Crosstalk between signal transduction pathways provides a complex intracellular avenue for fine tuning of hormone-induced signals. Over the last few years, we have studied the signaling mechanisms of three locust adipokinetic hormones (AKHs), which control mobilization of energy reserves from insect fat body as fuels for flight and transduce their signals via adenylyl cyclase-and phospholipase C- (PLC) dependent pathways. In this study, we examine possible crosstalk between these signaling routes. We show that cAMP does not affect basal and AKH-stimulated inositol phosphate (InsP(n)) production. Incubation of fat body with aluminium fluoride, an activator of G proteins, increased InsP(n) levels by 77%, whereas cholera toxin and pertussis toxin were ineffective. This implies that fat body PLC is not activated by Gβγ, but possibly by G(q)α. The involvement of this G protein in AKH signaling was demonstrated by our observation that the GPAntagonist-2A, which antagonizes G(q), attenuated glycogen phosphorylase activation by AKH-I. As plasma membrane Ca2+ channels may constitute another target for cAMP-mediated modulation, we studied the type of channels involved in AKH signaling using a variety of L-, N- and T-type Ca2+ channel inhibitors. None of these blocked AKH-induced glycogen phosphorylase activation, suggesting that voltage-dependent Ca2+ channels do not mediate AKH-induced Ca2+ influx.
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