title: Electric potential differences in the isolated guinea-pig placenta.
abstract: Knowledge of the magnitude of the electric potential differences between the maternal and fetal circulations and the trophoblast is necessary to describe transport of ions into and out of the trophoblast as it occurs in placental transfer of charged molecules. The value of the electric potential difference is also of significance in describing the transport of neutral molecules when their transport is coupled to electrogenic co-transport systems. We developed a method to obtain the values of these potential differences, in the isolated guinea-pig placenta perfused on both sides with an artificial medium. A positively charged ion that carries a radioactive label is allowed to equilibrate between the trophoblast and its circulations. The intracellular equilibrium concentration can be calculated and, because the extracellular concentration is known, the potential difference can be obtained with the Nernst equation. Rapid equilibrium is obtained by charging the trophoblast by means of perfusion of the placenta with the ion at a high concentration, followed by reduction of the concentration in the medium until equilibrium is observed. This is done in both a continuous and discontinuous manner. In addition to measurements of the potential differences, their origin was investigated. It was shown that at least part of the potential difference is generated by the action of transcellular Na--K exchange, because depolarization could always be obtained by decreasing the transmembrane Na and K gradients. Mean values obtained were delta psi F = 71 +/- 21 mV (+/- SD) for the potential difference between the fetal circulation and the trophoblast and delta psi m = 64 +/- 16 mV for the potential difference between the maternal side and the trophoblast with the cell interior negative.
mesh_terms: D000818:Animals; D002794:Choline / Q000378:metabolism; D005260:Female; D006096:Gramicidin / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008431:Maternal-Fetal Exchange / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects; D010042:Ouabain / Q000494:pharmacology; D010477:Perfusion; D010920:Placenta / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011247:Pregnancy; D012964:Sodium / Q000378:metabolism; D013757:Tetraethylammonium Compounds / Q000378:metabolism; D014327:Trophoblasts / Q000378:metabolism*
chemical_list: D007473:Ion Channels; D013757:Tetraethylammonium Compounds; D006096:Gramicidin; D010042:Ouabain; D012964:Sodium; D002794:Choline; D011188:Potassium