Mechanisms of ion transport during primary urine formation by the Malpighian tubule of Drosophila melanogaster were analyzed through measurements of fluid secretion rate, transepithelial ion flux, basolateral membrane

The excretory system of insects consists of the Malpighian tubules and hindgut. The tubules secrete fluid by generation of a transepithelial osmotic gradient, which results from the active transport of Na+, K+ and Cl– into the lumen (Phillips, 1981). This primary urine is modified in the lower (proximal) Malpighian tubule and/or the rectum by reabsorption of useful molecules and water. The physiology of ion transport by the Malpighian tubules has been extensively investigated in a number of insect species. Current models propose that cations are transported through the transcellular pathway, but several different transport pathways for anions have been described in tubules from different species. Anion transport may involve paracellular pathways or transcellular pathways, either through the same cell type as for cations or through a different cell type (Beyenbach, 2003; Ianowski et al., 2002; Linton and O’Donnell, 1999). Ion transport in tubules is driven primarily by a vacuolartype H+-ATPase, which generates a proton gradient across the apical membrane. This gradient, in turn, energizes apical amiloride-sensitive K+/H+ and/or Na+/H+ exchange, driving net movement of K+ and Na+ from cell to lumen and, in some species, generating a large positive transepithelial potential that drives passive transepithelial Cl– transport (Maddrell and O’Donnell, 1992). The basolateral membrane transport systems involved in fluid secretion by Malpighian tubules differ among species. Transport of both K+ and Cl– across the basolateral membrane during secretion of Na+-rich fluid by blood-feeding insects is driven by the Na+ electrochemical potential. K+, Cl– and Na+ ions are transported across the basolateral membrane through a Na+-driven Na+:K+:2Cl– cotransporter in Rhodnius prolixus (Ianowski and O’Donnell, 2001; Ianowski et al., 2002). KCl is subsequently reabsorbed in the lower tubule (Haley and O’Donnell, 1997). In the mosquito, Aedes aegypti, both Na+ channels and Na+:K+:2Cl– cotransport have been implicated in fluid secretion (Hegarty et al., 1991; Williams and Beyenbach, 1984). In contrast, Malpighian tubules in species that are not blood feeders, secrete K+-rich fluids. In the ant Formica polyctena, several basolateral ion transporters have been proposed, including K+ channels, a K+:Cl– cotransporter and a Na+:K+:2Cl– cotransporter (Leyssens et al., 1993b, 1994). K+ The Journal of Experimental Biology 207, 2599-2609 Published by The Company of Biologists 2004 doi:10.1242/jeb.01058