THE PHARMACOLOGY OF THE LIPID-MOBILIZING RESPONSE TO ADIPOKINETIC HORMONE FAMILY PEPTIDES IN THE MOTH, MANDUCA-SEXTA

The biological activities of the nonapeptide Manduca adipokinetic hormone were investigated in the tobacco hornworm. In adult moths injection of Manduca adipokinetic hormone caused an increase in haemolymph lipid concentration, while in fifth-instar larvae the peptide activated fat body glycogen phosphorylase. The dose of peptide required to cause a 50% response was 0.3 pmol (equivalent to about 0.12 ng g-1) in the adult insect, and 0.07 pmol (about 0.008 ng g-1) in a fifth-stage larva. Thus larvae were more sensitive to the hormone than adult insects. The pharmacology of the adipokinetic response in adult Manduca was investigated using naturally occurring adipokinetic hormone family peptides. Of the 11 peptides assayed only six elicited significant lipid-mobilization at a dose of 50 pmol. Of these, prawn red pigment-concentrating hormone and Blaberus hypertrehalosaemic hormone were the least active and did not promote significant hyperlipaemia below this high dose. Locust adipokinetic hormone 1, Carausius hypertrehalosaemic factor 11 and cockroach myoactive peptide 11 were all active at low doses, but were partial agonists incapable of eliciting full adipokinetic responses at doses up to 50 pmol. Other than Manduca adipokinetic hormone, only the decapeptide Heliothis hypertrehalosaemic hormone was a full agonist. This peptide was as active as Manduca adipokinetic hormone itself. The adult Manduca fat body adipokinetic hormone receptor appears to have more specific structural requirements than the Orthopteran receptors studied previously. In addition to the standard adipokinetic hormone framework of pGlu1, Phe4, Trp8, and the C-terminal amide, the residues Leu2, Thr3, Ser6 and Gly9 all seem to be important for full adipokinetic activity in Manduca. Unlike the Orthopteran receptors, the Manduca receptor does not require the residue Pro6, preferring Ser6. A tentative model of the Manduca adipokinetic hormone receptor is proposed. A competitive adipokinetic assay showed that two adipokinetic hormone family peptides that are partial agonists (Carausius hypertrehalosaemic factor II and locust adipokinetic hormone 1) were capable of significantly reducing the lipid mobilizing response to coinjected Manduca adipokinetic hormone. This result shows that antagonists to insect peptide hormones arc possible, and suggest that in adult Manduca fat body a single receptor population may be responsible for mediating the adipokinetic response.