Estrogen and progesterone interactions in the rabbit uterus in vivo after steroid administration

After a single injection of Estradurin (polyestradiolphosphate), rabbit plasma 17 beta-estradiol (E2) levels rose to a maximum in 3 days and were maintained for at least a 13-day period. The uterine E2 concentration rose slowly so that at day 12 the tissue/plasma E2 ratio was similar to that in untreated controls. The gain in tissue weight after the Estradurin injection correlated well with the increase in tissue E2 concentration. In a group of rabbits to which progesterone (P) was administered 7 days after Estradurin injection, the E2 concentration in plasma did not change significantly. However, the tissue E2 concentration showed a significant decrease after four daily injections of P. When P injections were halted (progesterone withdrawal) for 2 days, the tissue E2 levels rose and were greater than those found in the group not receiving P. These levels were significantly higher than those found in animals which had received daily injections of P for 2 or 4 days. These results indicate that P is able to suppress the accumulation of E2 by uterine tissue, in agreement with published evidence that P is able to reduce the levels of cytoplasmic E2 receptors.