title: Apical H(+)/base transporters mediating bicarbonate absorption and pH(i) regulation in the OMCD.
abstract: The outer medullary collecting duct (OMCD) plays an important role in mediating transepithelial HCO transport [J(HCO(3)(-))] and urinary acidification. HCO absorption by type A intercalated cells in the OMCD inner stripe (OMCD(is)) segment is thought to by mediated by an apical vacuolar H(+)-ATPase and H(+)-K(+)-ATPase coupled to a basolateral Cl(-)-HCO exchanger (AE1). Besides these Na(+)-independent transporters, previous studies have shown that OMCD(is) type A intercalated cells have an apical electroneutral EIPA-sensitive, DIDS-insensitive Na(+)-HCO cotransporter (NBC3); a basolateral Na(+)/H(+) antiporter; and a basolateral Na(+)-K(+)-ATPase. In this study, we reexamined the Na(+) dependence of transepithelial Na(+) transport in the OMCD(is) and determined the role of apical NBC3 in intracellular (pH(i)) regulation in OMCD(is) type A intercalated cells. Control tubules absorbed HCO at a rate of approximately 13 pmol. min(-1). mm(-1). Lowering luminal Na(+) from 140 to 40 mM decreased [J(HCO(3)(-))] by approximately 15% without a change in transepithelial potential (V(te)). Furthermore, 50 microM EIPA (lumen) also decreased [J(HCO(3)(-))] by approximately 13% without a change in V(te). The effect of lowering luminal Na(+) and adding EIPA were not additive. These results demonstrate that [J(HCO(3)(-))] in the OMCD(is) is in part Na(+) dependent. In separate experiments, the pH(i) recovery rate after an NH prepulse was monitored in single type A intercalated cells with confocal fluorescence microscopy. The pH(i) recovery rate was approximately 0.21 pH/min in Na(+)-containing solutions and decreased to approximately 0.16 pH/min with EIPA (50 microM, lumen). In tubules perfused/bathed without Na(+), luminal Na(+) addition resulted in a pH(i) recovery rate of approximately 0.36 pH/min, whereas the Na(+)-independent recovery rate was approximately 0.16 pH/min. EIPA (50 microM, lumen) decreased the Na(+)-dependent pH(i) recovery rate to approximately 0.07 pH/min. The Na(+)-independent recovery rate was decreased to approximately 0.06 pH/min by bafilomycin (10 nM, lumen) and to approximately 0.10 pH/min using Schering 28080 (10 microM, lumen). These findings indicate that NBC3 contributes to pH(i) regulation in OMCD(is) type A intercalated cells and plays only a minor role in mediating [J(HCO(3)(-))] in the OMCD(is).
authors.affiliation: Department of Physiology and Biophysics, College of Medicine, University of South Florida, Tampa, Florida 33612, USA.
authors.forename: Kay-Pong
authors.initials: KP
authors.lastname: Yip
authors.forename: Shuichi
authors.initials: S
authors.lastname: Tsuruoka
authors.forename: George J
authors.initials: GJ
authors.lastname: Schwartz
authors.forename: Ira
authors.initials: I
authors.lastname: Kurtz
mesh_terms: D000136:Acid-Base Equilibrium / Q000502:physiology*; D000143:Acids / Q000378:metabolism; D000584:Amiloride / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D017506:H(+)-K(+)-Exchanging ATPase / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007685:Kidney Tubules, Collecting / Q000166:cytology / Q000378:metabolism*; D018613:Microscopy, Confocal; D006180:Proton-Translocating ATPases / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism; D027982:Sodium-Bicarbonate Symporters / Q000378:metabolism*
chemical_list: D000143:Acids; D001639:Bicarbonates; D027982:Sodium-Bicarbonate Symporters; D000584:Amiloride; D012964:Sodium; D017506:H(+)-K(+)-Exchanging ATPase; D006180:Proton-Translocating ATPases; C039614:ethylisopropylamiloride