Cyclic AMP mediates the elevation of proline by AKH peptides in the cetoniid beetle, Pachnoda sinuata

The role of cyclic nucleotides in the transduction of the hyperprolinaemic and hypertrehalosaemic signal of the endogenous neuropeptide Mem-CC was investigated in the cetoniid beetle Pachnoda sinuata. Flight and injection of Mem-CC into the haemocoel of the beetle induce an increase of cAMP levels in the fat body of the beetle. This increase is tissue-specific and does not occur in brain and flight muscles. An elevation of cAMP levels was also found when in vitro preparations of fat body tissue were subjected to Mem-CC. Elevation of the cAMP concentration after injection of Mem-CC is time- and dose-dependent: the maximum response is measured after 1 min, and a dose of 25 pmol Mem-CC is needed. Injection of cpt-cAMP, a cAMP analogue which penetrates the cell membrane, causes a stimulation of proline synthesis but no mobilisation of carbohydrate reserves. The same is measured when IBMX, an inhibitor of phosphodiesterase, is injected. cGMP seems not to be involved in synthesis of proline nor carbohydrate release, because injection of cpt-cGMP has no influence on the levels of proline, alanine and carbohydrates in the haemolymph. Although glycogen phosphorylase of the fat body is activated by Mem-CC in a time- and dose-dependent manner, it cannot be stimulated by cpt-cAMP. The combined data suggest that cAMP is involved in regulation of proline levels by Mem-CC but not in regulation of carbohydrates. Octopamine has no effect on metabolites in the haemolymph and is not capable of activating glycogen phosphorylase, indicating that it is not involved in the regulation of substrates in this beetle. Furthermore, the requirements of the receptor of Mem-CC are different for eliciting a hypertrehalosaemic and a hyperprolinaemic effect, respectively, suggesting that differentiation in signal transduction begins at the receptor level.