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

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 microgram oxytocin (i.v.) on day 4 and plasma was collected to assay 13,14-dihydro-15-keto PGF2 alpha. The concentration of progesterone and oestradiol administered had no effect on the concentration of 13,14-dihydro-15-keto PGF2 alpha following oxytocin administration (P > 0.05). However, the group that was not pretreated exhibited a small but significant 13,14-dihydro-15-keto PGF2 alpha 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 microgram 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 PGF2 alpha 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.(ABSTRACT TRUNCATED AT 250 WORDS)