Signal transduction of adipokinetic hormones involves Ca 2+ fluxes and depends on extracellular Ca 2+ to potentiate cAMP-induced activation of glycogen phosphorylase

Abstract

Adipokinetic hormone (AKH)-induced mobilization of insect fat body glycogen occurs through activation of glycogen phosphorylase. In the migratory locust, signal transduction of AKH-I, -II and -III has been shown to involve the formation of cAMP. In the present study, we show that both the elevation of fat body CAMP levels and the activation of phosphorylase by the three AKHs in vitro depend on the presence of extracellular Ca 2+; in the absence of Ca 2+ in the medium, no phosphorylase activation occurs, whereas a concentration of at least 1.5 mM Ca 2+ in the medium is required for maximal activation by each of the hormones. Furthermore, we show that AKH-I, -II and -III increase the influx of extracellular calcium into the fat body, as well as the efflux of cytosolic calcium from the fat body into the medium within 1 min of incubation. Although the time courses of their effects and the maximal responses to massive doses (40 nM) of the three hormones do not differ, AKH-III induces the highest increase in Ca 2+ efflux when applied in a physiological dose (4 nM). No difference in the levels of Ca 2+ influx induced by 4 nM of the hormones was observed. Quantitative analysis of the data suggests that the AKH-induced influx is larger than the efflux, implying a net rise in the fat body Ca 2+ concentration.