Hormonal regulation of fat body glycogen phosphorylase activity in larval Manduca sexta during starvation

The hormonal regulation of fat body glycogen phosphorylase activity in Manduca sexta larvae was studied. During the first 3 hr of starvation the corpora cardiaca (CC) release a glycogen phosphorylase-activating hormone (GPAH). The haemolymph of 24-hr-starved larvae seems to contain increased levels of GPAH, but after 48 hr the titre can be assumed to be as low as prior to starvation. Abdominal stretch receptors do not appear to be involved in the regulation of GPAH release from the CC. Phosphorylase activation can be prevented by the injection of glucose or by feeding the animals with agar containing various carbohydrates. These treatments seem to prevent the release of GPAH from the CC rather than the action of GPAH on the fat body. The physiological signal which initiates peptide release remains unclear.