Central role of the apical membrane H+-ATPase in electrogenesis and epithelial transport in Malpighian tubules

The effects of bafilomycin A(1), a blocker of V-type H+-ATPases, were inves tigated in Malpighian tubules of Aedes aegypti. Bafilomycin A(1) reduced ra tes of transepithelial fluid secretion and the virtual short-circuit curren t (vI(sc)) with an IC50 of approximately 5 mu mol l(-1). As vise decreased, the electrical resistance increased across the whole epithelium and across the apical membrane, indicating effects on electroconductive pathways. Baf ilomycin A(1) had no effect when applied from the tubule lumen, pointing to the relative impermeability of the apical membrane to bafilomycin A(1). Th us, bafilomycin A(1) must take a cytoplasmic route to its blocking site in the proton channel of the Ht-ATPase located in the apical membrane of princ ipal cells. The inhibitory effects of bafilomycin A(1) were qualitatively s imilar to those of dinitrophenol in that voltages across the epithelium (V- t), the basolateral membrane (V-bl) and the apical membrane (V-a) depolariz ed 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, bu t also the basolateral membrane. Furthermore, electrical coupling offers a balance between electroconductive entry of cations across the basolateral m embrane and extrusion across the apical membrane to support steady-state co nditions during transepithelial transport.