Characterization and physiological variation of estrogen receptors in rabbit corpora lutea throughout pregnancy and pseudopregnancy: the effect of hysterectomy and sustained estradiol treatment.

Previous studies suggest that regression of the rabbit corpus luteum is associated with a uterine-induced loss of responsiveness to estradiol. To determine if this is due to loss of estrogen receptor, cytoplasmic and nuclear estrogen receptors were measured in pseudopregnant, hysterectomized-pseudopregnant and pregnant rabbits throughout luteal life. Estrogen receptor levels were higher in corpora lutea than in nonluteal tissue and were generally higher in nuclei compared to cytosol. Estrogen receptor levels were low on Day 3, increased 2- to 3-fold by Day 6-8, reached peak levels by Days 8-10, and then gradually decreased in a pattern similar to the pattern of serum progesterone typical of each group. Hysterectomy was not associated with elevated cytoplasmic or nuclear estradiol receptor levels. When hysterectomized rabbits were treated with estradiol-filled Silastic implant on Day 1, nuclear estradiol receptor levels fell by Day 20 to levels seen in untreated hysterectomized rabbits. Despite substantial losses in nuclear estrogen receptor, serum progesterone remained elevated on Days 16 and 20. Thus, the ability of estradiol to maintain serum progesterone in hysterectomized rabbits did not correlate directly with the level of estrogen receptor.