Apical proton secretion by the inner stripe of the outer medullary collecting duct

The inner stripe of outer medullary collecting duct (OMCDis) is unique amon g collecting duct segments because both intercalated cells and principal ce lls secrete protons and reabsorb luminal bicarbonate. The current study cha racterized the mechanisms of OMCDis proton secretion. We used in vitro micr operfusion, and we separately studied the principal cell and intercalated c ell using differential uptake of the fluorescent, pH-sensitive dye, 2',7'-b is(2-carboxyethyl)-5(6)-carboxyflu (BCECF). Both the principal cell and int ercalated cell secreted protons, as identified as Na+/H+ exchange-independe nt intracellular pH (pH(i)) recovery from an intracellular acid load. Two p roton transport activities were identified in the principal cell; one was l uminal potassium dependent and Sch-28080 sensitive and the other was lumina l potassium independent and luminal bafilomycin A(1) sensitive. Thus the OM CDis principal cell expresses both apical H+-K+-ATPase and H+-ATPase activi ty. Intercalated cell Na+/H+ exchange-independent pH(i) recovery was approx imately twice that of the principal cell and was mediated by pharmacologica lly similar mechanisms. We conclude 1) the OMCDis principal cell may contri bute to both luminal potassium reabsorption and urinary acidification, role s fundamentally different from those of the principal cell in the cortical collecting duct; and 2) the OMCDi, intercalated cell proton transporters ar e functionally similar to those in the principal cell, raising the possibil ity that an H+-K+ - ATPase similar to the one present in the principal cell may contribute to intercalated cell proton secretion.