FIGURE 7.

Proposed model for NHE3 transit pool and “storage pool” in OK cell apical domain with Ca²⁺-induced dissociation of NHE3 from NHERF2. NHERF2 and a pool of apical NHE3 co-localize in OK cells under basal conditions, and some NHE3 co-precipitates with NHERF2. The amount of NHE3 that is immobile (fixed to the microvillar cytoskeleton) decreases within 10 min of Ca²⁺ elevation (FRAP) and NHE3 and NHERF2 dissociate by 1–2 min (FRET, co-precipitation). We assume but have not proved that the freed up NHE3 is at least part of the apical pool of NHE3 that is endocytosed in response to elevated Ca²⁺. Note that the lack of dissociation of NHE3-NHERF1 with elevated Ca²⁺ identifies a Ca²⁺-insensitive pool of NHE3 present in the microvilli. NHE3 that binds NHERF2 defines both the pool of NHE3 stored in the apical domain under basal conditions (storage pool), which is likely to be inactive, and the transit pool of NHE3, which moves from the microvillus to the inter-microvillus clefts as part of stimulated endocytosis.