A common mechanism for activation of the Na+/H+ exchanger by different types of stimuli

The mechanism of activation of Na+/H+ exchanger by various stimuli was studied in the human epidermoid carcinoma cell line A431 and in peripheral blood mononuclear cells (PBM). Intracellular pH (pHi) was measured by using the fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Stimulation of A431 cells by epidermal growth factor (EGF), bradykinin (BK), phorbol-12-myristate 13-acetate (PMA), and osmotic shrinkage resulted in exchanger activation. In PBM, activation of Na+/H+ exchanger was induced by concanavalin A (Con A) and phytohemagglutinin (PHA), as well as PMA and osmotic shrinkage. Inhibition of protein kinase C inhibited only PMA-stimulated exchanger activation in both cell types. When osmotic shrinkage was applied after exposure of the cells to any agonist, augmentation of exchanger activation by osmotic stress was observed. These findings suggest that various stimuli activate Na+/H+ exchanger through different mechanisms. Kinetic analysis demonstrated that activation of the exchanger by any type of stimulus resulted in modification of the apparent affinities for intracellular H+ (H+i) and intracellular Na+ (Na+i) in opposite directions. While there is an increased apparent affinity for H+i, the apparent affinity for Na+i decreases. This finding suggests that in A431 cells this phenomenon serves as a common mechanism for activation of Na+/H+ exchanger by different stimuli.