Experimentally induced subnormal or normal luteal phases in sheep: reproductive hormone profiles and uterine sex steroid receptor expression

This study investigated if ewes expected to have subnormal luteal phases (SNLP) present a different pattern of uterine oestrogen receptor (ER) and progesterone receptor (PR) expression at the expected time of premature luteolysis. The concentrations of uterine ER, PR and ERα mRNA, and the steroid ovarian hormone were determined in anoestrous ewes treated with either gonadotrophin-releasing hormone (GnRH) to develop a SNLP (n = 16), or progesterone + GnRH to develop a normal luteal phase (NLP; n = 16). The ER, PR and ERα mRNA concentrations were measured using binding and solution hybridisation assays, while the hormone level concentration was measured by radioimmunoassay. In all ewes, a luteinising hormone- and follicle-stimulating hormone-synchronised surge was found. The SNLP group had lower preovulatory oestradiol levels than the NLP group. On Day 5, the SNLP group had lower progesterone levels, and higher uterine ER, PR and ERα mRNA concentrations than the NLP group. While in the SNLP group the receptor expression increased from Days 1 to 5, in the NLP group the receptor expression decreased. The results suggest that the induction of steroid receptor expression in the uterus and the hormonal environment found in the experimental SNLP group at the expected time of premature luteolysis may be involved in the mechanisms causing SNLP.