Adipokinetic hormone-induced influx of extracellular calcium into insect fat body cells is mediated through depletion of intracellular calcium stores.

Adipokinetic hormone I (AKH I) needs extracellular Ca2+ for its activating action on glycogen phosphorylase in locust fat body in vitro. TMB-8 reduces this AKH effect significantly, indicating that for a major part, hormone action also requires the mobilization of Ca2+ from intracellular stores. Using 45Ca2+, AKH was shown to stimulate both the influx and the efflux of Ca2+. Thapsigargin also enhances the influx of extracellular Ca2+ into the fat body cells, indicating that the stimulating effect of AKH on Ca2+ influx may be mediated through depletion of intracellular Ca2+ stores as well. AKH is known to enhance cAMP levels in locust fat body. We show that elevation of cAMP with forskolin or theophylline leads to activation of glycogen phosphorylase, both in the presence and in the absence of extracellular Ca2+. The present data are discussed in an attempt to elucidate further the mechanism underlying transduction of the hormonal signal in locust fat body.