Antagonistic control of fluid secretion

by diuretic hormones. The need for diuretic hormones in blood feeders such as Rhodnius prolixus is obvious: however, diuretic activity is also present in the corpora cardiaca of two tenebrionid beetles inhabiting arid environments, the desert beetle Onymacris plana and the mealworm Tenebrio molitor (Nicolson and Hanrahan, 1986; Nicolson, 1992). In vivo experiments indicate that the fluid secreted by stimulated Malpighian tubules of Onymacris plana is directed to the midgut for eventual recycling to the haemolymph (Nicolson, 1991). On the basis of these findings, Nicolson (1991) suggested that such diuretic hormones act as ‘clearance hormones’, because they elicit rapid clearance of metabolic waste from the haemolymph without loss of precious water. Insect diuretic hormones include serotonin (Barrett and Orchard, 1990; Maddrell et al., 1991) and diuretic peptides of at least three groups: the corticotropin-releasing-factor (CRF)related peptides, the smaller kinins (Coast, 1996) and the more recently discovered calcitonin-related peptides (Furuya et al., 2000b; Coast et al., 2001). The 13 known CRF-related peptides (Baldwin et al., 2001) share various degrees of homology with vertebrate CRF and appear to act through the second messenger cyclic AMP to increase the rate of cation transport (Audsley et al., 1993; Beyenbach, 1995; O’Donnell et al., 1996). They are the only family of diuretic peptides for which unequivocal evidence is available that they serve a hormonal function (Patel et al., 1995). Insect kinins make up the second major family of diuretic peptides. Kinins were initially isolated on the basis of their myotropic activity on the hindgut of the cockroach Leucophaea maderae and range in length from six to 13 amino acid residues (Coast, 1996). They have a highly conserved COOH-terminal pentapeptide sequence and appear to act through an increase in intracellular Ca2+ concentration, which increases the anion permeability of the Malpighian tubules (O’Donnell et al., 1998). Only two calcitonin-like peptides have been fully identified to date; their signaltransduction pathway appears to be complex, largely involving elevation of intracellular Ca2+ concentrations in Locusta 493 The Journal of Experimental Biology 205, 493–501 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB3733