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Am J Physiol Renal Physiol 276: F847-F856, 1999;
0363-6127/99 $5.00
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Vol. 276, Issue 6, F847-F856, June 1999

Pathways for HCOminus 3 exit across the basolateral membrane in rat thick limbs

Françoise Leviel, Dominique Eladari, Anne Blanchard, Jean-Stéphane Poumarat, Michel Paillard, and René-Alexandre Podevin

Laboratoire de Physiologie et Endocrinologie Cellulaire Rénale, Faculté de Médecine Broussais-Hôtel Dieu, Institut National de la Santé et de la Recherche Médicale, Unité 356, 75270 Paris, France

We studied the pathways for HCO-3 transport in basolateral membrane vesicles (BLMV) purified from rat medullary thick ascending limbs (MTAL). An inward HCO-3 gradient in the presence of an inside-positive potential stimulated the rate of 22Na uptake minimally and did not induce a 22Na overshoot, arguing against the presence of electrogenic Na+-HCO-3 cotransport in these membranes. An inside-acid pH gradient stimulated to the same degree uptake of 86Rb+ (a K+ analog) with or without HCO-3. Conversely, applying an outward K+ gradient caused a modest intracellular pH (pHi) decrease of ~0.38 pH units/min, as monitored by quenching of carboxyfluorescein; its rate was unaffected by HCO-3, indicating the absence of appreciable K+-HCO-3 cotransport. On the other hand, imposing an inward Cl- gradient in the presence of HCO-3 caused a marked pHi decrease of ~1.68 pH units/min; its rate was inhibited by a stilbene derivative. Finally, we could not demonstrate the presence of a HCO-3/lactate exchanger in BLMV. In conclusion, the presence of significant Na+-, K+-, or lactate-linked HCO-3 transport could not be demonstrated. These and other data suggest that basolateral Cl-/HCO-3 exchange could be the major pathway for HCO-3 transport in the MTAL.

medullary thick ascending limb; bicarbonate; lactate; membrane vesicles


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