Fig. 9. H⁺-ATPase-dependent base secretion in H⁺-ATPase-rich cells. (A) Basal base secretion. Most of the H⁺-ATPase is in cytoplasmic vesicles. (i) Continuous recycling with the basolateral membrane. (B) Upregulation of base secretion under blood alkalosis. (ii) Short-term response: microtubule-dependent relocation of the pre-existent cytoplasmic pool to the basolateral membrane. (iii) In the longer term, there is also upregulation of H⁺-ATPase synthesis. (B) Model for base secretion in H⁺-ATPase-rich cells. P, Pendrin; CA, carbonic anhydrase; CC, chloride channel; a, apical side; b, basolateral side; N, nucleus.