Summary

Forskolin stimulates rapid fluid secretion by the Malpighian tubules of Rhodnius prolixus at concentrations above 5x10–6 mol l-1. In the presence of a threshold concentration of forskolin, the tubules are 30–50 times more sensitive to 5-hydroxytryptamine (5-HT) than in its absence. Similar synergism is seen between 5-HT and extracts of the mesothoracic ganglionic mass (which is rich in the peptide diuretic hormone, DH) and between 5-HT and samples of haemolymph, also rich in peptide DH, from fed insects 1–2 h after feeding. The dose-response curves for mixtures of forskolin and 5-HT and of peptide DH and 5-HT are all very steep, approximately five times steeper than for any one stimulant alone. Forskolin, 5-HT and extracts of the ganglionic mass all stimulated adenylate cyclase from broken membrane preparations from the Malpighian tubules in a dose-dependent manner and at doses similar to those required to stimulate fluid secretion by intact tubules. Mixtures of ganglionic extract and 5- HT stimulated adenylate cyclase activity in a synergistic fashion. Injections into fifth-instar Rhodnius, 24 h before feeding, of 5,7-dihydroxytryptamine, which is known to block or reduce 5-HT release, caused delays in the onset of the consequent diuresis or prevented it altogether. This is consistent with the proposal that the rapid onset of diuresis after feeding is caused by the simultaneous release of 5-HT and peptide DH acting synergistically.