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.

Evidence that the conceptus contributes to the inhibition of follicular growth in the ewe

The present study was designed to determine the contribution of the gravid uterus and the conceptus in the inhibition of antral follicular growth. All antral follicles were counted and measured on day 48 post mating in ovaries of pregnant ewes (Group 1, n = 5) and in ovaries of ewes from which the uterus (Group 2, n = 5) or the conceptus (Group 3, n = 5) had been surgically removed on days 32-33 post mating. The secretory activity of the corpora lutea was maintained after removal of the uterus or the conceptus until slaughter of ewes. No difference was found between the three groups of ewes in the mean (+/-SEM) total number (non-atretic and atretic) of follicles per ewe, being 59.8 +/- 7.6, 35.0 +/- 7.6 and 50.8 +/- 4.6, respectively for Groups 1, 2 and 3. The mean (+/-SEM) number of follicles 2 mm or more in diameter was greater (P < 0.01) in Group 2 ewes (3.4 +/- 0.5) and Group 3 ewes (4.2 +/- 0.5) than in Group 1 ewes (1.6 +/- 0.4). Of all follicles counted which were 2 mm or larger, 90% in Group 1 were atretic, this value being higher (P < 0.05) than in Group 2 (68%) and Group 3 (55%). The mean (+/-SEM) diameter of the largest (F1) and second largest (F2) follicles was greater (P < 0.01) in Group 2 (F1, 4.0 +/- 0.4; F2, 3.2 +/- 0.4) and in Group 3 (F1, 3.9 +/- 0.2; F2, 3.3 +/- 0.2) than in Group 1 (F1, 2.6 +/- 0.1; F2, 2.2 +/- 0.1). The mean diameter (+/-SEM) of the largest non-atretic follicles was greater (P < 0.01) in Group 2 (4.0 +/- 0.4) and Group 3 (3.9 +/- 0.2) than in Group 1 (1.7 +/- 0.3). In conclusion, the results from this in vivo experiment provide evidence that the conceptus prevents the development of large healthy follicles by inducing atresia during the first trimester of pregnancy. The inhibition of follicular growth would reduce oestradiol production and permit the maintenance of luteal function during early pregnancy.

Evidence that a non-steroidal factor from ovine placenta inhibits aromatase activity of granulosa cells in vitro

Previous studies in vivo provide evidence that extra-ovarian factors, currently unknown but nevertheless present and associated with pregnancy, directly prevent the growth of follicles beyond a diameter of 2 mm during the last trimester of pregnancy in the ewe. In the present study, the effect of charcoal-treated extract from sheep placenta on total aromatase activity, as determined by the conversion of [3H] androstenedione to estradiol and measurement of [3H] water, was investigated using primary culture of human granulosa cells in serum-free medium as a model. Addition of different doses (50, 150, 300 and 600 micrograms protein) of cotyledons extract of day 110 of pregnancy produced a dose-dependent diminution of granulosa cell aromatase activity in the absence of FSH. The maximal inhibition of aromatase activity occurred at a dose of 600 micrograms. These results showed that the cotyledons of late pregnancy contain a non-steroidal factor that inhibits basal aromatase activity in granulosa cells. Extracts prepared from cotyledons of days 90 and 110 of pregnancy but not extracts of days 50 and 70 significantly reduced basal aromatase activity in a dose-dependent manner. These results suggest that the production of the aromatase inhibiting factor increases with the advance of pregnancy. The aromatase inhibiting activity was lost after heating (80 degrees C, 30 min) or after treatment with trypsin (1 mg/ml) of cotyledons extract of day 110 of pregnancy, demonstrating the protein nature of the aromatase inhibiting factor. In conclusion, these experiments demonstrate that ovine placenta contains a heat- and trypsin-sensitive factor likely to be a protein which inhibits granulosa cell aromatase activity.

Evidence that a non-steroidal factor from corpus luteum of pregnant sheep inhibits aromatase activity of ovarian follicles in vitro

Porcine and ovine follicular tissues were used to investigate, in vitro, the effect of charcoal-treated aqueous extract from ovine corpora lutea of pregnancy on aromatase activity as determined by the conversion of [3H]testosterone to oestradiol by follicular walls and measurement of 3H2O release. Extract (500 micrograms protein) prepared from corpora lutea of day 112 of pregnancy but not extract (500 micrograms) prepared from ovine fetal cotyledonary tissue obtained at a similar time significantly decreased (P < 0.02) aromatase activity of pig follicles in the absence of FSH. These results demonstrate that a non-steroidal factor in the corpora lutea of late pregnancy directly inhibits aromatase activity. When the effects of different doses (300, 600 or 1200 micrograms) of luteal extract from corpora lutea of day 100 of pregnancy on aromatase activity of pig follicles were studied, the dose by treatment (presence or absence of FSH) interaction was not significant. Luteal extract dose at 300 micrograms did not affect aromatase activity but a significant decrease in activity occurred at 600 micrograms of luteal extract (600 versus 300 micrograms, P < 0.02). There was no further significant increase in the inhibitory effect with 1200 micrograms luteal extract. When the effects of 600 micrograms luteal extract from corpora lutea of days 15, 75 or 100 of pregnancy on aromatase activity of pig follicles were studied, a significant (P < 0.05) stage of pregnancy effect was detected, but the stage of pregnancy by treatment (presence or absence of FSH) interaction was not significant. No effect was noted with day 15 or day 75 luteal extract.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a gonadal nonsteroidal factor that specifically inhibits release of luteinizing hormone in ewes

Bilaterally ovariectomized ewes were used to investigate the effect of systemic administration (i.v.) of charcoal-treated aqueous luteal extracts from ovine corpora lutea on plasma concentrations of pituitary gonadotrophins. Jugular blood samples were taken every 15 min at least 5 h before (control period) and 5 h after (treatment period) injection. In Expt 1, the administration of luteal extract from corpora lutea of days 70-76 of pregnancy, but not of the extract prepared from muscular tissue, resulted in a significant decrease of mean concentrations of luteinizing hormone (LH) (P < 0.02) and frequency of LH pulses (P < 0.01). Plasma follicle-stimulating hormone (FSH) concentrations were not affected by injections of either extract. These findings provide the first demonstration of the presence of a nonsteroidal factor in the corpus luteum of midpregnancy that selectively suppresses the secretion of LH. In Expt 2, mean concentrations of LH and FSH and frequency of LH pulses were unaffected by injections of luteal extracts from ovine corpora lutea of days 10-12 of the oestrous cycle or day 15 of pregnancy. These data suggest that some factor(s), probably from the fetoplacental endocrine unit, is required to ensure the production of a significant quantity of the luteal LH-inhibiting factor after day 15 of pregnancy. In Expt 3, treatment of luteal extract from corpora lutea of day 70 of pregnancy with proteolytic enzymes destroyed the LH-inhibiting activity, suggesting the proteic nature of the luteal LH-inhibiting factor. In Expt 4, plasma concentrations of LH were not affected by injection of charcoal-treated extract prepared from fetal cotyledonary tissue of days 110-120 of pregnancy suggesting that the LH-inhibiting factor exclusively originates from the corpus luteum during pregnancy. These experiments provide the first direct evidence for the existence of a potent nonsteroidal factor of luteal origin that specifically inhibits pulsatile secretion of LH, without influencing FSH release in female animals. We propose the term LH-release-inhibiting factor (LH-RIF) to describe this activity.

Role of the corpus luteum of pregnancy in controlling pituitary gonadotrophin secretion during the early post-partum period in the ewe

No difference was found between 5 intact ewes and 5 ewes from which the CL had been excised at Day 70 of pregnancy in the plasma concentration of progesterone at Day 140, and concentrations of progesterone remained below 0.2 ng/ml during the first 20 days post partum. Plasma concentrations of LH, frequency and amplitude of LH pulses were low at Day 140 and increased considerably, particularly in the CL-excised ewes, as early as Day 5 post partum. No significant differences were found between the two groups of ewes in the mean plasma concentrations of FSH for any of the 5 stages examined. Taken together, these results suggest that some factor, other than progesterone, associated with the CL of pregnancy is involved in the inhibition of pulsatile LH secretion during the early post-partum period.