Increased sensitivity and accumulation of estradiol in the rat oviduct during early pregnancy

We have previously reported that a single injection of estradiol-17 beta (E2) given on day 3 of pregnancy (P3) is far more effective for accelerating oviductal transport in the rat, than treatment given on day 1 (P1). In order to quantify this change, dose-response curves were established for six different doses of E2 (range 0.031 to 1.00 micrograms per animal) given on P1, P2 or P3. In addition, a possible mechanism was explored by comparing the plasmatic and oviductal levels of E2 between 30 and 180 min following treatment with E2 on P1 or P3. As the interval from ovulation to treatment was increased, the transport of a larger number of embryos was accelerated and a smaller dose was required. The minimal effective dose decreased 30-fold from P1 to P3, the oviducts accumulated 20% to 90% more E2 on P3 than on P1, tissue levels were 6- to 48-fold higher than plasma levels and the latter did not differ between P1 and P3. It is concluded that the oviduct exhibits increased sensitivity and responsiveness to E2 on P3 and this is associated with greater accumulation of the hormone in the organ, not attributable to higher E2 plasma levels.