title: Mechanisms of H+/HCO3- transport in the medullary thick ascending limb of rat kidney.
abstract: The rat MTAL secretes protons into the tubular fluid and thus absorbs bicarbonate at substantial rates. Yet the cellular mechanisms of H+/HCO3- transport in the rat MTAL remain largely unsettled. We have performed intracellular pH recovery studies with use of the fluorescent probe BCECF in suspensions of rat MTAL fragments. Luminal H+ secretion occurs by two mechanisms (each responsible for 50% of the normal pHi recovery rate): (1) an electroneutral Na+/H+ antiporter that has an Na-Km of about 11 mM and is inhibited by amiloride (Ki = 2.8 x 10(-5) M); (2) a primary H+ pump that is inhibited by 10(-4) M NEM and 10(-4) M omeprazole, but not by 10(-4) M vanadate or removal of external K. These results suggest the presence of a vacuolar H(+)-ATPase rather than a H(+)-K(+)-ATPase. Basolateral HCO3 exit occurs predominantly by a Cl(-)- and Na(+)-independent electroneutral K+/HCO3- symporter, that has an HCO3-Km of about 17 mM, and is partially inhibited by 10(-4) M DIDS. Basolateral HCO3- efflux was not accompanied by variations of membrane potential monitored with the Em-sensitive fluorescent probe DIS-C3-5, and was not affected by maneuvers that depolarize the cells. It was strongly inhibited by cellular K depletion and dependent on transmembrane K gradient. We conclude that the rat MTAL should secrete protons through both Na+/H+ antiporter and H(+)-ATPase, and that basolateral HCO3- exit should occur through an electroneutral K+/HCO3- symporter.
authors.affiliation: Laboratoire de Physiologie et Endocrinologie Cellulaire rénale, Université Pierre et Marie Curie, Hôpital Broussais et Centre Biomédical des Cordeliers, INSERM CJF 88-07, Paris, France.
authors.forename: P
authors.initials: P
authors.lastname: Borensztein
authors.forename: F
authors.initials: F
authors.lastname: Leviel
authors.forename: M
authors.initials: M
authors.lastname: Froissart
authors.forename: P
authors.initials: P
authors.lastname: Houillier
authors.forename: J
authors.initials: J
authors.lastname: Poggioli
authors.forename: E
authors.initials: E
authors.lastname: Marty
authors.forename: M
authors.initials: M
authors.lastname: Bichara
authors.forename: M
authors.initials: M
authors.lastname: Paillard
mesh_terms: D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D004563:Electrochemistry; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007679:Kidney Medulla; D008138:Loop of Henle / Q000378:metabolism*; D006180:Proton-Translocating ATPases / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers
chemical_list: D001639:Bicarbonates; D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D006180:Proton-Translocating ATPases