution of ligand between the two binders. The present findings suggest that the effects of passive immunization against progesterone are associated with perturbation of tissue concentrations of steroid in the target organ as a result of high antibody concentrations in the circulation.     

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.     

Effects of a luteolytic dose of oestradiol benzoate on uterine oxytocin receptor concentrations, phosphoinositide turnover and prostaglandin F-2 alpha secretion in sheep

Administration of oestradiol-17 beta benzoate on Days 9 and 10 of the oestrous cycle resulted in episodic secretion of PGF-2 alpha (as indicated by elevated circulating concentrations of 13,14-dihydro-15-ketoprostaglandin F-2 alpha) and a decline in circulating progesterone. Release of PGF-2 alpha began 35 +/- 3 h after first injection of oestrogen and progesterone concentrations declined from 42 +/- 3 h. Secretion of oxytocin, which was first observed 26 +/- 3 h after oestrogen treatment, preceded secretion of PGF-2 alpha; 69% of pulses of oxytocin coincided with episodes of PGF-2 alpha secretion. Uterine oxytocin receptor concentrations were raised in ewes treated with oestrogen, increases occurring in caruncular endometrium and myometrium by 12 h after treatment and in intercaruncular endometrium by 24 h. Raised receptor concentrations were followed at 24 h by increases in the incorporation of [3H]inositol into phosphatidylinositol and in the hydrolysis of labelled tissue phosphoinositides in response to oxytocin in slices of caruncular endometrium incubated in vitro. The following sequence of events is therefore suggested to occur at oestrogen-induced luteolysis: induction of the oxytocin receptor; increased turnover of phosphoinositides; onset of episodic secretion of PGF-2 alpha; and functional luteolysis.     

Invitro uptake of estradiol and progesterone in the hamster uterus relative to the time of embryo transfer and stage of development

The normal time for cleavage to the 2- and 8-cell stages of development of naturally ovulated hamster embryos is 25–27.5 and 59–61 h respectively, after ovulation and mating. The corresponding values for PMS-hCG treated hamsters were 33–35.5 and 62–64 h respectively. When 2-cell embryos, obtained by this timing method, were transferred to the oviductal bursa on Day 2 of the cycle, and when 8-cell embryos were transferred to the uterus on Day 3 of the cycle, implantation rates of 61.5 and 64.0% respectively, were obtained. The current study was conducted to determine the effects of transfer on estradiol and progesterone uptake by the uterine tissue on Day 14 after mating. A 2-fold increase in estradiol uptake was observed in superovulated, nonpregnant uteri when compared with nonsuperovulated animals. This level was also significantly higher than with nonsuperovulated pregnant animals and of animals receiving 2-cell embryo transfers. Estradiol uptake increases of 3.2 and 1.2-fold were noted for animals receiving 8-cell embryo transfers in naturally and superovulated groups respectively. Superovulation resulted in increased progesterone uptake. Transfer of 2- and 8-cell embryos resulted in a 15.8 and 109.9% increase in progesterone uptake respectively in naturally ovulated hamsters. Similar values for superovulated hamsters were 16.2 and 87.6% respectively. The 8-cell embryos, however, were transferred about four hrs prior to the time for normal 8-cell cleavage and this, coupled with increased estradiol uptake by the embryos themselves, resulted in an elevated estradiol and progesterone uptake in the uterine tissue even when measured on Day 14 of pregnancy. The degree of increase was less with superovulated animals receiving 8-cell embryos, reflecting higher levels of estradiol and progesterone uptake in control tissues. This could account for the slight delay in development of superovulated embryos.     

Demonstration that progesterone 'blocks' uterine activity in the ewe in vivo by a direct action on the myometrium

Intrauterine pressure was monitored in vivo in oestrogen-treated ovariectomized ewes before, during and after treatment with progesterone (50 mg s.c./day for 3 days). Progesterone reversibly reduced the frequency and amplitude of myometrial activity and abolished uterine reactivity to oxytocin (i.v.) and PGF-2alpha (intrauterine infusion). The rate of rise of intrauterine pressure during active pressure cycles was significantly reduced. These results confirm that the action of progesterone on the ovine myometrium is comparable to the classic progesterone 'block'. The intrauterine infusion of PGF-2alpha (10 microgram/min), which elicited a marked mechanical response in the control animals, failed to stimulate the progesterone-'blocked' uterus, suggesting that the inhibition produced by progesterone is due to a direct action of the hormone on the uterine muscle and not to an indirect mechanism operating through endometrial prostaglandin output.     

Effects of hysterectomy and uterine decidualization on in vivo levels of prostaglandins in the corpus luteum of adult pseudopregnant rats

A possible role of the uterus in regulating content of luteal prostaglandins (PGs) was investigated. Pseudopregnancy was induced in adult virgin female rats by mating them with vasectomized male rats. On Day 5 of pseudopregnancy, decidualization of the uterus was induced or hysterectomy was performed. As controls, intact pseudopregnant animals with a luteal phase of 13 +/- 1 days were used. Measurements of in vivo tissue levels of PGF2 alpha, PGE2, and 6-keto-PGF1 alpha were performed by RIA after homogenization and extraction procedures in CL of pseudopregnancy and remainder of ovaries on Days 5, 13, and 19. Serum levels of progesterone and 20 alpha-dihydroprogesterone were determined by RIA. In hysterectomized animals, PGF2 alpha levels increased 2.5-fold in corpora lutea on Day 13 compared with levels on Day 5 of pseudopregnancy, but were still lower than in control rats undergoing functional luteolysis on Day 13. Decidual-tissue-bearing rats exhibited low levels of PGF2 alpha on Day 13 of pseudopregnancy. On Day 19, when luteolysis had occurred in decidual-tissue-bearing and hysterectomized rats, as judged by plasma levels of progestins, luteal content of PGF2 alpha was elevated to a similar level as that in control animals undergoing functional luteolysis on Day 13. When data pooled from control, decidual-tissue-bearing and hysterectomized rats were analyzed, a highly significant inverse correlation (r = -0.72, n = 46, p less than 0.001) between luteal PGF2 alpha content and ratio of plasma progestins was found.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of progesterone and oestradiol in the regulation of uterine oxytocin receptors in ewes.

The interaction between oestrogen and progesterone in the regulation of the uterine oxytocin receptor in sheep was evaluated by measuring the binding of oxytocin to membrane preparations of caruncular and intercaruncular endometrium and myometrium. Ovariectomized ewes were assigned in groups of five to each cell of a 4 x 2 factorial design. The four treatments were (a) vehicle (maize oil) for 12 days, (b) progesterone (10 mg day-1) for 9 days, (c) progesterone for 9 days followed by maize oil until day 12 and (d) progesterone for 12 days. The two oestradiol treatments consisted of the administration of implants in the presence or absence of oestradiol. The ewes were killed on day 10 (group b) or day 13 (groups a, c and d) for collection of uterine tissues. The response of the caruncular and intercaruncular endometrium to the treatments was similar. In the absence of oestradiol, treatment with progesterone continuously for either 9 or 12 days reduced the concentration of the oxytocin receptor in comparison with both the control and the progesterone withdrawal group (in which values were similar). The presence of oestradiol reduced the receptor concentrations in control and both 9- and 12-day continuous progesterone treatment groups, but enhanced the concentration in the progesterone withdrawal group. The myometrial oxytocin receptors responded in a similar way to those in the endometrium to progesterone treatment alone, but the addition of oestradiol produced no further effect. In conclusion, progesterone and oestradiol caused downregulation of the endometrial oxytocin receptor. On the other hand, progesterone withdrawal, similar to that which occurs during luteolysis, increased receptor density in the presence of oestradiol. Progesterone may influence the response of the myometrium to oxytocin by causing a reduction in receptor density.     

Uterine blood supply as a main factor involved in the regulation of the estrous cycle--a new theory

The paper presents a new theory on the physiological mechanism of initiation of luteolysis, function of endometrial cells and protection of corpus luteum. This theory is based on previous studies published by the authors and their coworkers on the retrograde transfer of PGF2alpha in the uterine broad ligament vasculature during the estrous cycle, early pregnancy and pseudopregnancy. The studies were focused on cyclic changes in uterine blood supply and the apoptosis of endometrial cells. Moreover, the results of many other authors are cited. The statements of the theory are as follows: 1. The initiation of luteolysis is a consequence of regressive changes in the endometrium which are due to the reduction of the uterine blood supply below the level necessary to provide for the extended needs of active endometrium. 2. During the luteal phase, both a considerable increase in uterine weight and a decrease in blood flow through the uterine artery, resulting from increasing progesterone concentration, reduce the uterine blood supply. In comparison to the volume of blood flowing to the porcine uterus during the estrus period, only 30-40% of the blood volume is determined on day 12 of the estrous cycle. The uterine weight at that time is 40-60% larger than that in the early luteal phase. Thus, due to the considerable constriction of uterine blood vessels, there is a discrepancy between the requirement for oxygen and other factors transported by blood and the possibility of supplying the uterus with these substances. After reaching the threshold of uterine blood supply level, which in pigs takes place around day 12 of the estrous cycle, regressive changes and PGF2alpha release from endometrial cells occurs. 3. Estrogens and progesterone are the major factors affecting blood flow in vessels supplying the uterus. The factors that modulate, complement and support vasodilation and vasoconstriction are: PGE2, LH, oxytocin, cytokines, neurotransmitters and other local blood flow regulators. In some animal species these modulators, especially those of embryonic origin, may be crucial for the status of uterine vasculature. 4. During early pregnancy, the action of embryo signals (estrogens, cytokines), endometrial PGE2 as well as LH results in the relaxation of the uterine artery (pigs: day 12) and, consequently, in an increase in uterine blood supply. This reaction of the maternal recognition of pregnancy effectively prevents regressive changes in well developed endometrial cells to occur. 5. Local uptake and retrograde transfer of PGF2alpha into the uterine lumen during early pregnancy protects corpus luteum from PGF2alpha luteolytic action. 6. During the period of regressive changes resulting from the limited uterine blood supply, endometrial cells restrain PGF2alpha synthesis. They are, however, still capable of releasing prostaglandin when uterine blood supply is improved after the embryo appears in the uterus. This potential capability for PGF2alpha synthesis was demonstrated in in vitro studies when endometrial cells collected during its regressive phase were incubated in medium and stimulated by LH and oxytocin. 7. Prostaglandin F2alpha pulses in venous blood flowing from the uterus do not confirm pulsatile secretion of PGF2alpha. The pulses may result from the pulsatile excretion of PGF2alpha with venous blood according to the rhythmic uterine contractions associated with oxytocin secretion. 8. The results supporting this concept are presented and discussed in due course. The critique of Bazer and Thatcher's theory on exocrine versus endocrine secretion of prostaglandin F2alpha during the estrous cycle is also depicted.     

Suppression of prostaglandin F-2 alpha release and delay of luteolysis after active immunization against oxytocin in the goat

Active immunization against oxytocin significantly prolonged the oestrous cycle in 3 out of 4 goats; the mean (+/- s.e.m.) cycle length was 29.1 +/- 1.7 days (n = 12) compared to 19.4 +/- 0.6 days (n = 9) in control animals. During Days 10-21 of the cycle in the 3 responsive goats, peripheral plasma concentrations of progesterone and oxytocin were steady and those of 13,14-dihydro-15-keto-prostaglandin F-2 alpha were very low (50-100 pg X ml-1) with no marked pulsatile activity. The major effect of immunization would appear to be suppression of the synthesis of the uterine luteolysin PGF-2 alpha, thus confirming that endogenous oxytocin has a facilitatory role in luteolysis via prostaglandin production.     

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.     

Effects of induced endometritis on the life-span of corpora lutea in pseudopregnant rabbits and incidence of spontaneous uterine infections related to fertility of breeding does

A significant percentage of rabbit does fail to become pregnant after AI. We hypothesized that uterine infections induced by the insemination procedure are related to delayed luteolysis and high progesterone concentrations noted to present at the time of AI. The rabbits, randomly assigned to 4 groups (3 animals/group), were given 0.8 microgram GnRH analogue (Day 0) just prior to infusing the uterus with sterile extender (control group) or with extender inoculated with 0.5, 1, and 2 x 10(6) Pasteurella multocida (treated groups). The effects of treatments on functional life-span of CL were assessed by evaluating plasma progesterone from Day 0 to Day 23 of pseudopregnancy. In treated rabbits, the progesterone profiles closely overlapped those found in controls until approximately Day 14. Thereafter, they varied greatly between animals, but luteolysis was delayed by at least 5-6 d and developed less rapidly than in controls. On Day 21, progesterone concentrations were higher than normal in 4 treated does. In a field survey, vaginal swabs were collected at the time of the second AI from 114 non-pregnant rabbits and those positive to bacteriological culture, were killed humanely 16 d later to collect uterine swabs. Positive uterine swabs were found only in 19 of the 34 does having a positive vaginal swabs and all of them were not pregnant. The most frequent pathogen isolated was S. aureus (50%), followed by E. coli (37.5%) and P. multocida (12.5%). We demonstrated that uterine infection increases the life-span of CL in non-pregnant does and that infections of the genital tract system are quite common among does on breeding farms, probably related to using AI.     

In vivo oxytocin release from microdialyzed bovine corpora lutea during spontaneous and prostaglandin-induced regression

The release of luteal oxytocin during spontaneous and prostaglandin-induced luteolysis was investigated in cows. A continuous-flow microdialysis system was used in 11 cows to collect dialysates of the luteal extracellular space between Days 12 and 24 postestrus. Seven cows were untreated and were expected to exhibit spontaneous luteolysis during sampling, whereas 4 cows received prostaglandin F(2alpha) (PGF(2alpha)) systemically between Days 13 and 15 to induce luteolysis during sampling. Oxytocin was detectable in the dialysate of all cows before Day 16 postestrus and occurred as 2 or 3 discrete pulses per 12-h sampling period. For non-PGF(2alpha)-treated cows, dialysate oxytocin content began to decline spontaneously on Day 15 postestrus and was undetectable by Day 17 postestrus. Oxytocin decay curves preceded onset of serum progesterone decline by at least 72 h and were not related temporally with onset of progesterone decline within cow. Exogenous PGF(2alpha) (25 mg, i.m.) produced a 10-fold increase in dialysate oxytocin within 1 h (1.9 +/- 0.3 pg/ml to 20.8 +/- 3.0 pg/ml; P < 0. 01). Dialysate oxytocin then declined to pretreatment concentrations within 2 h and was undetectable within 8 h posttreatment. A second PGF(2alpha) injection given 20 h after the first did not result in a measurable increase in dialysate oxytocin, probably because luteolysis was underway. Although robust luteal oxytocin release was observed after treatment with a pharmacological dose of PGF(2alpha), the lack of detectable oxytocin secretion during spontaneous luteolysis suggests that the contribution of luteal oxytocin in the cow may be less than that proposed for the ewe.