Effect of plasma concentrations of ovarian steroids on their passage into uterine lumen in rabbits.

Significant increment of blood-borne ovarian steroids are found in the uterine lumen 1 h after mating. Is the transfer of ovarian steroids into the uterus determined by the peripheral blood concentrations of the ovarian steroids? To answer this question, rabbits, ovariectomized 24 h earlier, were infused over a 1-h period with either estradiol (E2; 0.7 and 7.0 micrograms/h), progesterone (P4; 74 and 740 micrograms/h), or testosterone (T; 0.45 and 4.5 micrograms/h). E2, P4, and T were determined in the tissue and flushings of the uterus and tissue and flushings of the esophagus and plasma. A different group of rabbits was infused with E2 (0.7 micrograms), P4 (74 micrograms), and T (0.45 microgram/h) combined. The increase in plasma steroid concentration after infusion of either E2, P4, or T was reflected in an elevation of these steroids in the uterine lumen, albeit not in the same ratios as found in plasma. The simultaneous infusion of E2, P4, and T blocked completely the passage of T and decreased (P less than 0.05) the passage of E2 into the lumen of the uterus. Treatments did not affect the steroid concentration in the wall of the uterus. It was concluded that the content of E2, P4, or T in the uterine flushing increased when E2, P4, or T was infused individually. However, when these steroids were infused together, there was selective inhibition of the transfer process.