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Insect diuretic peptides differ chemically and, even within a single species, several forms of diuretic peptide may be present (Wheeler and Coast, 1989). Thus, in Locusta migratoria, diuretic peptides have been described as AVP-like, ACTH-like, and as novel peptides. Only two insect diuretic peptides have been fully characterised, one from Locusta (Proux et al., 1987), and the other from Manduca sexta (Kataoka et al., 1989). They have no structural analogy to each other, but each has similarities with some vertebrate peptides. Fractionation of aqueous extracts of the corpora cardiaca of Acheta domesticus yields a number of peptides which stimulate fluid secretion by isolated Malpighian tubules (Coast and Wheeler, 1989). One peptide, Acheta peptide-I (AP-I), is associated with a major UV absorbing peak, and was selected for sequence analysis.

In locusts, adipokinetic hormones have well defined actions: they mobilise diacylglycerols from the fat body and stimulate oxidation of lipids by the flight muscles (Goldsworthy and Wheeler, 1989). Their actions presumably depend on interactions with specific receptors in the tissues, but there have been no direct studies of AKH receptors. Some information about receptors can, however, be inferred from biological assay data. For most actions, AKH-I and to a lesser extent AKH-II are both effective agonists, but AKH-II stimulates cAMP accumulation in the fat body more than AKH-I (Goldsworthy et al., 1986), and AKH-II releases carbohydrate from the fat body, whereas AKH-I does not (Loughton and Orchard, 1981). These differences in actions suggest that AKH-I and -II may act via different receptors at the fat body, although their high degree of cross reactivity in other assays indicates that they may also stimulate the same receptors. The availability of synthetic naturally occurring analogues of adipokinetic peptides has prompted an examination of their structure/activity relationships.

The structures of 3 locust adipokinetic hormones are known: the decapeptide adipokinetic hormone I is common to all species of locust investigated so far, but the octapeptide AKH-II’s of Locusta (AKH-IIL) and Schistocerca (AKH-IIS) differ by a single amino acid (see Table 1). In addition to these locust peptides, structurally related but often functionally different peptides from other insect species have been sequenced. These include the adipokinetic peptide from Heliothis zea (Jaffe et al., 1986), which is identical to that in Manduca sexta (Ziegler et al., 1985); hypertrehalosaemic peptides I and II from Periplaneta americana (sequenced independently by various groups; see Goldsworthy et al., 1986a); and hypertrehalosaemic factor II from Carausius morosus (Gade and Rinehart, 1986). Together with a crustacean peptide, Red Pigment Concentrating Hormone, these peptides apparently represent a group of closely related molecules, the AKH/RPCH family: they possess striking sequence and structural similarities, including blocked amino and carboxy terminals. We have compared the activities of these peptides in the locust hyperlipaemic assay to give an insight into the structural features necessary for the fit of the AKH molecule to its receptor on the fat body.

Adipokinetic hormones (AKH) I and II are released into the haemolymph during long-term flight in locusts and have several well defined sites of action: they increase the rate of release of diacylglycerols from the fat body and stimulate the uptake and utilisation of these lipids in the flight muscles (see Goldsworthy, 1983)• Adipokinetic hormones also promote indirect effects such as haemolymph lipoprotein transformations (Goldsworthy et al., 1985). Lipids released from the fat body are transported as part of lipoproteins; in resting locusts the majority of haemolymph diacylglycerol is carried as part of lipoprotein Ayellow, but after hormone injection Ayellow becomes lipid-loaded and associates with non-lipid carrying apoproteins to become lipoprotein A+, which carries 10 to 15X as much lipid as Ayellow (Mwangi and Goldsworthy, 1977; Van Der Horst et al., 1981a,b; Wheeler and Goldsworthy, 1983a,b).