Fig. 5. The V-type H⁺ ATPase powers transepithelial NaCl and KCl secretion in Malpighian tubules of the yellow fever mosquito. Only conductive transport pathways are shown to illustrate diverse voltage-dependent transport mechanisms driven by the V-type H⁺ ATPase located at the apical membrane. Intraepithelial current generated by the proton pump is carried by H⁺ across the apical membrane, by Cl^- through the paracellular pathway (septate junctions, sj), and by K⁺ and Na⁺ across the basolateral membrane. Under control conditions, K⁺ channels account for as much as 64% of the conductance of the basolateral membrane (Beyenbach and Masia, 2002). Claudin-like proteins are hypothesized to define the Cl^- selectivity of the paracellular, septate junctional pathway (Beyenbach, 2003).

The 2H⁺/cat⁺ antiporter in the apical membrane remains to be identified. Mosquito natriuretic peptide and its second messenger cyclic AMP decrease the resistance to Na⁺ entry across the basolateral membrane (Beyenbach, 2001). The diuretic peptide leucokinin decreases the resistance of the paracellular pathway for Cl^- (Beyenbach, 2003). E_p, electromotive force of the V-type H⁺ ATPase; R, resistance; I, current; p, pump, cat⁺, cation. E_p and R_p form a proton current generator.