Uterine blood flow and timing of blastocyst implantation in the guinea pig.

The temporal and direct relationships between serum estradiol (E) and progesterone (P) levels and uterine blood flow (UBF) and intrauterine oxygen tension (IU PO2) were examined in guinea pigs between days 1 and 8 of pregnancy. Both UBF and IU PO2 were measured in situ using a noninvasive electromagnetic blood flow monitor and needle oxygen electrode, respectively. Serum P levels remained relatively constant throughout the first 8 days of pregnancy. In contrast, serum E levels declined to basal levels between days 1 and 3, and then subsequently rose between days 4 and 7 before again falling to basal levels by day 8. The changes in UBF and IU PO2 paralleled those of serum E levels, both exhibiting a rise between days 5 and 6. The direct effect of oil, E, or P injections on UBF and IU PO2 measurements in ovariectomized animals indicated that E induced a dramatic rise in both uterine parameters, whereas both measurements remained at basal levels after oil or P treatment. The results of this study indicate that UBF and IU PO2 levels are directly regulated by the cyclic fluctuations in serum E. The temporal relationship between the days 4 to 7 rise in serum E, UBF, and IU PO2 levels and the timing of blastocyst implantation suggest that these events are involved in uterine preparation for nidation in the guinea pig.