Ion conductances in the microvillous and basal membrane vesicles isolated from human placental syncytiotrophoblast.

Experiments were performed to characterize the ionic conductances in microvillous and basal membranes from human placenta. Microvillous and basal membranes were prepared from term placental tissue by homogenization, magnesium precipitation, differential and sucrose density gradient centrifugation. The relative permeabilities of sodium, potassium and chloride were measured using the bi-ionic potential technique which employs a fluorescent probe [diS-C3-(5)] which partitions into membranes in a potential-dependent manner. The permeabilities of sodium and chloride relative to potassium were determined by measuring their effects on a known membrane potential produced by a potassium gradient. In microvillous membranes PNa/PK = 0.25 and PClPK = 0.19 while in basal membranes, PNa/PK = 1.31 and PCl/PK = 0.03. Measurements of chloride permeability relative to sodium confirmed these results. The cation conductances were inhibited by quaternary ammonium compounds. Addition of tetramethylammonium altered the relative permeabilities in a pattern suggesting a block of potassium conductance while tetraethylammonium appeared to block both sodium and potassium conductances.