DIFFERENTIAL INDUCTION OF INOSITOL PHOSPHATE-METABOLISM BY 3 ADIPOKINETIC HORMONES

Many (in)vertebrates simultaneously release several structurally and f unctionally related hormones; however, the relevance of this phenomeno n 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 coex ist. We now demonstrate disparities in the signal transduction of thes e hormones. Massive doses of the AKHs stimulated total inositol,phosph ate (InsP(n)) production in the fat body biphasicly, but time courses were different. Inhibition of phospholipase C (PLC) resulted in attenu ation of both InsP, synthesis and glycogen phosphorylase activation. T he AKHs evoked differential formation of individual [H-3]InsP(n) isome rs (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]I nsP(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. Th ese subtle differences in InsP(n) metabolism, together with earlier fi ndings on differences between the AKHs, support the hypothesis that ea ch AKH exerts specific biological functions in the overall syndrome of energy mobilization during flight. (C) 1997 Elsevier Science Ireland Ltd.