The failure of prolactin to initiate lactogenesis in the ewe.

Infusion of exogenous prolactin (NIH-P-S11) at 1 mg/h for 10 h into ewes pretreated for 30 days with oestradiol benzoate and progesterone was unable to initiate milk secretion. Similarly primed ewes injected with 10 microgram thyrotrophin releasing hormone also failed to lactate but all ewes injected with dexamethasone (10 mg daily for 5 days) after oestrogen plus progesterone treatment secreted copious quantities of milk. The results suggest that prolactin, itself, is not capable of initiating lactogenesis in the ewe.     

Artificial induction of lactation in ewes: the relative importance of oxytocin and the milking stimulus.

Eight ovariectomized and four intact ewes were given oestrogen plus progesterone to develop the mammary glands. The intact ewes (group A) and four ovariectomized ewes (group B) then received four injections each day of 1 i.u. syntocinon for 5 days whereas the other four ovariectomized ewes (group C) received placebo injections of 0-9% saline. Milking commenced the day after the last of these injections. Yields of mammary secretion on the first day of milking--representing secretion accumulated during the injection of syntocinon or saline--for ewes in groups A and B were significantly higher than yields from group C (control) ewes. After 12 days of milking, yields of secretion from group A ewes were significantly higher than yields from group B ewes which in turn were significantly higher than yields from ewes in group C.     

Prolactin enhances uteroglobin gene expression by uteri of immature rabbits

The effect of prolactin on uteroglobin production by immature rabbits was studied with neonatal (1 day old) and juvenile (14 days old) does. The animals were divided into 11 treatment groups for each age category and exposed to a 9-day injection protocol. Each day the animals received a subcutaneous injection of oestradiol-17 beta and/or ovine prolactin and/or progesterone, or were sham-injected. Juvenile animals, which received 100 micrograms oestradiol/kg 24 h-1, plus progesterone or plus prolactin and progesterone, produced detectable amounts of uteroglobin in the uterine secretions (0.034 +/- 0.010 mg uteroglobin/mg total protein and 0.098 +/- 0.03 l mg uteroglobin/mg total protein, respectively). None of the animals in the other juvenile treatment groups or any of the neonatal groups produced uteroglobin. From this survey it was apparent that uteroglobin secretion could be induced by exogenous oestradiol and progesterone in rabbits treated as early as 14 days of age, and that the added supplementation of prolactin enhanced the response to the ovarian steroids. As a result, additional juvenile animals were injected with 100 micrograms oestradiol +/- prolactin + progesterone and the effects of these two treatments were quantitated as follows: uteroglobin mRNA levels by slot-blot hybridization; endometrial surface area by computerized image analysis; and oestrogen, progesterone and prolactin receptors by immunocytochemistry. Prolactin modified the response of the juvenile rabbit uterus to oestradiol + progesterone for all parameters tested.     

The importance of prolactin for initiation of lactation in the pregnant ewe

A single injection of ergocryptine (0.5 mg/kg liveweight) given to ewes 0.5-20 days prepartum or two injections (0.5 mg/kg liveweight per injection) given c. 30 and 10 days prepartum reduced concentrations of plasma prolactin to negligible (less than 5 ng/ml) values for 4 weeks after parturition, but did not affect concentrations of growth hormone and placental lactogen. Milking of treated ewes had no effect on concentrations of plasma prolactin during the first 4 weeks of lactation, but concentrations of growth hormone were increased during the 10-20 min period after milking. The half-life of prolactin in plasma was estimated as 21 min. In spite of the dramatic effect of ergocryptine on plasma prolactin all treated ewes secreted copious quantities of milk of normal composition. Mean daily yields of ewes treated with ergocryptine were not significantly different (P greater than 0.05) from those of untreated control ewes, but the mean +/- s.e.m. of total milk production over the first 3 weeks of lactation for ergocryptine-treated ewes was significantly lower (P less than 0.05) than that of control ewes (9.5 +/- 1.11 v. 14.1 +/- 1.20 kg milk). The results suggest that prolactin is not an essential component of the lactogenic and galactopoietic complexes of hormones in the ewe.     

Artificial induction of lactation in cattle by use of dexamethasone trimethylacetate.

Injections of dexamethasone trimethylacetate initiated lactation in nulliparous Ayrshire heifers previously given a series of injections of oestradiol benzoate plus progesterone to develop mammary glands. Essentially normal lactation occurred following injection of 20 mg/day dexamethasone for 3 days, whereas injection of 40 mg/day for 4 days initiated secretion of smaller volumes of milk-like fluid containing relatively high levels of lipid. Milking alone failed to initiate lactation.     

Control of oestrus in cattle using progesterone coils

Four experiments were conducted to determine the effect of length of treatment, stage of cycle at start of treatment and administration of oestradiol benzoate or progesterone at the start of treatment with intravaginal progesterone coils on oestrous response and fertility. In Experiment 1, the number of heifers in oestrus was affected neither by injection of 5 mg oestradiol benzoate alone or with 200 mg progesterone nor by length of treatment. More heifers (P < 0.05) were in oestrus on day 2 after treatment following a 12-day treatment compared to a 9-day treatment.In Experiment 2, heifers between days 17 and 20 of the oestrous cycle received an injection of either 5 mg oestradiol benzoate alone or with 200 mg progesterone at the start of a 9-day treatment with progesterone coils. Neither the number of heifers in oestrus nor the pattern of onset were affected after treatment. In Experiment 3, heifers between days 0 and 3 of the oestrus cycle received progesterone coils for 9, 12 or 14 days. In addition, animals received (i) no further treatment, (ii) a gelatin capsule adhered to the coil containing 10 mg oestradiol benzoate (iii) a gelatin capsule adhered to the coil containing 200 mg progesterone. Following a 9- or 12-day treatment period heifers receiving the coil with the oestrogen capsule had a high oestrous response ( compared to , P < 0.05). When oestrogen was not given, there was a significant linear effect of duration of treatment on the number in oestrus (9 days, ; 12 days ; 14 days, ; P < 0.05).In Experiment 4, post-partum cows were used to compare a 9- and 12-day treatment period and half the animals in each group received either 5 mg oestradiol benzoate and 200 mg progesterone at the start of treatment or a 10 mg gelatin capsule adhered to the coil. The length of treatment affected the number of heifers in oestrus since were in oestrus after a 12-day treatment period compared with after a 9-day period (P < 0.001). There were no significant differences in the number of cows in oestrus after injection of oestrogen and progesterone ( ) or after the use of the gelatin capsule ( ).     

The effect of oestrogen on selective transfer of IgG1 into mammary secretion of ewes

Two separate experiments were conducted to examine the effects of exogenous oestrogen on selectivetransfer of IgG1 into mammary secretion of ewes. In one experiment, non-pregnant ewes were induced to lactate artificially by first developing mammary glands with injections of progesterone plus low doses of oestrogen then triggering milk secretion with either glucocorticoid or high doses of oestrogen. In the other experiment, lactating ewes were injected with oestrogen each day for 6 days. The results of the experiments suggest that oestrogen affects selective transfer of IgG1 into mammary secretion of the ewes. Moreover, the results show that, in the absence of high levels of oestrogen in blood, the magnitude of the selective transfer of IgG1 into mammary secretion is related inversely to the synthetic activity of the glandular epithelium.     

Effect of progesterone and oestradiol benzoate on oestrous behaviour and secretion of luteinizing hormone in ovariectomized fallow deer (Dama dama).

Eighteen ovariectomized fallow deer does and two adult bucks were used to investigate the effect of exogenous progesterone and oestradiol benzoate on oestrous behaviour and secretion of luteinizing hormone (LH). In Expts 1 and 2, conducted during the breeding season (April-September), does were treated with intravaginal Controlled Internal Drug Release (CIDR) devices (0.3 g progesterone per device) for 12 days and differing doses of oestradiol benzoate administered 24 h after removal of the CIDR device. The dose had a significant effect on the proportion of does that exhibited oestrus within the breeding season (P less than 0.001), the incidence of oestrus being 100% with 1.0, 0.1 and 0.05 mg, 42% for 0.01 mg and 0% for 0.002 mg oestradiol benzoate. There was a significant log-linear effect of dose on the log duration of oestrus, which was 6-20, 2-14, 2-12 and 2 h after treatment with 1, 0.1, 0.05 and 0.01 mg of oestradiol benzoate, respectively. Dose had a significant effect on the peak plasma LH concentration (P less than 0.01), mean (+/- s.e.m.) surge peaks of 27.7 +/- 2.3, 25.9 +/- 1.8 and 18.6 +/- 3.4 ng/ml being observed following treatment with 1, 0.1 and 0.01 mg oestradiol benzoate respectively. In Expt 3, also conducted during the breeding season, progesterone treatment (0 vs. 6-12 days) before the administration of 0.05 mg oestradiol benzoate had a significant effect on the incidence of oestrus (0/6 vs. 10/12, P less than 0.05), but not on LH secretion. The duration of progesterone treatment (6 vs. 12 days) had no effect on oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The passage of spermatozoa through the cervix of ovariectomized ewes treated with progesterone and oestrogen.

Thirty spayed ewes were used in a 2 x 3 split-plot factorial experiment (n = 5) in which oestrus was induced with 30 or 90 mug oestradiol benzoate (OB) following a period of progesterone priming. They were inseminated 36 hr after oestrogen and the numbers of spermatozoa in the caudal, mid- and cranial regions of the cervix (sub-plots) were estimated 1, 12 and 24 hr later. At each interval of time and for each region of the cervix, fewer spermatozoa were recovered from the ewes treated with 30 mug OB than from those treated with 90 mug (P less than 0-05).     

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.     

Cervical mucus and oestrous behaviour responses to oestrogens in sheep: Progesterone-oestrogen relationships and role of progesterone pre-treatment

The final dose of progesterone (5, 10, 20 mg) and time to oestrogen injection relative to the final dose of progesterone (24–72 h) had no significant effect on the production of cervical mucus measured 24 h after the injection of 30 μg oestradiol benzoate (ODB). However, there were significant effects on the behavioural oestrous responses (time from injection of oestrogen to onset of oestrus and duration of oestrus). Time to onset of oestrus increased from 18 to 27.8 h with increasing dose of progesterone (P < 0.001) and decreased from 24.8 to 20 h with increasing time to oestrogen injection (P < 0.05). Conversely, the duration of oestrus decreased from 36.2 to 23.8 h with increasing dose of progesterone (P < 0.001) and increased from 29 to 39 h with increasing time to oestrogen injection (P < 0.01).Ovariectomized ewes became refractory to ODB as measured by the cervical mucus response after the fifth sequential daily injection of 20 μg oestradiol benzoate. Progesterone priming was not required to restore subsequent sensitivity to oestrogen treatment. However, there was a positive linear relationship between length of recovery period and level of response to subsequent treatment.It was concluded that: (1) progesterone pre-treatment or priming is not necessary in the cervical mucus bioassay in ovariectomized ewes; and (2) a period of 8–16 days is needed between assays for normal sensitivity to be regained.     

Endometrial protein secretion during early pregnancy in entire and ovariectomized ewes

The secretion and synthesis of protein in vitro by explants of endometrium were examined in entire ewes during the first 10 days of the oestrous cycle and during an equivalent interval in ovariectomized ewes which received injections of oestradiol and progesterone. The schedule of steroid injections given was designed to simulate endogenous ovarian secretion of progesterone during the luteal phase before oestrus, of oestradiol around oestrus and of progesterone during the luteal phase after oestrus. The rate of protein synthesis and tissue RNA:DNA and protein:DNA ratios in intercaruncular and caruncular endometrium were generally higher in entire than in ovariectomized ewes. In ovariectomized ewes oestradiol increased these activities at 2-4 days after oestrus, whereas progesterone preceding oestradiol caused increases at oestrus, but not thereafter. In entire ewes and in ovariectomized ewes receiving the full steroid treatment regimen, protein secretion was high at oestrus and declined markedly during the next 4-6 days. In ovariectomized ewes not receiving progesterone before oestradiol, secretion increased between 4 and 6 days after oestrus, or during the equivalent stage of treatment in ewes which did not show oestrus. The omission of this progesterone did not modify secretion by caruncular endometrium. Oestradiol increased protein secretion by both tissues. The data suggest that progesterone given before oestradiol (or its equivalent in entire ewes) inhibits the secretion, at about 4-7 days after oestrus, of uterine proteins which may impair embryo development in ovariectomized ewes which do not receive this progesterone.     

Effects of prolactin on conceptus survival and uterine secretory activity in pigs

Hypoprolactinaemia was induced by bromocriptine (CB154; 100 mg/day) which decreased circulating prolactin by 40% (P less than 0.06), but did not affect conceptus survival at Day 25 when administered on Days 10-16 when compared to saline:ethanol-treated control gilts. Bromocriptine or vehicle was administered to cyclic gilts on Days 10-11, oestradiol valerate was injected on Day 11 and uterine flushings were collected on Day 12. Total recoverable protein and uteroferrin in uterine flushings were not affected by treatment. However, leucine aminopeptidase activity (P less than 0.02) and total recoverable Ca2+, Na+, K+ and Cl- (P less than 0.05) were decreased in uterine flushings of gilts that received bromocriptine, suggesting that hypoprolactinaemia decreased general secretory activity of the endometrial epithelium and modulated ionic changes, respectively, in the uterine environment of pigs. Subcutaneous administration of pig prolactin (1 mg/12 h) increased (P less than 0.001) serum prolactin 4.5-fold. The interaction between hyperprolactinaemia and progesterone, without oestrogen, on components of uterine flushings were determined using gilts that received progesterone (200 mg/day) and prolactin or saline on Days 4-14 after ovariectomy on Day 4. On Day 15, there were no differences (P greater than 0.05) in any of the uterine secretory components measured. Hyperprolactinaemia (1 mg pig prolactin on Days 6-11) enhanced overall uterine secretory response on Day 12 to oestradiol (5 mg) administered on Day 11 compared to gilts that received 1 ml saline on Days 6-11 of the oestrous cycle. Total recoverable protein and leucine aminopeptidase activity were greater (P less than 0.05) for oestradiol-treated gilts, but effects of prolactin were not significant. Total recoverable glucose (P less than 0.01), PGF-2 alpha (P less than 0.02), uteroferrin (P less than 0.01) and specific activity of uteroferrin (P less than 0.001) were increased by prolactin and oestradiol, but not oestradiol alone. Calcium (P less than 0.05), chloride (P less than 0.05) and potassium (P less than 0.01) were increased in response to oestradiol. These results indicate an interaction between oestradiol and prolactin, but not progesterone and prolactin, which enhances secretion of some products of the pig uterine endometrium.     

Ovarian steroid involvement in endogenous opioid modulation of LH secretion in seasonally anoestrous mature ewes

In June, 16 mature ewes were ovariectomized and allocated to four groups: 1, saline; 2, naloxone; 3, progesterone implant plus naloxone; 4, oestrogen implant plus naloxone. Steroids were implanted at the time of ovariectomy. At 5 days after ovariectomy, the animals were intravenously infused with saline for 8 h and naloxone (50 mg/h) in saline for 8 h the following day. Three intact ewes were given naloxone in a similar way. During infusions and for 8 h on the day after naloxone, jugular venous blood samples were taken every 15 min and assayed for LH. Naloxone resulted in significant increases in mean LH concentration (P less than 0.01), LH episode frequency and episode height (P less than 0.05) in Group 3 ewes, but was without effect in any other group. These results provide evidence that the progesterone status of the ewe affects its response to naloxone, that progesterone negative feedback on LH release may be mediated by an opioid system, and that increased oestradiol negative feedback during seasonal anoestrus is unlikely to work via increased opioid inhibition of LH.     

Early induction of parturition and initiation of lactation in sheep

Dexamethasone and estradiol benzoate were used to induce parturition in ewes at about day 120 of gestation as part of a program to reduce the time taken to progeny-test carpet-wool rams by evaluating the birthcoats of their offspring. Ten ewes received 5 injections of 12 mg dexamethasone over 2.5 days commencing on day 117. Eight lambed 3.1 +/- 0.53 days after the final injection. Of 14 ewes which received 20 mg estradiol benzoate on day 118, three delivered lambs, 2.0 +/- 0.41 days after injection. All lambs were born dead or died within 2 hours of birth. Following parturition all ewes came into lactation. The dexamethasone group produced more colostrum, with higher total solids content than the estradiol group. However, the volume was less than in a control group which lambed at full term. It was concluded that dexamethasone could be used successfully to induce parturition at 120 days and that onset of lactation was similar to that which occurs at full term.     

The secretion of prolactin in intact and lutectomized pregnant ewes. Effect of the anti-progesterone steroid RU 486.

To study the role, if any, of luteal factors in the control of prolactin secretion during the last two thirds of pregnancy in the ewe, we examined: a) the effect of RU 486 administration on prolactin secretion on days 97, 112 and 131 of pregnancy in five intact ewes and in five ewes from which the corpus luteum (CL) was removed on day 78 of pregnancy; and b) the secretory patterns of prolactin on days 60, 80, 100 and 120 of pregnancy in five intact ewes and in five ewes from which the CL was removed on day 70 of pregnancy. In a pilot experiment, we showed that daily i.v. injections (from day 91 to day 105 of pregnancy) of RU 486 at a dose of 50 mg caused a marked release of prolactin, without any effect on the secretion of progesterone and progression of pregnancy. In experiment 1, a single i.v. injection of 50 mg of RU 486 resulted in a significant (P < 0.01) increase in plasma prolactin concentrations on any day of pregnancy examined in the intact and lutectomized ewes. The prolactin responses (the maximum concentrations, the time to maximum concentrations and the area under the response curves) were not different between the two groups in any stage of pregnancy examined. In the two groups, spontaneous parturition occurred at term with alive lambs. There was no difference between the two groups in gestation length and lamb birth weight. In experiment 2, we showed that plasma concentrations of prolactin fluctuated in a pulsatile manner during the last two-thirds of pregnancy. The mean prolactin concentrations, the frequency and the amplitude of prolactin pulses were not significantly different between the intact and the lutectomized ewes in any stage of pregnancy examined. In conclusion, these experiments demonstrated that the ovine CL of pregnancy is not involved in the control of prolactin secretion in the ewe. The stimulation of prolactin secretion by the RU 486 is probably due to its anti-progesterone action exerted at the level of the receptor. The placental progesterone plays a central role in the control of prolactin secretion during the last two-thirds of pregnancy.     

Prolactin as a factor in the uterine response to progesterone in rabbits

The direct effect of prolactin on uteroglobin production and on uterine endometrial oestrogen and progesterone receptor concentrations was tested by using ovariectomized rabbits (at least 12 weeks) treated with prolactin; prolactin + progesterone; prolactin + oestradiol + progesterone; oestradiol + progesterone; or progesterone alone. Prolactin treatment produced a significant (P less than 0.05) increase in the concentration of cytosolic oestrogen and progesterone receptors, restoring the concentrations to values found at oestrus. However, the concentration of nuclear receptors remained low. In the remaining treatment categories there was no significant (P greater than 0.05) increase in the concentration of oestrogen and progesterone receptors compared with those in ovariectomized controls. However, the sequential treatment of ovariectomized animals with prolactin + progesterone stimulated uteroglobin production to a concentration equal to that found in intact rabbits on the 5th day of pregnancy. This was not achieved by prolactin or progesterone alone or with oestradiol. These results suggest that prolactin acts as an essential factor in the rabbit uterine response to progesterone, perhaps by the modulation of progesterone receptor activity.     

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

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

Critical progesterone requirement for maintenance of pregnancy in ovariectomized rats

The minimum progesterone concentration required to maintain the pregnancy was studied by varying doses of progesterone given subcutaneously to rats ovariectomized on Day 8 of pregnancy. Injecting 3 mg progesterone plus 200 ng oestradiol benzoate daily provided serum progesterone values between 25.4 +/- 7.0 and 35.2 +/- 6.2 ng/ml throughout Days 10-19 which were significantly lower than normal levels (P less than 0.05), but resulted in 93.6% of fetal survival on Day 19 which was not significantly different from 93.3% in the control group. Injecting 2 mg progesterone plus 200 ng oestradiol benzoate daily gave progesterone values between 13.2 +/- 4.6 and 19.0 +/- 6.2 ng/ml and could not maintain fetal viability to Day 19 (14.2%, P less than 0.05 compared with control group). Critical times to supplement progesterone in rats ovariectomized on Day 8 or Day 15 were studied by varying the time of progesterone implantation after ovariectomy. Progesterone implants were administered 8, 12 and 24 h after ovariectomy on Day 8 and 24, 36 and 48 h after ovariectomy on Day 15. On Day 8, progesterone replacement could be delayed to 8 h but not 12 h, while on Day 15, progesterone replacement could be delayed up to 36 h but not 48 h after ovariectomy without affecting fetal survival.     

Effect of ovine trophoblast protein-1, oestrogen and progesterone on oxytocin-induced phosphatidylinositol turnover in endometrium of sheep

In Exp. 1, endometrium was collected from Day-15 cyclic ewes and effects of oTP-1, oxytocin and oTP-1 + oxytocin, in various temporal relationships, on phosphatidylinositol (PI) turnover were determined. Co-treatment of endometrium with oTP-1 and oxytocin inhibited stimulatory effects of oxytocin, while treatment with oTP-1 before and during oxytocin administration had no effect. Turnover of PI was unaffected by oTP-1 alone. In Exp. 2, ovariectomized ewes were treated with progesterone (50 mg/day) for 10 days and then oestrogen (100 micrograms/day) for 2 days and endometrium was collected. Oxytocin stimulated PI turnover in endometrium, but oTP-1 had no effect alone or in combination with oxytocin. In Exp. 3, ovariectomized ewes were treated with corn oil (1 ml/day), oestrogen (50 micrograms/day), progesterone (50 mg/day) or progesterone + oestrogen for 10 days and endometrium was collected. Oxytocin stimulated PI turnover only in ewes that received progesterone. oTP-1 alone had no effect on PI turnover, while co-treatment of endometrium with oxytocin and oTP-1 stimulated PI turnover in ewes treated with progesterone, but not progesterone and oestrogen. Pretreatment of endometrium with oTP-1 stimulated PI turnover when ewes were treated with progesterone or progesterone + oestrogen. Pretreatment of endometrium with oxytocin and then treatment with oTP-1 inhibited PI turnover compared to treatment with oxytocin alone. In Exp. 4, ovariectomized ewes were treated as in Exp. 2. Catheters were placed into the uterine horns and ewes received oTP-1 into one horn and serum into the other twice daily on Days 10-12 of steroid treatment. Endometrium collected on Day 13 was used to measure PI turnover and received either no treatment or oxytocin. Oxytocin stimulated PI turnover in endometrium of these ewes and in-vivo treatment of the ewes with oTP-1 had no effect on PI turnover. These results indicate that antiluteolytic effects of oTP-1 are not mediated by inhibiting effects of oxytocin on phosphatidylinositol turnover if oxytocin receptors are present and that uterine responsiveness to oxytocin is progesterone dependent.