Functionally and morphologically, Madin-Darby canine kidney (MDCK) cells resemble intercalated cells of urinary epithelia. Experiments were performed on domes of confluent MDCK monolayers to test for apical H+ secretion. Apical application of 10(-3) mol/l amiloride or of Na(+)-free solution significantly reduced the limiting pH gradient across the dome epithelium (delta pHd) consistent with inhibition of apical Na+/H+ exchange. Short-circuit current (SCC) measurements disclosed an acetazolamide-sensitive, (basolateral to apical) positive transepithelial current stimulated by 10(-7) mol/l aldosterone and inhibited by acidification of apical medium to pH = 4.5. Histochemical evaluation of carbonic anhydrase (CA) activity revealed cytoplasmic and apical-membrane-bound CA particularly in dome-forming cells. Apical substitution of Na+ by K+ increased delta pHd, whereas a reduction of K+ concentration to 0.5 mmol/l or addition of barium or omeprazole (10(-5) mol/l) to the apical superfusate reduced delta pHd by at least 75%. Aldosterone-stimulated SCC was completely abolished by the apical application of barium. We conclude that besides Na+/H+ exchange MDCK cells can express an apically located H(+)-K+ pump stimulated by aldosterone and inhibited directly by the anti-ulcer agent omeprazole or indirectly, either by blocking apical K+ recycling or by interfering with the CA-dependent intracellular formation of H+ ions.