Suppression of pulsatile luteinizing hormone secretion by gonadotrophin-releasing hormone antagonist does not affect episodic progesterone secretion or corpus luteum function in ewes

Abstract

Progesterone secretion has been observed to be episodic in the late luteal phase of the oestrous cycle of ewes and is apparently independent of luteinizing hormone (LH). This study investigated the effects of suppressing the pulsatile release of LH in the early or late luteal phase on the episodic secretion of progesterone. Six Scottish Blackface ewes were treated i.m. with 1 mg kg-1 body weight of a potent gonadotrophin-releasing hormone (GnRH) antagonist on either day 4 or day 11 of the luteal phase. Six ewes received saline at each time and acted as controls. Serial blood samples were collected at 10 or 15 min intervals between 0 and 8 h, 24 and 32 h, and 48 and 56 h after GnRH antagonist treatment and daily from oestrus (day 0) of the treatment cycle for 22 days. Oestrous behaviour was determined using a vasectomized ram present throughout the experiment. Progesterone secretion was episodic in both the early and late luteal phase with a frequency of between 1.6 and 3.2 pulses in 8 h. The GnRH antagonist abolished the pulsatile secretion and suppressed the basal concentrations of LH for at least 3 days after treatment. This suppression of LH, in either the early or late luteal phase, did not affect the episodic release of progesterone. Daily concentrations of progesterone in plasma showed a minimal reduction on days 11 to 14 after GnRH antagonist treatment on day 4, although this was significant (P < 0.05) only on days 11 and 13. There was no effect of treatment on day 11 on daily progesterone concentration, and the timing of luteolysis and the duration of corpus luteum function was unaffected by GnRH antagonist treatment on either day 4 or day 11. These results indicate that the episodic secretion of progesterone during the luteal phase of the oestrous cycle in ewes is independent of LH pulses and normal progesterone secretion by the corpus luteum can be maintained with minimal basal concentrations of LH.

Physiological and practical effects of progesterone on reproduction in dairy cattle

“…It seems clear that the LH surge is essential for ovulation and luteinization of the ovulated follicular cells into the luteal cells, however, the role for LH pulses, after the LH surge, is still controversial. Consistent with our results, other researchers have also found that treatment with a GnRH antagonist eliminated pulsatile LH secretion but had little or no effect on circulating P4 and CL function (Baird, 1992;McNeilly et al, 1992). In contrast, when sheep are hypophysectomized on day 5 after estrus and CL recovered on day 12, circulating P4, luteal P4, and weight of CL are lower than expected for a day 12 CL but similar to a day 5 CL (Kaltenbach et al, 1968; Denamur et al, 1973;Farin et al, 1990).…”

Mechanisms Controlling the Function and Life Span of the Corpus Luteum

“…However, the effect on secretion of progesterone is not as dramatic as that observed in primates, and treatment of cows with a GnRH antagonist after the corpus luteum is fully developed had no effect on secretion of progesterone (283). Treatment of sheep during development or maintenance of the corpus luteum with GnRH antagonist had very little effect on the secretion of progesterone (225). Thus pulsatile release of LH appears necessary for normal luteal function in primates and for luteal development in cows.…”

Homozygosity for a single base-pair mutation in the oocyte-specific GDF9 gene results in sterility in Thoka sheep

“…The limit of detection was 0.2 pg/ml and intra-and interassay CV were !12%. Progesterone was measured by RIA without extraction as described previously (McNeilly et al 1992). The limit of detection was 0.2 pg/ml and intra-and interassay CV were !11%.…”