Developmental changes in the response of larval Manduca sexta fat body glycogen phosphorylase to starvation, stress and octopamine

Fasting or starvation of 1(st)- and 2(nd)-day fifth instar Manduca sexta la rvae leads to rapid activation of fat body glycogen phosphorylase. Under fe eding conditions, 21-29% of the phosphorylase was found in the active form. However, after only one hour of starvation, the active form increased to 5 5-65%. In larvae on the 3(rd)-day there was a slower increase in the activa tion, requiring three hours of starvation to reach a maximum of 60-65%. No activation was observed in 4(th)-day larvae after three hours of starvation . When 1(st)- or 2(nd)-day larvae were decapitated, the time-course of acti vation of glycogen phosphorylase was very similar to that observed in intac t insects. However, activation of glycogen phosphorylase following decapita tion was only observed in 1(st)- and 2(nd)-day larvae. In 2(nd)-day larvae, octopamine promoted activation of glycogen phosphorylase and 100-pmol of o ctopamine promoted maximum activation. Higher amounts of injected octopamin e caused a decrease in activation. The injection of 100 pmol of octopamine caused a 50-55% activation of phosphorylase within 30 minutes. The simultan eous injection of the alpha-adrenergic receptor antagonist phentolamine wit h octopamine blocked the octopamine effect in 1(st)- and 2(nd)-day feeding larvae. However, the activation of glycogen phosphorylase observed in ligat ed/decapitated larvae on the 1(st)- and 2(nd)-day was not abolished by inje ction of phentolamine. All of these data suggest that factors other than ad ipokinetic hormone and octopamine may be involved in the activation of glyc ogen phosphorylase during fasting or starvation in the early part of the fi fth larval stage of M. sexta. (C) 2000 Elsevier Science Ltd. All rights res erved.