| The organic anion salicylate is a naturally occurring plant secondary metabolite that affects growth in some insect species. This study examines the cellular mechanism of salicylate transport by insect Malpighian tubules and gut. The analysis of salicylate uptake and transport across the renal epithelia was based on fluxes measured by a radioisotope tracer technique using [14C]salicylate and also by means of salicylate selective microelectrodes. My results show that salicylate uptake and transport across the Malpighian tubule epithelium of Drosophila melanogaster is active, Na+-dependent and inhibited by ring structure monocarboxylic acids. The maximum rate of transport (Jmax 1) and the half saturation concentration (K t) are 12.6 pmol tubule-1 min-1 and 30.7 mumol 1-1 for basolateral uptake, and 1.1 pmol tubule-1 min-1 and 50.0 mumol 1-1 for transepithelial transport, respectively. The transepithelial transport of salicylate is suppressed by ouabain, low K+ or K+-free saline. Hence, this transport system resembles that previously described for the prototypical organic anions p-aminohippuric acid (PAH) and fluorescein in D. melanogaster and other insect species, respectively. In addition, the transepithelial transport of salicylate by the Malpighian tubules increases in response to increases in fluid secretion rate produced by leucokinin I, cAMP or cGMP. Chronic exposure to dietary salicylate of larvae increases dramatically the fluid secretion rate and the transepithelial transport of salicylate by the Malpighian tubules. It is also demonstrated that the salicylate transport system, with variation in its physiological characteristics, is present in a wide range of insects from different orders, except in Rhodnius prolixus. The chromatographic analysis of the secreted fluid shows that in some species the Malpighian tubules can both excrete and metabolize salicylate. It is proposed that in some species that encounter high levels of salicylate or related compounds in the diet, this transport system might help to cope with the elimination of such toxic molecules from the haemolymph.;1 Abbreviations are listed in Appendix 1. |
