title: C1/HCO3 exchange in human placental brush border membrane vesicles.
abstract: 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.
authors.affiliation: Department of Pharmacology, State University of New York Health Science Center, Syracuse 13210.
authors.forename: S M
authors.initials: SM
authors.lastname: Grassl
mesh_terms: D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D000165:Acridine Orange; D000838:Anions; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D002712:Chlorides / Q000378:metabolism*; D004563:Electrochemistry; D005260:Female; D005456:Fluorescent Dyes; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008871:Microvilli / Q000378:metabolism*; D010920:Placenta / Q000648:ultrastructure*; D011247:Pregnancy; D014634:Valinomycin / Q000494:pharmacology
chemical_list: D000838:Anions; D001639:Bicarbonates; D002712:Chlorides; D005456:Fluorescent Dyes; D014634:Valinomycin; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D000165:Acridine Orange; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid