Endometrial transport generates the maternal-fetal electrical potential difference in guinea pigs

These studies examined the transport characteristics of the uterine endometrium with respect to the origin and mechanism of generation of the maternal-fetal electrical potential difference (PD) in pregnant guinea pigs. Late-gestation animals were used in two experimental preparations. In vivo, a sealed uterine pouch that preserved blood flow to the endometrium was prepared by removal of the fetus, placenta, and fetal membranes from the uterus and replacement with Earle's solution, a balanced electrolyte solution. In vitro, sections of uterine wall comprised of myometrium and endometrium without fetal membranes were mounted in Ussing chambers. Transuterine PDs (fetal side negative) were indistinguishable in vivo and in vitro, averaging 29.6 +/- 4.5 and 32.6 +/- 6.1 (95% confidence interval) mV in the respective preparations. Both values are within the range of maternal-fetal PD measured in intact guinea pigs, indicating that the fetoplacental unit is not essential in generating an intrauterine PD. The maternal-fetal PD, therefore, is likely a passive result of the fetus and placenta being immersed in fluids at the intrauterine potential. In vitro, both PD and short-circuit current (Isc) were completely inhibited by ouabain (10(-3) M) at the serosal (maternal) side of the uterine wall but unaffected by the inhibitor from the luminal (fetal) side. Amiloride (10(-5) M) and valinomycin (10(-5) M) caused decreases in the PD when added to the luminal side, both in vivo and in vitro, and were both ineffective from the serosal side in vitro. Isc was reduced 83% from 315 +/- 24 to 53 +/- 6 (SE) microA/cm2 after luminal amiloride (5 x 10(-4) M), indicating that Na+ is the predominant ion actively transported.(ABSTRACT TRUNCATED AT 250 WORDS)