Loss of uterine luminal fluid in the rat: relative importance of changing peripheral levels of estrogen and progesterone.

Experiments were conducted to determine the relative importance of the decline in circulating estrogen levels, as opposed to the increase in progesterone levels, in the hormonal control of the loss of luminal fluid from the uterus of the rat, an event which usually occurs early on the morning of estrus. Silastic capsules containing crystalline estradiol-17beta (E2) were implanted subcutaneously in ovariectomized immature rats. Approximately 64 h later, the capsules were either removed (E2 withdrawal) to mimic the fall in peripheral estrogen levels occurring at proestrus, or left in place (continuous E2), and, at the same time, progesterone or oil was administered. Fifteen hours later, irrespective of whether E2 was withdrawn or not, 2 mg progesterone reduced uterine luminal fluid accumulation to levels usually seen in intact animals on the morning of estrus; however, the loss of fluid occurred slightly sooner if E2 was withdrawn when progesterone was administered. By itself, E2 withdrawal resulted in only a small decrease in uterine luminal fluid by 15 h, even though serum E2 levels had fallen to less than 1.5 pg/ml by this time. This loss was brought about by escape of the fluid through the cervix rather than by its reabsorption from the lumen. The dose-response relationships between progesterone and uterine luminal fluid accumulation indicated that when E2 was withdrawn, a smaller amount of progesterone brought about the loss of uterine luminal fluid accumulation than when E2 was continuous. These results suggest that increased progesterone secretion on the afternoon of proestrus is essential for the loss of uterine luminal fluid, and that the decline in estrogen secretion at this time may be of importance by allowing the progesterone to be more effective.