The inner stripe of outer medullary collecting duct (OMCD_is) is unique among collecting duct segments because both intercalated cells and principal cells secrete protons and reabsorb luminal bicarbonate. The current study characterized the mechanisms of OMCD_is proton secretion. We used in vitro microperfusion, and we separately studied the principal cell and intercalated cell using differential uptake of the fluorescent, pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Both the principal cell and intercalated cell secreted protons, as identified as Na⁺/H⁺ exchange-independent intracellular pH (pH_i) recovery from an intracellular acid load. Two proton transport activities were identified in the principal cell; one was luminal potassium dependent and Sch-28080 sensitive and the other was luminal potassium independent and luminal bafilomycin A₁ sensitive. Thus the OMCD_is principal cell expresses both apical H⁺-K⁺-ATPase and H⁺-ATPase activity. Intercalated cell Na⁺/H⁺ exchange-independent pH_i recovery was approximately twice that of the principal cell and was mediated by pharmacologically similar mechanisms. We conclude 1) the OMCD_is principal cell may contribute to both luminal potassium reabsorption and urinary acidification, roles fundamentally different from those of the principal cell in the cortical collecting duct; and 2) the OMCD_is intercalated cell proton transporters are functionally similar to those in the principal cell, raising the possibility that an H⁺-K⁺-ATPase similar to the one present in the principal cell may contribute to intercalated cell proton secretion.

intracellular pH; 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein; intercalated cell; proton-adenosinetriphosphatase; proton-potassium-adenosinetriphosphatase; Sch-28080; bafilomycin A₁

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