Ovum Transport in Pregnant, Pseudopregnant, and Cyclic Rats and Its Relationship to Estradiol and Progesterone Blood Levels

The time of passage of ova from the oviduct to the uterus and the systemic plasma levels of estradiol and progesterone were investigated in pregnant and pseudopregnant rats, and in rats with regular 4-day cycles. The relationships between ovum transport rates and steroid levels in plasma in these three physiologic conditions were examined. The day following proestrus was designated Day 1 in the three groups. The number and distribution of ova were similar in the three physiologic conditions during the first 3 days, but significant differences appeared thereafter. The major shift in the distribution of ova between oviduct and uterus took place between Days 3 and 4 in cycling rats and between Days 4 and 5 in pregnant and pseudopregnant rats. The total number of ova recovered decreased significantly on Day 4 in cycling rats, on Day 5 in pseudopregnant rats, and remained unchanged in pregnant animals. Differences in plasma estradiol levels did not reach statistical significance. Progesterone concentration in plasma was significantly higher in pseudopregnant than in pregnant rats on Days 2 through 5, and it was significantly lower in cycling rats in comparison with the other groups on Days 3 and 4. The estradiol/progesterone ratio differed between groups since Day 1. The results of this study provide definite evidence that the time of passage of ova from the oviduct to the uterus is subject to physiologic regulation in the rat and support the concept that postovulatory changes in sex steroid levels in plasma may contribute to this regulation.