Effect of inhibitors on macropinocytosis and NHE activity. (A) DIC (left) and TMR-dextran epifluorescence images (middle) of islands of A431 cells incubated in the absence (Untreated) or presence of EGF as detailed in Materials and methods. Arrowheads point to dextran-filled macropinosomes. After determination of macropinocytosis, cells were fixed and stained with rhodamine-phalloidin to visualize actin (left). Arrowheads point to the aspect of the cell not in contact with neighboring cells. Bar, 10 µm. (B) Quantification of macropinocytosis in control and HOE-694-treated cells. Data are means ± SE of ≥5 separate experiments. (C) Effect of 10 µM HOE-694 on Na⁺-induced recovery of pH_c after an acid load. NHE activity initiated where indicated by reintroduction of Na⁺. Results are representative of 3–4 similar experiments. (D) Concentration dependence of the effect of HOE-694. NHE activity was measured as in C and rates were calculated from the slopes from Na⁺-induced pH_c recovery curves. Data are means ± SE of three experiments. Where missing, error bars are smaller than symbol.