THE CONCENTRATION-DEPENDENCE OF CRF-LIKE DIURETIC PEPTIDE - MECHANISMS OF ACTION

The mechanism of action of synthetic CCRF-DP, the corticotropin-releas ing factor (CRF)-related diuretic peptide of the salt marsh mosquito C ulex salinarius, was investigated in isolated Malpighian tubules of th e yellow fever mosquito Aedes aegypti, A low concentration of CCRF-DP (10(-9 )mol l(-1)) caused a small but insignificant increase in transe pithelial secretion of NaCl and fluid, but significantly reduced trans epithelial voltage and resistance without a change in short-circuit cu rrent, pointing to the stimulation of passive Cl- transport through th e paracellular pathway as the principal mechanism of a mild diuresis, Significant changes in voltage and resistance but not in short-circuit current were duplicated by the ionophore A23187 (0.4 mu mol l(-1)), s uggesting Ca2+ as a second messenger at 10(-9 )mol l(-1) CCRF-DP, A hi gh concentration of CCRF-DP (10(-7) mol l(-1)) significantly increased transepithelial secretion of NaCl and fluid and significantly increas ed short-circuit current, pointing to the stimulation of active Nat tr ansport through the transcellular pathway as the mechanism of a strong diuresis. This effect was mimicked by dibutyryl-cAMP, suggesting cAMP as a second messenger at 10(-7 )mol l(-1) CCRF-DP, Dibutyryl-cGMP had no effects. These results suggest dose-dependent, receptor-mediated e ffects of CCRF-DP that target discrete transport pathways via discrete second messengers: low concentrations of CCRF-DP cause a mild diuresi s, apparently via Ca2+-mediated effects on paracellular Cl- transport, and high concentrations cause a strong diuresis via cAMP-mediated eff ects on active transcellular Na+ transport in addition to the effects on the paracellular pathway.