Extracellular Na dependence for native NHE3 in OK cells and native NHE1 in mouse NHE1 wild-type skin fibroblasts. (A) Extracellular Na dependence of native NHE3-induced proton fluxes at intracellular pH (pH_i) 6.0 (left) and 7.2 (right) in OK cells. Note biphasic shape of Na dependence of NHE3 transport at pH_i 6.0 and sigmoidal shape at pH_i 7.2, similar to that observed for NHE1 in CHO cells (see Fig. 4) and NHE1 wild-type mouse skin fibroblasts (see below). (B) Extracellular Na dependence of native NHE1-induced proton fluxes at intracellular pH (pH_i) 6.0 (left) and 7.2 (right) in wild-type NHE1 mouse skin fibroblasts. Note biphasic shape of Na dependence of NHE1 transport at pH_i 6.0 and sigmoidal shape at pH_i 7.2. Magnitude of Na-induced proton fluxes and extracellular Na dependence are very similar to CHO cells, which express natively NHE1 on the plasma membrane (see Fig. 4). No proton fluxes were detected in NHE1 knockout mouse fibroblasts or in CHO-derived AP-1 cells, which are devoid of plasmalemmal NHE1 (not depicted).