Intrauterine Oxygen Tension during the Estrous Cycle in the Guinea Pig: Its Relation to Uterine Blood Volume and Plasma Estrogen and Progesterone Levels

Abstract

This study was undertaken to determine: the oxygen tension (PO2) within the lumen of the guinea pig uterus throughout the estrous cycle and during the preimplantation period; the relationships between intrauterine PO2, uterine blood volume (UBV) and endogenous plasma estradiol-17.beta. (E) and progesterone (P) levels and the effects of exogenous ovarian steroid hormones on uterine blood volume and intraluminal PO2. Mean arterial and venous PO2 remained constant throughout the cycle. Mean intrauterine PO2 did not differ significantly from venous PO2 except at the times of ovulation and uterine capacity for implantation. Intrauterine PO2 increased rapidly to a peak during day 0 (proestrus-estrus) and returned to venous levels by day 1. A 2nd dramatic increase in PO2 began on day 4, reached a peak on day 6 and declined to venous levels by day 8. Uterine blood volume remained at basal levels throughout the cycle except for dramatic increases at .apprx. the times of ovulation and uterine receptivity to the blastocyst. Also temporally correlated with the increases in intrauterine PO2 and UBV were elevations in plasma E concentration. Estrogen was low throughout the cycle except on days 0 and 6. Plasma P peaked on day 0, returned to basal levels on day 1 and was elevated on days 2-7. Intrauterine PO2 increased following ovariectomy; exogenous estrogen administered to ovariectomized animals markedly elevated PO2; P suppressed the postovariectomy increase in intraluminal PO2, maintaining PO2 at venous levels; E + P briefly increased PO2. Ovarian steroid treatments in ovariectomized animals resulted in changes in intrauterine PO2 concomitant with those of UBV. Synchronous changes in plasma ovarian steroid levels, UBV and intrauterine PO2 suggest that intraluminal O2 concentration is regulated partly by estrogen and P via alterations in uterine hemodynamics. Intrauterine PO2 is low until the anticipated time of conceptus entry, but increases rapidly thereafter to maximum levels at the time of uterine receptivity to the ovum. The coincidence of peaks in PO2 with ovulation and implantation suggests that increased intrauterine PO2 may play a role in sperm viability, conceptus development and implantation.