The fluid secreted by the upper segment of Malpighian tubules of Rhodnius prolixus during diuresis

tubules of Rhodnius prolixus during diuresis consists of approximately 100 mmol l–1 NaCl and 80 mmol l–1 KCl (Maddrell and Phillips, 1975). Secretion of ions and osmotically obliged water by tubules of this and other species is driven primarily by an apical vacuolar-type H+-ATPase (Wieczorek et al., 1989; Maddrell and O’Donnell, 1992). It is suggested that electrogenic transport of H+ from the cell to the lumen energizes amiloride-sensitive exchange of cytoplasmic K+ and/or Na+ for luminal H+. Entry of Na+, K+ and Cl– through a basolateral Na+-K+-2Cl– cotransporter has been proposed for tubules of Rhodnius prolixus on the basis of the effects of bumetanide, Na+-free saline and Cl–-free saline on fluid secretion and transepithelial potential (O’Donnell and Maddrell, 1984; Ianowski and O’Donnell, 2001). Na+-K+-2Cl– cotransport has also been implicated in basolateral entry of ions into Malpighian tubules of other species, including Aedes aegypti (Hegarty et al., 1991), Formica polyctena (Leyssens et al., 1994), Manduca sexta (Audsley et al., 1993; Reagan, 1995), Teleogryllus oceanicus (Xu and Marshall, 1999) and Locusta migratoria (Al-Fifi et al., 1998). In vertebrates, the cation-Cl– cotransport superfamily includes two Na+-K+-Cl– (NKCC) isoforms, one Na+-Cl– (NCC) isoform (bumetanide-insensitive, K+-independent) and four K+-Cl– (KCC) isoforms. The Na+-K+-Cl– cotransporter is an electroneutral transporter with a stoichiometry of 1Na+:1K+:2Cl– in the overwhelming majority of cases (Haas and Forbush, 2000). Nevertheless, a cotransporter with a stoichiometry of 2Na+:1K+:3Cl– has been described (Russell, 1983). The direction of net ion transport by the Na+-K+-2Cl– cotransporter may be into or out of the cell depending on the sum of the chemical potential gradients of the transported ions, and the transporter can be inhibited by loop diuretics such as furosemide or bumetanide (Haas and Forbush, 2000). Several alternative routes for Cl– and/or K+ entry have been proposed in tubules of other insects, including a K+-Cl– cotransporter in Drosophila melanogaster (Linton and O’Donnell, 1999), Ba2+-sensitive K+ channels in Formica 1645 The Journal of Experimental Biology 205, 1645–1655 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4060