Single‐cell analysis of the intracellular pH and its regulation during the monocytic differentiation of U937 human leukemic cells

Monocytic differentiation of U937 cells induced by retinoic acid is accompanied by a 0.2‐pH‐unit cell alkalinisation. The effect of retinoic acid on intracellular pH (pHi) develops slowly and it precedes the differentiation of the cells by 24 h. Heterogeneity in cellular pHi values was assessed using flow cytometry. It was higher at the differentiated stage than at the undifferentiated stage. It was reduced under conditions of clamped pHi values. Two membrane mechanisms allow U937 cells to recover from an intracellular acidosis. These are the Na+/H+ exchange system and a Na+‐dependent HCO3−/Cl− exchange system. The increase in the pHi observed after monocytic differentiation resulted from a twofold increase in the maximum activity of the Na+/H+ exchange system with no change in the activity of the bicarbonate‐dependent system. The properties of interaction of the Na+/H+ exchanger of U937 cells with Na+, Li+, amiloride and its derivatives were defined and appeared to be unique to human leukemic cells.