C1/HCO3 exchange in human placental brush border membrane vesicles

Membrane transport pathways for transplacental transfer of CO2/HCO3 were investigated by assessing the possible presence of a Cl/HCO3 exchange mechanism in the maternal-facing membrane of human placental epithelial cells. Cl/HCO3 exchange was tested for in preparations of purified brush border membrane vesicles by 36Cl tracer flux measurements and determinations of acridine orange fluorescence changes. Under 10% CO2/90% N2 the imposition of an outwardly directed HCO3- concentration gradient (pHo 6/pHi 7.5) stimulated Cl- uptake to levels approximately 2-fold greater than observed at equilibrium. Maneuvers designed to offset the development of ion gradient-induced diffusion potentials (valinomycin, Ko = Ki) significantly reduced HCO3- gradient-driven Cl- uptake but concentrative accumulation of Cl- persisted. Early time point determinations performed in the presumed absence of membrane potential suggests the reduced level of HCO3- gradient-driven Cl- uptake resulted from a more rapid dissipation of the HCO3- concentration gradient. Concentrative accumulation of Cl- was not observed in the presence of a pH gradient alone under 100% N2, suggesting a preference of HCO3- over OH- as a substrate for transport. As monitored by acridine orange fluorescence the Cl- gradient-dependent collapse of an imposed pH gradient (pHo 8.5/pHi 6) was accelerated in the presence of CO2/HCO3 when compared with its absence, indicating coupling of HCO3- influx to Cl- efflux. Increasing concentrations of the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid were observed to cause a stepwise reduction in HCO3- gradient-driven Cl- uptake (I50 approximately 25 microM) further suggesting the presence of a Cl/HCO3 exchange mechanism. The results of this study provide evidence for a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl/HCO3 exchange mechanism in the maternal-facing membrane of human placental epithelial cells. The identification of an ion-coupled HCO3- transport pathway in placental epithelia may suggest functional roles in mediating transplacental transfer of CO2 as well as maintenance of fetal acid/base balance.