Tetraethylammonium and nicotine transport by the Malpighian tubules of insects
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We examined transepithelial transport of the prototypical type I organic cation (OC) tetraethylammonium (TEA) and the plant alkaloid nicotine by the isolated Malpighian tubules (MTs) of nine insect species from six orders. Isolated tubules were exposed to radiolabelled forms of either TEA or nicotine in the bathing (basal) fluid. Luminal (apical) secreted fluid was collected and TEA or nicotine concentration was determined. Active net transport of nicotine from bath to lumen was observed by the MTs of all the insects studied. TEA was also transported from bath to lumen in MTs of all species except Rhodnius prolixus and Aedes aegypti. MTs of both of these blood feeders did not show active transport of TEA under normal physiological conditions. Transport of TEA but not nicotine increased during the moult in the MTs of Rhodnius, but the concentrations of TEA in the secreted fluid were still consistent with passive accumulation in response to the lumen-negative transepithelial potential. Nicotine transport by Rhodnius MTs was inhibited by the type II OC quinidine, a known p-glycoprotein inhibitor, but not by the type I OCs N-methylnicotinamide or cimetidine. Taken together, the results suggest that active transport of OCs by the MTs is common among species from different orders and that transepithelial TEA and nicotine transport occur through separate pathways.
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