Bicarbonate-induced activation of taurocholate transport across the basal plasma membrane of human term trophoblast.
The efflux of [14C]taurocholate from previously loaded vesicles, obtained from basal plasma membrane of human trophoblast, was studied. Apparent Km (620 microM) and Vmax (1.79 nmol.min-1.mg protein-1) values were similar to those found in influx experiments (Marin et al., Gastroenterology 99: 1431-1438, 1990). Transmembrane gradients of both bicarbonate (100 mM) and unlabeled taurocholate (0.5 mM) accelerated [14C]taurocholate efflux. The bicarbonate-induced effect was not abolished by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and K(+)-valinomycin voltage clamp. Neither was it mimicked by 5,5'-dimethyloxazolidine 2,4-dione (DMO) or by other organic (taurine, glycine, lactate, or acetate) or inorganic (Cl-, SCN-, HPO24-, or SO24-) anions, and it was not sensitive to carbonic anhydrase inhibitors. No effect of bicarbonate was observed either in the absence of gradient or in the presence of a cis-directed gradient. Bicarbonate-induced transstimulation was related to an increase in the value for the apparent Vmax (+30%). Study of the stoichiometry suggests that the most probable coupling ratio is one, bicarbonate: taurocholate. In summary, these results provide evidence for the existence of a bicarbonate-driven anion exchange in the basal plasma membrane of the human term placental trophoblast.
el-Mir
M Y
MY
Department of Physiology and Pharmacology, University of Salamanca, Spain.
Eleno
N
N
Serrano
M A
MA
Bravo
P
P
Marin
J J
JJ
D000086:Acetazolamide / Q000494:pharmacology; D001639:Bicarbonates / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002250:Carbon Radioisotopes; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D010920:Placenta / Q000502:physiology; D011247:Pregnancy; D011865:Radioisotope Dilution Technique; D013656:Taurocholic Acid / Q000378:metabolism*; D014327:Trophoblasts / Q000378:metabolism*
D001639:Bicarbonates; D002250:Carbon Radioisotopes; D013656:Taurocholic Acid; D000086:Acetazolamide