The Concentration-Dependence of Crf-Like Diuretic Peptide: Mechanisms of Action Available to Purchase

The mechanism of action of synthetic CCRF-DP, the corticotropin-releasing factor (CRF)-related diuretic peptide of the salt marsh mosquito Culex salinarius, was investigated in isolated Malpighian tubules of the yellow fever mosquito Aedes aegypti. A low concentration of CCRF-DP (10⁻⁹ mol l⁻¹) caused a small but insignificant increase in transepithelial secretion of NaCl and fluid, but significantly reduced transepithelial voltage and resistance without a change in short-circuit current, pointing to the stimulation of passive Cl⁻ transport through the 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 μmol l⁻¹), suggesting Ca²⁺ as a second messenger at 10⁻⁹ mol l⁻¹ CCRF-DP. A high concentration of CCRF-DP (10⁻⁷ mol l⁻¹) significantly increased transepithelial secretion of NaCl and fluid and significantly increased short-circuit current, pointing to the stimulation of active Na⁺ transport 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 effects of CCRF-DP that target discrete transport pathways via discrete second messengers: low concentrations of CCRF-DP cause a mild diuresis, apparently via Ca²⁺-mediated effects on paracellular Cl⁻ transport, and high concentrations cause a strong diuresis via cAMP-mediated effects on active transcellular Na⁺ transport in addition to the effects on the paracellular pathway.