REGULATION OF FAT-BODY GLYCOGEN-PHOSPHORYLASE ACTIVITY DURING REFEEDING IN MANDUCA-SEXTA LARVAE

In 12-h-starved larvae of the tobacco hornworm, Manduca sexta, fat body glycogen phosphorylase was quickly inactivated when insects were refed with normal diet and agar which contained 3% sucrose. Only the first 2 min of refeeding were necessary to induce enzyme inactivation. During this short period, larvae did not ingest enough sucrose to increase the hemolymph glucose concentration. This may indicate that the gut released a hormone(s) which directly or indirectly led to the inactivation of fat body glycogen phosphorylase. Inactivation of the enzyme could also be induced by injection of glucose (30 mg) into the hemolymph of starving M. sexta larvae suggesting that there may be separate control from a neuroendocrine site such as the brain or the corpora cardiaca. Trehalose was less effective. Bovine insulin (2 and 4-mu-g/starved larva) did not induce phosphorylase inactivation over 20 min or decrease hemolymph carbohydrate or lipid concentrations within 60 min. It is, therefore, necessary to screen insect tissues for substances which could bring about inactivation of fat body glycogen phosphorylase.