Differential induction of inositol phosphate metabolism by three adipokinetic hormones

Many (in)vertebrates simultaneously release several structurally and functionally related hormones; however, the relevance of this phenomenon is poorly understood. In the locust e.g. each of three adipokinetic hormones (AKHs) is capable of controlling mobilization of carbohydrate and lipid from fat body stores, but it is unclear why three AKHs coexist. We now demonstrate disparities in the signal transduction of these hormones. Massive doses of the AKHs stimulated total inositol,phosphate (InsP(n)) production in the fat body biphasicly, but time courses were different. Inhibition of phospholipase C (PLC) resulted in attenuation of both InsP, synthesis and glycogen phosphorylase activation. The AKHs evoked differential formation of individual [H-3]InsP(n) isomers (InsP(1-6)), the effect being most pronounced for InsP(3). 40 nM of AKH-I and -III induced a substantial rise in total InsP(n) and [H-3]InsP(3) at short incubations, whereas the AKH-II effect was negligible. At a more physiological dose of 4 nM, the AKHs equally enhanced Ins(1,4,5)P-3 levels. The InsP(3) effect was most prolonged for AKH-III. These subtle differences in InsP(n) metabolism, together with earlier findings on differences between the AKHs, support the hypothesis that each AKH exerts specific biological functions in the overall syndrome of energy mobilization during flight. (C) 1997 Elsevier Science Ireland Ltd.