Regulation of fat body glycogen phosphorylase in larval tobacco hornworm,Manduca sexta

Activation and inactivation of fat body glycogen phosphorylase was investigated in ligated abdomens of larval Manduca sexta and in vitro. After maximal activation through Manduca adipokinetic hormone (AKH) or chilling, inactivation of glycogen phosphorylase commenced as soon as the stimulus for the activation was removed indicating that the enzyme system in the fat body is fine-tuned to low phosphorylase activities which is necessary to allow glycogen synthesis. In intact ligated abdomens phosphorylase can be activated repeatedly by either stimulus showing that the fat body system does not lose its responsiveness. It was impossible to achieve complete conversion of the inactive form of phosphorylase into the active form even after administration of AKH and simultaneous chilling. © 1992 Wiley-Liss, Inc.