The role of intracellular pH in ligand internalization

Internalization of EGF and transferrin measured as the rate of uptake of ¹²⁵l‐labeled ligands was compared in the cell line CCL39 and a mutant derivative, PS120, lacking the Na⁺/H⁺ antiport system. No significant alteration was detected between the two cell lines. In contrast, pretreatment of the mutant cells PS‐120 with 20 mM NH₄Cl for 30 min to decrease persistently intracellular pH resulted in an increase in ¹²⁵I‐EGF and ¹²⁵I‐transferrin uptake by 60% and 25%, respectively. However, similar NH₄Cl pretreatment of the parental cell line, CCL‐39, which only affected intracellular pH very transiently did not cause an increase of ligand uptake. The binding of ¹²⁵I‐EGF to CCL‐39 and PS‐120 cells with or without NH₄Cl pretreatment showed that NH₄Cl pretreatment did not affect EGF binding in either CCL‐39 or PS‐120 cells. Since cells regulate intracellular pH by ion transport systems, we also examined the role of Na⁺, K⁺‐ATPase. Ouabain, an inhibitor of Na⁺, K⁺‐ATPases, showed no effect on ¹²⁵I‐EGF uptake in either of the cell types with or without NH₄Cl pretreatment. Taken together, these results suggest that the plasma membrane‐bound Na⁺/H⁺ antiport, a major pH_i‐regulating system in vertebrates, indirectly plays a role in ligand internalization through regulation of intracellular pH.