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 activa ting 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 intra cellular stores. Using Ca-45(2+), AKH was shown to stimulate both the influx and the efflux of Ca2+. Thapsigargin also enhances the influx o f extracellular Ca2+ into the fat body cells, indicating that the stim ulating effect of AKH on Ca2+, influx may be mediated through depletio n of intracellular Ca2+ stores as well. AKH is known to enhance cAMP l evels in locust fat body. We show that elevation of cAMP with forskoli n 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 un derlying transduction of the hormonal signal in locust fat body.