Regulation of fat body glycogen phosphorylase activity in adult Manduca sexta

Fat body glycogen phosphorylase was activated very slowly when adult tobacco hornworms, Manduca sexta, were starved. By contrast, chilling led to a significant activation of the enzyme within 2 h. This is a direct effect, since enzyme activation can also be induced in vitro. Inactivation of glycogen phosphorylase occured very rapidly in intact animals and in vitro showing that the fine-tuning of the glycogen breakdown/synthesis enzyme system favours glycogen synthesis in these animals. Activation of fat body glycogen phosphorylase in adult M. sexta at 0°C in vitro could be prevented by supplementing the saline with glucose; trehalose was ineffective. This effect of glucose was dose-dependent between 2 and 6mg/ml. The same action could not, however, be detected in larval fat body. When adult fat body was incubated at 0°C for up to 4h, no progressive activation of glycogen phosphorylase was observed in the absence of glucose suggesting that glucose is not the main regulator of enzyme activity. Glucose, however, may be a potent modulator of the glycogen phosphorylase/glycogen synthase system.