LOCUST ADIPOKINETIC HORMONES - CARRIER-INDEPENDENT TRANSPORT AND DIFFERENTIAL INACTIVATION AT PHYSIOLOGICAL CONCENTRATIONS DURING REST AND FLIGHT

Since concomitant release of structurally related peptide hormones wit h apparently similar functions seems to be a general concept in endocr inology, we have studied the dynamics of the lifetime of the three kno wn adipokinetic hormones (AKHs) of the migratory locust, which control flight-directed mobilization of carbohydrate and lipid from fat body stores. Although the structure of the first member of the AKHs has bee n known for 20 years, until now, reliable data on their inactivation a nd removal from the hemolymph are lacking, because measurement require s AKHs with high specific radioactivity. Employing tritiated AKHs with high specific radioactivity, obtained by catalytic reduction with tri tium gas of the dehydroLeu(2) analogues of the AKHs synthesized by the solid-phase procedure, studies with physiological doses of as low as 1.0 pmol per locust could be conducted. The AKHs appear to be transpor ted in the hemolymph in their free forms and not associated with a car rier protein, despite their strong hydrophobicity. Application of AKHs in their free form in in vivo and in vitro studies therefore now has been justified. We have studied the degradation of the three AKHs duri ng rest and flight. The first cleavage step by an endopeptidase is cru cial, since the resulting degradation products lack any adipokinetic a ctivity. Half-lives for AKH-I, -II and -III were 51, 40, and 5 min, re spectively, for rest conditions and 35, 37, and 3 min, respectively, d uring flight, The rapid and differential degradation of structurally r elated hormones leads to changes in the ratio in which they are releas ed and therefore will have important consequences for concerted hormon e action at the level of the target organ or organs, suggesting that e ach of the known AKHs may play its own biological role in the overall syndrome of insect flight.