Basolateral Membrane Sodium-independent CI-/HCO3 Exchanger in Rat Inner Medullary Collecting Duct Cell

Previous studies have shown that the middle third of the rat inner medullary collecting duct (IMCD-2) secretes protons de-spite the absence of intercalated cells, the cell thought to se-crete protons in other portions of the collecting duct. A new cell, the IMCD cell, is the predominant cell in IMCD-2. The mechanism responsible for base exit in the IMCD cell was characterized by measuring cell pH of isolated perfused tu-bules with 2',7'-bis-(carboxyethyl)-5,6-carboxyfluorescein. Re-duction of bath HCO3 caused a significant and reversible de-crease in cell pH, whereas a similar change in luminal HCO3 had a significantly smaller effect, indicating that the HCO3/ H+ permeability of the basolateral membrane is much larger than the apical membrane. The rate of cell acidification in-duced by reduction in bath HCO3, a measure of basolateral HCO3 transport, was significantly decreased in the absence of bath and lumen Cl. Decreases in bath Cl caused a significant and reversible increase in cell pH, which was not changed significantly by complete removal of Na from perfusate and bath, but was significantly inhibited by basolateral 4'5'-diiso-thiocyanostilbene-2,2'-disulfonic acid. A chemical voltage clamp did not inhibit the rate of cell alkalinization after bath Cl removal, indicating that Cl-/HCOj exchange is not via paral-lel C1 and HCO3 conductances. Cell pH was measured in sin-gle cells by low-light-level imaging to show that most cells contain the chloride-dependent HCO3 pathway. We conclude that the rat IMCD cell possesses a basolateral Na-independent Cl-/HCO3 exchanger which may serve as the base exit step for transepithelial proton secretion. (J. Clin. Invest. 1990. 85:1959-1966.) anion exchanger * 2',7'-bis-(carboxyethyl)-5,6-carboxyfluorescein (BCECF)- cell p