Effects of progesterone pretreatment on the oxytocin receptor concentration and the response to oxytocin during the simulated early luteal phase in the ovariectomized ewe

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Authors:
A. P. Beard
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M. G. Hunter
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DOI:
https://doi.org/10.1530/jrf.0.1020057
Volume/Issue:
Volume 102: Issue 1
Page Range:
57–63
Article Type:
Research Article
Online Publication Date:
Sep 1994
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A steroid-treated ovariectomized ewe model was used to investigate the role of progesterone pretreatment in the control of functional oxytocin receptor concentrations during the early luteal phase. Ovariectomized ewes (n = 28) were injected with oestradiol for 2 days (final injection = day 0) with or without progesterone pretreatment (progestagen sponge for 10 days). Ewes were then given high or low concentrations of progesterone combined with high, low or zero concentrations of oestradiol in a pattern known to simulate the early luteal phase profile (n = 4 per group). Ewes were given 1 μg oxytocin (i.v.) on day 4 and plasma was collected to assay 13,14-dihydro-15-keto PGF2α. The concentration of progesterone and oestradiol administered had no effect on the concentration of 13,14-dihydro-15-keto PGF2α following oxytocin administration (P > 0.05). However, the group that was not pretreated exhibited a small but significant 13,14-dihydro-15-keto PGF2α response in comparison with the equivalent pretreated group (P < 0.05). In a subsequent study, ewes were divided into groups pretreated and not pretreated with progesterone; both groups were given oestrous concentrations of oestradiol and high concentrations of progesterone and oestradiol together. On day 0, 2, 3 or 4, ewes from each group (n = 3, 3, 4 and 4, respectively) were given 1 μg of oxytocin i.v., and the endometrium was collected to measure the binding of oxytocin receptors. Oxytocin caused a significant (P < 0.05) increase in the concentration of 13,14-dihydro-15-keto PGF2α in all ewes on day 0 but not on days 2, 3 or 4. Oxytocin receptor concentrations were maximal on day 0 and basal by day 4. The decline in receptor concentrations occurred more rapidly in the progesterone-pretreated than in the ewes that were not pretreated. This study has shown that progesterone pretreatment alters the subsequent steroid hormone control of oxytocin receptor concentrations, and has identified the delayed decline in oxytocin receptor concentrations as the potential cause of premature luteolysis in ewes that are not pretreated.

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Print ISSN:
1470-1626
Online ISSN:
1741-7899