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H. Sakurai, B. M. Adams and T. E. Adams

The effect of duration of a simulated follicular phase on gonadotrope responsiveness was assessed in orchidectomized sheep (wethers). The oestrogenic and hypothalamic inputs characteristic of the ovine follicular phase were simulated by continuous infusion of oestradiol (5 μg h−1 in 10% (v/v) ethanol-saline) and circhoral delivery of GnRH (200 ng per hourly pulse) for 0, 6, 12, 24, 48 or 96 h (n = 6 wethers per group). Responsiveness increased (P < 0.05) with increasing duration of simulated follicular phase. In a second experiment, responsiveness was assessed 96 h after initiation of infusion of oestradiol in wethers receiving hourly pulses of GnRH or saline. Concurrent administration of GnRH reduced (P < 0.05) the magnitude of the oestradiol-induced increase in gonadotrope responsiveness. In a companion study, anterior pituitary tissue was collected 96 h after the start of infusion of oestradiol and circhoral delivery of GnRH or saline. Pituitary stores of LH and tissue concentrations of GnRH receptor and mRNA encoding the GnRH receptor were increased (P < 0.05) by oestradiol infusion. The magnitude of these oestradiol-induced responses was not affected (P > 0.05) by concurrent GnRH treatment. Tissue concentrations of FSH and mRNA encoding the FSHβ subunit were decreased (P < 0.05) by oestradiol infusion. This suppressive effect of oestradiol was not reversed by GnRH. These results indicate that oestradiol stimulation, but not concurrent delivery of GnRH, is essential for full expression of surge-like secretion of LH. In addition, the oestradiol-induced increase in gonadotrope responsiveness during the simulated follicular phase is sustained throughout the period of oestradiol delivery.

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T. E. Adams, B. M. Adams and J. G. Watson

Summary. Blood samples were collected every 15 min for 6 h during the follicular (1 day before oestrus), and early (Days + 1 to + 3), mid- (Days + 4 to + 8), and full (Days + 9 to +14) luteal phases of the oestrous cycle. Serum concentrations of immunoactive LH were measured by radioimmunoassay. The biological activity of serum LH was determined by an in-vitro bioassay that uses LH-induced testosterone production from mouse interstitial cells as an endpoint. Only ovine and bovine LH and hCG had appreciable activity in this bioassay. The temporal pattern of secretion of bioactive LH paralleled the secretory pattern of immunoactive LH at all stages of the ovine oestrous cycle. However, the secretory pattern itself varied regularly through the oestrous cycle. The frequency of secretory excursions of LH was highest during the follicular phase (6·2 ± 0·9 pulses/6 h) and was progressively reduced through the luteal phase (1·1 ± 0·1 pulses/6 h during full luteal phase). Conversely, amplitude of secretory excursions of immunoactive LH was low during the follicular phase (0·79 ± 0·08 ng/ml) and significantly (P < 0·05) increased during the mid- and full luteal phases (1·49 ± 0·10 and 2·37 ± 0·20 ng/ml, respectively). The biopotency of LH (bioactive LH/immunoactive LH) at the peak of secretory excursions was 1·00 ± 0·03 during the follicular phase, 0·66 ± 0·02 during the mid-luteal phase, and 1·18 ± 0·19 during full luteal activity. The biopotency of LH was markedly reduced during the preovulatory surge of gonadotrophin. During the full luteal phase each pulse of LH secretion was associated with an abrupt increase in secretion of progesterone. These results indicate that both the quantitative and qualitative character of serum LH varies through the oestrous cycle of the sheep.

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B. M. Adams, H. Sakurai and T. E. Adams

The effect of oestradiol on tissue concentrations of GnRH receptor and mRNA encoding GnRH receptor was assessed in orchidectomized sheep (wethers) made deficient in GnRH by passive immunization. Wethers were assigned to one of four groups (n = 6 animals per group). Animals in groups 2 and 4 received ovine anti-GnRH sera (200 ml; i.v.) at passive immunization, while antisera against the carrier protein was administered to wethers in groups 1 and 3. Oestradiol was delivered as a continuous infusion (2 μg h−1) to wethers in groups 1 and 2. Animals in groups 3 and 4 were infused with vehicle alone. Anterior pituitary tissue was collected at the end of the 48 h infusion. Anti-GnRH sera induced a rapid reduction in the serum concentration of LH. Continuous delivery of oestradiol resulted in a twofold increase (P < 0.05) in tissue concentration of GnRH receptor. This oestradiol-induced response was manifest even in wethers in which endogenous GnRH had been neutralized by passive immunization. Conversely, infusion of oestradiol increased (P < 0.05), and intravenous administration of anti-GnRH sera decreased (P <0.05), concentrations of mRNA encoding GnRH receptor in pituitary tissue. When delivered in combination, anti-GnRH sera reduced (P <0.05), but did not eliminate, the oestradiol-induced augmentation of steady-state concentrations of mRNA encoding GnRH receptor. These data demonstrate that the basal concentration of mRNA encoding GnRH receptor is dependent on continued GnRH stimulation. In contrast, the oestradiol-induced increase in steady-state concentration of mRNA encoding GnRH receptor is manifest even in the absence of GnRH.

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T. E. Adams, J. F. Quirke, J. P. Hanrahan, B. M. Adams and J. G. Watson

Summary. Rates of ovulation differed significantly (P < 0·01) among ewes of the different genetic lines. However, of the reproductive characteristics studied, only progesterone concentration at the height of luteal function, duration of oestrus, and interval from onset of oestrus to peak of the preovulatory gonadotrophin surge showed significant positive association with rate of ovulation. The pattern of secretion of LH during the periovulatory period did not differ in the Galway and Finnish Landrace breeds. The total amount of LH secreted during the preovulatory surge did not differ amongst lines. Similarly, no difference in the plasma concentration of LH at the height of the preovulatory surge was noted among Galway and reference Finnish Landrace lines. However, the concentration of LH at the height of the surge was significantly (P < 0·05) reduced in the selected Finnish Landrace line. Plasma concentrations of FSH during the preovulatory period were significantly (P < 0·05) elevated in the breed (Galway) with the lowest prolificacy. When contrasted with either of the Finnish Landrace lines, the magnitudes of the preovulatory surge of FSH and the secondary surge of FSH were significantly greater (P < 0·05) in Galway ewes. These results suggest that genetic difference in rate of ovulation among sheep breeds is not tightly coupled to quantitative differences in plasma concentration of gonadotrophic hormones during the periovulatory period.

Keywords: gonadotrophins; periovulatory period; ovulation rate; prolific sheep

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N. R. Adams, S. Atkinson, G. B. Martin, J. R. Briegel, R. Boukhliq and M. R. Sanders

During earlier studies we observed that ewes housed and sampled intensively to measure pulses of LH in plasma had a higher ovulation rate than similar ewes housed outside. In Expt 1, we pursued this observation by testing whether the increase was due to effects of housing or collection of blood samples. Ewes sampled at intervals of 4 h for 2 days before progestagen sponge removal and 2 days after sponge removal, and every 20 min for 12 h the day before sponge removal and every 10 min for 4 h on the day of sponge removal had a higher ovulation rate than ewes that were not sampled (1.72 versus 1.41; P < 0.05). The ovulation rate of the ewes housed indoors but not sampled was similar to that of ewes that remained in the paddock (1.43). In Expt 2, we studied the effects of blood sampling in three groups of 20 ewes sampled every 20 min for different periods of 24 h. Ewes from all three groups were sampled the day before sponge removal (day − 1) and, in addition, one group of ewes was sampled for the previous 48 h (i.e. days −3 to −1) and another group was sampled on day − 8. The frequency of LH pulses was lower (P < 0.05) in ewes sampled for the first time on day − 1 compared with the frequency of LH pulses in groups also sampled earlier in the cycle (day −8 or days −3 and − 2). In ewes sampled on days −3 to −1, the frequency of LH pulses was low for the first 24 h and then increased. Changes in the mean concentration of FSH followed a pattern similar to that of LH pulse frequency. The ovulation rate was increased in ewes sampled on day −8 (P < 0.01) compared with a control group of 40 unsampled ewes. We conclude that collection of blood samples from Merino ewes to measure LH pulses can change the frequency of those pulses and the mean concentration of FSH, and these changes can be accompanied by an altered ovulation rate.