Fig. 3. Mechanism of transepithelial NaCl and KCl secretion in Malpighian tubules of the yellow fever mosquito Aedes aegypti under control conditions. (A) Phenotypic model. K⁺ enters principal cells from the peritubular Ringer bath (or hemolymph) via K⁺ channels located in the basolateral membrane. Na⁺ enters via cotransport with K⁺ and Cl^-. Na⁺ and K⁺ are extruded from the cell across the apical membrane via K⁺/H⁺ transport and Na⁺/H⁺. The proton gradient driving the antiport is generated by an electrogenic V-type H⁺-ATPase located in the apical membrane. The Cl^- conductance of the basolateral membrane (channel) is at present hypothetical to allow an exit mechanism for steady-state intracellular Cl^- concentrations. (B) Electrical model. Outward positive current generated by the ATP-driven V-type H⁺-ATPase returns to the cytoplasmic face of the pump via the paracellular shunt pathway (R_sh) and the basolateral membrane (R_bl). (C) Mitochondrion that produces ATP for the V-type H⁺-ATPase is densely packed in the microvillus of the brush border. E, electromotive force; V, voltage; R, resistance; a, apical membrane; bl, basolateral membrane; t, transepithelial. For further details, see Beyenbach (1995, 2001) and Beyenbach et al. (2000).