Characterization of salicylate uptake across the basolateral membrane of the Malpighian tubules of Drosophila melanogaster

The organic anion salicylate is a plant secondary metabolite that can protect plants against herbivores. Transport of salicylate across the basolateral membrane of the Malpighian tubules of Drosophila melanogaster was studied using a radioisotope tracer technique. The uptake of [14C]salicylate by the Malpighian tubules was active, saturable and Na+-dependent; the maximum uptake rate (Jmax) and the half saturation concentration (Kt) were 12.6 pmol tubule−1 min−1 and 30.7 μmol l−1, respectively. In contrast to organic anion transport by vertebrate renal tissues, salicylate uptake was not trans-stimulated by glutarate (0.01–1.0 mmol l−1) or cis-inhibited by high concentrations (5 mmol l−1) of various α-keto acids (glutaric acid, α-ketoglutaric acid, succinic acid, and citric acid). Changes in basolateral membrane potential or physiologically relevant changes in bathing saline pH did not affect the rate of [14C]salicylate uptake. Ring-structure monocarboxylic acids (benzoic acid, nicotinic acid, gentisic acid, unlabelled salicylic acid, α-cyano-4-hydroxycinnamic acid, probenecid, fluorescein, and P-aminohippuric acid) strongly inhibited [14C]salicylate uptake rate. In contrast, short-chain monocarboxylic acids had little (butyric acid) or no effect (lactic acid, pyruvic acid, and propionic acid). Our results suggest that salicylate uptake across the basolateral membrane of D. melanogaster Malpighian tubules is mediated by a non-electrogenic, α-cyano-4-hydroxycinnamic acid-sensitive, Na+:salicylate cotransport system.