An evaluation of the role of cyclic AMP as an intracellular second messenger in Malpighian tubules of the house cricket, Acheta domesticus

Exogenous cAMP, cAMP analogues and forskolin all stimulated fluid secretion by the isolated Malpighian tubules of Acheta domesticus . IBMX potentiated the effects of 8-bromo cAMP and forskolin, and the diuretic activity of extracts of corpora cardiaca. The effects of forskolin and corpora cardiaca on cAMP production were determined on isolated tubules both in the presence and absence of IBMX. Such treatments increased intracellular levels of cAMP, with this response paralleling or preceding the increase in fluid secretion. Fluid secretion increased with intracellular cAMP concentrations up to about 8 μM, but further increases in cAMP were not associated with any further increases in fluid secretion. The adenylyl cyclase activity of Malpighian tubule plasma membrane preparations was investigated and shown to be stimulated in a dose-dependent manner by extracts of whole heads. Results are discussed in relation to the role of cAMP as a second messenger in the Malpighian tubules of Acheta , and consideration is given to the establishment of a convenient assay for diuretic peptides which stimulates adenylyl cyclase activity.