Acid incubation causes exocytic insertion of NHE3 in OKP cells

Incubation of opossum kidney clone P (OKP) cells in acid media(pH 6.8) causes activation of Na⁺/H⁺ exchanger3 (NHE3) at 6, 12, and 24 h. OKP cell NHE3 protein abundance was increased by 45% at 24 h of acid incubation but wasunaffected at 3-12 h. By contrast, apical membrane NHE3, measured by surface biotinylation, increased approximately twofold at 6, 12, and24 h, mirroring the increase in activity. Acid incubation caused a76% increase in exocytic insertion of NHE3 into the apical membranebut had no effect on endocytic internalization at 6 h. LatrunculinB, an inhibitor of microfilament organization, inhibited theacid-induced increases in apical membrane NHE3, exocytic insertion ofNHE3, and NHE3 activity at 6 h. These studies demonstrate two mechanisms for acid-induced increases in NHE3 activity. Beginning at6 h, there is an increase in apical membrane NHE3 that is due tostimulated exocytic insertion and is required for increased NHE3activity. At 24 h, there is an additional increase in total cellular NHE3.