Regulation of Intracytoplasmic pH and “Apparent” Intracellular pH in Alveolar Macrophages

Abstract

An approach for determining the permeability of cell and subcell membranes to H⁺/HCO^-₃ has been applied to alveolar macrophages. The approach uses analysis of substrate ratios which are compartment specific and involved in pH-dependent reactions. By determining the changes in substrate pair ratios produced by pertubations of CO₂, the effects of intracellular acidosis and alkalosis can be determined. By measuring substrate pair ratios at a constant PCO₂ and variable H⁺/HCO^-₃ it is possible to determine the permeability of the macrophage plasma membrane to these ions qualitatively. Measurements of intracellular lactate/pyruvate ratios showed that the alveolar macrophage plasma membrane is permeable to H⁺/HCO^-₃. Previous studies have shown that alveolar macrophages are freely permeable to Cl^-. pH_i was determined from measurements of pH_e and Cl^-_i/Cl^-_e following changes of external PCO₂ at a constant HCO^-₃ concentration and following changes in pH_e produced by changes of external HCO^-₃ concentration at a constant PCO₂. The data show that the relationship between pH_i and pH_e is: pH_i = 0.973 pH_e - 0.027 (r = 0.99. P < 0.001). Thus at pH_e = 7.40. macrophage intracellular pH_i = 7.17. Alveolar macrophages appear to be permeable to H⁺/HCO^-₃ and thus resemble erythrocytes, platelets, and lymphocytes and not brain and muscle cells.