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W. Haresign and G. E. Lamming

Summary. Pretreatment of seasonally anoestrous Clun Forest ewes with 750 i.u. PMSG or 50 μg oestradiol benzoate 24 or 7 h respectively before a single injection of 150 μg synthetic LH-RH significantly increased the release of LH compared to that after injection of 150 μg LH-RH alone. Total LH release in the two 'combined' treatments was approximately 70% of that found at a natural oestrus, compared to 25% for LH-RH alone. All but one of the treated ewes ovulated, but only those pretreated with PMSG consistently produced corpora lutea capable of elevating peripheral plasma progesterone concentrations although these were lower than those at natural mid-cycle. These progesterone concentrations were, however, comparable to those during the natural cycle when corrected for the higher metabolic clearance rate found during anoestrus.

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STELLA PICKWORTH and G. E. LAMMING

Summary.

Rabbits ovariectomized 24 hr post coitum (p.c.) were injected i.m. with 2 mg or 4 mg 17α-hydroxyprogesterone caproate daily. Pre-natal development was compared with that of control litters at 9 and 29 days, 5 days and 7 days p.c. The 2 mg treatment supported 26·4% pre-natal survival to 29 days p.c., most mortality occurring between 7 and 9 days. The 4 mg dose rate permitted 71 ·0% survival to 29 days p.c. which was not significantly different from controls (81·5%). Blastocyst size at 5 days p.c. was not significantly affected by treatment but both ovariectomized groups showed slight retardation at 7 days when stage of attachment was assessed by observation and by analysis of blastocyst fluid for lactic acid. Attachment was least advanced in the group receiving the lower dose rate.

In 29-day foetuses the degree of ossification of the femur was positively related to foetus weight. In control litters deposition of calcium and total ash components and relative decrease in water content were also further advanced in the larger foetuses, suggesting a general association of size and maturity.

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G. E. Lamming and G. E. Mann

We have investigated changes in endometrial oxytocin receptor concentrations and prostaglandin F release in response to exogenous oxytocin treatment in ovariectomized cows treated with progesterone and oestradiol, and made comparisons with similar treatment in cyclic cows. In long-term ovariectomized cows, endometrial oxytocin receptors were present (300 fmol mg−1 protein), but no prostaglandin F was released in response to oxytocin treatment until after the administration of progesterone. Subsequent administration of a concentration of oestradiol sufficient to induce oestrus resulted in the downregulation of these receptors and the loss of oxytocin responsiveness, which did not reappear within 20 days in the absence of further hormone treatment. When induced oestrus was followed by further treatment with luteal phase concentrations of progesterone and oestradiol, both oxytocin receptors and oxytocin-stimulated release of prostaglandin F reappeared by day 16 after oestrus, in a pattern similar to that seen during the luteal phase of cyclic cows. These results demonstrate how progesterone and oestradiol control the development and responsiveness of endometrial oxytocin receptors in cows, and provide a valuable model in which to investigate further the precise control of the oxytocin receptor in this species.

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G. E. Mann and G. E. Lamming

Mated cows that subsequently undergo luteolysis have significantly lower plasma concentrations of progesterone than do cows in which pregnancy is maintained. Two groups of ovariectomized cows were treated with progesterone and oestradiol to recreate luteal phases exhibiting the extremes of plasma concentrations of progesterone (6.0 ± 0.4 and 12.4 ± 0.8 ng ml−1) normally seen in intact cows. The effect of this difference in progesterone on the development of the luteolytic signal was determined by monitoring the basal and oxytocin-stimulated plasma concentrations of the principal metabolite of prostaglandin F, PGFM on days 12–16 of the simulated luteal phases. Basal PGFM concentrations were similar in the low and high progesterone groups on all days. The mean plasma concentration of PGFM showed a larger increase following oxytocin in the low progesterone group on days 13 and 14 (P < 0.05) and on days 15 and 16 (P < 0.1). These results demonstrate that a low plasma concentration of progesterone results in the development of a stronger luteolytic signal, and provide an explanation for the fact that cows with lower plasma concentrations of progesterone are more prone to embryo loss.

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G. E. Lamming and B. J. McLeod

Summary. Plasma LH concentrations were monitored in 6 Hereford × Friesian suckled cows at about 80 days post partum, before and during a 14-day period of continuous s.c. infusion of GnRH (20 μg/h). Blood samples were collected at 10-min intervals on Days −2, −1, 1, 2, 3, 4, 7, 10, 13 and 14 (Day 1 = start of infusion). Plasma LH concentrations rose from mean pretreatment levels of 1·3 ± 0·20 ng/ml to a maximum of 17·1 ± 3·09 ng/ml within the first 8 h of GnRH infusion, but returned to pretreatment levels by Day 2 or 3. In 4/6 animals, the initial increase was of a magnitude characteristic of the preovulatory LH surge. In all animals, an i.v. injection of 10 μg GnRH, given before the start and again on the 14th day of continuous infusion, induced an increase in LH concentrations but the increase to the second injection was significantly (P < 0·01) less (mean max. conc. 6·4 ± 0·76 and 2·3 ± 0·19 ng/ml). Mean LH concentrations (1·0 ± 0·08, 1·1 ± 0·08 and 0·9 ± 0·06 ng/ml) and LH episode frequencies (3·3, 4·3 and 3·2 episodes/6 h) did not differ significantly on Days −2,7 and 13. However, the mean amplitude of LH episodes was significantly lower (P < 0·05) on Day 13 (1·3 ± 0·10 ng/ml) than on Day −2 (1·8 ± 0·16 ng/ml). Therefore, although the elevation in plasma LH concentrations that occurs in response to continuous administration of GnRH is short-lived and LH levels return to pre-infusion values within 48 h of the start of infusion, these results show that the pituitary is still capable of responding to exogenous GnRH, although the LH response to an i.v. bolus injection of GnRH is reduced. In addition, this change in pituitary sensitivity is not fully reflected in endogenous patterns of episodic LH secretion.

Keywords: GnRH infusion; pituitary sensitivity; LH; cows

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A. P. Beard and G. E. Lamming

The control of temporal changes in oxytocin receptor concentrations and oxytocin-induced 13,14-dihydro-15-keto PGF (PGFM) release was examined in ewes. One week after ovariectomy, 36 ewes were administered fluorogesterone acetate for 10 days followed by oestradiol (3 × 16 μg day−1) for 2 days (pretreatment cycle). Day 0 was designated as the time of the final 'oestrous' oestradiol injection. Ewes were then treated for up to 12 days with progesterone (24 mg day−1 maximum) with or without oestradiol (both hormones administered in 1 ml of corn oil i.m. at 8 h intervals) in a pattern known to simulate natural plasma profiles of the oestrous cycle. The three treatments were zero oestradiol, low oestradiol (12 μg day−1 maximum), and high oestradiol (36 μg day−1 maximum). Subgroups of four ewes from each treatment group were given 1 μg of oxytocin (i.v.) on day 10, 11 or 12 of the simulated cycle, and endometrial oxytocin receptor concentrations were determined in samples collected within 3 h of oxytocin administration. On day 10 only one ewe in each group exhibited a PGFM response to oxytocin, and the mean response was unaffected by the concentration of oestradiol administered. On days 11 and 12 there was a significant effect of oestradiol concentration (P < 0.05) on the pattern of PGFM release in response to oxytocin, the high oestradiol concentration causing a rapid increase in the concentration of PGFM following oxytocin administration. On day 12 the oestradiol concentration was positively correlated with the PGFM mean response (P < 0.01). Oxytocin receptor concentrations were positively correlated with the concentration of PGFM released on days 11 and 12. We conclude that the quantitative effect of oestradiol is mediated principally through the oestrogenic stimulation of uterine oxytocin receptors, although additional effects on post-receptor events cannot be excluded. These results demonstrate that the oestradiol concentration affects the timing, the magnitude and the pattern of the PGF response to oxytocin in progesterone-treated ovariectomized ewes. In the natural cycle, a high oestradiol concentration may therefore be associated with an earlier onset of luteolysis.

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Stella Acritopoulou, W. Haresign and G. E. Lamming

Summary. Treatment of 18 cyclic Clun Forest ewes with two i.m. injections of ICI 80,996, given 9 days apart and without reference to stage of the oestrous cycle, synchronized ovulation in all ewes at a mean time interval of 73·1 ±1·6 (s.e.m.) h from the second injection. The interval from the LH peak to ovulation was 22·6 ± 0·7 h and this is comparable to previously reported figures for a natural oestrus.

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J. H. Payne and G. E. Lamming

In unilateral pregnant ewes, obtained by surgically transecting one uterine horn to isolate it from the rest of the pregnant uterus, the isolated horn develops endometrial oxytocin receptors by day 16 despite being in a hormonal environment equivalent of that of a pregnant ewe. This study examines uterine prostaglandin production in these animals. On day 16 of pregnancy, ewes were anaesthetized and a uterine vein draining each horn and the jugular vein cannulated. Venous blood samples were taken at 10 min intervals for 1 h before and 1 h after an i.v. injection of oxytocin (1 μg). Uterine vein samples were assayed for PGF and PGE2 and jugular blood samples assayed for 13,14-dihydro,15-keto-PGF. All ewes showed high plasma concentrations of 13,14-dihydro,15-keto-PGF after the oxytocin injection. PGF increased in the uterine vein draining the nonpregnant transected horn of a unilaterally pregnant ewe following oxytocin treatment, whereas there was no increase in concentration in plasma draining the pregnant horn. Basal plasma PGF concentrations from the uterine vein draining the pregnant horn were significantly higher than concentrations obtained from the uterine vein of the transected horn while PGE2 concentrations did not vary between the two horns. After oxytocin injection, there was a significant increase in plasma PGE2 concentration in venous blood collected from both sides. These results indicate that (1) oxytocin receptors present in the isolated nonpregnant horn can be stimulated by oxytocin to produce PGF, despite the steroid environment being that of a pregnant ewe; (2) the presence of the conceptus locally inhibits the endometrial PGF production following the challenge but enhances baseline production; (3) PGE2 production does not differ between the pregnant and nonpregnant uterine horns of ewes with transected uteri and (4) oxytocin causes a bilateral increase in venous PGE2 concentrations draining pregnant and nonpregnant uterine horns.

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T. J. Parkinson and G. E. Lamming

Summary. Plasma progesterone and LH secretion patterns were examined in 18 mature dairy cows during the oestrous cycle and after insemination. Blood samples were collected every 15 min for 8 h per day on Days 3, 5, 6, 7, 8, 9, 10, 12, 14, 16, 17, 18, 19, 20 and 21 of the oestrous cycle, then, in the same cows, at the same times during early pregnancy. PGF-2α secretion rates (as determined by plasma PGFM concentrations) were also monitored on Days 14, 16 and the day of, or equivalent to, luteal regression. Mean daily plasma progesterone concentrations were similar until Day 16 in cyclic and pregnant cows, after which values in non-pregnant animals declined. Regression analysis indicated that progesterone concentrations were best described by a quadratic expression with fitted maximum values on Day 13 in non-pregnant animals but values increased linearly over the whole period to Day 21 in pregnant cows. The frequency, amplitude and area under the curve of LH episodes showed no significant differences between cyclic and pregnant animals. In pregnant cows, the amplitude and area under the curve of progesterone episodes increased linearly between Days 8 and 21, although no such increase occurred in cyclic cows. Low-level PGFM episodes were present in cyclic and pregnant cows on Days 14 and 16 after oestrus, and high amplitude episodes occurred in non-pregnant cows during luteal regression. Pregnant cows showed a significant depression of the amplitude, but not the frequency of episodes at the expected time of luteal regression. These results confirm that the corpus luteum of pregnancy secretes an increasing amount of progesterone per se and per unit of LH until at least Day 21 after mating. They further suggest that the corpus luteum of the cyclic cow may experience small episodes of PGF-2α and be subjected to initial degenerative changes by Day 14 after oestrus, some time before the onset of definitive luteolysis.

Keywords: early pregnancy; cow; LH/progesterone relationships

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N. B. HAYNES and G. E. LAMMING

Summary.

Unilateral ovariectomy of mature female rats 62 to 88 hr after copulation resulted in a significant reduction in embryo survival in the ipsilateral uterine horn when compared to the contralateral horn. The effect was not apparent when oestradiol-17β was injected into the ovarian site at the time of ovariectomy.