Maternal-fetal potential difference in the rat is generated by the placenta.

Electrical potential difference, PD, was recorded between maternal blood on one side and uterine lumen (transuterine PD), amniotic fluid (amniotic fluid PD), or vitelline blood of the fetus (maternal-fetal PD) on the other side in rats on day 21 of gestation. The values obtained are 2.4 +/- 6.0 mV (mean +/- s.e.), 11.3 +/- 4.5 mV and 9.6 +/- 4.1 mV, respectively. Maternal-fetal PD recorded over a period of 12 min decreased slowly. It increased transiently when the placenta was separated from the uterus, then it decreased more rapidly than in the corresponding controls afterwards. Maternal-fetal PD did not change significantly when the sac of fetal membranes containing the fetus was exposed intact from the uterus. In other experiments the PD was recorded in vitro between vitelline blood of the fetus which had been removed from the uterus together with intact sac of fetal membranes and a bath of Ringer solution. The PD recorded when only the exposed surface of the placenta (the surface by which the placenta had been attached to the uterus) was immersed in Ringer fluid, was the same as that recorded earlier between maternal and fetal blood in the same fetus. The PD decreased rapidly when the placenta was cooled, and it increased again when the placenta was rewarmed. The PD decreased to near 0 mV when the whole sac of fetal membranes was immersed in Ringer. On the basis of these observations it is concluded that the maternal-fetal PD in the rat is generated by the placenta.