D.J. Van der Horst

Insect flight muscle metabolism

The flight of an insect is of a very complicated and extremely energy-demanding nature. Wingbeat frequency may differ between various species but values up to 1000 Hz have been measured. Consequently metabolic activity may be very high during flight and the transition from rest to flight is accompanied by an increase of 50-100-fold in metabolic rate. Small mammals running at maximal speed and flying birds achieve metabolic rates exceeding resting levels by only 7-14-fold. The exaggerated metabolic rate during insect flight is not accompanied by an oxygen debt, which implies -apart from metabolic adaptations- ample availability of oxygen in the organs responsible for flight. Metabolic rate therefore can be estimated, apart from the depletion of fuel depots, by rates of oxygen consumption.

Effects of the adipokinetic hormone on the release and turnover of haemolymph diglycerides and on the formation of the diglyceride-transporting lipoprotein system during locust flight

Abstract When the release of the adipokinetic hormone in Locusta migratoria during flight is prevented by high concentrations of haemolymph trehalose or sucrose, neither the characteristic elevation of the haemolymph diglyceride concentration nor the formation of the flight specific diglyceride-carrying haemolymph lipoproteins is observed. Nevertheless, although the transport capacity is restricted to the lipoproteins present in the resting condition, the transport rate of diglycerides through the haemolymph is elevated considerably. Continuation of flying activity in such locusts injected with extracts of corpora cardiaca and flying under trehalose or sucrose loads, resulted in a rapid increase of both the content and the turnover of the haemolymph diglycerides. This was accompanied by the formation of the specific haemolymph lipoprotein system capable of accepting the elevated lipid. It is suggested that the primary effect of the increasing adipokinetic hormone titre at the onset of normal flight is to elicit the changes in association of haemolymph protein components already present in the resting locust to form the specific lipoproteins which allow both the enhanced concentration and transport of diglycerides required for optimum energy supply for the flight muscles. In addition, the hormone may have a moderatory function in controlling fat body lipolytic processes.

Role of Lipids in Energy Metabolism

Most reviews of lipid metabolism in insects have covered all classes of lipid compounds. Since lipids are generally defined as substances poorly soluble in water but soluble in organic solvents, the authors had to deal with compounds with divergent physiological functions, e.g., phospholipids and pheromones. The subject of this chapter limits the lipoidal substances to be discussed to those that provide a direct source of metabolic energy, though we recognize that other important lipid classes, for example those contributing to (sub)cellular components, are also essential for metabolism.

A dual function of alcohol dehydrogenase in Drosophila

Alcohol dehydrogenase (ADH) of Drosophila not only catalyzes the oxidation of ethanol to acetaldehyde, but additionally catalyzes the conversion of this highly toxic product into acetate. This mechanism is demonstrated by using three different methods. After electrophoresis the oxidation of acetaldehyde is shown in an NAD-dependent reaction revealing bands coinciding with the bands likewise produced by a conventional ADH staining procedure. In spectrophotometric measurements acetaldehyde is oxidized in an NAD-dependent reaction. This activity is effectively inhibited by pyrazole, as specific inhibitor of ADH. By means of gas chromatographic analysis a quick generation of acetate from ethanol could be demonstrated. Our conclusion is further supported by experimental results obtained with either purified ADHF enzyme or genotypes with or without ADH, aldehyde-oxidase, pyridoxal-oxidase and xanthine-dehydrogenase activity. These results are discussed in relation to ethanol tolerance in the living organism in particular with respect to differences found between ADH in Drosophila melanogaster and D. simulans, and in relation to the possible implications for the selective forces acting on ADH-polymorphism.

Insect adipokinetic hormones

Peptides with adipokinetic (and usually carbohydrate-mobilizing) potency have been demonstrated in various insects, including Locusta migratoria, Schistocerca gregaria, Manduca sexta, Danaus plexippus and Periplaneta americana. As far as characterized by now the adipokinetic factors are blocked peptides, consisting of eight to ten amino acid residues. In locusts the adipokinetic hormones are synthesized in the glandular lobe of the corpus cardiacum and released into the haemolymph in response to flight stimuli. This release is under direct control of neurons, the cell bodies of which are located in the lateral areas of the protocerebrum, while their axons run via the nervi corporis cardiaci II into the glandular lobe. Hormone release is modulated by axons present in the nervi corporis cardiaci I as well as by the haemolymph trehalose concentration. Trehalose apparently exerts its influence via a neuronal network present in the corpus cardiacum. The fat body is the main target organ of the adipokinetic hormones, which are involved in both mobilization and release of flight substrates from fat body stores, i.e., trehalose from glycogen and diacylglycerol from triacylglycerol. Lipid release is accompanied by haemolymph lipoprotein conversions.

Dynamics of energy substrates in the haemolymph of Locusta migratoria during flight

In the two-fuel system for flight of the migratory locust, the haemolymph carbohydrate concentration falls during flight periods of up to 1 hr, the decrease being greater in case the pre-flight carbohydrate level is higher. The increase in the lipid concentration from the onset of flight is virtually independent of the initial lipid concentration. Flight intensity affects these changes in substrate concentrations: the carbohydrate level decreases more rapidly if flight speed is higher, whereas the increase in lipid concentration is delayed at higher flight speeds. Respiratory carbon dioxide production is elevated rapidly during flight and reaches over eight times the resting level. From the rate of 14CO2 production after labelling of the haemolymph diglyceride pool it is concluded that diglycerides contribute to providing the energy for flight from the earliest stage of flying activity; diglyceride oxidation increases until maximum utilization is attained after some 45 min of flight. The decline in haemolymph carbohydrate concentration due to flying activity results in a decrease of haemolymph osmolarity. Free amino acids, particularly taurine, increase markedly in the haemolymph during flight; yet their concentration only partially counterbalances the fall in haemolymph osmolarity.

Turnover of locust haemolymph diglycerides during flight and rest

Abstract The constant, elevated diglyceride concentration in the haemolymph of adult male Locusta migratoria during sustained tethered flight (14.1 ± 1.2 mg/ml haemolymph) was established to be a steady state, in which the rate of diglyceride mobilization matched the rate of utilization. Pulse-labelling of the haemolymph diglycerides with [1- 14 C]-oleic acid in steady state flight conditions results in a turnover time of about 1 hr for the diglyceride pool, whereas in animals rested after flight values close to 8 hr were obtained. So, during flight there is a sharp increase in lipid transport, the initial turnover rate in the resting animal (0.3–0.5 mg diglyceride/hr) reaching 3.4 mg diglyceride/hr at the steady state flight level. As identical turnover rates were obtained using [1- 14 C]-palmitic acid or [1- 14 C]-linolenic acid, apparently there is no preferential utilization of diglycerides with a specific fatty acid composition in flight muscle energy metabolism. The characteristic diglyceride fatty acid spectrum at the steady state flight level remained remarkably constant during flight performances lasting for 6 hr. The above findings indicate homogeneity of the haemolymph diglyceride pool and suggest random utilization of fatty acids.

Dynamics in the haemolymph trehalose pool during flight of the locust, Locusta migratoria

In adult male Locusta changes in level and specific radioactivity of the haemolymph trehalose pool were studied during flight and rest after pulse-labelling with [U-14C]-trehalose. In the initial period of flight, consumption of haemolymph trehalose is high (about 7.3 mg/animal/hr), but there is little mobilization of trehalose from body carbohydrate stores. After 30 min of flight, however, steady state conditions in the haemolymph trehalose pool are reached in which the turnover rate of the (constant) pool amounts to 2.4 mg/animal/hr. At rest, turnover rate of the haemolymph trehalose pool is significantly lower (0.5 mg/animal/hr). It is concluded that during sustained tethered flight (when diglyceride is the predominant energy source) trehalose contributes substantially to flight muscle energy metabolism since 23% of the energy supply originates from carbohydrate oxidation.

Hormone-induced rearrangement of locust haemolymph lipoproteins The involvement of glycoprotein C2

Formation of lipoprotein A ÷ and elevation of lipoprotein fraction O in locust (Locusta migratoria migratorioides) haemolymph as induced by adipokinetic hormone (AKH) includes the participation of non-lipid carrying proteins (fraction C), which was examined in more detail. By using gel filtration chromatography, the rather heterogenous C-proteins were resolved into three protein fractions, only one of which (C2) appeared to be actually involved in the lipoprotein reassociation. The changes in amino acid composition of the elevated lipoprotein fractions as compared with those from the lipoproteins in the resting situation are accounted for by the contribution of the rather specific amino acid composition of this C2-fraction. Polyacrylamide gel electrophoresis (PAGE) indicates that the C2-protein is migrating as only one band; SDS-PAGE revealed that the C2-protein consists of one single polypeptide chain with an approximate molecular weight of 20,000. This chain is also recovered in the subunit structure of the lipoprotein fractions induced by AKH-injection (A ÷, OATH) in contrast with that of the lipoprotein fractions in resting haemolymph. Unlike the other C-proteins, protein C2 displayed immunoreactivity with antiserum raised against lipoprotein A +. From carbohydrate analyses, C 2 appeared to be a glycoprotein containing approx. 12.5% carbohydrate. In vivo pilot studies on the dynamics of C2-proteins using 3H-labelled glycoprotein C2 gave evidence for the incorporation of radiolabel into both A ÷ and OATH. Possible functions of the involvement of the glycoprotein to A ÷ formation are discussed. Key Word Index: Locusta migratoria migratorioides, lipoproteins, lipid transport, energy supply, adipokinetic hormone, glycoprotein

THE RECYCLING OF PROTEIN COMPONENTS OF THE FLIGHT-SPECIFIC LIPOPHORIN IN LOCUSTA MIGRATORIA

Abstract The metabolism of locust lipophorin A + during lipid delivery to the flight muscle and lipid loading at the fat body was studied in vitro . Protein C 2 was shown to be released upon hydrolysis of lipophorin A + -carried diacylglycerol by the flight muscle lipoprotein lipase. This in vitro released protein C 2 was shown to reassociate with lipophorin A y upon hormone-induced lipid mobilization from fat body in vitro . These results demonstrate the reversibility of the association of protein C 2 with lipophorin A y and support the shuttle function of the protein components of locust lipophorin A + in lipid transport.