Estrogen action in the corpus luteum.

The luteotropic action of estradiol has been studied in the rabbit and rat, and it is proposed that in both species, estradiol may be the "ultimate" luteotropic hormone. The acute dependence of the rabbit corpus luteum upon estradiol is illustrated by the rapid decline in serum progesterone after withdrawal of estradiol (removal of Silastic implant containing 17 beta-estradiol) and by the restoration of serum progesterone to normal values when the estradiol implant is replaced 24 hours later. The identification and characterization of a cytoplasmic and nuclear estrogen receptor in the rabbit corpus luteum and the translocation of the cytoplasmic receptor suggest that estradiol may be acting as it does in other estrogen target tissues. The administration of LH antiserum in pregnant rats causes rapid decreases in luteal estradiol and serum progesterone concentrations, which can be prevented by the administration of low doses of testosterone. This luteotropic effect of testosterone is attributed to an action of estradiol which is formed within the corpus luteum via the luteal aromatase. The rabbit corpus luteum, which has no aromatase, is totally dependent upon estradiol produced by the ovarian follicles and LH is essential to maintain follicular estradiol synthesis. The rat corpus luteum, which is rich in aromatase activity, may produce its own estrogen from an androgen precursor synthesized within the luteal tissue. The essential role of LH may be the stimulation of the synthesis of androgen precursor. As a working hypothesis it is proposed that in these two species LH is necessary to stimulate the synthesis of estradiol which then acts to sustain progesterone secretion.