The activity of hNHE3 is stimulated by Nedd4-2 knockdown. The effect of Nedd4-2 knockdown on NHE3 activity was determined on PS120/hNHE3 (A), PS120/ratNHE3 (B), and PS120/rbNHE3 (C) cells. NHE3 activity was determined fluorometrically using the intracellular pH-sensitive dye BCECF-AM as described under “Experimental Procedures.” The cells were incubated in NH4+, washed with TMA, and Na+-dependent pH recovery was initiated by addition of Na+ buffer. NHE3 activity was expressed as the rate of Na+-dependent intracellular pH recovery (upper panel), and representative traces of Na-dependent pHi recovery are shown below. Results are presented as the means ± S.E. n ≥ 9. *, p < 0.01 compared with shCon. The rates of Na+-dependent pHi recovery were determined in SK-CO15 cells transfected with shNedd4-2 (D) or shNedd4-1 (E). F, NHE3 activity in Caco-2bbe/rbNHE3 cells transfected with shNedd4-1 or shCon was determined. n ≥ 9.