Transtrophoblast and microvillus membrane potential difference in mature intermediate human placental villi

Single mature intermediate villi from term placentas after normal gestation and vaginal or cesarian delivery were identified microscopically and mounted in a bath for conventional micro-electrode studies. With the application of strict selection criteria, the following observations on electrical potential difference (PD) were made. 1) With the tissue bathed in Earle's medium (37 degrees C) the PD across the syncytiotrophoblast microvillus membrane with respect to the bath was not normally distributed. The median PD was -22 mV (range -12 to -60 mV, n = 200). This fell to -6 mV after prior incubation for 4 h with cyanide (3 mM) or iodoacetate (2 mM) but was not altered by short-term application of these agents or by 0.1 mM ouabain. Substituting Na+ in Earle's medium with choline had no effect on PD, but replacing Cl- with gluconate caused a depolarization of 6 mV (P < 0.002). Increasing KCl in the bath fluid revealed an apparently low microvillus membrane K+ conductance. The low microvillus membrane PD may reflect a low Na(+)-K(+)-ATPase activity and/or a low membrane permeability to K+. 2) The transtrophoblast PD measured by insertion and withdrawal of the electrode was significantly different from zero [P < 0.003, median -3 mV (range 0 to -15 mV), n = 11]; and PD measured by insertion of the electrode into the villus core and beyond was -6 mV (significantly different from zero P < 0.003, range -2.5 to -10 mV, n = 6). If a similar PD were to exist in vivo, it could have a significant influence on ion transport across the placenta at term.