Fat body fructose-2,6-bisphosphate content and phosphorylase activity correlate with changes in hemolymph glucose concentration during fasting and re-feeding in larval Manduca sexta

Fasting of second-day fifth instar larval Manduca sexta leads to a rapid de crease in hemolymph glucose concentration from 3.39+/-0.29 to 0.33+/-0.06 m M in 1 h, along with a decrease in the fructose-2,6-bisphosphate content in the fat body (from 5.92+/-0.31 to 2.80+/-0.47 nmol fructose-2,6-bisphospha te/g fat body in 3 h) and activation of fat body glycogen phosphorylase (fr om 16% to 55-65% phosphorylase a). During re-feeding an increase in the glu cose level in the hemolymph was observed (from 0.36+/-0.05 to 3.91+/-0.36 m M in 3 h), along with an increase in the fructose-2,6-bisphosphate level in the fat body (from 2.88+/-0.47 to 6.66+/-0.42 nmol fructose-2,6-bisphospha te/g fat body in 3 h) and inactivation of fat body glycogen phosphorylase ( from 56% to 16% phosphorylase a). These data are consistent with the hypoth esis that a decrease in hemolymph glucose both activates fat body glycogen phosphorylase and causes a decrease in fat body fructose-2,6-bisphosphate c ontent. Both of these changes would favor conversion of stored glucose to t rehalose in the fat body. When second-day larvae were decapitated, the chan ges in hemolymph glucose and fat body fructose-2,6-bisphosphate were very s imilar to those observed in fasting whole insects. These data are consisten t with a direct role for glucose in controlling carbohydrate metabolism in Manduca sexta. (C) 2001 Elsevier Science Ltd. All rights reserved.