Roles of NHE1 and PDGFR-α in pH_i regulation in NIH3T3 cells during interphase and growth arrest. (A–C) Original tracings of pH_i recovery measurements in the absence and presence of 5 µM EIPA. 10 mM NH₄Cl was present in the Ringer’s solution as indicated by the horizontal line. Cells grown in the presence of serum (A), cells which had been serum starved for 48 h (B), and serum-starved cells treated with 50 ng/ml PDGF-AA for 1 h before the experiment (C) are shown. It may be noted that the rate of slow acidification in the presence of NH₄Cl, representing NH₄⁺ influx via either K⁺ channels or other transporters with substrate affinity for NH₄⁺ (Boron, 2004), is reduced in the growth-arrested cells in the absence but not in the presence of PDGF-AA. This indicates that these transporters are regulated during the cell cycle and also means that the total acidification after NH₄Cl removal is less in the growth-arrested cells. However, this does not affect the comparison between the recovery rates in the absence and presence of EIPA. (D) Summary of the pH_i recovery rates under the conditions shown. The rate of pH_i recovery (in pH units/minute) was calculated from the slope of the initial linear part of the curve after NH₄Cl removal. Data shown are representative traces (A–C) or mean ± SEM of three to five experiments per condition. Data were analyzed using parametric or nonparametric ANOVA, and the level of significance is shown (*, P < 0.05; **, P < 0.01; ***, P < 0.001). Ctrl., control.