Central Role Of the Apical Membrane H⁺-Atpase in Electrogenesis and Epithelial Transport in Malpighian Tubules Available to Purchase

The effects of bafilomycin A₁, a blocker of V-type H⁺-ATPases, were investigated in Malpighian tubules of Aedes aegypti. Bafilomycin A₁ reduced rates of transepithelial fluid secretion and the virtual short-circuit current (vI_sc) with an IC₅₀ of approximately 5 μmol l⁻¹. As vI_sc decreased, the electrical resistance increased across the whole epithelium and across the apical membrane, indicating effects on electroconductive pathways. Bafilomycin A₁ had no effect when applied from the tubule lumen, pointing to the relative impermeability of the apical membrane to bafilomycin A₁. Thus, bafilomycin A₁ must take a cytoplasmic route to its blocking site in the proton channel of the H⁺-ATPase located in the apical membrane of principal cells. The inhibitory effects of bafilomycin A₁ were qualitatively similar to those of dinitrophenol in that voltages across the epithelium (V_t), the basolateral membrane (V_bl) and the apical membrane (V_a) depolarized towards zero in parallel. Moreover, V_bl always tracked V_a, indicating electrical coupling between the two membranes through the shunt. Electrical coupling allows the H⁺-ATPase to energize not only the apical membrane, but also the basolateral membrane. Furthermore, electrical coupling offers a balance between electroconductive entry of cations across the basolateral membrane and extrusion across the apical membrane to support steady-state conditions during transepithelial transport.