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Proton Transport in Human Placental Microvillous Membrane Vesicles

Proton Transport in Human Placental Microvillous Membrane Vesicles

The change of the acridine orange absorbance was used to monitor the formation and/or dissipation of a pH gradient in microvillous membrane vesicles (MVV) isolated from human term placenta. Under Na<sup>+</sup> efflux conditions, an acidification of the intravesicular space occured and it was completely inhibited by 0.1 mM amiloride. Under K<sup>+</sup> efflux conditions, an acidification of the intravesicular space occured and it was potentiated by valinomycin or FCCP. An inwardly directed chloride gradient also induced a minor intravesicular acidification, but it was not observed in voltage-clampled MVV. The initial rate of the dissipation of a pH gradient was accelerated by pulse injections of Na<sup>+</sup> in a saturable manner and Li<sup>+</sup> could replace Na<sup>+</sup>. The kinetic parameter of Na<sup>+</sup> in placental Na<sup>+</sup>/H<sup>+</sup> exchange was similar to that of renal Na<sup>+</sup>/H<sup>+</sup> exchange. Amiloride was a inhibitor of directly coupled Na<sup>+</sup>/H<sup>+</sup> exchange and its IC<Sub>50</sub> in placental MVV was about 14-fold higher than that in renal brush border membrane. These results indicate that Na<sup>+</sup>/H<sup>+</sup> exchanger exists in human placental MVV and that its kinetic characteristics is similar to that of renal Na<sup>+</sup>/H<sup>+</sup> exchanger but its pharmacological characteristics is different. In placental MVV K<sup>+</sup>, H<sup>+</sup>, and, relatively minor chloride conductances are present. The magnitude of Cl<sup>-</sup>/OH<sup>-</sup> exchange, even though it exists, seems to be smaller than that of Na<sup>+</sup>/H<sup>+</sup> exchange.

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