Effect of NHE inhibition and of cytosolic pH on EGF receptor autophosphorylation. (A) Immunoblot analysis of tyrosine phosphorylation (P-Tyr) of EGF-R (M_w 170 kD) in A431 cells incubated for 5 min with or without EGF in Na⁺-rich buffer, with HOE-694 in Na⁺-rich buffer or in NMG⁺-rich buffer. Blot is representative of four experiments. (B) Quantitation of the effect of HOE-694 or NMG⁺ on EGF-R autophosphorylation, obtained by scanning immunoblots like the one in A (black bars). Data are means ± SE of 4–7 experiments. The effect of the same agents/conditions on macropinocytosis is shown for comparison (open bars). (C) Quantification of EGF-R phosphorylation in cells stimulated in Na⁺-rich medium or clamped with nigericin/K⁺ at the indicated pH (black bars). Data are means ± SE of 3–4 experiments. Data were normalized to controls in Na⁺-rich medium; normalized macropinocytosis is shown for comparison (open bars). ***, P < 0.001.