AFLUID Apr. 45/4

Weiner, I. David, Amy E. Frank, and Charles S. Wingo. Apical proton secretion by the inner stripe of the outer medullary collecting duct. Am. J. Physiol. 276 (Renal Physiol. 45): F606–F613, 1999.—The inner stripe of outer medullary collecting duct (OMCDis) 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 OMCDis 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, 28,78-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Both the principal cell and intercalated cell secreted protons, as identified as Na1/H1 exchange-independent intracellular pH (pHi) 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 OMCDis principal cell expresses both apical H1-K1-ATPase and H1-ATPase activity. Intercalated cell Na1/H1 exchange-independent pHi recovery was approximately twice that of the principal cell and was mediated by pharmacologically similar mechanisms. We conclude 1) the OMCDis 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 OMCDis intercalated cell proton transporters are functionally similar to those in the principal cell, raising the possibility that an H1-K1ATPase similar to the one present in the principal cell may contribute to intercalated cell proton secretion.