The role of calcium in the activation of glycogen phosphorylase in the fatbody of the fruit beetle, Pachnoda sinuata, by hypertrehalosaemic hormone

The role of calcium in the mediation of the hypertrehalosaemic signal of th e endogenous neuropeptide Mem-CC was investigated in vitro and in vivo in t he cetoniid beetle Pachnoda sinuata. The presence of Mem-CC increases the i nflux of extracellular Ca-45(2+) into the fat body as well as the efflux of Ca-45(2+) from pre-loaded fat body into the incubation medium. Extracellul ar calcium is essential to exert maximal activation of the fat body glycoge n phosphorylase by saturating doses of Mem-CC (0.3 nM). This effect of extr acellular Ca2+ is dose-dependent: maximal activation of glycogen phosphoryl ase by Mem-CC is achieved at calcium concentrations of approximately 1.2 mM and the ED50 was calculated to be 0.6 mM. Both, thimerosal and thapsigargi n caused a stimulation of carbohydrate metabolism in the fat body, suggesti ng that a release of calcium from the endoplasmic reticulum is involved in this process. However, neither entry of extracellular calcium nor the relea se from the endoplasmic reticulum are sufficient alone for a full activatio n of the phosphorylase. The results of the present study suggest that calci um from extracellular as well as from intracellular sources is part of the second messenger system for the transduction of the hypertrehalosaemic sign al of Mem-CC in the fat body of P. sinuata. (C) 2001 Elsevier Science B.V. All rights reserved.