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Uterine blood flow, pH, and pCO2 during nidation in the guinea pig: ovarian regulation.

The regulation of uterine blood flow (UBF), intrauterine pH (IUpH), and carbon dioxide levels (IUpCO2) by ovarian steroids during the nidatory period was examined in the guinea pig to determine if changes in the intrauterine environment are associated with cyclic changes in ovarian function. Between days 4 and 8 (day 0 = ovulation) of pregnancy, guinea pigs were prepared for the simultaneous in situ measurement of UBF, IUpH, and IUpCO2, and the changes in these parameters were compared to serum levels of 17 beta-estradiol (E2) and progesterone (P). The same parameters were evaluated in ovariectomized guinea pigs treated with oil, 1 or 10 micrograms E2, 2 mg P, or 1 microgram E2 plus 2 mg P (P2E1). UBF rose between days 4 and 5 to peak at the expected time of nidation on day 6 before declining to basal levels by day 8. Similar fluctuations were observed in IUpCO2 and serum E2 levels, while IUpH fluctuated in a manner opposite to that of IUpCO2 levels. Serum P levels remained constant between days 4 and 8. Both P and E2 treatments elevated IUpH but depressed IUpCo2, compared with levels in ovariectomized oil-treated controls. An elevation in UBF rates was also observed in E2-treated animals. Combined P2E1 treatment elevated UBF and IUpCO2 above oil-treated control levels and lowered IUpH. These data indicate that a causal relationship exists among UBF, IUpCO2, and serum E2 fluctuations during the nidatory period in the guinea pig and that these parameters are inversely related to changes in IUpH. It is suggested that ovarian regulation of the intrauterine environment may have a direct effect on the timing of blastocyst implantation in the guinea pig.