CALCIUM-DEPENDENT ACTION OF HYPERTREHALOSEMIC HORMONE ON ACTIVATION OF GLYCOGEN-PHOSPHORYLASE IN COCKROACH FAT-BODY

Signaling mechanisms for Blaberus discoidalis hypertrehalosemic hormon e (Bld-HrTH)-dependent glycogen phosphorylase activation were investig ated in vitro using fat body of the tropical cockroach, B. discoidalis . Brief treatment of fat bodies with Bld-HrTH in the absence of extrac ellular Ca2+ showed a significant activation of phosphorylase. Althoug h extracellular Ca2+ was required for a full activation of phosphoryla se by Bld-HrTH, stimulation in the absence of extracellular Ca2+ sugge sted that intracellular Ca2+ was also involved in Bld-HrTH signal tran sduction. Thapsigargin and thimerosal mobilize Ca2+ from intracellular stores by different mechanisms, and both chemicals stimulated phospho rylase activities as effectively as a maximum dose of Bld-HrTH. Bld-Hr TH likely induces intracellular Ca2+ release followed by extracellular Ca2+ entry across the plasma membrane. Inositol-1,4,5-trisphosphate ( InsP(3)) levels were greatly increased by Bld-HrTH in a time- and dose -dependent manner, suggesting that InsP(3) might be a Ca2+-mobilizing intracellular second messenger for Bld-HrTH.