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Adjustment of amino-acid-induced cytoplasmic pH decrease by the Na+/H+ exchange system in human lymphocytes has been studied using a fluorometric technique to monitor the intracellular pH change. When the interior of lymphocytes is acidified by addition of nigericin to medium, cytoplasmic pH is immediately corrected toward its resting value. This recovery of the cytoplasmic pH depends on extracellular Na+ and is inhibited by amiloride. A temporary (less than 2 min) decrease in the cytoplasmic pH, followed by a slow recovery phase, was observed in incubation with 1.0 mM leucine in Na+-containing medium. This leucine-dependent decrease of cytoplasmic pH persisted longer when amiloride was added to the medium. Cytoplasmic pH recovery from the leucine-induced acidification depends on external Na+ concentration. Amiloride-sensitive Na+/H+ exchanger was stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in the lymphocytes and preincubation of the cells with TPA partially prevented the leucine-induced cytoplasmic acidification. We conclude that human peripheral lymphocytes are provided with an amino acid-H+ cotransport system, which is cooperatively coupled to the amiloride-sensitive Na+/H+ exchanger to correct the cytoplasmic pH anomaly.
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