Apolipophorin-III and adipokinetic hormone in lipid metabolism of larval Manduca sexta

Abstract

The hemolymph lipid levels were measured and the density of lipophorin was determined during late larval development in Manduca sexta. During the feeding phase of the 4th and 5th instar larvae the lipid level in hemolymph remained largely unchanged at less than 2 mg/ml. During the molt from 4th to 5th instar, the hemolymph lipid level increased, but decreased after feeding restarted in the 5th instar. In wandering larvae and prepupae the hemolymph lipid level increased from about 2 to nearly 10 mg/ml. The density of lipophorin from feeding larvae was found to be 1.148 g/ml with minor amounts of lipophorin having a lower density of about 1.128 g/ml and sometimes a small amount with a density of 1.174 g/ml. In molting larvae, however, the density was clearly lower, 1.116 g/ml. In wandering larvae of all ages, two predominant forms of lipophorin were observed; the density of these forms was 1.132 g/ml and 1.177 g/ml. Rarely, one or three different forms of lipophorin were observed.

While the lipophorin of feeding larvae contains only apoLp-I and II (and lipids), the lipophorin of molting larvae contains in addition apoLp-III. ApoLp-III is seldom present in lipophorin from wandering larvae. According to our current models, lipophorin can take up only a certain amount of diacylglycerol before it needs apoLp-III for surface stabilization. Injection of 1 pmol of M. sexta AKH into feeding larvae increased the hemolymph lipid level, decreased the density of lipophorin to 1.125 g/ml and resulted in the association of apoLp-III with lipophorin. Cardiacectomy did not prevent feeding larvae from developing to wandering larvae. The major lipophorin of cardiacectomized larvae of the second day of wandering, however, had a density of 1.171 g/ml and the minor lipophorin had a density of 1.130 g/ml, which was unlike the situation in untreated or sham operated wandering larvae. These results demonstrate for the first time that in larvae AKH not only controls the activation of fat body glycogen phosphorylase, as reported earlier, but also the mobilization of lipids. Our results also suggest that apoLp-III has a function in larval lipid metabolism.