Regulation of Intracellular pH in Tumor Cell Lines: Influence of Microenvironmental Conditions¹

The effect of microenvironmental factors on the regulation of intracellular pH (pH_i) in MGH U1 cells and EMT-6 cells was studied using the fluorescent pH probe 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxy-fluorescein. Na⁺/H⁺ exchange and Na⁺-dependent Cl⁻/HCO⁻₃ exchange were found to be present in both cell types. The activity of both exchangers was dependent on pH_i, with low levels of activity at neutral pH and an increase in activity as pH_i fell. The level of extracellular pH (pH_e) also influenced the operation of the exchangers, with a fall in activity as pH_e was reduced over the range 7.4–6.6. This effect was more marked for the Na⁺-dependent Cl⁻/HCO⁻₃ exchanger than for the Na⁺/H⁺ antiporter, suggesting that under conditions of reduced pH_e the Na⁺/H⁺ antiporter is the major mechanism for regulation of pH_i. Neither 6 h of radiobiological hypoxia nor variations in the extracellular [Ca²⁺] over the range 1–6 mm had an effect on the regulation of pH_i, while extracellular lactate (5–10 mm) caused a small, concentration-dependent decrease in the combined activity of both exchangers. We conclude that under the microenvironmental conditions found in some regions of tumors, Na⁺/H⁺ exchange may be the major method of regulation of pH_i.