Bafilomycin A 1 is a potent inhibitor of urine formation by Malpighian tubules of Drosophila hydei: Is a vacuolar-type ATPase involved in ion and fluid secretion?

Abstract

The effects of bafilomycin A 1, 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-Cl), N-ethylmaleimide (NEM), vanadate and ouabain on urine formation by isolated larval Malpighian tubules of Drosophila hydei were studied. Of these reagents, bafilomycin was the most effective inhibitor of the secretory activity of the tubules. When added to the bathing saline, the secretory rates of fluid, K + and Na + were rapidly decreased in a dose-dependent manner, at 1 μM bafilomycin to about 40–50%. NBD-Cl and NEM at concentrations of 10 and 50 times higher than bafilomycin, respectively, showed inhibitory effects comparable to bafilomycin, but their time course of inhibition was characterized by a progressive decline of secretion rates. When vanadate was added, the fluid secretion persisted for about 15 min and then decreased dose-dependently, at 1 mM vanadate to about 34%. In contrast to bafilomycin, which enhanced the Na + concentration of the secreted fluid slightly to a relatively stable value (at 1 μM to about 120%), both vanadate (1 mM) and ouabain (0.1 mM) increased the Na + concentration steadily to about 200 and 250%, respectively. Contrary to vanadate, the inhibitory effect of ouabain on fluid secretion within the experimental time (45 min) was relatively small. Considering the different effects of bafilomycin and vanadate on fluid and ion secretion a new working hypothesis for a transport model is proposed.