Absence of short-term regulation over gluconeogenesis by glucose in the insect Manduca sexta L.

In vivo gluconeogenesis from (3-13 C)alanine was evident in terminal instar Manduca sexta larvae from the selective fractional 13C enrichment in trehalose, a disaccharide of glucose and the major blood sugar of insects. De novo glucose synthesis was observed in insects fed a low carbohydrate diet for 1 or more days. Gluconeogenesis was not inhibited by a single injection of glucose nor by short-term feeding on glucose-supplemented diet. Reduced fractional 13C enrichment in trehalose was demonstrated upon glucose administration, but was explained by isotopic dilution following direct synthesis of trehalose from the unlabeled glucose. Isotopic dilution was also quantified by analysis of the 13C labeling pattern in trehalose synthesized following injection of (1,2-13C2)glucose. The results suggest the absence of short-term regulation over gluconeogenesis by glucose and may partially explain why blood sugar level in M. sexta and other insects fluctuates over a wide concentration range. Although glucose had no observable effects on gluconeogenesis, injection of or feeding glucose resulted in a significantly increased activity of the pentose phosphate pathway.