title: An apical K(+)-dependent HCO(3)- transport pathway opposes transepithelial HCO(3)- absorption in rat medullary thick ascending limb.
abstract: Absorption of HCO(3)(-) in the medullary thick ascending limb (MTAL) is mediated by apical membrane Na(+)/H(+) exchange. The identity and function of other apical acid-base transporters in this segment have not been defined. The present study was designed to examine apical membrane HCO(3)(-)/OH(-)/H(+) transport pathways in the rat MTAL and to determine their role in transepithelial HCO(3)(-) absorption. MTALs were perfused in vitro in Na(+)- and Cl(-)-free solutions containing 25 mM HCO(3)(-), 5% CO(2). Lumen addition of either 120 mM Cl(-) or 50 mM Na(+) (50 microM EIPA present) had no effect on intracellular pH (pH(i)). Lumen Cl(-) addition also had no effect on pH(i) in the presence of 145 mM Na(+) or in the nominal absence of HCO(3)(-)/CO(2). Thus there was no evidence for apical Cl(-)/HCO(3)(-) (OH(-)) exchange, Na(+)-dependent Cl(-)/HCO(3)(-) exchange, or Na(+)-HCO(3)(-) cotransport. In contrast, in tubules studied in Na(+)- and Cl(-)-free solutions containing 25 mM HCO(3)(-), 5% CO(2) and 120 mM K(+), removal of luminal K(+) induced a rapid and pronounced decrease in pH(i) (DeltapH(i) = 0.56 +/- 0.06 pH U). pH(i) recovered following lumen K(+) readdition. The initial rate of net base efflux induced by lumen K(+) removal was decreased 85% at the same pH(i) in the nominal absence of HCO(3)(-)/CO(2), indicating a dependence on HCO(3)(-)/CO(2) and arguing against apical K(+)/H(+) exchange. A combination of the apical K(+) channel blockers quinidine (0.1 mM) and glybenclamide (0.25 mM) had no effect on the lumen K(+)-induced pH(i) changes, arguing against electrically coupled K(+) and HCO(3)(-) conductances. The effect of lumen K(+) on pH(i) was inhibited by 1 mM H(2)DIDS. In addition, lumen addition of DIDS increased transepithelial HCO(3)(-) absorption from 10.7 +/- 0.7 to 14.9 +/- 0.7 pmol x min(-1) x mm(-1) (P < 0.001) and increased pH(i) slightly in MTAL studied in physiological solutions (25 mM HCO(3)(-) and 4 mM K(+)). Lumen DIDS stimulated HCO(3)(-) absorption in the absence and presence of furosemide. These results are consistent with an apical membrane K(+)-dependent HCO(3)(-) transport pathway that mediates coupled transfer of K(+) and HCO(3)(-) from cell to lumen in the MTAL. This mechanism, possibly an apical K(+)-HCO(3)(-) cotransporter, functions in parallel with apical Na(+)/H(+) exchange and opposes transepithelial HCO(3)(-) absorption.
authors.affiliation: Department of Medicine, University of Texas Medical Branch, Galveston, TX 77555, USA.
authors.forename: Bruns A
authors.initials: BA
authors.lastname: Watts
authors.forename: David W
authors.initials: DW
authors.lastname: Good
mesh_terms: D000042:Absorption; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D001692:Biological Transport; D027963:Chloride-Bicarbonate Antiporters / Q000502:physiology*; D002712:Chlorides / Q000493:pharmacokinetics; D017136:Ion Transport; D007679:Kidney Medulla / Q000502:physiology*; D008297:Male; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D027982:Sodium-Bicarbonate Symporters / Q000502:physiology*
chemical_list: D001639:Bicarbonates; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides; D027982:Sodium-Bicarbonate Symporters; D011188:Potassium