Signal transduction in isolated fat body from the cockroach Blaptica dubia exposed to hypertrehalosaemic neuropeptide

Hypertrehalosaemic hormones stimulate trehalogenesis while inhibiting glycolysis in cockroach fat body. Signal transduction of the hypertrehalosaemic peptide Bld HrTH was examined in isolated fat body of the Argentine cockroach Blaptica dubia with respect to its effects on the increase in trehalose production and decrease in the content of the glycolytic activator fructose 2,6-bisphosphate in the tissue. Cyclic AMP does not seem to be involved in these processes as the cAMP analogue cpt-cAMP and the phosphodiesterase inhibitor IBMX, which both permeate cell membranes had no effect on either parameter. Octopamine at physiological concentrations (10(-7) mol.l-1) was also ineffective but at 10(-5) mol.l-1 or above, octopamine stimulated trehalose production although the content of fructose 2,6-bisphosphate in fat body was not affected. Both calcium entry and the release of Ca2+ from intracellular stores seem to be involved in the action of the hormone. If Ca2+ was omitted from the incubation medium, the hormone stimulated trehalose production less, though still significantly, whereas the hormone effect on fructose 2.6-bisphosphate was completely abolished in the absence of extracellular Ca2+. With Ca2+ present in the medium, the effect of the hormone on fructose 2.6-bisphosphate could be fully mimicked by the calcium ionophore A23187, suggesting that calcium entry is a decisive step in this signalling pathway. Trehalose production, on the other hand, was increased by thimerosal and thapsigargin which increase cytosolic Ca2+ from intracellular stores, whereas thimerosal in the absence of extracellular Ca2+ increased rather than decreased the content of fructose 2,6-bisphosphate, thus dissociating the two effects, which are normally coordinated by the hormone. Trehalose production and the content of fructose 2,6-bisphosphate were not significantly affected by mepacrine and mellitin, which are known to inhibit, respectively stimulate, phospholipase A2. Our data suggest that the effects of Bld HrTH on the stimulation of trehalose production and reduction of fructose 2,6-bisphosphate content in fat body are mediated by Ca2+, but that different signalling pathways are involved, suggesting that the two processes, although they are functionally linked, could be regulated separately.