Fig. 6. Model of transepithelial electrolyte and fluid secretion in isolated Malpighian tubules of A. aegypti. The tubule consists of principal and stellate cells in a ratio of 5:1 (Beyenbach, 2003). Transepithelial secretion of Na⁺ and K⁺ is transcellular, and secretion of Cl^– is paracellular and/or through stellate cells under control conditions. The transepithelial Cl^– flux is largely paracellular through septate junctions under diuretic conditions stimulated by leucokinin (Yu and Beyenbach, 2004). The V-type H⁺-ATPase located at the apical membrane powers transcellular and paracellular transport via electrical coupling (Beyenbach, 2003). The Na/K ATPase may be present in stellate cells, but its contribution to transepithelial electrolyte and fluid secretion appears to be minor or negligible in view of a high V-type H⁺-ATPase activity and a low Na/K ATPase measured in Aedes Malpighian tubules (Weng et al., 2003). Control data for the Tenmo ADFa experiment of Fig. 1 are shown. The Cl^– channel in the basolateral membrane of principal cells is hypothetical to allow the exit of Cl^– such that the epithelial cell remains in steady state. V_bl, basolateral membrane voltage; V_a, apical membrane voltage; V_t, transepithelial voltage; R_pc, input resistance, principal cell; R_t, transepithelial resistance.