Changes in nuclear estrogen receptor level in the rat oviduct during ovum transport

The purpose of this study was to assess variations in the concentration of nuclear estrogen receptor in the oviduct during normal embryo transport and to determine its temporal relationship with the passage of eggs to the uterus. Groups of 6--8 rats were sacrificed at different intervals from 0800 h on Day 1 to 1700 h on Day 4 of pregnancy to determine the concentration of nuclear estrogen receptor in the oviduct, the plasmatic estrogen level and the location of eggs in the genital tract. No significant variations in the amount of nuclear estrogen receptor were observed throughout the first 3 days. However, on Day 4 of pregnancy, concomitant with an elevation of plasma estrogen level there was a significant increase in nuclear estrogen receptor which started 4 to 5 h before the first ova began to reach the uterus. The receptor concentration attained a 3-fold elevation over previous days and returned to basal levels when 30% of the ova had been transferred to the uterus. These results provide evidence that estrogen exerts a direct action on the oviduct during embryo transport and that the passage of embryos to the uterus is preceded by a well-defined increment of estrogenic action on the oviduct. This temporal relationship is in keeping with the concept that estrogen receptor mediated events taking place in the oviduct may be involved in timing the transport of ova through the tubouterine junction.