The major stimulant of ileal fluid reabsorption in Locusta migratoria and Schistocerca gregaria corpora cardiaca, ion-transport peptide (ITP), had no stimulatory action on fluid secretion by isolated Malpighian tubules

secretion by the Malpighian tubules of a KCl-rich primary urine, containing most small solutes found in the haemolymph, followed by selective reabsorption of essential substances in the hindgut as required for homeostasis. Secretion by Malpighian tubules is driven by a proton gradient (maintained by an apical V-type ATPase secreting protons into the lumen), which in turn drives a luminal K+/H+ antiporter (Zhang et al., 1994). Cl− follows passively, moving via a paracellular or transcellular shunt and a basal Na+/K+/Cl− cotransporter (Hegarty et al., 1991; Wang et al., 1996; O’Donnell et al., 1998). Fluid reabsorption in the anterior (ileum) and posterior (rectum) hindgut is largely driven by an apical electrogenic Cl− pump and is associated with passive movement of K+ as the major cation. Most of the primary urine is recovered in the hindgut, particularly under dehydrating conditions, and this recycling of fluid concentrates substances destined for excretion. Rapid progress has been made in isolating and sequencing the neuropeptides that control secretion by Malpighian tubules in a variety of insects (for a review, see Coast, 1996). These include corticotropin-releasing factor (CRF)related diuretic peptides and the insect kinin family of neuropeptides (Coast, 1995). CRF-related peptides are Cterminally amidated peptides 30–47 amino acid residues in length that act through cyclic AMP to stimulate the Na+/K+/Cl− cotransporter (Audsley et al., 1993) and to open basal Na+ channels (Clark et al., 1998). In contrast, kinins are small (6–15 amino acid residues) amidated peptides that are believed to stimulate Cl− secretion by opening a cellular or paracellular shunt (Pannabecker et al., 1993; O’Donnell et al., 1998) to produce an increase in the rate of Na+/KCl transport. These two families of diuretic peptides act synergistically in controlling primary urine production, permitting maximal stimulation at lower concentrations of hormone than would otherwise be required (Coast, 1995). The situation is less clear for neuropeptide stimulants of hindgut reabsorption (for a review, see Phillips et al., 1998). However, the major stimulant of ileal reabsorption from the brain and nervous corpus cardiacum (NCC), ion-transport peptide (ITP; a peptide 72 amino acid residues in length with an amidated C terminus) has been isolated, sequenced, synthesised and its biological actions on hindgut transport processes elucidated (Audsley et al., 1992a; Meredith et al., 1996; King et al., 1999). The ileum is functionally analogous to the proximal convoluted tubule of vertebrate kidneys: ITP stimulates iso-osmotic fluid reabsorption by fourfold in this locust segment by stimulating electrogenic Cl− transport and increasing K+ conductance (Audsley et al., 1992a). ITP also causes a submaximal stimulation (40 %) of rectal Cl− transport. 3195 The Journal of Experimental Biology 202, 3195–3203 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB2392