Fig. 7. Hypothetical transport model of NaHCO₃ secretion and HCl absorption across the serotonin-stimulated cell population in the isolated and perfused anterior stomach of mosquito larvae (Aedes aegypti) proposed to be reflected in the outside negative transepithelial voltage (V_te). (A) Na⁺-independent part. Metabolic CO₂ is hydrated and dissociates, accelerated by carbonic anhydrase (CA), into H⁺ and HCO_3^-. H⁺ are pumped by V-ATPases across the basolateral membrane to the hemolymph, resulting in hyperpolarization of the cellular electrical potential and in high cellular HCO_3^-. Cl^-/HCO_3^- exchange across the luminal membrane is driven by the high cellular HCO_3^-. To explain the electrogenic nature of the overall process, anion channels are proposed to be present in the apical membrane, allowing Cl^- recycling and/or electrogenic secretion of HCO_3^- driven by the cellular negativity. Cl^- channels in the basolateral membrane allow transcellular absorption of Cl^- ions (cf. Boudko et al., 2001a). (B) Na⁺-dependent part. In addition to energization of HCO_3^- secretion via apical anion exchange/channels (see A), the V-ATPase is proposed to energize transapical NaHCO₃ secretion via electrogenic Na⁺/2–3HCO_3^- symporters. Na⁺/H⁺ exchangers in the basolateral membrane are considered to supply the cells with Na⁺ and to support the V-ATPases to drive H⁺ across the basolateral membrane. Paracellular secretion of Na⁺/absorption of Cl^- driven by V_te is proposed to guarantee mass transport. See Discussion for further details.