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  • Author: J. MARTINET x
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Twice-daily intramuscular injections of 0·5 mg oestradiol benzoate, starting on Day 3 of the cycle, were able to prolong the life of the corpora lutea in sheep, as judged by their weight, their DNA and RNA content, and the progesterone concentration in ovarian vein blood. This luteotrophic effect persisted even when the pituitary stalks of the animals were sectioned, but it was abolished when the animals were hypophysectomized.

Four possible sites of oestrogen action were considered; the hypothalamus, the pituitary, the corpus luteum and the uterus. It was concluded that oestrogen is luteotrophic principally because of its action on the uterus, where it seems to interfere with the normal luteolytic mechanism.

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The functional activity of ovine CL was assessed by their weight, DNA and RNA content, and the concentration of progesterone in ovarian venous blood.

If sheep were hysterectomized on Days 9 to 12 of the oestrous cycle, the CL were maintained in a fully functional state until at least Day 60, but their activity had begun to decline by Day 128 to 135. When hysterectomized sheep were hypophysectomized, there was a significant decline in luteal activity within 48 hr, regardless of whether or not the pituitary stalk and pars tuberalis were left intact. The CL had almost completely stopped secreting progesterone within 4 days of hypophysectomy.

Hysterectomized, hypophysectomized animals were therefore used in a series of experiments to test the luteotrophic properties of sheep pituitary gonadotrophins. Doses of up to 5 mg FSH/day were unable to prevent complete luteal regression; similarly, doses of up to 5 mg LH/ day were also without effect. When mixtures of FSH and LH were given, the results were no better. However, prolactin in doses of up to 1000 i.u./ day was invariably able to maintain functional CL for 12 days, although at a considerably reduced level of activity. When prolactin was combined with a small dose of LH (0·25 mg/day), the CL were maintained at a level of activity comparable to that seen in hysterectomized animals before hypophysectomy. The slight synergistic action of FSH with prolactin was probably due to LH contamination. No further beneficial effects were obtained by adding FSH to the prolactin—LH mixture.

We conclude that prolactin and LH are both necessary for the maintenance of the ovine CL, and that these two hormones together make up the `luteotrophic complex'. But whilst prolactin on its own has some luteotrophic activity, LH by itself is completely ineffective.

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K. H. Al-Gubory, M. R. Blanc and J. Martinet

Summary. 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.

Keywords: ewe; pregnancy; post partum; corpus luteum; pituitary gonadotrophins

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K. H. Al-Gubory, J. Martinet, M. R. Blanc, J. C. Poirier and A. Solari

Summary. 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.

Keywords: ewe; corpus luteum; luteinizing hormone; follicle-stimulating hormone; LH-release-inhibiting factor

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K. H. Al-Gubory, M. R. Blanc, J. C. Poirier, A. Solari and J. Martinet

Summary. Concentrations of LH and FSH were measured in blood samples collected from the jugular vein at 20-min intervals for 7 h (09:00–16:00 h) on Days 60, 80, 100 and 120 of pregnancy in 5 intact ewes and 5 from which the CL had been excised on Day 70. In the 5 intact ewes, plasma LH concentrations remained low and unchanged between Days 60 and 120. During this period, pulsatile release of LH occurred irregularly and infrequently. Removal of the CL resulted in an increase in the basal values of LH and in the frequency and amplitude of LH pulses. Concentrations of FSH were relatively constant in all stages of pregnancy examined and were similar in both groups of ewes. These results show that (1) LH concentrations are low during the second half of pregnancy; and (2) LH, but not FSH, increases after CL excision, presumably by removing some luteal factor inhibitor of LH secretion.

Keywords: ewe; pregnancy; corpus luteum; FSH; LH

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A. Schirar, Y. Cognié, F. Louault, N. Poulin, C. Meusnier, M. C. Levasseur and J. Martinet

Summary. The post-partum secretion of LH, FSH and prolactin was monitored in 15 suckling and 6 non-suckling Préalpes du Sud ewes lambing during the breeding season by measuring plasma hormone concentrations daily at 6-h intervals and also weekly at 20-min intervals for 6 h from parturition to resumption of regular cyclic ovarian activity. There was a constant phenomenon in the resumption of normal patterns of FSH and LH secretion: there was a rise in FSH values culminating on average on Day 4 post partum and returning subsequently to values observed during the oestrous cycle, and concurrently an increase in the frequency and amplitude of LH pulses more progressive in suckling than in non-suckling ewes which led to an elevation of LH mean concentrations and occurrence of an LH surge. Since neither the FSH secretory pattern nor FSH mean values differed between suckling and non-suckling ewes, the results suggested that LH pulsatile pattern was a major limiting factor for the resumption of normal oestrous cycles. Before regular oestrous cycles resumed other changes in preovulatory LH surges also occurred: (i) they increased in duration and probably in amplitude; (ii) they were preceded by an acceleration in LH pulse frequency and a large decrease in FSH values as in normal cyclic ewes; and (iii) at least in non-suckling ewes they occurred concurrently with a prolactin surge.

Keywords: ewe; post partum; LH; FSH; prolactin; suckling; breeding season

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A. Schirar, Y. Cognie, F. Louault, N. Poulin, M. C. Levasseur and J. Martinet

Summary. In Préalpes de Sud ewes after an autumn lambing, the mean post-partum interval to first LH surge was 10 ± 1 days and 17 ± 1 days for non-suckling and suckling ewes, respectively. Post-partum interval to first luteal phase, estimated from plasma progesterone concentrations, was similar in non-suckling and suckling ewes (27 ± 1 days and 28 ± 5 days, respectively). Interval to first oestrus was shorter in non-suckling (22 ± 2 days) than in suckling ewes (35 ± 2 days) but these first oestrous periods were followed by short luteal phases in 60% (12/20) of non-suckling ewes and in only 7% (2/29) of suckling ewes. Finally, suckling slightly postponed the resumption of the first oestrus followed by a normal oestrous cycle (37 ± 1 days versus 31 ± 2 days) because progesterone, essential for oestrus expression, was secreted mainly during normal luteal phases in 70% (21/30) of suckling ewes and during short cycles in 95% (21/22) of non-suckling ewes. Therefore, the primary consequence of suckling is to regulate the conditions of resumption of cyclic ovarian activity after parturition.

Keywords: ewe; post partum; breeding season; suckling; ovarian activity; oestrous activity

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M. Evrard-Herouard, M. P. de la Llosa-Hermier, J. Martinet, P. Mauleon, P. de la Llosa and C. Hermier

Summary. The LH binding properties (determined using tritiated methylated LH) and the in-vitro steroidogenic activity of CL from ewes in the oestrous cycle or early pregnancy (Day 18) were compared.

No significant alteration in the K d values was observed. However, the number of sites was maximal at Day 10 of the cycle and in early pregnant animals which had not been pregnant for at least 3 months (dry ewes). Non-lactating or suckling ewes had half the numbers of binding sites. The increase of the number of receptor sites was accompanied by a steroidogenic response at lower LH concentration.

During incubation or superfusion for 5 h, a refractoriness to LH stimulation appeared after 1 h with high LH concentrations and after 3 h with low concentrations. The opposite effect of the addition of indomethacin or PGF-2α suggests the intervention of PGs in this phenomenon.