Characterization of transepithelial transport of salicylate by the Malpighian tubules of Drosophila melanogaster and the effects of changes in fluid secretion rate

Abstract.  A radioisotope tracer technique is used to study mechanisms and regulation of transepithelial transport of the plant allelochemical salicylate by the Malpighian tubules of Drosophila melanogaster . Transepithelial transport of salicylate is nearly abolished in Na⁺-free saline, and inhibited by ouabain, low K⁺ or K⁺-free bathing saline. In addition, the carboxylates probenecid, unlabelled salicylate, fluorescein, and p -aminohippuric acid (PAH) significantly inhibit transepithelial transport of salicylate. The sulphonates taurocholate and phenol red also inhibit transepithelial transport of salicylate, whereas amaranth has no effect. Stimulation of fluid secretion by cAMP, cGMP or leucokinin I increases transepithelial transport of salicylate, particularly when the concentration of salicylate in the bathing saline is high. The correlation between the fluid secretion rate and transepithelial transport of salicylate shows that 64% of the changes in salicylate transport can be explained on the basis of changes in fluid secretion rate. The results show that naturally-occurring plant secondary metabolite salicylate is transported into the lumen of the Mapighian tubules of D. melanogaster by a mechanism similar to that previously described for the prototypical organic anions PAH and fluorescein. In addition, the transepithelial transport of salicylate increases in response to increases in fluid secretion rate.