SIGNAL-TRANSDUCTION OF ADIPOKINETIC HORMONES INVOLVES CA2-INDUCED ACTIVATION OF GLYCOGEN-PHOSPHORYLASE( FLUXES AND DEPENDS ON EXTRACELLULARCA2+ TO POTENTIATE CAMP)

Adipokinetic hormone (AKH)-induced mobilization of insect fat body gly cogen occurs through activation of glycogen phosphorylase. In the migr atory locust, signal transduction of AKH-I, -II and -III has been show n 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 phos phorylase by the three AKHs in vitro depend on the presence of extrace llular Ca2+; in the absence of Ca2+ in the medium, no phosphorylase ac tivation occurs, whereas a concentration of at least 1.5 mM Ca2+ in th e medium is required for maximal activation by each of the hormones. F urthermore, we show that AKH-I, -II and -III increase the influx of ex tracellular calcium into the fat body, as well as the efflux of cytoso lic calcium from the fat body into the medium within 1 min of incubati on. Although the time courses of their effects and the maximal respons es to massive doses (40 nM) of the three hormones do not differ, AKH-I II induces the highest increase in Ca2+ efflux when applied in a physi ological dose (4 nM). No difference in the levels of Ca2+ influx induc ed by 4 nM of the hormones was observed. Quantitative analysis of the data suggests that the AKH-induced influx is larger than the efflux, i mplying a net rise in the fat body Ca2+ concentration.