The presence of a Na(+)-H+ exchanger in isolated tracheal epithelial cells has been tested. Sodium uptake in tracheocytes was found to increase when the intracellular medium was made acidic. Micromolar 5-(N-ethyl-N-isopropyl) amiloride (EIPA) concentrations reduced the sodium uptake in acidified cells. Intracellular pH was studied using the fluorescent indicator, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). The rate of recovery from an acidifying pulse was partially dependent on the presence of extracellular sodium. EIPA decreased the initial rate of pH recovery from acidification in the presence but not in the absence of external sodium. These results suggest the existence of a Na(+)-H+ exchanger in sheep tracheocytes, and of some other Na(+)-independent proton extruding mechanism(s). Proton secretion into the tracheal lumen was studied in whole tracheae bathed in bicarbonate-free solutions by a 'pH stat' technique. The rate of proton secretion was significantly decreased by the addition of EIPA to the luminal solution, suggesting the involvement of an apically located Na(+)-H+ exchanger in proton secretion in sheep trachea.

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