Serum stimulation occurs in NHE3 WT, NHE3 PBMIImut, and NHE3 Δ4. Serum (10% FBS) similarly stimulated wild-type NHE3 activity (Vmax) and that in NHE3 PBMIImut and NHE3 Δ4. The experiment was repeated three times, and results are shown as mean ± S.D. Error bars represent S.D. p values are comparisons of the 10% FBS effect in NHE3 WT, NHE3 PBMIImut, and NHE3 Δ4, respectively (unpaired t test).